Volcanic tsunamis and prehistoric cultural transitions in Cook Inlet

Journal of Volcanology and Geothermal Research 176 (2008) 377–386
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
Volcanic tsunamis and prehistoric cultural transitions in Cook Inlet, Alaska
James Begét a,⁎, Cynthia Gardner b, Kathleen Davis a
a
b
Department of Geology and Geophysics and Geophysical Institute, University of Alaska, Fairbanks, AK, 99775-5780, United States
U.S. Geological Survey, David A. Johnston Cascades Volcano Observatory, 1300 SE Cardinal Court, Bldg. 10, Suite 100,Vancouver, WA 98683-9589, United States
A R T I C L E
I N F O
Article history:
Accepted 24 January 2008
Available online 3 June 2008
Keywords:
Augustine Volcano
paleotsunami deposits
volcanic tsunamis
debris avalanches
geo-archeology
volcanic hazards
A B S T R A C T
The 1883 eruption of Augustine Volcano produced a tsunami when a debris avalanche traveled into the
waters of Cook Inlet. Older debris avalanches and coeval paleotsunami deposits from sites around Cook Inlet
record several older volcanic tsunamis. A debris avalanche into the sea on the west side of Augustine Island
ca. 450 years ago produced a wave that affected areas 17 m above high tide on Augustine Island. A large
volcanic tsunami was generated by a debris avalanche on the east side of Augustine Island ca. 1600 yr BP, and
affected areas more than 7 m above high tide at distances of 80 km from the volcano on the Kenai Peninsula.
A tsunami deposit dated to ca. 3600 yr BP is tentatively correlated with a southward directed collapse of the
summit of Redoubt Volcano, although little is known about the magnitude of the tsunami. The 1600 yr BP
tsunami from Augustine Volcano occurred about the same time as the collapse of the well-developed
Kachemak culture in the southern Cook Inlet area, suggesting a link between volcanic tsunamis and
prehistoric cultural changes in this region of Alaska.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Volcanic eruptions can cause natural disasters that have significant
effects on human societies over large areas (Torrence and Grattan,
2002). Volcanic phenomena such as lava flows, volcanic mudflows,
debris avalanches, and pyroclastic flows can bury and damage
extensive areas around volcanoes (Sheets and Grayson, 1979; Blong,
1984). Volcanic tsunamis can cause extensive damage at greater
distances from eruptions than other volcanic processes and cause
casualties in coastal areas where people are completely unaware of
what is happening at the volcano. The 1883 Krakatau eruption in
Indonesia, for example, produced large tsunamis that caused almost
complete destruction and more than 36,000 fatalities in coastal areas
of Java and Sumatra up to 100 km from the volcano, with more
scattered damage and fatalities as far away as Sri Lanka, more than
2500 km from the volcano (Latter, 1981).
Augustine Volcano in the southern Cook Inlet area of Alaska also
erupted in 1883 and also produced a tsunami, but the wave and its
effects were much smaller than those at Krakatau. Historical accounts
and paleotsunami deposits show the 1883 wave was about 6–8 m high
in areas 80 km from the volcano and affected widely separated coastal
sites over an area of 10,000 km2 around southern Cook Inlet (Beget
⁎ Corresponding author.
E-mail address: [email protected] (J. Begét).
0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2008.01.034
and Kowalik, 2006). By a stroke of luck no fatalities resulted from the
Augustine volcanic tsunami because Cook Inlet has very large tides,
ranging from 6–10 m, and the 1883 tsunami occurred near low tide
(Kienle et al., 1987).
Paleotsunami deposits and erosional features can be used to
reconstruct tsunami histories (Rhodes et al., 2006). The sedimentary
deposits and erosional features produced by historic and prehistoric
tsunamis can be used to reconstruct wave heights and the extent of
inland inundation (Dawson and Shi, 2000; Carey et al., 2001). Most
research on tsunami deposits has been done on sediments left by
waves in tidal marshes and back beach areas, but tsunami waves may
also carry marine and beach sediments into terrestrial lakes and peats
(Bondevik et al., 1997, 1998, 2003). In both settings, tsunamis produce
distinctive layers of sediment entrained from beaches and other
coastal environments that record deposition from one or more waves
that reach inland areas beyond the limits of normal wave activity
(Tuttle et al., 2004).
