Recent structural evolution of the Cumbre Vieja volcano, La Palma

Transcription

Journal of Volcanology and Geothermal Research 94 Ž1999. 135–167
www.elsevier.comrlocaterjvolgeores
Recent structural evolution of the Cumbre Vieja volcano, La
Palma, Canary Islands: volcanic rift zone reconfiguration as a
precursor to volcano flank instability?
S.J. Day
a
a,b,)
, J.C. Carracedo
b,c
, H. Guillou d , P. Gravestock
b
Benfield Greig Hazard Research Centre, UniÕersity College London, Gower Street, London WC1E 6BT, UK
b
Department of Geography and Geology, Cheltenham and Gloucester College of Higher Education, UK
c
Estacion
de Canarias, CSIC, La Laguna, Tenerife, Spain
´ Volcanologica
´
d
Centre des Faibles RadioactiÕites,
´ CEA-CNRS, France
Received 10 May 1999
Abstract
The Cumbre Vieja volcano is the youngest component of the island of La Palma. It is a very steep-sided oceanic island
volcano, of a type which may undergo large-scale lateral collapse with little precursory deformation. Reconfiguration of the
volcanic rift zones and underlying dyke swarms of the volcano is used to determine the present degree of instability of the
volcano. For most of its history, from before 125 ka ago to around 20 ka, the Cumbre Vieja volcano was characterised by a
triple Ž‘‘Mercedes Star’’. volcanic rift zone geometry. The three rift zones were unequally developed, with a highly
productive south rift zone and weaker NE and NW rift zones: the disparity in activity was probably due to topographicgravitational stresses associated with the west facing Cumbre Nueva collapse structure underneath the western flank of the
Cumbre Vieja. From 20 ka to about 7 ka, activity on the NW volcanic rift zone diminished and the intersection of the rift
zones migrated slightly to the north. More recently, the triple rift geometry has been replaced at the surface by a
N–S-trending rift zone which transects the volcano, and by E–W-trending en echelon fissure arrays on the western flank of
the volcano. The NE rift zone has become completely inactive. This structural reconfiguration indicates weakening of the
western flank of the volcano. The most recent eruption near the summit of the Cumbre Vieja, that of 1949, was accompanied
by development of a west facing normal fault system along the crest of the volcano. The geometry of this fault system and
the timing of its formation in relation to episodes of vent opening during the eruption indicate that it is not the surface
expression of a dyke. Instead, it is interpreted as being the first surface rupture along a developing zone of deformation and
seaward movement within the western flank of the Cumbre Vieja: the volcano is therefore considered to be at an incipient
stage of flank instability. Climatic factors or strain weakening along the Cumbre Nueva collapse structure may account for
the recent development of this instability. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Cumbre Vieja volcano; volcanic rift zones; volcanic vents
)
Corresponding author. Tel.: q44-171-504-2212; fax: q44-171-380-7193; E-mail [email protected]
0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 1 0 1 - 8
136
S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
1. The Cumbre Vieja volcano, La Palma: a highly
active and potentially unstable oceanic island volcano
The Cumbre Vieja volcano forms the southern
third of the island of La Palma. La Palma and the
adjacent island of El Hierro are the youngest islands
in the Canarian archipelago and are presently in a
‘‘shield building’’ phase of activity comparable to
the present activity of the island of Hawaii ŽCarracedo et al., 1998; Carracedo et al., 1999b-this
volume.. The Cumbre Vieja rises to almost 2 km
above sea level Ž6 km above the surrounding ocean
floor. and has a subaerial area of 220 km2 and a
subaerial volume of about 125 km3 , yet the oldest
dated rocks within it are only about 125 ka old
ŽGuillou et al., 1998.. There is also an unknown but
probably at least comparable volume of rock in the
submarine part of the volcano, which has grown
southwards from the preexisting northern part of the
island ŽFig. 1.. It is likely that activity of the volcano
began significantly before 125 ka. Carracedo et al.
Ž1999a, 1999b-this volume. discuss the stratigraphy
and geochronology of the Cumbre Vieja and its
Fig. 1. Simplified geological map of La Palma.
relationship to the older parts of the island of La
Palma. A particularly important feature of the latter
is the presence of an older collapse scar, the Cumbre
Nueva collapse scar, upon which the Cumbre Vieja
is partly built. This contribution complements these
stratigraphic and geochronological studies by considering the structural evolution of the Cumbre Vieja.
The structural study of the Cumbre Vieja described here was carried out in order to address
concerns raised by the 1949 eruption of the volcano
ŽBonelli Rubio, 1950., during which west facing
fault ruptures developed along the crest of the volcano. Following the recognition that giant lateral
collapses are a common feature of oceanic island
volcanoes, as discussed below, the question has arisen
of whether this faulting might be a precursor to a
future giant lateral collapse of the western flank of
the Cumbre Vieja ŽCarracedo, 1994, 1996a,b.. This
paper also seeks to identify the timing and time
scales of any structural changes that have taken place
within the Cumbre Vieja edifice in the past, using
the results of detailed mapping ŽCarracedo et al.,
1997a. and highly accurate radiometric dating ŽGuillou et al., 1998.. The work was coupled with geodetic monitoring of the volcano ŽMoss et al., 1999-this
volume..
2. Precursors to lateral collapse at island volcanoes: is Kilauea typical?
Since the collapse of the northern flank of Mt. St.
Helens at the start of the eruption of 18th May 1980,
the lateral collapse of the flanks of large volcanoes
has come to be recognised as a major process in their
development and a severe volcanic hazard. The hazards are especially great in the case of lateral collapses at oceanic island volcanoes, both because of
the exceptionally large volumes of these collapses
and because they have the potential to generate giant
tsunami with runup heights of hundreds of metres at
distances of hundreds of kilometres ŽMoore, 1964;
Moore and Moore, 1984.. In view of these potential
consequences, the identification of the long term
precursors to giant lateral collapses has become a
critical problem to be addressed by studies of oceanic
island volcanoes. Attention has focused upon active
137
volcanoes where neotectonic structures, seismicity
and geodetic data indicate lateral deformation that
may be precursory to a future lateral collapse. Of
these by far the best known and intensively studied,
partly because it is deforming so rapidly, is Kilauea
Volcano, Hawaii ŽSwanson et al., 1976; Holcomb,
1987; Lipman et al., 1988; Clague and Denlinger,
1994; Denlinger and Okubo, 1995..
Although Kilauea is commonly regarded as the
type example of an unstable oceanic island volcano,
it is in many respects atypical. The Hawaiian volcanoes are larger but less steep than many oceanic
island volcanoes, with heights of up to 10 km above
the ocean floor and slopes averaging 5–108. In contrast, volcanoes such as the Cumbre Vieja; Teide, on
Tenerife; and Pico do Fogo ŽFogo island, Cape
Verde islands. rise 6 to 8 km above the surrounding
ocean floor but have average slopes between 158 and
more than 208. The maximum average subaerial
slope of Pico do Fogo is no less than 288. Profiles of
the Cumbre Vieja, Fogo and Kilauea, at different
scales but all with no vertical exaggeration, are
compared in Fig. 2. The greater slope angles make
these islands intrinsically less stable, and also imply
substantial structural differences between them and
Kilauea.
The south flank of Kilauea is also atypical in that
it shows semi-continuous, partly incremental Žcoseismic. seaward movement which continues through
intereruptive periods ŽSwanson et al., 1976.. This
probably reflects the persistence of magma and ductile, high temperature cumulates in the deeper parts
of the Kilauean rift zones ŽDecker, 1987; Clague and
Denlinger, 1994.. In contrast, examination of the San
Andres fault system on El Hierro indicates that there
was at most a few tens of metres of slip on this fault
system before sudden slip of about 300 m in an
aborted lateral collapse event ŽDay et al., 1997.. This
implies that steep sided oceanic island volcanoes can
become prone to catastrophic flank failure after only
a little precursory deformation. However, such failure is only likely to occur during eruptions or intrusion events ŽElsworth and Day, 1999-this volume..
This is in marked contrast both to Kilauea and to
many stratovolcanoes, where deformation is significant even in intereruptive periods ŽVan Wyk de
Vries and Francis, 1997. and collapse may occur
without magmatic activity ŽSiebert et al., 1987..
138
Fig. 2. Topographic profiles through the Cumbre Vieja Žalong lines A–AU and B–BU in Fig. 1., Pico do Fogo and Kilauea-Mauna Loa
compared. Note that the profiles are drawn at different scales but that all have no vertical exaggeration.
It is therefore important to recognise more subtle
features that indicate that an oceanic island volcano
such as the Cumbre Vieja is evolving towards, or
already in, a state of potential catastrophic lateral
instability. In this contribution, we make particular
use of the distribution and orientation of volcanic
vents. As advocated by Nakamura Ž1977., the distribution of volcanic vents on the flanks of a volcano
reflects the distribution of underlying feeder dykes
and other intrusions, which are in turn controlled by
the stress field in the volcano as originally shown by
Anderson Ž1935.. In many cases, these vents are
concentrated into volcanic rift zones, and as a first
approximation the structural evolution of such a
volcano can be studied by considering the evolution
of its volcanic rift zones. It should be noted that the
term ‘‘ volcanic rift zone’’ is used here in the strict
structural sense advocated by Walker Ž1993. rather
than the looser topographic sense. As discussed below, the topographic expression of a rift zone may be
greatly complicated or even suppressed altogether
where it is developed on preexisting topography such
as the flank of an earlier volcano: for example, the
SW rift zone of Kilauea volcano would be excluded
by the topographic definition because it does not
form a distinct ridge but instead rests upon the flank
of Mauna Loa ŽHolcomb, 1987..
