The Canary Islands: an example of structural control on the growth

Jo u ~ o f volc~ology.
armgeomermmresem~
ELSEVIER
Journal of Volcanologyand Geothermal Research 60 ( 1994) 225-241
The Canary Islands: an example of structural control on the growth
of large oceanic-island volcanoes
J.C. C a r r a c e d o
Volcanological Station of the Canary Islands, Spanish Research Council (CSIC), P.O. Box 195, 38206 La Laguna, Spain
(Received August 16, 1993; revised version accepted December 16, 1993)
Abstract
Dike complexes, which are increasingly accepted as a common feature in the growth of most oceanic volcanoes,
are well represented in the Canary Islands, where their deep structure can be readily observed through hundreds
of infiltration galleries excavated for water mining. These intrusive complexes have their surficial representation
as narrow, clearly aligned clusters of emission centers that, cumulatively, form steep topographic ridges. In the
subsoil, a narrow band of tightly packed parallel dikes runs through the center of the structure. These volcanotectonic features behave as true active polygenetic volcanoes and show clear rift affinities. The geometry of these
rift zones is either single or three-branched. The two-branched stage, probably transitional, has not been observed.
The rift zones play a key role in the mass wasting and destruction of mature oceanic volcanoes. Cumulative gravitational stresses related to the growth of the volcanic edifices increase their instability. More ephemeral mechanisms associated with intense eruptive phases, such as dike wedging, increase of slope angles and strong local
seismicity associated with magma movement can finally trigger massive landslides. Massive landslides, enhanced
by later erosion, may be the explanation for the origin of numerous horseshoe-type valleys and calderas in the
Canary Islands. The "least-effort" geometry of complex rift zones seems to fit some mechanism of magma-induced
upwelling, such as a hotspot, in the explanation of the genesis of the Canarian Archipelago. The rift zones play a
major role in the distribution of historic volcanism in the Canary Islands and, therefore, in their volcanic hazards
assessment.
1. Introduction and geological framework
The Canarian Archipelago is a volcanically active alignment of seven islands situated in a band
200 by 500 km off the African continental margin opposite Cape Juby. Fuerteventura, one of the
eastern islands, is only 100 km from the African
coast (Fig. 1, inset). The magmatic and volcanic
evolution of the archipelago has been influenced
by the oceanic-continental transitional nature of
the lithosphere on which the islands developed
(Dash and Bosshard, 1968; Bosshard and Mc-
Farlane, 1970; Banda et al., 1981; Hoernle et at.,
1991), the effect of the Atlas tectonics
(Schmincke, 1976, 1982) and the near stationary nature of the African plate in this region since
the Miocene, about 10 m m / y r for the last 60 Ma
according to Duncan (1981) and Morgan
( 1983 ). The African plate was at least stationary
during the subaerial growth stage of these islands.
The eastern Canary Islands m a y have been active since the Late Cretaceous (Le Bas et at.,
1986). Volcanoes were formed through multiple
volcanic cycles (Carracedo, 1979; Schmincke,
0377-0273/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSD10377-0273 ( 94 )00006-3
1 C. Carracedo/Journal of Volcanology and Geothermal Research 60 (1994) 225-241
226
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Fig. 1. Map of the Canarian Archipelago showing the location and age of historic volcanic eruptions. The index map shows in
vertical striped pattern the islands with historic (last 500 yr) volcanism, in horizontal striped pattern, those with Quaternary
volcanism, and in empty pattern, those without Quaternary volcanism. The oldest published radiometric ages from subaerial
volcanics of each island are indicated in the inset in parentheses. K/Ar ages of La Palma are from Abdel-Monem et al. ( 1972 ),
of Gomera and Fuerteventura from Feraud ( 1981 ), of Tenerife from Ancochea et al. (1990), of Lanzarote from Coello et al.
(1992) and of Gran Canaria from McDougall and Schmincke (1976). The oldest subaerial volcanism of Hierro is not yet well
determined, but it is older than 0.7 Ma since a stratigraphically intermediate volcanic series has reverse geomagnetic polarity
(Carracedo et al., in prep. ).