Here we report on paleotsunami deposits and erosional features
that suggest at least four volcanic tsunamis were produced from two
different volcanoes in the Cook Inlet area of Alaska during the last
3600 years. Archeological studies indicate the native people living in
the Cook Inlet area during this time interval inhabited coastal villages
and depended on marine resources for survival (Klein, 1997). We show
that one of the prehistoric volcanic tsunamis in southern Cook Inlet
occurred at approximately the same time as a significant cultural
break in the late Holocene archeological record of the Cook Inlet area,
and we suggest the tsunami played a role in this cultural transition.
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2. The 1883 debris avalanche and tsunami from Augustine Volcano
The 1883 volcanic tsunami from Augustine Volcano provides a
model for understanding the magnitude and possible effects of
prehistoric tsunamis on the early inhabitants of Cook Inlet, Alaska.
The tsunami was generated on October 6, 1883, when a portion of the
summit of Augustine Volcano collapsed northward into the sea.
Augustine Island is uninhabited, so the nearest observations of the
eruption were made from English Bay (modern Nanwalek), located
80 km northeast of the volcano (Fig. 1). An eyewitness account of the
volcanic tsunami produced by the eruption was recorded in the daily
log of the Alaska Commercial Company trading post at English Bay
(Alaska Commercial Company, 1883):
“At this morning at 8:15 o'clock, 4 tidal waves flowed with a
westerly current, one following the other at the rate of 30 miles
p. hour into the shore, the sea rising 20 feet above the usual level.
At the same time the air became black and foggy, and it began
to thunder. With this at the same time it began to rain a finely
Powdered Brimstone Ashes, which lasted for about 10 minutes,
and which covered everything to a depth of over 1/4 inch…
the rain of ashes commencing again at 11 o'clock and lasting all
day.”
Accounts of the 1883 debris avalanche collected by archeologists and
oral historians from the descendants of native Alaskans living in villages
affected by the 1883 tsunami are consistent with the written account
(Pratt Museum, 2004). There were no reported fatalities from the 1883
tsunami, but the tsunami flooded coastal dwellings and washed away
small boats. Cook Inlet has some of the largest tides on earth, and the
1883 Augustine tsunami occurred during a falling tide, when water
levels were several meters below high tide level (Fig. 2). The 20 ft (ca.
6 m) waves observed at English Bay mainly affected areas near the shore,
and little damage occurred in the small coastal village that existed at that
time. The distribution of 1883 paleotsunami deposits shows the tsunami
washed over the southernmost part of the low-lying sand spit occupied
by the village of English Bay (Beget and Kowalik, 2006).
Contemporary scientists reported the 1883 tsunami (Davidson,
1884), but didn't fully understand how the tsunami was generated at
Augustine Volcano. Modern geologic studies on Augustine Island (Kienle
et al.,1987; Siebert et al.,1989; Siebert et al.,1995; Waitt and Beget,1996;
Waitt and Beget, in press) have shown that edifice failure during the
1883 eruption generated a debris avalanche with a volume of ca. 0.5 km3
that flowed down the north flank of the volcano towards the shoreline of
Augustine Island and formed Burr Point (Fig. 2). Former seacliffs
showing the position of the 1883 shoreline today lie 2 km inland from
the coast. The former cliffs have been largely buried by hummocky 1883
avalanche debris deposits ca. 10–15 m thick and by more recent
pyroclastic flows. The 1883 debris avalanche flowed an additional 4 to
5 km into the sea (Beget and Kienle, 1992; Beget and Kowalik, 2006).
The debris avalanche displaced enormous amounts of water from
Cook Inlet as it flowed into the sea. Numerical modeling studies indicate
that the tsunami was formed at and above the leading edge of the debris
avalanche as it flowed beneath the sea, in a manner similar to the way
local tsunami waves were observed forming above submarine landslides
during the 1964 Good Friday earthquake near Valdez and Seward, Alaska.