Many components may contribute to the overall
stress field controlling the positions and orientations
of volcanic rift zones wsee discussion in McGuire
Ž1996.x. The result may be a complex stress field
which varies both laterally and vertically, and
changes with time. It is therefore necessary to have
detailed stratigraphic and geochronological control
upon the history of changes in vent distribution and
also to exploit the additional information provided
by vent orientations. It has long been recognised that
elongate fissure vents are orientated along the trend
of the underlying feeder dykes. Tibaldi Ž1995.
showed that the orientations of elongate scoria cones
and the positions of features such as low points on
the rims of the summit craters of the cones could
also be used to infer dyke trends. Dyke trends can
also be deduced where multiple vents formed in the
same eruption can be identified and linked, either
from historical records or from careful stratigraphic
mapping of the eruption products, particularly pyroclastic sequences. We have used methods similar to
Tibaldi’s in the present work, but emphasise in
addition the importance of en echelon fissure and
vent sets, since these can be used to infer changes in
the orientation of the stress field along the trajectory
of propagation of the feeder dykes, as discussed
further below.
3. Changes in the distribution and orientation of
volcanic vents on the Cumbre Vieja volcano
3.1. OÕerall geometry and stratigraphic subdiÕision
The Cumbre Vieja volcano is dominated by sequences of basic to intermediate alkaline lavas and
139
pyroclastic units including scoria and spatter cones,
phreatomagmatic lithic–scoria–ash breccias and airfall lapilli beds. It also contains a number of phonolite domes and lavas which are scattered over the
volcano. The topography of the volcano is dominated
by a north–south ridge which runs almost the entire
subaerial length of the volcano and also extends
offshore for a few kilometres to the south of the
island before bending to a south easterly direction
ŽMasson, pers. commun... The highest part of this
ridge ŽFig. 3. is everywhere above 1700 m elevation
for a distance of some 5 km north to south, with a
smaller but nevertheless strongly elongate central
area generally above 1900 m elevation around the
phonolite dome Nambroque. However, this highest
area is formed of sequences of superimposed scoria
and spatter cones, with subordinate lavas and phonolite domes, similar to much of the rest of the axial
ridge and many other parts of the volcano. There is
no central summit crater or feeder complex. Although the scattered phonolitic units may have been
fed by small and probably transient magma pockets
within the volcanic edifice, the near ubiquitous occurrence of lithospheric xenoliths and other petrological evidence indicates that the basic and intermediate magmas are erupted directly from reservoirs in
the oceanic lithosphere, below the base of the La
Palma edifice ŽKluegel et al., 1997; Hansteen et al.,
1998; Kleugel, 1998..
The existence of this summit ridge led to the
suggestion that the Cumbre Vieja has only one rift
zone and that the volcano is merely a southward
overgrowth from the older volcanoes which form the
north of La Palma ŽAfonso, 1974; Ancochea et al.,
1994.. However, consideration of the vent density
distribution ŽCarracedo, 1994, 1996a,b. on the Cumbre Vieja indicates that less topographically distinct
NW and NE rift zones are also present on the flanks
of the volcano, and that the Cumbre Vieja may have
a triple rift or ‘‘Mercedes Star’’ ŽCarracedo, 1994.
rift structure, like most other Canarian volcanoes.
The vent distribution evidence alone also permits a
further alternative, that of a single dominant volcanic
rift together with a sparse radial swarm of feeder
dykes or vents centred on the highest parts of the
summit ridge around Nambroque ŽFig. 3..
Fig. 3 is a simplified version of the geological
map of the Cumbre Vieja ŽCarracedo et al., 1997a.,
140
Fig. 3. Geological map of the Cumbre Vieja.
in which the following main stratigraphic units can
be distinguished:
Ž1. A cliff-forming sequence of lavas and pyroclastic units intruded by phonolite cryptodomes and
lava domes. These rocks predate the end of formation of the palaeocliffs, formed in a period of intense
coastal erosion associated with low relative sea level.
Rare dykes intruded into this sequence are exposed
in the high cliffs along the western coast of La
Palma. Lava flows at the top of the cliffs can be
traced inland and demonstrated to post date many of
the scoria cones, lava flows and phonolite lava domes
exposed high on the flanks of the volcano and along
the summit ridge. The younger units on the volcano
therefore form a thin veneer, less than 100 m thick,
on the cliff-forming sequences.
Ž2. A platform-forming sequence, including
phonolites and pyroclastic rocks but dominated by
lavas which have built up a series of lava deltas at
the coast. This group of rocks is very widespread,
especially in the northeast of the volcano, but only
forms a thin veneer on the older rocks.
Ž3. Within the platform-forming sequence it is
possible to distinguish young lavas and scoria cones,
with associated pyroclastic units and at least one
group of phonolite cryptodomes. These units are
morphologically very fresh, with little vegetation
cover, and include both sub-historic and known historic eruptive units. Parts of the coastal platform,
especially on the west coast of the volcano, are
formed by lavas of this unit. The individual eruptive
units belonging to this group are discussed further in
Section 3.4, below.
These distinctions are primarily made on the relationships of the rocks to coastal erosion features and
the obvious freshness of the most recent vents.
Nowhere on the volcano are marked compositional
changes or breaks in activity Žindicated by laterally
extensive terrestrial unconformities, weathering horizons or distal ashfall sequences. present within the
sequences. Nevertheless, precise radiometric dating
ŽGuillou et al., 1998. has shown that these three
units have chronological significance. The cliff-forming sequence formed between about 125 ka and 20
ka; most of the sequences exposed in palaeo-seacliffs
up to 700 m high along the west coast were emplaced between 125 ka and about 80 ka, although the
whole of the sequences exposed in the lower eastern
141
palaeocliffs are less than about 40 ka old. The older
platform-forming sequence formed between 20 ka
and about 8 ka; all dated units of the younger
sequence are less than 7 ka old. The chronological
significance of these stratigraphic units means that
the distributions and orientations of volcanic vents
and intrusions in the three units can be compared in
order to investigate the structural evolution of the
volcano through time.
3.2. Volcanic Õents and dykes in the cliff-forming
sequence (125–20 ka)
As noted above the cliff-forming sequence forms
by far the largest part of the volume of the Cumbre
Vieja edifice and also a significant part of the present surface area. Sufficient outcrop of cliff-forming
series rocks from all parts of the subaerial Cumbre
Vieja volcano exists to constrain the distribution of
volcanic vents at this period in its evolution. Unfortunately, most of these vents are either partly buried
or sufficiently reduced by erosion for evidence of the
alignment of the underlying dykes to be doubtful at
best. A few exceptions to this rule, including elongate clusters of vents which are inferred to be cogenetic, are indicated in Fig. 4. In addition, a number
of WNW- to NW-trending dykes are exposed in the
west coast cliff section. These provide important
direct evidence for the orientations of feeder dykes
to the upper part of the cliff-forming sequence in this
area. Occurrences of scoria cones and surtseyan tuff
rings in these cliffs also give some indication of vent
distributions in the older, otherwise buried parts of
the cliff forming sequence.
The vents of the cliff-forming series occur in
several parts of the volcano. Six sectors can be
defined for the purposes of description, converging
upon the highest parts of the edifice around Nambroque. The approximate boundaries of these sectors,
indicated by letters A–F, are shown in Fig. 4.
Sector A forms the axis or crest of the Cumbre
Vieja from the south tip of the island to the summit
region. Exposures of cliff-forming series rocks in
this part of the volcano are dominated by scoria and
spatter cones, with subordinate lapilli and phreatic
and phreatomagmatic ash units, a few phonolite
domes and only very rare lavas. The pit crater of
Hoyo Negro, formed in 1949 ŽFigs. 6 and 14. and
142
Fig. 4. Distribution of volcanic vents and dykes during cliff-forming series activity. Sectors A–F discussed in text.
still some 150 m deep, exposes at least five discrete
but superimposed scoria and spatter cones of the
cliff-forming series, and only one small lava flow.
Vent elongations, where observed, indicate north–
south alignment of feeder dykes in Sector A. It
appears to represent a very well developed volcanic
rift zone.
Sectors B and C, on the flanks of the southern
part of the Cumbre Vieja, are not very well exposed
except in the coastal palaeocliffs. However, the latter
include the highest palaeocliffs in the Cumbre Vieja
and provide excellent sections through the cliff-forming sequences in this part of the volcano. In strong
contrast to sectors D and E, further north, neither
scoria cones nor tuff rings nor dykes are exposed in
these cliffs. The sequences are formed by lavas
derived from the crest of the volcano ŽSector A..
Thus, no dykes were emplaced radially to the summit area of the volcano in directions to the SE and
SW.
Sector D, to the west and north-west of the Nambroque area, is a broad zone with a large number of
scoria and spatter cones in addition to a few phonolite domes and the coastal dykes at El Remo. Where
scoria cone elongation directions can be reliably
determined, they indicate underlying feeders trending
WNW to NW throughout the sector. Flow banding
and flow folds within the Los Campanarios phonolite
dome in Sector D are also directed to the NW along
bearing 300 Žapproximately., suggesting a similar
alignment of the underlying feeder. The concordant
WNW to NW trend of feeder dykes, exposed and
inferred, is developed throughout Sector D. This is
despite the WSW bearing of the southern part of the
sector from the summit region of the volcano around
Nambroque. The dykes exposed in the cliffs at El
Remo are consistently oriented at an angle of 20 to
408 northwards of the bearing that they would be
expected to have if they were radial to the summit of
the volcano.
Apparent north–south alignments of volcanic
vents in Sector D, such as the group Montana
Todoque–Montana de la Laguna–Montana de Triana–Montana de Gazmira along the coast in the
extreme north–west of the sector, are formed by
cones of demonstrably different ages. In the case of
the group of four cones noted above, Montanas de
Triana and Gazmira Žat the northern end of the
143
apparent trend. are substantially more degraded and
weathered than the other two. The other two cones
show distinct NW–SE elongation indicative of
broadly NW-trending feeder dykes.