1982; Cantagrel et al., 1984; Ancochea et al.,
1990) with different evolutionary histories on
each island. The islands can be separated into
three categories (Fig. 1, inset) according to their
eruptive activity: ( 1 ) the islands ofTenerife, La
Palma, Lanzarote and probably E1 Hierro (Hern~indez Pacheco, 1982) which have had eruptions in historic time ( < 500 yr) and are thus, by
definition, volcanically active; eruptive activity
on La Palma and Lanzarote indicates that volcanism is presently active along the entire alignment of the archipelago; (2) the islands of Fuerteventura and Gran Canaria with Quaternary
volcanism; and (3) the island of Gomera, where
no evidence of Quaternary eruptions has been
found (Cantagrel et al., 1984), although longer
periods without eruptions on Lanzarote, Gran
Canaria and Fuerteventura were followed by renewed activity.
2. Main morphological and structural features of
the insular edifices
Two relevant volcano-tectonic features (Fig.
2 ) characterize the islands morphologically: ( 1 )
rift-type clusters of aligned eruptive vents or rift
J.C. Carracedo I Journal of Volcanology and Geothermal Research 60 (I 994) 225-241
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Fig. 2. Schematic representation of the main volcanic features related to volcanic activity in the Canarian Archipelago. 1 = rift
zones; 2 = tensional-stress fields in the rift zones, indicated by empty arrows; 3 = caldera-type depressions of possible rift zonegenerated landslide origin, in black arrows; 4 = incipient faulting, in black triangles; 5 = central-type Teide volcano. See text for
discussion.
zones; and
depressions.
(2)
valleys
or
caldera-type
2.1. Rift zones
Rift zones are rift-type volcano-tectonic features that form a tight cluster of recent emission
centers piled up along narrow dorsal ridges
known locally in Spanish as "dorsales". The term
"rift-type" indicates that these active volcanic
structures show some of the main features of Hawaiian rifts (forceful injection of dikes, extension, etc. ), but lack some important ones such as
direct connection with a shallow underlying
magma chamber, downdropped caldera, etc.
(Carracedo et at., 1992 ). The Canarian and Hawaiian archipelagos show many similarities in
their growth, the main difference being in scale,
in the rate of plate motion and in the eruptive
rates and volumes. These differences impose important constraints on the frequency of erup-
tions, eruptive rates and volumes, aspect ratios,
etc., in their respective rift zones.
At erosional windows, a "coherent" (Walker,
1992) swarm of feeding dikes can be observed,
with increasingly dense packing with depth and
towards the axis of the rift zones. These coherent
dike swarms form the inner structure of the
ridges, readily observable in some of the islands
through hundreds of infiltration galleries (Plan
Hidrol6gico de Tenerife, 1989) that cross these
features at different depths and altitudes (Figs.
3 and 5).
Morphologically, the Canarian rift zones can
be separated into two main types: ( 1 ) very steep
slope, high-aspect-ratio rift zones, such as the
Cumbre Vieja on La Palma (Fig. 4), or the
northeastern ridge of Tenerife and the northeastern and northwestern ridges of E1 Hierro (Figs.
2 and 5 ), which are probably related to eruptive
patterns of high frequency; and (2) low-aspectratio rift zones, which are well represented by the
228
J. C. Carracedo / Journal o f Volcanology and Geothermal Research 60 (1994) 225-241
NE-SW RIFT ZONE
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infiltration galleries
(water-mining
excavation tunnel)
Fig. 3. Infiltration galleries, locally known as "galerias" (2 × 2 m, near horizontal tunnels up to 8 km long, excavated for the
purpose of intercepting groundwater) in Tenerife. U p p e r inset shows a cross-section of the northeastern rift zone and the infiltration galleries that cross its deep structure (from the Plan Hidrol6gico Insular de Tenerife, 1989 ).