The numerical modeling shows that tsunamis generated at Augustine
Island traveled at speeds of up to 100 km/h and took about 50 min to
travel from Augustine Island to English Bay (Beget and Kowalik, 2006).
The eyewitness account from English Bay notes that a minor ash
fall occurred at the same time as the tsunami arrived. This ash fall
cannot have been a product of the same eruption that generated the
tsunami, as ash would have traveled more slowly than the tsunami
wave. Ash eruptions tracked during the 2006 eruption of Augustine
Fig. 1. Augustine and Redoubt Volcanoes have produced large debris avalanches and lahars that traveled into Cook Inlet and generated tsunamis. The western side of Cook Inlet is
largely uninhabited, while on the eastern side a series small towns and villages occur along the coastline of the Kenai Peninsula. The road network (solid lines) and the state ferry
system (dashed line) are restricted to the east side of the inlet, and remote sites beyond the road and ferry networks can only be accessed by small boat, air taxi, and helicopter.
J. Begét et al. / Journal of Volcanology and Geothermal Research 176 (2008) 377–386
379
Fig. 2. Oblique view of Augustine Volcano and Augustine Island from the northwest, created using Google Earth imagery. The 1883 debris avalanche, outlined by solid lines, formed
Burr Point on the north side of Augustine Island. A ca. 450-year-old debris avalanche formed West Island, delineated by dashed lines. Northeast Point was formed by a debris
avalanche (dotted lines) ca. 1600 yr BP. Each of these debris avalanches traveled an additional 4–6 km into Cook Inlet.
volcano, for example, took from 1 to 6 h to reach inhabited areas on
the Kenai Peninsula depending on wind speed and direction (Power
et al., 2006). The minor ash fall at English Bay that began at about the
same time as the tsunami lasted only ten minutes and is interpreted as
having been generated by a small precursory eruption a few hours
before the tsunamigenic events (Fig. 3).
Fig. 3. Reconstruction of events at Augustine Volcano plotted on a maregramme (tide plot) retrodicted for Seldovia, Alaska on October 6, 1883. English Bay and Augustine Island are
both located farther down Cook Inlet, and the tide would have fallen slightly more at these sites than at Seldovia. A small precursory eruption (A) occurred at Augustine Island at
about 5:00 in the morning. About 7:30 in the morning (B) a debris avalanche from Augustine Volcano flowed into Cook Inlet, generating a tsunami. At about the same time a large
explosive eruption occurred. At 8:15 am (C) the tsunamis generated at Augustine Island arrived at English Bay, at about the same time as ash erupted at 5:00 in the morning. The tide
was at ca. 2–2.5 m, so the ca. 6 m high tsunami (shown on the marregram as sharp peaks superimposed on the falling tide) overtopped low-lying areas of the spit occupied by the
village at English Bay. The tsunami propagated through other parts of Cook Inlet. At 11:00 am the volcanic ash erupted at ca. 7:30 in the morning and in subsequent eruptions began to
fall at English Bay (D), and continued to fall through the day.
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J. Begét et al. / Journal of Volcanology and Geothermal Research 176 (2008) 377–386
immediately after the tsunami (Alaska Commercial Company, 1883).
The 1883 ash deposits at English Bay typically form a gray layer 1–
3 cm thick (Beget and Kowalik, 2006). Distal 1883 ash deposits were
first recognized at Skilak Lake, some 200 km north of Augustine (Beget
et al., 1994), and are now known to occur at multiple localities in
southern Cook Inlet (Fig. 4). The 1883 tsunami deposits and Augustine
ash deposits at English Bay are in turn overlain by the 1912 Katmai
tephra. The two tephras have distinctive glass chemistries, and their
identities were confirmed by microprobe glass analysis and correlation with prior analyses of the 1883 Augustine and 1912 Katmai
tephras (Beget et al., 1994).
Distal 1883 Augustine tsunami deposits have also been found
along the Cook Inlet shoreline just east of Iliamna Volcano, where they
occur more than a meter above the high tide line. The 1883 Augustine
and Katmai 1912 tephras are not present in this area, but the deposits
have been dated by dendrochronology (Anders and Beget, 1999). Cores
from tidal lagoons near Homer also preserve 1883 tsunami deposits,
but suggest the waves did not extend above the high tide line. The
three localities where the 1883 deposits have been found agree well
with computer models showing that tsunami waves generated at
Augustine volcano are amplified in these specific regions due to the
influence of local bathymetry and coastal geomorphology (Troshina,
1996; Beget and Kowalik, 2006).