In Sector E, on the eastern and NE flank of the
volcano, elongation directions of individual scoria
cones and chains of vents such as Montana Hoya
Camacho appear to be more variable, changing from
NE in the northern part of the sector to easterly or
even ESE in the south of Sector E. The elongation
directions and vent chains are consistently close to
the dip direction of the slope, but also tend to be
offset slightly to the north of the local dip direction
throughout the sector by an angle of a few tens of
degrees.
Sector F in the north of the volcano lies between
the ill-defined northern margin of Sector D and the
much older rocks exposed in the west-facing Cumbre
Nueva collapse scar. The southern part of the sector
is covered by very young lava flows but it appears to
be characterised by a distinctly lower density of
earlier scoriarspatter cones than characterises Sector
D. Scoria cones in the north of the Sector F are very
deeply weathered and morphologically degraded and
may in fact be satellite vents of the older Bejenado
edifice to the north ŽCarracedo et al., 1999a., projecting through a thin Cumbre Vieja sequence. Furthermore, gravel pits in the northern part of Sector F
expose thick sequences of alluvial sediments which
appear, along with lava flows to the south, to have
ponded within and filled a depression between the
Cumbre Nueva collapse scar and a barrier to the
west along the eastern edge of Sector D. The nature
of this barrier is discussed further below.
In summary, the activity of the Cumbre Vieja
volcano during this period can be defined by six
sectors, with an alternating pattern of vent Žand
underlying feeder dyke.-rich and vent-poor sectors.
Of the three sectors with abundant vents and inferred
feeder dykes, the very narrow N–S zone ŽSector A.
has consistently N–S-trending elongate vents and
vent alignments; the broad NW zone ŽSector D. has
WNW- and NW-trending elongate vents, vent alignments and dykes throughout, even in those parts
which lie west and SSW of the summit area of the
volcano; and only the NE zone ŽSector E. shows a
pattern that could possibly be considered to be partially radial, although NE- to ENE-trending vent
144
alignments are present throughout the zone including
those areas due east of the summit. This sectoral
pattern indicates that despite the disparity in topographic expressions, the structure of the volcano
during the cliff-forming period is best described in
terms of a ‘‘Mercedes Star’’ ŽCarracedo, 1994,
1996a,b. triple volcanic rift system. Possible reasons
for the disparity in productivity implied by the much
greater topographic expression of the N–S rift zone
as compared to the other rift zones are discussed
further below.
3.3. Volcanic Õents in the early platform-forming
sequence (20–7 ka)
Early platform-forming series rocks occur in all
parts of the Cumbre Vieja. Their distribution is
shown in Fig. 5, from which it is apparent that the
most extensive platform-forming sequences and the
greatest numbers of eruptive vents are present in the
southern and NE parts of the volcano. Rocks of this
age in the south of the volcano may be underrepresented in Fig. 5 because of covering by younger
lavas. The distributions of volcanic vents of this age
can be considered in terms of the three rift zones
defined above in the cliff-forming series of rocks,
but two significant differences are apparent, in the
NE and NW rift zones.
The vents of the southern, N–S aligned rift zone
from Nambroque southwards maintain their concentration in Sector A along the crest of the ridge. N–S
elongation of many of these vents is also apparent.
The NE rift zone contains many vents of this age
and also has the most continuous coastal lava platform in any part of the volcano. The vents show
consistent ENE to NE elongations and alignments of
multiple vents. The elongation directions and vent
alignments again tend to strike slightly obliquely to
the overall slope. However, the region of more easterly trending vents in the south of Sector E as
defined in Section 3.2 and Fig. 4 appears to have
been inactive after about 20 ka before present. In
contrast the cover of platform forming lavas further
north, although thin, is almost complete and the
northern limit of Cumbre Vieja rocks is entirely
formed by platform forming lava flows. It therefore
appears that the axis of the NE rift zone may have
shifted significantly to the north after 20 ka ago.
In contrast to the intense activity of the NE rift
zone during this period, the NW rift zone, although
marked by a number of vents spread over a broad
area of sectors D and F Žas defined in Section 3.2.,
did not produce as great an area of platform forming
lavas as in the NE rift. In particular, the distal part of
the NW rift zone appears to have become completely
inactive. Few vents of this age occur in the south of
Zone D indicating that the southern side of the NW
rift zone may also have become inactive. Vent elongation directions Žsee also Fig. 7, below. do, however, indicate that the underlying feeder dykes were
to the NW and WNW, as before 20 ka. These vent
elongation directions, as in the NE rift zone, are
commonly oblique to the local topographic slope
direction.
In comparison with the distribution of activity in
the period prior to 20 ka, the intersection of the three
rift zones of the Cumbre Vieja volcano appears to
have migrated northwards by at most 2 or 3 km in
the period from 20 ka to 7 ka and the NW volcanic
rift zone appears to have undergone a marked decline in activity, although without a clear change in
its geometry. These subtle structural changes may
have been precursors to the more radical changes in
the geometry of the volcano from about 7 ka onwards.
3.4. Distribution of historic and sub-historic (post 7
ka) Õolcanic Õents
The distribution of the very youngest, morphologically freshest volcanic vents on the Cumbre Vieja
volcano is markedly different from those of earlier
periods and is shown in Fig. 6.
These vents include a number of prehistoric scoria
and spatter cones and associated lava flows, characterised by almost entirely unmodified morphologies
and at most partial vegetation cover. The young ages
of a number of these eruptive units have been confirmed by K–Ar andror C-14 radiometric dating
ŽGuillou et al., 1998, except for La Malforada wunpublished C-14 age of 1050 a B.P.; Carracedo et al.,
1999a. and Montana Quemada, previously dated using C-14 ŽHernandez Pacheco and Valls, 1982.x, as
shown in Table 1: locations of the vents listed in this
table are shown in Fig. 6. The majority of vents in
Fig. 5. Distribution of volcanic vents during early platform-forming series activity. Sectors A–F discussed in text.
145
146
Fig. 6. Distribution of sub-historic and historic volcanic vents.
this youngest group are, however, the products of
historic eruptions, in 1585, 1646, 1677, 1712, 1949
and 1971. Some of these eruptions involved multiple
vents separated by distances of up to 3 km and it
should be borne in mind that, without the historical
data, these vents would most probably have been
mapped as the products of separate eruptions.
Perhaps the most immediately apparent feature of
activity in this most recent stage of the history of the
Cumbre Vieja is the complete absence of eruptions
on the NE rift zone, in strong contrast to the intense
activity in that region in the preceding period. Al-
though minor intrusive activity at depth cannot be
excluded, this implies a very drastic reduction in the
rate of dyke-related extension across this rift zone. In
contrast, activity on the southern, N–S rift zone
follows the same pattern developed in previous periods, with most vents on the axial ridge. Recent and
historic activity in the general area of the old NW
volcanic rift zone has occurred on two distinct vent
alignment trends ŽFig. 7..
The prehistoric vents of La Barquita, Birigoyo
and Montana Quemada lie on an approximately N–S
trend. La Barquita and Birigoyo show definite vent
Table 1
Ages of pre-historic eruptive vents emplaced after the rift-zone
reorganisation. See Fig. 6 for locations of vents. See text for
further discussion and references
Eruptive vent
Dating method
AgeU Ž2 s errors.
Montana Quemada
Nambroque II-Malforada
Las Indias
Montana de Fuego
C-14
C-14
K–Ar
K–Ar
C-14
Birigoyo
K–Ar
420"60 a
1050"95 b
3 ka"2 ka
3 ka"2 ka
3255"140 b
3350"50 c
6 ka"3 ka
a
C-14 age quoted by Hernandez Pacheco and Valls Ž1982.;
confirmed by Guanche Žaboriginal. reports recorded by Spanish.
b
C-14 age determined by Kruger Analytical, USA.
c
C-14 age determined by CEA-CNRS, Gif-sur-Yvette, France.
elongation and rim breaches along a trend bearing
345, or slightly west of north. The eruptive fissure of
Montana Quemada is a N–S aligned elongate trough
implying a N–S-striking feeder dyke. The development of broadly N–S aligned vents in this northernmost part of the Cumbre Vieja suggests that the N–S
rift zone is propagating northwards from the older
summit region around Nambroque, thereby bisecting
the volcano.
The other group of vents occurs on the western
flank of the volcano and is represented by vents of
the 1585, 1712 and 1949 eruptions. These groups of
historic vents are distinctive in a number of ways:
Ž1. They form highly elongate fissures or fissure
alignments, with relatively little near vent constructional relief in the form of scoria or spatter cones
Žalthough the 1585 eruption also involved the emplacement of a number of juvenile phonolite domes,
Fig. 9.. The Llanos del Banco vent of the 1949
eruption is a sinuous trough, composed of multiple
overlapping vents, some 800 m long ŽFig. 8.; the
Jedey vent complex formed in the 1585 eruption is
about 1.5 km long and contains a dozen or more
overlapping vents ŽFig. 9.; and the 1712 eruption
involved seven or more discrete vents along an
arcuate but mainly WNW aligned trend extending
over a distance of some 3 km ŽFigs. 3 and 6..
Ž2. The overall trend of these fissures or fissure
alignments varies from WNW Žin the 1712 eruption.;
to slightly north of west Žthe Jedey vent complex.; to
slightly south of west Žthe Llanos del Banco vent.. In
147
the case of the 1585 and 1949 vent complexes, these
trends are markedly to the south of the WNW to NW
alignments of vents in the old NW rift zone.
Ž3. Within all three groups of vents, individual
vents Žor, in the case of the 1585 eruption complex,
three trends within the vent complex as shown in
Fig. 9. are arranged en echelon. Elongation directions of these individual vents are always offset to
the southwards Žor anticlockwise. with respect to the
overall trend of the vent alignment, and adjacent
vents are offset dextrally with respect to one another
as viewed along the alignment trend ŽFigs. 8 and 9..