NE-SW central volcanic lineament of Lanzarote
and are probably the result of less frequent eruptions of higher rates that extend over wide areas
without producing a well-defined, prominent
topographic ridge.
Two main types of rift zones can be defined
considering their geometry: single, as on La
Palma or Lanzarote, or triple, as on Tenerife and
El Hierro (Fig. 2 ). In the latter complex scheme,
a stratovolcano-type central volcano of differentiated (trachytic-phonolitic) magmas (TeidePico Viejo volcanic complex) has developed at
the triple junction on the more evolved island of
Tenerife, whereas on El Hierro, an island at an
earlier stage of building, this central activity is
not present.
2.2. Caldera-type depressions
Arcuate, horseshoe-shaped depressions are
frequently associated with well-developed, highaspect-ratio rift zones in the Canaries. Depressions at Orotava and Giiimar are perpendicular
to the axis of the northeastern dorsal ridge of Tenerife (Fig. 2 ). Similar features seem to have developed - - and new ones may be in progress - in relation with the very active southern rift zone
of La Palma (see Figs. 2 and 8). Caldera-form
embayments are quite common in the Canaries
(see Fig. 2). When they are related to complex
triple dorsal arrangements, the caldera-type
depressions are consistently located at the june-
J.C. Carracedo / Journal of Volcanology and Geothermal Research 60 (1994) 225-241
229
Fig. 4. View of the rift zone of Cumbre Vieja, extremely fast-growing in the last few thousand years, south of La Palma. Most
slope angles exceed 30%. In the foreground, the "old" shield island volcano (Plio-Pleistocene) is distinctly separated from the
recent rift zone.
tion of the rift zones, generally between the two
most active ones (Fig. 2).
The Cafiadas caldera on Tenerife and El Golfo
embayment on E1 Hierro are good examples.
Similar embayments in older volcanic series, like
the Taganana embayment on Tenerife or Jandia
on Fuerteventura, may have a similar significance, although erosive dismantling renders the
relationship between rift zones and depressions
less evident.
vary from 1 to 237 yr with a mean value of about
30 yr for the entire archipelago (Carracedo and
Rodriguez Badiola, 1991 ). The spatial distribution of recent volcanism is more easily investigated. Recent (late Pleistocene-Holocene) as
well as historic emission centers are clearly associated with the rift zones previously described
(Fig. 5 ). Very few of the Holocene volcanoes and
none of the historic ones (Fig. 1 ) are located
outside these volcano-tectonic features.
3. Time and space distribution of recent
volcanism in the Canary Islands
4. Genesis of Canarian rift zones
The analysis of the historic volcanism in the
Canary Islands (last 500 yr) does not reveal any
significant pattern since inter-eruptive periods
The above-mentioned characteristics of the rift
zones portray long-lasting, volcano-tectonic features that constitute the key factor controlling the
development of some insular edifices, possibly
CANARY ISLANDS
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Fig. 5. Concentration of Quaternary emission centers in the rift zones. In the island of Tenerife, a complex, "Mercedes"-type
stellate rift zone arrangement has formed. Single rift zone have developed in La Palma (fast-growing, high aspect ratio) and
Lanzarote (slow-growing, low aspect ratio ). Contours interval on Tenerife 800 m; on La Palma 500 m.
J. C. Carracedo / Journal of Volcanology and Geothermal Research 60 (1994) 225-241
changing their location, rate of activity and configuration as the different islands evolved. Active rift zones, or possibly rejuvenated rifts in the
case of Lanzarote, are presently well outlined only
on those islands with important recent eruptive
activity. In fact, historic activity occurred only
on the islands having rift zones, which seems to
suggest that these features developed in places
where magmatism is presently more active and
crustal conditions favor the evolution of steady
magma plumbing systems that facilitate sustained eruptive activity. This observation is in
accord with the speculations of Vogt and Smoot
(1984) that magmatism has to be frequent and
intense enough for the rift zones to stay hot
(thermal memory ) to influence and localize successive injections.