Fig. 4. Generalized isopach map of the 1883 Augustine tephra fall. Squares show the
locations of groups of measured sections of tephra deposits on Augustine Island, and at
English Bay, and occurrences of the tephra in multiple sediment cores from Beluga
Lagoon near Homer and Skilak Lake in the central Kenai Peninsula that were used to
constrain the isopach pattern.
The contemporary account also records a much larger ashfall
“commencing again at 11 o'clock and lasting all day” (Alaska
Commercial Company, 1883). We suggest this significant ash fall,
beginning about two hours and 45 min after the tsunami arrived,
records a major explosive eruption associated with or occurring soon
after the debris avalanche that generated the tsunami. This interpretation implies that the 1883 debris avalanche and a coeval(?)
explosive eruption occurred at about 7:30 in the morning, consistent
with typical travel times for ash clouds and the numerical modeling of
tsunami propagation times from Augustine Island (Fig. 3).
Our hypothesis that a debris avalanche and a coeval explosive
eruption occurred very close in time at Augustine Volcano in 1883 is
consistent with the observation that explosive eruptions often
accompany large debris avalanches (Siebert, 1984). For instance,
during the well-known eruption of Mt. St. Helens in 1980 the debris
avalanche exposed the volcanic conduit and triggered a lateral blast
and then a plinian eruption (Voight et al., 1983; Glicken, 1996). The
fact that the account of events at English Bay is geologically reasonable
and consistent with modern models of the behavior of volcanoes and
numerical models of tsunami generation and propagation attests to
the reliability of this important historic record of the 1883 Augustine
eruption and tsunami (Waitt and Beget, in press).
Seasonal high tides at English Bay reach about 6.7 m above sea
level, and low tides fall between −1.2 to +2.1 m, while the surface of
the spit ranges up to 8.5 m elevation (Waythomas and Waitt, 1998;
Waythomas, 2000). The 1883 paleotsunami deposits occur at elevations up to about a meter above the high tide line, and record waves
that only overtopped the part of the sand spit bordering the tidal
channel (Beget and Kowalik, 2006). This agrees with the retrodicted
tide level calculated for the morning of October 6, 1883 as sea level at
the time of the tsunami would have been about 2–2.5 m (Fig. 3), and a
wave ca. 6 m high would overtop only the lowest portions of the spit
at English Bay.
The 1883 paleotsunami deposits on the spit at English Bay can be
precisely dated because they are directly overlain by 1883 volcanic ash
from Augustine that contemporary reports indicate was deposited
3. The West Island debris avalanche and tsunami ca. 450 years ago
A large debris avalanche occurred on the western flank of
Augustine Island about 450 years ago (Siebert et al., 1989; Siebert
et al., 1995; Waitt and Beget, in press) and formed West Island (Fig. 2).
This debris avalanche was accompanied by a large explosive eruption
that dispersed ash towards the Kenai Peninsula northeast of Augustine
Island (Stihler et al., 1992; Beget et al., 1994).
Debris avalanche deposits around much of the perimeter of West
Island have been strongly modified by tsunamis (Fig. 5). While the
center of the island preserves typical smooth-sided conical and
complex hummocks, hummocks around most of the margin of the
Fig. 5. Aerial photograph taken on January 6, 2006, showing the ca. 450 year old debris
avalanche deposits on West Island. Hummocks in the center of West Island are
unmodified, but many hummocks in a zone 100–300 m wide around the margin of the
island have been partially or completely eroded by tsunamis, especially on the south
and west parts of the island. The boundary between the unmodified debris avalanche
deposits and the area affected by tsunamis is abrupt and distinct in most areas (dotted
line), although a few outliers of relatively well preserved hummocks occur in the area
affected by waves, and some water erosion has occurred in low lying areas within the
center of West Island. Also shown are nearby parts of Augustine Island and the lower
flank of Augustine Volcano.