In the case of the 1585 eruption complex at Jedey,
vents within each trend shown in Fig. 9 were linked
by simultaneous events. The phonolite domes and
heterogenous hybrid lavas were erupted at the vents
Fig. 7. Comparison of vent elongation directions during the pre-7
ka and post-7 ka periods in the NW rift zone, summit region and
adjacent areas.
148
It should also be noted that this phenomenon is
confined to the western flank of the volcano and is
not observed in older vents anywhere on the volcano.
Vents of this period at the crest of the volcano ŽFigs.
6 and 7. do not show en echelon segmentation and
have a broadly N–S elongation direction, as noted
above. These vents were fed by dykes which retained
a N–S alignment Žand associated east–west extension and host rock s 3 . throughout their ascent to the
surface.
3.5. Analysis of the changes in Õent distribution and
orientation in terms of changing stress field components
Fig. 8. Geological sketch map of the 1949 Llano del Banco vents,
showing inferred feeder dyke orientations.
along the central trend whilst those to north and
south erupted only basic magmas. This implies the
simultaneous existence of three separate but overlapping en echelon feeder dykes beneath this vent complex.
Ž4. In contrast to the slight obliquity of many
earlier vents to the local slope, best seen in the NE
rift zone ŽSections 3.2 and 3.3., the individual vents
in these three eruptions closely follow the local slope
direction.
These features indicate that the feeder dykes to
these eruptions show a consistent pattern of rotation
and segmentation with depth. This is depicted in Fig.
10. At depth, the feeder dykes to these eruptions
appear to strike NW or WNW, along the trend of the
old NW rift system. The extension direction Žand
implied minimum principal stress s 3 in the host
rock. associated with dyke emplacement is broadly
NE–SW, or NNE–WSW. As the dykes propagate
towards the surface the extension direction and implied s 3 in the rocks through which the dyke tips
are moving consistently rotate into a N–S direction,
parallel to the local topographic contours, resulting
in rotation of the propagation plane and segmentation of the dykes in the sense observed.
The variations in the stresses developed in different parts of the volcano during the most recent
Žpost-7 ka. period can be understood in terms of the
stresses which are predicted to develop within an
edifice which is dominated by topographic-gravitational loads; in other words, by the uneven distribution of its own weight. McGuire and Pullen Ž1989.
showed that in a ridge like edifice these stresses have
a characteristic pattern ŽFig. 11.. Across the crest of
the ridge, extensional stresses are developed due to
the partially unbuttressed weights of the flanks
pulling in opposite directions. Dykes will therefore
be emplaced parallel to the ridge crest in this region.
In contrast, on the flanks themselves downslope
compression is developed and any extension will
occur in the direction parallel to the topographic
contours. Dykes emplaced into the flanks of the
edifice will therefore be aligned downslope.
The occurrence of the downslope trending fissures
in en echelon arrays ŽFig. 10. can also be understood
in terms of this model if the old triple rift geometry
has persisted at depth in the form of dyke swarms.
Dykes propagating upwards from this region, with its
radially symmetric stress field and tangential minimum principal stress directions ŽCarracedo, 1994.,
into the region immediately beneath the flank of the
volcano would experience a rotation of s 3 and
therefore a rotation of their preferred alignment into
Žrespectively. contour parallel and downslope directions. In contrast, N–S aligned dykes propagating
upwards directly beneath the crest of the ridge would
experience no change in principal stress directions in
the host rock and thus no change in orientation.
149
Fig. 9. Geological sketch map of the 1585 Jedey vents showing lava flows and inferred feeder dyke orientations. The area of this map also
contains complex sequences of pyroclastic deposits erupted from the numerous vents active during the eruption.
This model does, however, raise a problem with
respect to the distribution of vents and vent alignment pattern earlier in the history of the volcano.
Prior to about 7 ka ago, as discussed above, a triple
rift pattern was established at the surface of the
volcano and numerous vents were emplaced in directions oblique to the local topography, most notably
in the NE rift zone but also in the NW rift zone. This
pattern was developed throughout the earlier history
of the volcano. The absence of major weathering
horizons, distal ash layer packages or erosional unconformities that can be correlated across sectors of
the volcano ŽCarracedo et al., 1999b-this volume.
indicates that at no earlier time in the history of the
volcano has a rift zone abandonment, comparable to
the recent abandonment of the NE rift zone, taken
place. Although crosscutting ŽNE- and East-trending.
sets of vents exist at the top of the cliff forming
series in the southern part of Sector E ŽSection 3.2.,
no sets of en echelon fissures comparable to those
150
Fig. 10. 3-D perspective sketch showing inferred subsurface geometries of recent dykes beneath the N–S crest of the Cumbre Vieja and
under the western flank of the volcano.
developed in the historic west flank eruptions have
been found. The abandonment of the NE rift zone
and other events since 7 ka therefore appear to be
unique in the history of the Cumbre Vieja.
Furthermore, the topography of the volcano has
changed little since about 20 ka ago when cliff
erosion ended. The magnitudes and orientations of
the topographic-gravitational components of the
Fig. 11. Sketch showing near-surface stress directions and fissure orientations in a volcanic ridge dominated by topographic-gravitational
stresses Žafter McGuire and Pullen, 1989..
stress field within the volcanic edifice can therefore
have changed little in the period of rift zone reorganisation. Whilst some influence of changes in the
topographic stresses over time upon the changes in
vent distribution and orientation patterns cannot be
excluded on the basis of this argument, it appears
that the primary cause of the reconfiguration of the
eruptive vent distribution and orientation pattern must
lie in changes in the other components of the overall
stress field.
The possibility that the reconfiguration may be
due to development or disappearance of a significant
magma reservoir within the volcanic edifice can be
excluded on two grounds. Firstly, as noted above
there appears to be no large magma reservoir within
the edifice, the bulk of the magmas ascending instead from lithospheric depths of the order of 7 to 11
km ŽHansteen et al., 1998.; and secondly, there is no
overall major change in the compositions of the
Cumbre Vieja rocks at the time of the reconfiguration. The one group of rocks whose genesis may
involve low pressure fractionation, the phonolites,
occur from at least 56 ka onwards ŽGuillou et al.,
1998.. The triple rift geometry developed through
the majority of the history of the volcano is therefore
best interpreted in terms of doming above a deep
magma body as proposed by Carracedo Ž1994;
1996a,b. for other Canarian volcanoes. The development of the triple rift geometry and the consequent
obliquity of the NE and NW rift zones to the local
topography is therefore critically dependent of the
efficient transmission of doming stresses from the
region at depth where they are developed to the
upper part of the volcano.
In view of the development of en echelon vent
geometries on the western flank of the Cumbre Vieja
Žand implied subsurface dyke segmentation., our
favoured explanation for the changes that have taken
place since about 7 ka on this flank of the volcano is
that the deep triple-rift stress field has become decoupled from the near surface stress field so that the
latter is now dominated by the essentially unchanged
topographic-gravitational stresses while the triple rift
stress field persists at depth ŽFig. 10; note the distinctly oblique dilation directions of the en echelon
fractures relative to the deeper part of the dyke
feeding them.. The development of decoupled stress
fields in these two adjacent regions implies that a
151
boundary zone has developed between them which is
structurally weak and thus inefficient at propagating
stresses from one to the other.
The extinction of the NE rift zone in contrast to
the development of en echelon fissure eruptions on
the west flank of the volcano implies an asymmetry
in the structure of the volcano from 7 ka onwards.
The east flank appears to have become a relatively
rigid buttress. The northward propagation of the
N–S rift zone can then be understood in terms of
synintrusive movement of the western flank of the
volcano away from this rigid buttress, the movement
being accommodated within the weakened zone. One
possible explanation for the extinction of the NE rift
zone and the northward propagation of the N–S rift
zone is therefore that the weakened region is to be
found only under the western flank of the Cumbre
Vieja. The location Žparticularly the depth., extent
and nature of this weakened region can be constrained by consideration of the pre-Cumbre Vieja
substrate upon which the volcano has grown, and the
faulting which took place along the crest of the
volcano during the 1949 eruption. These are examined in the following two sections.
4. The pre-Cumbre Vieja geology of southern La
Palma
As noted in Section 1, the Cumbre Vieja is only
the youngest component of the island of La Palma.
The north of the island is formed by an earlier shield
volcano, the Taburiente–Cumbre Nueva edifice
ŽCarracedo et al., in press.. In the last stages of
growth of this volcano a highly active N–S-trending
volcanic rift zone developed on its southern flank.
The remnants of the resulting topographic ridge form
the present day Cumbre Nueva ridge ŽFig. 1.. The
west flank and axis of the rift zone were removed
about 560 ka ago by a giant lateral collapse directed
to the south and west. Work presently in progress
ŽCarracedo et al., 1999b-this volume. indicates that
the axis of the Cumbre Nueva rift zone lay several
kilometres to the west of the present day ridge:
certainly, since the present Cumbre Nueva ridge is
composed of east dipping rift flank lavas with only a
few dykes, it must be significantly to the east of the
152
position of the old rift zone axis. The altitude of the
rift crest is therefore likely to have been 2000 to
2500 m above present sea level. The collapse scar,
whose floor is largely at or below present sea level
ŽCarracedo et al., 1999a. has since been partly infilled, mostly by the growth of the Cumbre Vieja
volcano. This has completely infilled the southern
part of the collapse scar and buried the southern end
of the Cumbre Nueva escarpment.
A major question in the geology of La Palma is
therefore that of the southward extents of the Cumbre Nueva rift zone and of the Cumbre Nueva col-
lapse structure beneath the Cumbre Vieja. In the
absence of subsurface geological data, both rift and
collapse structure have previously been assumed,
conservatively, to extend only a short distance south
of their present outcrop Že.g., Ancochea et al., 1994;
Carracedo, 1994.. However, there is a geometrical
problem associated with this interpretation which is
illustrated in Fig. 12.