The presence of these structures was first inferred by McFarlane and Ridley (1968) by
means of gravity measurements on Tenerife.
These authors explained the local Bouguer
anomaly showing a three-pointed star shape coinciding with topographic ridges as reflecting unusually high concentrations of dikes along fissure zones. The regular geometry of these major
fissure zones, at 120 ° to one another, could have
been generated by inflation of the volcano due to
magma intrusion. These zones controlled the
growth and triangular shape of Tenerife from the
earliest submarine growth stages. These geophysical interpretations were later confirmed by
geological observations at the surface and in the
subsoil (Navarro, 1974; Carracedo, 1975, 1979;
Arafia and Carracedo, 1978; Navarro and Coello,
1989).
5. A genetic model of Canarian rift zones
Three-armed patterns seem to be a common
scheme in the arrangement of volcanic vents in
oceanic volcanoes. The important role played by
narrow zones of concentrated volcanic activity
in controlling the shape and structure of oceanic
volcanoes has been reviewed for Hawaii (Fiske
and Jackson, 1972; Swanson et al., 1976; Nakamura, 1980; Wyss, 1980; Decker, 1987; Walker,
1992, among others), Gough Island (Chevalier,
231
1987), Marion and Prince Edward Islands
(Chevalier, 1986 ), Rrunion (Upton and Wadsworth, 1970; Chevalier and Bachelery, 198 l;
Lenat and Aubert, 1982 ) and for the Canary Islands (Carracedo et al., 1992). The complex
structure of rift zones - - topographic ridges of
aligned vents and swarms of feeder dikes - - has
recently been proposed as a common feature and
a key factor in the development of oceanic-island volcanoes by Walker ( 1992 ).
Updoming magma pressure, swell and eventual rupture and consequent injection of bladetype dikes is a synthesis of the repetitive process
that, by progressively increasing anisotropy,
forces the new dikes to wedge their path parallel
to the main structure (Carracedo, 1988; Carracedo et al., 1992). The result is a narrow, dense
swarm of parallel or subparaUel dikes. This model
can be applied to explain the origin of the Canarian rift zones with some peculiarities related to
crustal conditions under the rift zones and the
characteristics of the waning Canarian hotspot.
The existence in the Canarian Archipelago of
single or three-armed rifts can be related to the
local crustal structure of each island, mainly the
anisotropy of the crust due to the presence or absence of previous tectonic "fabric" signatures
(see Table 1 ).
The presence of coeval rift zones active at both
ends of the Canarian island chain (El Hierro at
the westernmost end and Lanzarote at the eastern end) suggests the activity of a hotspot that
has "spread" in at least partially detached plumes
or blobs (Hoernle and Schmincke, 1993 ) under
the different islands, probably because of the
quasi-stationary state of the African plate (Morgan, 1983; Holik et al., 1991 ).
The geometry of the complex Canarian rifts
with three branches separated by angles of 120 °
(see model in Fig. 6) suggests a "least-effort"
fracture as a result of magma-induced vertical
upwards loading (Luongo et al., 1991 ). In the
asymmetric model proposed by Walker (1992)
for the Hawaiian Islands instead of "least-effort" fracture caused by localized uplift, stresses
caused by the injection of dike wedges along a
collinear rift zone are relieved by the formation
of a new orthogonal rift; asymmetric dike injec-
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CRUSTAL STRUCTURE
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Primitive and
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magmas
Primitive, deep
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rift zones
Evolved magmas
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Table 1
A schematic model of the constraints exerted by the structural properties of the crust underneath the different islands of the Canarian Archipelago and
the different types of rift zones generated
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J.C. Carracedo / Journal of Volcanology and Geothermal Research 60 (1994) 225-241
233
central volcano
caldera
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feeder dikes
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ial
ism
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m
m
120° triple fracture
Fig. 6. A hotspot-based schematic model for the genesis of a complex "Mercedes"-type stellate rift zone, on one of the Canary
Islands. The concentration of the recent eruptive activity and the depressions that may have been generated by gravitational
slides are also indicated.