J. Begét et al. / Journal of Volcanology and Geothermal Research 176 (2008) 377–386
Island are eroded or destroyed. Hummocks nearest to the coast tend to
be the most highly modified, with many being eroded almost
completely away by waves that removed virtually all the fine-grained
material in the matrix facies, leaving only piles of the coarsest debris
and boulders (Fig. 6). The effects of wave modification grow less
intense towards the interior of West Island, but wave modification
locally occurs as much as several hundred meters inland from the
current coastline. The pattern and distribution of erosion can be used
to estimate the height of the tsunami waves that hit West Island. On
the south side of West Island fine sediment has been eroded from
hummocks at elevations up to 14 m above high tide at coastal sites,
while hummocks more than 20 m high in the interior of the island
appear unmodified. The wave erosion cannot be attributed to more
recent debris avalanches on the north side of Augustine, such as the
1883 avalanche and tsunami, because these avalanches were much
smaller. For instance, new field data on the height of 1883 tsunami
deposits on Augustine Island show the 1883 tsunami was not high
enough to produce the observed erosion around the margins of West
Island (Keskinen and Beget, 2006).
A more precise estimate of tsunami wave height can be made at a
site on the southwest shoreline of Augustine Island across the lagoon
from West Island, where wave erosion also eroded hummocks. This
portion of Augustine Island was buried by a debris avalanche ca.
1000 years ago (Beget and Kienle, 1992; Waitt and Beget, in press). A
series of volcanic ash and pumice deposits produced by Augustine
Volcano are intercalated with soil layers on top of the highest
hummocks, but the tephra and soil have been stripped away by wave
erosion at sites at lower sites near the coast. A distinct trim line
Fig. 6. (A) Central part of West Island showing unmodified, conical shaped hummocks
4–20 m high of the West Island debris avalanche. (B) Area on southwest coastline of
West Island where fine-grained material has been completely removed by wave erosion
from hummocks, leaving behind piles of cobble- and boulder-sized material that was
too coarse to be transported by the waves. Similar fines-depleted hummocks recording
erosion by tsunami waves occur around much of the margin of West Island, at distances
of 300 m from the modern seashore.
381
Fig. 7. Paleo-Alutiiq wooden box (scale in centimeters), dated to about 450 yr BP,
collected at the Karluk Archeologic site on Kodiak Island, and now in the collection of
the Alutiiq Museum and Archeological Repository, Kodiak Alaska. The box appears to
display an erupting volcano, with an eruption column, ash cloud, and volcanic ash
shown in the atmosphere above a typical volcanic cone. The illustration on the box also
displays a wave pattern next to the volcano, perhaps illustrating a volcanic tsunami
propagating away from the volcano. The box is about the same age as the West Island
debris avalanche, and may be a contemporary illustration of this event and the coeval
tsunami (Beget, 2000).
delineates the elevation of maximum wave run up in this region at
17 m above the high tide line. Because the tidal range is ca. 8 m in this
area, and it is not known if the debris avalanche occurred at high or
low tide, the height of run-up of the proximal West Island tsunami on
the southwest side of Augustine Island is estimated at 21 + 4 m.
The West Island debris avalanche buried the original shoreline on
Augustine Island and travelled at least 6 km farther into the waters of
Cook Inlet, where part of the avalanche forms West Island and part is
visible in bathymetry extending even farther beneath Cook Inlet.
Although tsunami waves impacted West Island and the southwest
side of Augustine Island, no distal paleotsunami deposits correlative
with the West Island tsunami have yet been found around lower Cook
Inlet (Waythomas, 2000; Waitt and Beget, in press).