The level crest of the present day Cumbre Nueva
ridge is at an elevation of about 1450 m. In addition,
the dips of the lavas at the crest of the ridge are
approximately perpendicular to its trend: there is
Fig. 12. Map showing predicted southern extents of 1500 m contour on Cumbre Nueva volcano prior to collapse, with variation according to
position of Cumbre Nueva rift zone axis indicated.
very little if any along ridge component of dip.
When allowance for post collapse erosion is made,
the ridge crest therefore closely approximates to the
position of the 1500 m contour at the time of the
collapse. The curvature of the Cumbre Nueva ridge
indicates that the original trend of this contour to the
south would have been to the SW, but nonetheless it
would not have curved back to the NW until it
intersected the axis of the Cumbre Nueva rift zone to
the west. Depending on the precise position of the
Cumbre Neuva rift zone axis, as shown in Fig. 12,
Cumbre Nueva rocks would have originally occurred
at elevations of 1500 m or more as far south as the
region immediately east of the village of Jedey, and
perhaps as far as El Remo on the present day coastline. Instead, younger Cumbre Vieja rocks occur at
around 1000 m elevation to the east of Jedey, and at
sea level at El Remo ŽFig. 3.. A minimum of between 500 m and 1500 m Žvertical thickness. of the
Cumbre Nueva sequence has therefore been removed
from these areas.
This indicates that the Cumbre Nueva collapse
extended further south than has previously been supposed. A possible geometry of the crest of the headwall is indicated in Fig. 12. Collapse structures with
a similar asymmetric scalloped geometry occur in
the adjacent island of El Hierro Žthe El Golfo collapse structure and the San Andres aborted collapse:
153
Carracedo, 1994; Carracedo et al., 1997b; Day et al.,
1997.. The presence of a southward extension of the
Cumbre Nueva collapse scar beneath some or all of
the westward flank of the Cumbre Vieja volcano is
also indicated by recent imaging sonar mapping of
pre-Cumbre Vieja debris avalanche deposits that extend further south along the SW submarine flank of
La Palma than can be accounted for by the previously inferred extent of the Cumbre Nueva collapse
ŽUrgeles and Masson, pers. comm...
The southern limit of the Cumbre Nueva collapse
scar must therefore underlie most if not all of the
region in which the en echelon vents have been
emplaced ŽSection 3.4; Figs. 6 and 7.. The exact
depth of the collapse structure below the surface and
the height of the buried collapse scar are not well
defined, but for present purposes the critical point is
the inferred presence of a west-dipping collapse scar
beneath the western flank of the Cumbre Vieja, at or
below present sea level ŽFig. 13.. Depending on its
position and orientation, this collapse scar may be
associated with one or more structurally weak lithological units: remnant debris avalanche deposits; a
collapse scar sediment fill sequence; and hyaloclastite units at the base of the filling volcanic
sequence. Direct evidence for the presence of post
collapse sediments comes from boreholes on the
northern side of the Cumbre Nueva collapse ŽCar-
Fig. 13. E–W cross-section through Fig. 12 Žalong line A–AU of Figs. 1 and 2. showing inferred position of collapse scar, collapse scar fill
sequence and original Cumbre Nueva topography.
154
racedo et al., 1999a.. Beneath the younger ŽBejenado
and Cumbre Vieja. lavas, these cross unconsolidated
and structurally weak alluvial fan breccias, clay matrix rich debris flow deposits and scree deposits
which directly overlie the unconformity representing
the floor of the collapse scar.
The postulated southern extension of the Cumbre
Nueva collapse structure therefore implies that structurally weak lithologies are present beneath the western flank of the Cumbre Vieja edifice. The presence
of such units would account for decoupling of deep
and shallow stress fields within the western half of
the Cumbre Vieja and thus for the geometry of the
structural asymmetry of the volcano that has developed since 7 ka ŽSection 3.4.. However, this hypothesis does not account on its own for the development
of this asymmetry only in this recent period: this is
discussed further below.
The presence of pre-Cumbre Vieja topography
beneath the volcano, on the scale shown in Figs. 12
and 13, may also account for the unequal development of the three rift zones throughout its history. As
noted above Žsee Fig. 11., McGuire and Pullen Ž1989.
show that topographic-gravitational stresses favour
emplacement of dykes along ridge crests, especially
adjacent to steep, unstable cliff faces. An example of
this is provided by the dykes across the back of the
Valle del Bove on Etna, running parallel to the top of
the headwall cliff face ŽMcGuire et al., 1991.. A
similar relationship would exist for the buried segment of the Cumbre Nueva collapse scar and dykes
feeding vents along the present line of the N–S rift
zone of the Cumbre Vieja. In contrast, dykes beneath
the NW rift zone would run obliquely across the
WSW dipping floor of the Cumbre Nueva collapse
scar, while those beneath the NE rift zone would run
obliquely across the intact eastern slopes of the older
volcano. In order to propagate to the surface they
would therefore have to overcome a component of
the downslope compressive stress indicated in Fig.
11. The overall effect of these various topographicgravitational stresses would therefore be to promote
activity on the N–S rift and partially suppress it on
the other two, throughout the history of the volcano.
Further suppression of the topographic expression of
the NW rift zone in particular can be attributed to the
development and infilling of a sediment and lava
trap between it and the Cumbre Nueva scarp to the
east and the older Bejenado volcano to the north
ŽSector F of Section 3.2..
5. The 1949 fault scarp: first surface rupture
associated with instability of the western flank of
the Cumbre Vieja?
The 1949 eruption of the Cumbre Vieja volcano,
lasting from 24th June 1949 to 30th July 1949,
involved two eruption sites: a N–S cluster of vents
in the summit region of the volcano on the N–S rift
system and, as discussed in Section 3.4, an en echelon vent system on the western flank of the volcano.
Amongst the syneruptive phenomena ŽBonelli Rubio,
1950. were locally intense seismicity and the development of surface ruptures, principally in the period
1str2nd July 1949, along a west-facing normal fault
system in the region between the two eruption centres. As discussed in Section 1, Carracedo Ž1994;
1996a; b. proposed that these faults might represent
an incipient stage of instability of the west flank of
the Cumbre Vieja, and a precursor to a flank collapse
in the future. They may also provide an explanation
of the structural reconfiguration of the volcano during the Holocene.
In view of the hazard implications of this, the
faults were mapped in detail by the first author with
the aims of determining:
1. their age relationships with respect to other events
in the eruption Žin conjunction with a reexamination of eyewitness accounts.;
2. their surface geometry including amounts and
directions of displacements, and thus inferring
their subsurface geometry;
3. whether or not they were associated with localised fumarolic activity that might indicate the
presence of shallow intrusions along their length;
4. whether the major surface ruptures identified by
Bonelli Rubio were in fact only a component of a
broader and more distributed deformation field
with deformation taking place on many smaller
and less obvious structures.
A map of the fault system showing displacements
measured on the mapped fault strands is shown in
Fig. 14; the chronology of the eruption based on the
accounts of Bonelli Rubio and others is summarised
in Table 2. It should be noted that the fault system
extends much further north than was thought —
Bonelli Rubio considered that the northern limit was
at the northern end of the Llanos del Agua — and
that the displacements along the fault have not previously been measured.
155
5.1. Field obserÕations of the 1949 fault system
Although vegetation growth and reworking of
1949 deposits have obscured the fault system in
places, the gaping fissures with vertical offsets of the
Fig. 14. Map of the 1949 fault system and eruption sites.
156
Table 2
Chronology of 1949 eruption, primarily based on Bonelli Rubio Ž1950., Martin San Gil Ž1960. and Monge Montuno Ž1981.; see text for
discussion of these and other accounts, which are in part contradictory
Date
Eruptive activity
Seismic activity
22nd February
–
25th March
–
21st June
22nd–23rd June
24th June
–
–
Start of eruptive activity at Duraznero Žcontinuing
at lesser intensity to 6th July.. Activity of
phreatomagmatic, vulcanian to strombolian type
Žactivity at Duraznero continuing at moderate level.
ŽEarliest date of seismicity recorded in Martin San Gil Ž1960.;
Bonelli Rubio does not mention any activity before 21st June.
Strong earthquakes in south of La Palma,
with damage to lighthouse.
Two strong earthquakes; many smaller felt earthquakes
Lesser seismic activity
Moderate seismicity, continuing to 6th July
1st, 2nd July
6th July
7th July
8th July
Strong vulcanian explosion at southernmost vent of
Duraznero, followed by diminution of activity Žnote:
other accounts have this diminution in activity not
occurring until 8th July.
–
Opening of Llano del Banco fissure and
commencement of eruption of lava at high rate
Žcontinuing until 26th July.
9th July–11th July
12th July
Opening of Hoyo Negro vents; Duraznero
vents remaining inactive. Explosive
Žmainly vulcanian. activity continuing at
Hoyo Negro until 22nd July
13th–14th July
21st–23rd July
30th July
End of explosive activity at Hoyo Negro Ž22nd July.
Brief resumption of eruptive activity at Hoyo Negro;
opening of Duraznero fissure north of Duraznero
crater and short but intense fire-fountain eruption
walls observed and photographed by Bonelli Rubio
on and after July 6th 1949 ŽBonelli Rubio, 1950. are
still recognisable along much of the length of the
fault system. The walls, especially where formed by
loose scoria rather than more cohesive spatter or
rubbly lava, have partially collapsed and filled the
fissure in many places. However, a narrow trough up
Two very strong earthquakes, felt throughout island Žstrongest
earthquakes of entire eruption.
Seismicity ceased temporarily after strong explosion
Strongly felt seismicity
Strong felt seismicity accompanying opening of vent Žnote: other
accounts have this vent opening occurring near-aseismically.