234
J.C. Carracedo / Journal of Volcanology and Geothermal Research 60 (1994) 225-241
tion would cause the existing rift zone to become
non-collinear. As stated previously, the twobranched rift zone stage has not been found in
the Canaries, and the three-branched stage consistently shows a symmetric pattern with angles
of 120°.
The present depth within the upper, lithosphere of the magmatic bodies that have generated the rift zones is difficult to estimate. The
brittle-ductile discontinuity at which the rifting
starts may have varied with the evolution of the
rift zones. Long-term location of the focal depth
of seismic events underneath the rift zones, now
in progress by means of a newly deployed seismic network (Carracedo et al., 1993 ), may help
to define the upper boundary of the bodies of
magma that sustain the rift zones.
6. The role of rift zones in triggering major
landslides
The erosive vs. tectonic origin of the more
prominent morphological escarpments of the
Canarian Archipelago (seacliffs, U-shaped valleys and calderas) has been controversial. Since
the work of Hausen (1962) in which the Canaries were explained as the remaining emerged
blocks of a sub-continent (the "table-land" formation ) that was fragmented and faulted, a clear
reluctance developed to apply tectonism in the
interpretation of escarpments in the Canaries.
Erosion or true collapse-caldera mechanisms
have been the common interpretations of these
conspicuous geological features. Arcuate head
and straight-walled valleys such as those of Gillmar and Orotava were, however, interpreted as
possible giant landslides (Bravo, 1962; Hausen,
1971; Ridley, 1971 ). Wide coastal embayments
like El Golfo, on northern E1 Hierro, have also
been interpreted as possible giant gravitational
landslides (Hausen, 1964, 1973; Bravo, 1982;
Holcomb and Searle, 1991 ).
Oahu-type, semicircular amphitheater calderas such as Taburiente (La Palma) and Las
Cafiadas (Tenerife) have also been interpreted
as open-toward-the-sea depressions generated by
giant landslides (Bravo, 1962; Hausen, 1961,
1969, 1971 ; Ridley, 1971; Navarro and Coello,
1989). In the case of the Las Cafiadas caldera, a
magmatic process, such as an explosive mechanism (Hausen, 1956; Machado, 1964; Ftister et
al., 1968 ) or a subsidence collapse (Arafia, 1971;
Booth, 1973) has also been postulated. Nevertheless, the main arguments to support the explosive and subsidence collapse models for the
Las Cafiadas caldera have been consistently refuted by Navarro and Coello ( 1989 ). The circular shape of the caldera is only apparent since
subsoil information shows an amphitheater open
toward the north under the Teide stratovolcano.
The voluminous pyroclastic deposits accumulated at the south of the island that Booth ( 1973 )
interpreted as plinian eruptions related to the
emptying and collapse of the caldera are, in fact,
the result of many eruptions, often separated by
discordances and paleosoils. The geophysical information published, gravimetric (Camacho et
al., 1988) or magnetotelluric (Astiz and Valentin, 1986) focused attention only on the outcropping part of the caldera and is far from conclusive. These arguments and the presence in the
subsoil of the caldera of an altered, plastically
deformable, clay-rich explosive breccia formation (the "fanglomerate" of Bravo, 1962) suggests a gravitational landslide origin for the Las
Cafiadas caldera, similar to those originating the
Orotava and Giiimar valleys.
Since the eruption of Mount St. Helens in
1980, massive landslides have gained acceptance
as a possible common and key factor in the masswasting destruction of mature, unstable volcanoes. Huge landslides associated with unstable,
rift-zone-bounded, unbuttressed flanks appear to
be responsible to a great extent for the destruction and loss of volume of most volcanic oceanic
islands (Duffield et al., 1981; Stieltjes, 1988;
Holcomb and Searle, 1991). This mechanism
seems to function in a similar way in large guyots
(Vogt and Smoot, 1984).