Native Alaskan peoples in Cook Inlet may have documented this
tsunami. A prehistoric artifact from Kodiak Island appears to be a
representation of an erupting volcano and a tsunami (Fig. 7). The
prehistoric wooden box panel, collected during archeological excavations at the Alutiiq village site of Karluk on Kodiak Island, is currently
on loan from the collection of Koniag, Inc. to the Alutiiq Museum and
Archaeological Repository in Kodiak. The box panel, measuring 8 by
17 cm, was recovered approximately 150 cm below the surface at the
Karluk archeological site on Kodiak Island. Although the box panel has
not been directly radiocarbon dated, it is associated with “house floor
three” described at the Karluk One site. Radiocarbon dates obtained
during excavation at the Karluk site suggest this stratigraphic level
dates to ca. 1550 A.D. (Jordan and Knecht, 1988). The West Island debris
avalanche and tsunami have been radiocarbon dated to ca. 1540 ± 110
A.D. (Siebert et al., 1989), and so are indistinguishable in age from the
artifact found at Karluk.
It is difficult to interpret the precise meaning of the image on the
wooden box, but clues to the meaning of such images can be derived
from ethnographically recorded practices. Among Yup'ik Eskimo, a
similar cultural group to the Alutiiq of Kodiak Island, such paintings
were used to illustrate stories based on actual events (Himmelheber,
1993). Objects used in public places, like wooden boxes, thereby
helped preserve knowledge of past events. Alutiiq myths and stories
include references to volcanic eruptions and earthquakes (Lantis,
1938). Minc (1986) suggests that events that threaten the survival of
the community are likely to be recorded. Large volcanic eruptions and
tsunamis certainly fall in this category.
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The box panel from Karluk shows several features that appear to be
illustrations of processes associated with erupting volcanoes and
tsunamis (Beget, 2000). The circular area above the triangular volcano
can be interpreted as an expanding eruption cloud and the two lines
connecting the cloud and the volcano summit as a narrow eruption
column. Small, dark spots to the right of the volcano may record
dispersal of volcanic ash and other ejecta. Finally, the round, doublehumped feature to the right of the volcano may illustrate a tsunami
wave propagating away from the volcano (Beget, 2000).
We speculate that the ancient artist who created this box panel
painting ca. 450 years ago was describing the West Island eruption of
Augustine Volcano in lower Cook Inlet. We note that the volcano
illustrated on the box closely resembles Augustine Volcano. While all
other volcanoes in the eastern Aleutian arc of the Alaska Peninsula and
south-central Alaska are surrounded by high mountains, only
Augustine Volcano is isolated and lies at sea level. In fact, the volcanic
cone of Augustine appears to rise directly from the sea when viewed
from a distance, and is very similar in shape and aspect to the
illustration of a volcano on the box panel painting.
If this interpretation is correct, the Karluk box comprises the
earliest known human record of a volcanic eruption and a tsunami in
the western hemisphere. Even if the box is not a depiction of the West
Island eruption of Augustine Volcano, the painting on the box by the
unknown Alutiiq artist appears to be a remarkably accurate depiction
of volcanic processes and volcanic hazards that characterized
prehistoric volcanic eruptions in Alaska and posed risks to the Alutiq
peoples who lived or traveled in the Cook Inlet area of Alaska.
4. The Northeast Point debris avalanche and tsunami ca. 1600 year
B.P.
A series of debris avalanches have occurred every few hundred
years on Augustine Volcano during the last two thousand years (Beget
and Kienle, 1992; Waitt and Beget, in press). Bathymetric data show
these debris avalanches travelled at least 4 km into the sea beyond the
modern shorelines. One of the largest of these events was the
Northeast Point debris avalanche (Fig. 2), emplaced shortly before
1610 ± 70 yr BP (Beget and Kienle, 1992). Numerical modeling shows
the Northeast Point debris avalanche was large enough to generate a
significant tsunami, especially if the avalanche occurred at or near
high tide. The Northeast Point debris avalanche entered the sea
traveling directly towards the Kenai Peninsula, and numerical models
show the resultant wave heights were amplified along the Kenai
Peninsula (Troshina, 1996).
No proximal tsunami deposits or erosional features were recognized during field mapping on the eastern flank of Augustine Island
(Waitt and Beget, 1996; Waitt and Beget, in press) because the east
coastline along this part of Augustine Island has been eroded back
hundreds of meters by wave action into cliffs 20–40 m high and there
is no preservation of coastal features or deposits from ca. 1600 yr ago
in this area today.