Weakly felt seismicity
Vent opening accompanied by two strong earthquakes
Frequent earthquakes; last on 14th accompanied by dilation of
fissure between Duraznero and Hoyo Negro
Intermittent, weak to moderate seismicity
Weak earthquakes at start of Duraznero fissure eruption
to some metres deep bounded by vertical walls is
preserved in places ŽFig. 15a,b.. A different geometry occurs where the fault system cuts fine grained
phreatomagmatic ashes and alluvially reworked ashes
from the prehistoric explosion craters of Crater El
Fraile and Llanos del Agua III, Žto the east and west
of the Duraznero fissure and Hoyo Negro, respec-
Fig. 15. Field photographs of the 1949 fault system. ŽA. View of the fault scarp Žca. 2 m high. at the northern end of the fault system, as
seen from the west. ŽB. Gaping fissure at the extreme southern end of the fault system, viewed down the length of the fault from the north.
Note Hoyo Negro lithic ash and 30th July lapilli deposits on hangingwall Žwestern. side of fault, and absence of these deposits on the
degraded footwall side. One metre length of tape for scale. ŽC. 1949 Fault-related fissures along western side of Llanos del Agua, cutting
yellow phreatomagmatic ash of prehistoric Crater El Fraile eruption. Fissures filled with grey Hoyo Negro lithic ash from early phase of
Hoyo Negro eruption and covered by undisturbed Hoyo Negro deposits.
157
158
tively.. In these areas, irregularly curved faults and
fissures filled with Hoyo Negro ash are exposed
where the deposits are cut by post-1949 erosional
gullies ŽFig. 15c. and the cliffs of the Hoyo Negro
crater. The generally good, and locally excellent,
preservation and exposure of the fault system allows
determination of displacements along its length with
an estimated accuracy of "20% above a detection
limit that in the erosional gullies and also in the
actively eroding cliffs of larger, pre-1949 barrancos
can be as little as a few centimetres but in the
wooded areas in the northern part of the fault system
is more probably as much as 0.5 m.
Although these limitations of the exposure should
be borne in mind it nevertheless appears that the
1949 fault system is a remarkably simple west facing
normal fault system, striking 165 overall Žparallel to
the trend of the recent northward extension of the
N–S rift system through Birigoyo and La Barquita;
see Figs. 6 and 7..
Vertical displacements reach a maximum of 4 m
in the centre and south of the fault system. Where
horizontal displacements can be estimated they are
typically half or less of the vertical displacement,
suggesting that the gaping fissures at the surface are
linked to a steeply dipping Žat 608 or more. fault at
greater depth. The displacements diminish gradually
towards the northern end of the fault but it terminates abruptly at its southern end. Here the displacement may be transferred along the northern wall of
the partially filled Crater El Fraile explosion crater:
either into a loose breccia fill in this crater comparable to that which is forming at present in the smaller
but comparable Hoyo Negro crater or, through a
transfer fault hidden beneath recent screes, into a
structure later occupied by the eruptive fissure system of the Duraznero vents ŽFig. 14..
A number of bends and jogs exist along the fault
system, which range in size from less than a metre to
hundreds of metres in scale. Measurement of the
offset in the horizontal plane of the corners of the
smaller bends indicate that movement was essentially dip-slip, without any significant strike-slip
component of movement. The larger bends occur
towards the southern end of the fault system, most
notably across the northern end of the Llanos del
Agua, a N–S elongate trough formed at least in part
by phreatic or phreatomagmatic explosions. The only
significant splays and antithetic faults along the fault
system are developed in the same area ŽFigs. 14 and
15c., perhaps as a result of local deformation along
the walls of the trough. The southward extension of
these N–S-trending antithetic faults along the Llanos
del Agua cannot be determined from present day
evidence because of their small displacement and
burial by Hoyo Negro deposits. However, Bonelli
Rubio’s contemporary photographs indicate that they
died out to the south over some tens of metres only.
The main fault can be traced continuously from
Llanos del Agua up the eastern wall of the Llanos
del Agua trough and across the northern side of
Hoyo Negro to its southern termination. Its displacement in this southern section remains more or less
constant both in displacement and amount. This is
despite the fact that it passes across the northern end
of a local, purely extensional fracture system, well
exposed in the southern wall of Hoyo Negro, which
is developed in the vicinity of the Duraznero eruptive fissure system. Well developed N–S-trending
dilational fissures were observed around the future
site of Hoyo Negro by Bonelli Rubio on 6th July
1949 but he clearly distinguished them from the
‘‘north west’’-trending faults: the further development of these dilational fissures later in the eruption
is discussed below.
The northern end of the fault system is marked by
the development of a number of small parallel faults
and fractures, with maximum displacements of the
order of 0.5 m. The development of more distributed
deformation is typical of fault tip lines ŽWalsh and
Watterson, 1988.. The surface trace of the fault dies
out about 700 m SE of the eastern end of the Llano
del Banco en echelon eruptive fissure on the flank of
the volcano.
Detailed examination of the areas on either side of
the fault trace were carried out in a search for other
faults either parallel, oblique or perpendicular to the
main fault trend. These investigations were concentrated in erosional gullies Žlocal name: barrancos.
where exposures up to hundreds of metres length
along the sides of the watercourses and several metres to tens of metres high are to be found. In the
gullies at the north end of the Llanos del Agua, in
Barranco de Los Llanos del Agua to the west and in
Barranco de Magdalena ŽFig. 14. these exposures are
of well bedded lapilli and scoria units and very well
bedded and laminated phreatic and phreatomagmatic
ash units. Faults and fractures with displacements of
tens of centimetres would be easily observed in the
former, whilst the latter would even display faults
with centimetric displacements, but with one exception no such faults were observed. The only fault
found cuts only the oldest rocks of the cliff-forming
sequence in this area: it appears to be associated with
the collapse of a phonolite dome and the formation
of the depression of Llanos de Sima ŽFig. 3..
Similarly well bedded units occur at less extensive outcrops in many places in the mapped area and
also do not show evidence for the development of
small scale fractures or faults. The majority of the
exposures in the barrancos run downslope ŽE–W.
but sinuosity in these and especially in the smaller
gullies provides some N–S-trending exposures in
addition to scattered flat or shallowly inclined
bedrock exposures in the floors of these ephemeral
watercourses. It can also be anticipated that significant distributed small scale fracturing or faulting in
the area of Fig. 14 would be located whatever its
orientation, but especially if it was on a similar trend
to the main fault system. In particular, there are no
east-facing normal faults which would, in conjunction with the west-facing faults, form an extensional
graben structure. The fault system therefore appears
to be a truly asymmetric structure, with a substantial
net downthrow to the west.
The lack of deformation in younger rocks of the
cliff-forming series and the whole of the platform
forming series in the area covered by Fig. 14 implies
not only that the mapped 1949 faults are the only
laterally extensive surface rupturing faults to have
formed in 1949, they are also the first such faults to
have formed in this, the summit region of the Cumbre Vieja, in certainly the past several thousand years
and probably in the last few tens of thousands of
years.
Despite the proximity of the 1949 fault system,
particularly at its southern end, to an eruptive fissure
system active close to the time of its formation, there
is no evidence for a concentration of fumarolic activity along it. Indeed, there is no evidence for contemporaneous fumarolic activity along the faults at all,
even in the fine grained palagonitic ash deposits
which are cut by the faults in Llanos del Agua.
Furthermore, Bonelli Rubio did not record any evi-
159
dence for fumarolic activity or other evidence for
localised gas emission when he examined the faults
in Llanos del Agua on 6th July 1949 and there are no
visible emissions in any of the published photographs, in contrast to the intense activity which
occurred around Hoyo Negro and Duraznero. In
contrast, Martin San Gil Ž1960. records low temperature or cold emissions of water vapour and CO 2 in
the early phases of the eruption from many sites
around Duraznero, at distances of more than a kilometre from the Duraznero vent. These may correspond to the N–S-trending dilational fracture system
noted by Bonelli Rubio Ž1950.. The west facing
normal fault system seems to have been isolated
from the gas venting sites.
The age relationships of the faults to the eruptive
units of the 1949 eruption, in particular to the laterally extensive Hoyo Negro ash and lithic breccia
deposits formed in phreatic and phreatomagmatic
explosions at Hoyo Negro from 12th July 1949
onwards Žsee Table 2., confirm Bonelli Rubio’s inference ŽBonelli Rubio, 1950. that the faults formed
on 1st andror 2nd July 1949 Žsee below. and certainly before 6th July when the surface ruptures were
observed and photographed in the field. The very
fresh appearance of the surface fissuring in the photographs together with the fact that they had not
previously been observed suggests that they had
formed very shortly before. The complete lack of
faulting or fracturing of the Hoyo Negro ash and
lithic breccia deposits, even where they drape the
fault scarps, imply that there was no movement on
the faults after 12th July, even during the activity at
Hoyo Negro in subsequent days and the later reopening of and brief eruption from the adjacent Duraznero fissure system at the end of the eruption
ŽTable 2; Fig. 14..
5.2. Seismicity during the 1949 eruption and the
timing of seismicity, fault rupture formation and
eruptiÕe episodes
There were no seismometers operating within the
Canary Islands during the 1949 eruption and thus no
instrumental data regarding the seismic activity associated with the eruption exists. However, many
earthquakes were felt in the island during the eruption and the timing of these events provides some
160
data relevant to the time of the formation of the 1949
surface fault rupture. The periods of most intense
activity are indicated in Table 2. Bonelli Rubio
Ž1950., Romero Ortiz Ž1951., Martin San Gil Ž1960.
and Monge Montuno Ž1981. provide varying and
partly contradictory accounts of the regions over
which seismicity was felt and present felt seismic
intensity maps ŽMercalli maps. constructed using the
procedures of the time. These procedures differed
markedly from those in use at the present day
ŽVinciguerra, pers. comm... Not too much reliance
can be placed upon these maps, particularly because
much of the affected region was at most lightly
populated in 1949 and therefore large parts of the
maps are based on very sparse data, but also because
the maps produced by different authors are also
contradictory.