In the Canaries, the model proposed for the
genesis of rift zones provides a new scheme that
may explain the sources of accumulative tensional stresses that can finally trigger mass
movements capable of creating the above-mentioned depressions. Extension stresses probably
Fig. 7. Satellite (Landsat mosaic assembled by the Spanish Geographic Institute) view of the El Golfo embayment on the island ofHierro, a prototype
of the typical Canarian windward-side coastal depressions resulting from marine erosion. Landslides and gravitational collapses triggered by extensional
stresses at the rift zones greatly enhance the erosion progression.
bO
I
¢%
236
J. C. Carracedo / Journal of Volcanology and Geothermal Research 60 (1994) 225-241
related to phases of intense eruptive activity tend
to build up until reaching the rupture threshold
that triggers massive landslides. These tensional
stresses are of three different types: ( 1 ) non-volcanic long-lasting stresses resulting from the
growth and progressive gravitational instability
of the volcanoes; (2) volcanic long-lasting cumulative stresses, such as (a) the wedging effect
of the dikes forcefully intruded between the parallel sheets of the intrusive complex, (b) the progressive increase in elevation and consequent in-
stability of volcanoes in intense magmatic phases,
and (c) progressive loading of the volcanoes by
new volcanic materials; and (3) volcanic
ephemeral stresses acting in the eruptive processes, such as (a) the strong local seismicity
originated by magma-fracting and forceful dike
intrusions and (b) the dynamically sustained increase in slope angles due to magma swelling. The
sum of these "coherent" tensional stresses may
reach a critical point, at which an eruptive event
may trigger release.
Santa Cruz
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FAULTS RELATED TO
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CANARy ISLANDS
A
Fig. 8. Calderas associated with the active rift zone of La Palma and directed towards its west flank. The enlarged area shows
arcuate en kchelon faults opened during the 1949 eruptive event (Bonelli Rubio, 1950) that may indicate the earliest stage of"
formation of a new caldera.
J.C. Carracedo /Journal of Volcanology and Geothermal Research 60 (1994) 225-241
The presence in the deep parts of the rift zones
of many open fractures, generally parallel to the
dikes - - key factor in underground water storage
on these islands (Navarro and Coello, 1989) - can be explained by the predominance of tearaway gravitational stresses formed during long
repose periods. When intense eruptive phases
start, the wedging of dikes may at first be compensated by the closing of some of those fractures by compression, thereafter starting the dilation wedge stresses that add to the ever-acting
gravitational loading.
The results of these tensional mechanisms vary
with the type of rift zone. In single structures,
landslides may occur perpendicularly to the rift
axis, either on one side of the axis (as in the
southern rift of La Palma, Figs. 2 and 8) or on
both sides (as in the northeastern rift of Tenerife). In complex three-branched rifts, generally
two of the branches concentrate the tensional
stresses and the less active one functions as a
buttress. This is clearly observed on Tenerife and
El Hierro, where northeastern and northwestern
branches have been more active than the southern branch, and the main mass slides were directed seaward in the opposite direction (as in
the Las Cafiadas or E1 Golfo depressions, see Fig.
2 ). Recently, Holcomb and Searle ( 1991 ) found
evidence in the GLORIA sonographs of a giant
gravitational slide in the Julan embayment,
southwestern E1 Hierro (Fig. 2 ), suggesting a period of intense activity of northwestern and
southern rift zones at a different stage of the volcanic evolution of the island.