The various authors do, however, agree on two
points: firstly, that the seismicity was markedly more
intense in the Cumbre Vieja than in the rest of the
island and, secondly, that no seismic activity was felt
on adjacent islands of the archipelago. These points
indicate that the sources of the earthquakes were
relatively shallow and most probably located within
the upper part of the volcanic edifice. For comparison, earthquakes which are felt only locally but are
also recorded instrumentally at Etna ŽLo Giudice and
Rasa,
` 1992. have instrumentally determined foci at
depths of at most 2 km. Bonelli Rubio Ž1950. and
Monge Montuno Ž1981. both argue that most events
occurred beneath the western flank of the volcano,
especially in the period from 1st to 2nd July 1949
discussed below; Romero Ortiz Ž1951. and Martin
San Gil Ž1960. centre the activity on the N–S rift
zone. The scarcity of reliable data may make it
difficult to distinguish between these alternatives.
For present purposes, however, some important
conclusions can be derived from the temporal data
presented in Table 2. The most intense seismicity
was felt in the few days Ž23rd–26th June. leading up
to the start of the eruption; on 1st and 2nd July Žthe
most intense seismicity of all, felt for a considerable
period, causing most damage and including individual earthquakes felt over most if not all of the
island.; on 7th and 8th July Žcoinciding with the
opening of the Llano del Banco fissure vents on the
western flank.; and on 12th–14th July Žcoinciding
with the explosive activity at Hoyo Negro.. It will be
noted that with the exception of the 1st–2nd July
activity all of these episodes either lead up to or
accompany discrete eruptive episodes. Thus, while
these other episodes of intense seismic activity can
plausibly be related to the propagation of dykes
toward the surface leading to the eruptions concerned, the more intense 1st–2nd July seismicity
cannot. This point and the definite constraint that the
surface ruptures had formed before 6th July 1949 led
Bonelli Rubio to conclude that the most likely date
for the movement to have occurred on the faults was
in the period of intense seismicity from 1st to 2nd
July, four days before the temporary cessation of
activity at Duraznero and six days before the opening of the Llano del Banco fissure vents. It follows
that the formation of the faults and the 1st–2nd July
seismicity can either be related to emplacement of an
intrusion beneath the surface which did not lead
immediately to an eruption, or to faulting within the
edifice related to movement of the western flank of
the volcano rather than to an intrusion episode.
5.3. Cause of the 1949 faulting and seismicity: shallow intrusion or flank faulting?
It is convenient to deal with the subsurface intrusion hypothesis first, since in the context of the
structural evolution of the Cumbre Vieja and the
possible hazard of a future lateral collapse this interpretation might be termed a ‘‘null hypothesis’’. The
most likely form that such an intrusion would take,
since the faults lie between the Duraznero and Llano
del Banco vents of the 1949 eruption, is that of a
dyke propagating between the two, most probably
from beneath Duraznero towards the Llano del Banco
since the former was active from 26th June to 6th
July whilst the latter did not appear until 8th July.
Since the 1949 faults are unique in the Cumbre
Vieja, and also because no intrusion event has been
monitored seismically or geodetically in the Canary
Islands, it is necessary to compare the 1949 faulting
and seismicity with dyke intrusion events on other
volcanoes. Those which are particularly relevant are
the 1983, 1985, 1989 and 1991 rifting events within
the SSE rift zone of Etna ŽMcGuire et al., 1991;
McGuire et al., 1996. and the frequent activity of the
rift zones of Kilauea ŽDecker, 1987; Holcomb, 1987..
In the case of the Etna rifting events the characteris-
tic structure, invariably developed, is a graben with
approximately equal displacements on inward facing
normal faults. This is despite the highly asymmetric
loads imposed on this part of Etna by the presence of
the Valle del Bove to the east. On Kilauea, particularly on the east rift zone, the characteristic fault
structures above shallow, laterally propagating dykes
are simple dilational fissures and symmetrical
grabens. The asymmetric 1949 surface ruptures are
therefore geometrically distinct from the structures
developed above propagating dykes or in other rift
faulting events on these volcanoes.
If a dyke were present beneath the 1949 fault
systems in the Cumbre Vieja, linking the two groups
of vents of the eruption, it would be expected that
the fault system would be linked, both in its geometry and kinematics, to these vent systems. In neither
aspect is this the case. At the northern end of the
fault system, the presence of a 165-trending magma
filled and therefore frictionless fracture would be
expected to result in significant strike-slip displacement on the fault system during the dilation of the
E–W-trending en echelon fractures which fed the
Llano del Banco eruptive activity. This is not observed. Furthermore, the broadly N–S extension on
the eruptive fissure system ŽFig. 8. is geometrically
incompatible with the westward Ždip-slip. displacement on the fault system. The sense of en echelon
offset of the fissures is actually the reverse of that
which would be expected if there were a dextral
offset across them to accommodate the movement on
the fault system. At the southern end the fault, if
mechanically linked via a dyke at depth to the
N–S-trending dilational fractures which extend
northwards from the Duraznero fissure through Hoyo
Negro ŽFig. 14., would be expected to show a gradient in displacement across this fracture zone or even
a reversal of sense of displacement. This is also not
observed.
Furthermore, a significant component of the displacement on the Duraznero–Hoyo Negro dilational
fissure system is likely to have taken place during
opening of the eruptive fissure along the west side of
Crater El Fraile on 30th July 1949: in particular the
main ‘‘dry’’ fissure to the north of this eruptive
fissure is not visible in photographs of its present site
taken on 28th July 1949 ŽMartin San Gil, 1960.. If
the fault system were directly linked to these late-di-
161
lating fissures it would be expected to record displacements after emplacement of the Hoyo Negro
ash and lithic breccia deposits, whereas in fact the
faults were not active after July 12th, the start of the
Hoyo Negro eruption ŽSection 5.1, above..
The presence of a dyke at shallow depths, a few
hundred metres or less, would result in surface manifestations of the proximity of magma including fumarolic activity or alteration, and explosion and
collapse pit craters. Decker Ž1987. notes the transient
development of fumaroles above propagating near
surface dykes in the East rift zone of Kilauea. None
of these features are or were observed along the
1949 fault system, either today or at the time of the
eruption. If on the other hand the dyke was deeper,
at a depth of more than about five hundred metres, a
further problem arises. As noted in Section 5.1, the
proportion of vertical to horizontal displacement
along the vertical walled gaping surface fissures
implies a fault dip at depth of about 608. The predicted position of an underlying dyke, if it is a
relatively deep structure, would therefore lie to the
west of the fault trace by a distance of some hundreds of metres and thus largely beneath the western
flank of the volcano. This is mechanically implausible as the dyke would then be perpendicular to the
downslope compressional forces indicated in Fig. 11
Žsee discussion in Section 3.4., and which appear to
control the orientations of the Llano del Banco and
other west flank fissures.
In conclusion, the 1949 fault system is unlike
other fault systems developed above shallow subsurface dykes; shows no geometric or kinematic evidence of direct mechanical linkage to the various
dilational fissures developed during the 1949 eruption as would be predicted by the shallow subsurface
dyke model; shows no evidence for the presence of a
dyke very close to the surface; and presents geometrical and mechanical problems if the postulated dyke
is present at greater depth. Finally, any model which
interprets the 1949 fault system as the product of
near surface intrusion emplacement must also explain why similar faulting has NOT occurred during
the many previous eruptions in the summit region of
the Cumbre Vieja. It should, however, be noted that
this discussion of a shallow connecting dyke does
not exclude the deeper subsurface linkage between
the ridge crest and flank eruption sites of the 1949
162
Fig. 16. Three possible geometries of inferred sub-surface flank deformation during the 1949 eruption. See text for discussion.
eruption that is implied by the alternation in eruptive
activity between the two sites ŽTable 2.. However,
these points merely require that the two vent systems
be linked at the depth of the magma reservoir feeding the eruption w7–11 km, as discussed above ŽSections 3.1 and 3.5.x.
Whilst in the absence of borehole or other direct
subsurface data the presence of a dyke at depth
beneath the 1949 fault system cannot be definitively
excluded, it would appear that an alternative explanation for the 1949 faulting must be sought. This
alternative hypothesis, as noted at the beginning of
this section, is that the 1949 fault system results from
incipient instability of the western flank of the Cumbre Vieja volcano, triggered by the eruption Žthrough
mechanisms discussed by Elsworth and Day, 1999,
and references therein.. All the available data, especially the westward dip of the fault plane inferred
above and the consistent displacement of the hangingwall Žwestern. side down and to the west, are at
least consistent with this hypothesis. However, more
data on the subsurface geometry of the structure are
required to confirm it.
The observations of felt seismicity described
above indicate very shallow sources: there is no
evidence that the fault represents the surface expression of a steeply dipping structure extending to
depth. Furthermore, if a deep steeply dipping structure were present it would be expected to interact
with the persistent magma reservoirs inferred by
Kluegel et al. Ž1997.; Kleugel Ž1998. and Hansteen
et al. Ž1998.. By analogy with Kilauea, ŽSection 2;
Decker, 1987; Clague and Denlinger, 1994. persistent activity on such a structure during intereruptive
periods would be expected but is not observed ŽMoss
et al., 1999-this volume.. The Mercalli intensity
maps of Bonelli Rubio Ž1950. and Monge Montuno
Ž1981., if accepted, strongly imply the presence beneath the western flank of the volcano of abundant
near surface faults or a through going fault structure.
Furthermore, the absence of surface fault ruptures on
the western flank implies that these structures, if
present, form a shallowly-dipping detachment fault
or deformation zone sub-parallel to the surface. The
geometry of this shallowly dipping zone is not well
constrained. Three possible alternatives are shown in
Fig. 16. In these models, the steep normal fault
represented by the surface rupture at the crest of the
163
Cumbre Vieja might represent the initial stages of
development of a headwall fault linked to the shallow detachment structure.