Nevertheless, instantaneous giant landslides
related to rift activity are easier to produce when
formations that can deform plastically are present in the subsoil, such as the altered, clay-rich
explosive breccias in the subsoil of Tenerife (the
"fanglomerate" of Bravo, 1962). In volcanic
suites without interlayered formations that can
deform plastically the opening of caldera-type
embayments, these may result by a different
mechanism. A long period in which the progression of erosion - - mainly marine, at the windward-side coasts - - is greatly enhanced by rifttriggered gravitational collapses of a lesser scale,
settlement faults, etc., may be a better explana-
237
tion for these structures. For example, although
the type locality for these features, the El Golfo
embayment on the island of E1 Hierro (Figs. 2
and 7), has been interpreted as being the result
of an instantaneous gigantic gravitational slide
(Hausen, 1964, 1973; Bravo, 1982; Navarro and
Soler, 1993 ), it may have mainly an erosive origin. A marine-beach and eolian sand-dune covering thick piedmont deposits resting on a mafine-cut platform have been found in boreholes
at present sea level at the foot of the escarpment
(Carracedo et al., in press). This morphological
and sedimentary sequence, characteristic of the
mature erosive stage of pre-Quaternary coastal
embayments in the Canary Islands (Jandia on
Fuerteventura, Famara on Lanzarote, etc.), is
usually present on the windward side of the islands where erosion is very active.
7. Implications of the existence of rift zones in
the genesis of Canarian volcanism
The genesis of the Canarian Archipelago has
been the subject of an intense debate since the
early 1970s, and is not completely resolved. The
great success of Wilson's (1973) hotspot or
mantle plume model in the explanation of the
origin of Hawaiian-type volcanic island chains
prompted many authors to test the model in the
Canarian Archipelago. Morgan ( 1971 ), Burke
and Wilson (1972), Wilson (1973), Middlemost (1973), Schmincke (1973, 1987), Vogt
(1974), Khan (1974), Carracedo (1979) and
Holik et al. ( 1991 ), among others, considered the
archipelago to be related to plume-generated
processes. An early analysis of African plate motion suggested that it was at a quasi-stationary
state for the last 25 Ma (Burke and Wilson,
1972), or with a motion evaluated by Morgan
(1983) at about 300 km for the last 20 Ma. Palaeomagnetic measurements in the Canaries
cannot differentiate geomagnetic poles from the
Early Miocene onwards in the polar wandering
curve (Watkins, 1973; Carracedo, 1979).
Abdel-Monem et al. (1971, 1972) provided
the first isotopic dates for the subaerial volcanics. Their oldest ages approximately fitted the
238
J. C. Carracedo /Journal of Volcanology and Geothermal Research 60 (1994) 225-241
hotspot model since a progression in age in the
direction of the expected plate motion was observed. Anguita and Hernfin ( 1975 ) preferred a
kinematic model based on membrane tectonics,
with a propagating fracture moving like a zipper
in a way that roughly conforms to the sea-floorspreading fabric. Newer geochronological data
(McDougaU and Schmincke, 1976; Carracedo,
1979; Feraud, 1981; Cantagrel et al., 1984; Ancochea et al., 1990; Coello et al., 1992 ) have substantially changed the earlier scheme (see present oldest subaerial ages in parentheses in lower
inset of Fig. 1 ). Although any kinematic model
probably should be based on more equivalent
ages (e.g. from the submarine building stages
when available), the presently available age data
still support a modified hotspot or propagating
fracture model.
If the three-branched geometry of the rift zones
in fact represents "least effort" angles of 120 °,
this suggests a vertical upwards loading related
to a local mantle upwelling (Burke and Dewey,
1973; Wyss, 1980; Luongo et al., 1991 ). This favors a hotspot genesis for the Canaries, in a process somewhat similar to that existing in Hawaiian-type island chains, although on a lesser
scale of plate motion, eruptive rates and volumes (Carracedo et al., 1992). The motion of
the African plate for the last 60 Ma (Morgan,
1983 ) would be compatible with a quasi-stationary plume that roughly fits the age scheme and
trend of the Canarian chain for the last 30-20
Ma, and which would presently be located under
the island of E1 Hierro (see fig. 10 in Holik et al.,
1991 ). Analysis of the geoid anomalies in the area
of the Canarian Archipelago led Filmer and
McNutt (1988 ) to infer the local lithosphere to
be very rigid, without any indication of shallow
reheating related to mantle plume activity. However, Holik et al. ( 1991 ) detected the presence
at the northern end of the archipelago of a chaotic seismic facies that they interpreted as volcanic in origin. They also detected a low-velocity
anomaly at the base of the crust, which they propose to reflect the signature of thermal rejuvenation and underplating of the oceanic crust as a
result of the earlier activity of the Canarian hotspot around 60 Ma.
8. Implications of the existence of rift zones in
volcanic hazards assessment
One of the main factors invoked as hindering
volcanic hazards assessment in the Canarian Archipelago (7 main islands, 1.7 million inhabitants, 6-7 million tourists per year) is the apparent spatial dispersion of volcanism, in which an
eruption may happen with the same statistical
probability at any point in the archipelago. Only
the age of previous volcanism has been generally
used as an inverse probability factor in the prediction of locations of future eruptions.
The main risk factors associated with volcanism in the Canary Islands (outlined in Fig. 2 )
can now be analysed taking into consideration the
presence of the rift zones. Since most of the Quaternary volcanism and all the historic eruptions
are located in the rift zones, they function, from
the point of view of volcanic surveillance and risk
mitigation, as polygenetic active volcanic edifices and constitute by far the most probable location of any future volcanic eruption in the archipelago. There is, therefore, a compelling
reason to focus volcanic surveillance on these active edifices (Carracedo et al., 1993).
Volcanic hazards associated with the basaltic
fissural-type eruptions of rift zones are generally
small in magnitude. Damage is usually caused by
tephra fall within a radius of a few hundred
metres around the vent and by small-volume and
low-velocity lava flows, which have courses that
are closely controlled by the topography. An exception is the 1730 eruption of Lanzarote, in
which 3-5 km 3 of volcanic products were emitted over 6 years, covering 23% of the island with
pyroclastic ejecta and lava flows (Carracedo et
al., 1992 ). In complex and evolved rift zones, as
on Tenerife, erupted magmas are progressively
more differentiated towards their junction (Soler and Carracedo, 1986). At the rift zones junction, lavas of trachytic-phonolitic composition
have been erupted in long-lasting central-type
composite volcanoes (Teide-Pico Viejo). Explosive eruptive mechanisms and even high-energy plinian episodes have occurred in this stratovolcano, active as recently as the 15th century.
The Teide complex is nested in the caldera of Las
J. C Carracedo /Journal of Volcanology and Geothermal Research 60 (I 994) 225-241
Cafiadas, in the unbuttressed northern flank of
the island (feature 5 in Fig. 2), bounded by rift
zones which have been highly active in recent
times. The growth of this stratovolcano (3718 m
high and 1700 m on the caldera floor), the instability towards the north coast and the dilation
stresses expected in the forceful emplacement of
dike-type conduits in any new fissure eruption
emplaced in these rift zones, pose a significant
hazard to the island and emphasize the need for
the study and surveillance of this central edifice
and its associated rift zones.
Rift zone-generated massive landslides seem to
be geographically common in the Canaries and
should also be considered to be a hazard. This is
illustrated by the opening in the 1949 eruption
in La Palma of caldera-rim-type, broadly curving circular faults (BoneUi Rubio, 1950), probably as a result of the stresses trying to tear away
the west flank of the southern rift zone (Fig. 8 ).
A mechanism of forceful emplacement of dikes
could be responsible for incipient displacement
of the unstable, unbuttressed flank of this rift
zone, which has had an extremely fast growth
rate, forming slope angles > 30% (Fig. 4). An
earlier but similar process may have generated
the slide calderas of Taburiente and Cumbre
Nueva to the north, a mass-wasting response to
the overgrowth of the rift zone, nearly 5 km high
from the sea floor (Figs. 4, 5b and 8 ). Associated
tsunamis (Moore and Moore, 1984) also pose a
related hazard to the islands nearby.
Acknowledgments
This work was supported by the Spanish DGICYT Project PB92-0119 ]. Thorough reviews and
comments by K. Hoernle, P.R. Vogt, G.P.
Walker, R.T. Holcomb and an anonymous reviewer are very gratefully acknowledged. Many
points in this paper were clarified by comprehensive discussions over many years with J.M.
Navarro.
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