Future geodetic measurements of slow intereruptive deformation if this exists ŽMoss et al., 1999-this
volume. or careful and accurate monitoring of shallow seismicity and ground deformation during future
eruptions may be required to directly constrain the
geometry and kinematics of any deformation of the
west flank of the volcano. The geometries shown in
Fig. 16 do, however, provide the basis for explanations of the structural development of the Cumbre
Vieja, as developed in the following section.
6. Towards an overall model for the structural
evolution of the Cumbre Vieja volcano during the
past 125 ka?
The various lines of evidence discussed in this
paper lead to a partial reconstruction of the structural
development of the Cumbre Vieja volcano which can
be summarised as follows:
Ž1. The volcano developed on a complex substrate
with considerable topography. This consisted of an
elongate N–S topographic ridge formed by collapse
to the west of one flank of a well developed volcanic
rift zone. The eastern flank was thus intact while the
western side of the ridge was formed by the steep
headwall of a collapse scar.
Ž2. The western flank of the Cumbre Vieja volcano has built up upon the collapse scar and may
therefore overlie a relatively weak collapse scar fill
sequence. This western flank of the volcano may
therefore have been inherently less stable throughout
its history but until the most recent stages of that
history there has been no evidence of actual instability.
Ž3. During most of its history the volcano has
been characterised by a triple or ‘‘Mercedes Star’’
volcanic rift system with South, NE and NW volcanic rifts and underlying dyke swarms. At no stage
in the history of the volcano has a central crater or
feeder complex developed. However, perhaps as a
result of the topographic-gravitational stresses set up
by the preexisting topography, the South rift zone
has always been the most active structure, with the
greatest volume of erupted products. Nevertheless,
164
the triple rift geometry remained stable for a period
of at least 100 ka and through the accumulation of
well in excess of 700 m of lavas and the erosion of
coastal cliffs up to 700 m high.
Ž4. During the latter stages of development of this
unequal triple rift system, from about 20 ka ago, the
intersection of the active rift systems may have
migrated slowly to the north over a distance of a few
kilometres. In the same period, whilst the NE rift
zone may have experienced its most intense activity,
the NW rift zone began to decline. These developments may have been precursory to the subsequent
more rapid structural evolution of the volcano.
Ž5. The more recent evolution of the volcano has
occurred without major changes in the topography of
the volcano: post-20 ka rocks are at most a hundred
metres or so thick and form a discontinuous veneer
on the older rocks, whilst coastal cliff erosion largely
ceased with the post glacial rise in sea level and the
formation of coastal lava platforms.
Ž6. Beginning at about 7 ka ago, the volcano
underwent a structural reorganisation with the following sequence of events: activity on the NE rift
zone ceased altogether; the South rift zone propagated northwards, bisecting the volcano; and Žperhaps only in the past 500 years. flank eruptions from
en echelon, E–W trending fissures occurred on the
west flank of the volcano in the region occupied by
the old NW rift zone. The geometry of these en
echelon fissure sets indicates that they are fed by
dykes that are broadly NW-trending, aligned along
the old NW rift zone trend, at depth but which rotate
and segment as the propagate towards the surface.
The near surface geometry of these dykes is controlled by topographic-gravitational stresses. The
overall rift zone reorganisation has involved a decoupling of the near surface stress field from the deeper
stress field under the western flank of the volcano,
implying development of a weak or compliant layer
under that flank. In the same period, the eastern side
of the volcano has acted as a Žrelatively. rigid buttress.
Ž7. The most recent eruption near the summit of
the volcano and involving an en echelon fissure
eruption on the flank of the volcano, that of 1949,
was also accompanied by development of a west
facing normal fault system along the summit ridge of
the volcano. The geometry and kinematics of this
fault system are inconsistent with the hypothesis that
it represents a structure developed above a near
surface dyke. Available data on the subsurface geometry of the fault system indicates seaward movement and incipient instability of the western flank of
the volcano. The fault may link into a shallow,
possibly seaward dipping detachment zone: if it exists this zone may consist of many small faults or a
larger through going structure. The fact that such
faulting has not occurred earlier in the history of the
volcano implies that it is linked to the most recent
stages of the structural evolution of the volcano.
The structural evolution of the Cumbre Vieja
volcano over the past 20 ka or so therefore points to
a progressive destabilisation of the western flank of
the volcano. The rapidity of the rift reorganisation
some 7 ka ago and the subsequent events, in comparison to the possible very slow migration of the rift
zone intersection in the preceding period suggests
that the process may be accelerating. A similar acceleration in the rate of structural reconfiguration may
be evident in the recent history of the Cha das
Caldeiras volcano, Fogo ŽDay et al., 1999-this volume..
The stability of the Cumbre Vieja volcano through
the period of growth prior to 20 ka raises a problem:
if weak lithologies have been present beneath the
western flank of the volcano since the early stages of
its growth, why has it not shown some degree of
instability and decoupling of deep and shallow stress
fields earlier in its history?
One possible answer to this may lie in the fact
that the stability of volcanoes depends upon the pore
fluid pressure distribution within them ŽElsworth and
Day, 1999-this volume, and references therein.. A
rise in the water table within the flank of a volcanic
edifice will tend to destabilise it, especially during
eruptions due to mechanical and thermal pore fluid
pressure increases. Between 20 ka the water table in
the Cumbre Vieja may have risen due to one or both
of two effects: the postglacial rise in sea level and,
perhaps more significantly, by a change in the local
climate. Evidence for Quaternary climate change in
La Palma will be considered elsewhere.
The alternative but not incompatible explanation
is that volcano may have been very slightly unstable
through much of its history but that with the growth
and continued very slow deformation of the volcano
it may have passed a critical point between 20 ka
and 7 ka ago. This critical point would occur when
deformation in the relatively weak zone under the
west flank of the volcano ŽFig. 16. increased to the
point at which strain weakening set in, whether due
to comminution and a reduction in cohesive strength
or to a reduction in permeability and thus increased
sensitivity to pore fluid pressure increases ŽDay,
1996..
This model implies that a deformation zone or
fault system has been developing under the west
flank of the Cumbre Vieja, perhaps close to the floor
of the Cumbre Nueva collapse scar, for at least the
past 7 ka. On this view, it would now extend at least
through the region within which the en echelon flank
fissure eruptions have taken place, and perhaps as far
north as the northward propagating tip of the N–S
rift zone. The first appearance of surface faulting in
the 1949 eruption, and only in a small section of the
volcano, is not incompatible with this, for the nucleation of faults in the subsurface and their subsequent
progressive growth towards the surface is well documented for tectonic faults Žsee Scholz, 1990 and
Yeats et al., 1997 for recent reviews; also Walsh and
Watterson, 1988.. In general, a fault will only rupture the surface when greater movement has already
taken place on it at depth; and the trace length of the
first surface rupture may only be a small fraction of
its total dimensions. An implication of a strain weakening model of this type is that further deformation
during future eruptions in the summit region or even
in intereruptive periods will cause the west flank of
the Cumbre Vieja to weaken further, causing an
acceleration in the deformation rate and a greater
susceptibility to large scale collapse. It should be
noted that less than 10 eruptions have taken place in
the summit region of the Cumbre Vieja since the
onset of the radical rift zone reorganisation and
therefore that the probability of emergence of the
fault system as a surface fault rupture in any one of
these eruptions is therefore quite high, even if assumed to be random. If, as argued here, the fault
system has grown progressively though time, the
emergence of the fault system only as recently as
1949 can be seen as a consequence of the evolution
of the volcano rather than as an improbable event.
The Cumbre Vieja volcano does not seem, however, to be in an extremely unstable state at present.
165
The eruption of 1971, at the extreme southern end of
the island and thus at a relatively low elevation and
perhaps to the south of the region of the Cumbre
Nueva collapse scar, does not seem to have caused
further slip on the surface fault ruptures. Geodetic
monitoring in progress since 1994 ŽMoss et al.,
1999-this volume. has not as yet detected significant
movement across the 1949 fault system and of the
west flank of the volcano more generally. The implication of these constraints upon a strain weakening
model is that it would predict that future seaward
movement on the 1949 fault system or elsewhere in
the west flank of the volcano is only likely to occur,
in the short term at least, during or immediately after
ŽElsworth and Day, 1999-this volume. eruptions in
the northern part of the Cumbre Vieja, in the general
region of the 1949 eruption vents.
In conclusion, it should be further emphasised
that there is much that remains unknown about the
structure and stability of the western flank of the
Cumbre Vieja, although as shown here the structural
evolution of the volcano as a whole can be understood in terms of a weakening and destabilisation of
this flank through the past 20 ka. It is perhaps
arguable that this hypothesis will not be fully tested
until a future eruption at or near the summit of the
volcano is adequately monitored. Such monitoring
will require a seismic network capable of accurately
locating earthquake foci and determining earthquake
focal mechanisms and a geodetic network covering
the whole of the western flank and summit region of
the volcano.
Acknowledgements
Fieldwork by the authors was financed by grants
from the Consejo Superior de Investigaciones Cientificas of Spain, the European Union, and NATO.
Production of the geological map of the Cumbre
Vieja upon which Fig. 3 is based was financed by
the Canarian Government. Kathryn Sharp provided
invaluable assistance and advice in the drafting of
this map. We gratefully acknowledge discussions
with Jane Moss, Bill McGuire, Derek Elsworth,
George Walker, Robin Holcomb and Francisco Perez
Torrado, and especially advice on the problems of
interpreting Mercalli Intensity maps from Sergio
166
Vinciguerra. We also greatly appreciate discussions
with participants in the 1997 La Palma workshop,
especially Andreas Kleugel, Thor Hansteen and Hubert Staudigel. Reviews by Geoff Wadge and Michel
Semet greatly improved the paper.
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Recent structural evolution of the Cumbre Vieja volcano, La Palma

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