Paleotsunami deposits dating to ca. 1600 yr BP were recognized at
three distal localities near Seldovia and Nanwaleck (Fig. 1). At
Nanwalek, layers of redeposited beach sand and volcanic ash layers
are preserved along the shore intercalated with terrestrial peat along
seacliffs up to 8 m above high tide line. A thin, continuous sand
horizon can be traced for more than 20 m along the outcrop. This
deposit records a transient event that transported beach sand at least
7 m above the high tide line into peat beds, and is interpreted as a
paleotsunami deposit (Fig. 8). The remarkable lateral continuity of this
deposit across the peat deposit is typical of paleotsunami deposits
found in lakes and peats (Dawson and Shi, 2000).
Radiocarbon dating of peat immediately underlying this paleotsunami sand horizon yielded 1620 ± 70 yr BP (B-190882) at one site, and
1650 ± 40 yr BP at a second site. These dates are indistinguishable from
the age of the Northeast Point debris avalanche, but do not correlate
with prehistoric earthquakes along the subduction zone in southcentral Alaska (Combellick, 1991).
The discovery of the ca. 1600 yr BP paleotsunami deposit at
Nanwalek proves that the tsunami reached an elevation at least 7
above the high tide line at this site. Given a tidal range in this area of
ca. 6 m, the minimum height of the 1600 yr BP tsunami at Nanwelak is
estimated at 7–13 m. Numerical modeling of tsunamis in Cook Inlet
suggests the tsunami was likely of similar height in other areas of
Fig. 8. (A) Sand layers (white bands) preserved in peat at Miller's Landing on Kachemak Bay, near Homer Alaska interpreted as paleotsunami deposits. (B) Coarse sand layer in peat
preserved near Nanwalek, Alaska. The deposit consists of beach sands redeposited in terrestrial peats at an elevation of 6 m above the high tide line. Radiocarbon dates indicate the
sand horizon was deposited about 1600 yr BP, correlative with the Northeast Point debris avalanche from Augustine Volcano. (C) Sand layer in peat directly overlain by volcanic ash
deposit from Redoubt Volcano at Millers Landing. Radiocarbon dates indicate the sand horizon was deposited about 3600 yr BP, correlative with the Crescent River lahar from
Redoubt Volcano. This sand horizon is the lower of the two sand layers shown in (A).
J. Begét et al. / Journal of Volcanology and Geothermal Research 176 (2008) 377–386
southern Cook Inlet, but progressively attenuated as it propagated into
upper Cook Inlet (Troshina, 1996; Beget and Kowalik, 2006).
Two more sites with paleotsunami sands were identified in
trenches dug in peats along the coast near Seldovia. Both Seldovia
sites lie near sea level, in areas where subsidence of 0.5–1.0 m
occurred during the historic 1964 Good Friday earthquake, so their
current elevation is not a reliable guide to the height of the 1600 yr BP
tsunami wave. The granulometry of all of these paleotsunami deposits
is virtually identical to nearby beach sand, and does not resemble local
fluvial sand (Davis, 2006). These are interpreted as additional deposits
of the tsunami produced by the Northeast Point debris avalanche from
Augustine Volcano.
5. Redoubt Volcano debris avalanche and tsunami ca. 3600 years
BP
Redoubt Volcano (3108 m) lies on the Alaska Peninsula about
130 km north of Augustine Island and is substantially larger and more
voluminous than Augustine Volcano (Fig. 1). Redoubt's largest
Holocene eruption occurred ca. 3600 yr BP when the summit of the
volcano collapsed down the south flank of the volcano and formed a
large debris avalanche (Beget and Nye, 1994). The resultant deposit
extends 30 km down the Crescent River valley to the modern
shoreline of Cook Inlet (Riehle et al., 1981). The proximal parts of
this deposit consist of isolated large blocks and groups of hummocks
typical of dry debris avalanches. The irregular hummocky surface of
the proximal debris avalanche gradually transforms at a distance of ca.
10 km from Redoubt Volcano into a distal flat-topped valley-filling
deposit that extends another 20 km to the modern coast. The distal
portion of the Crescent River debris avalanche consists mainly of
hydrothermally altered clays and other fine-grained debris (Riehle
et al., 1981).
The position of the shoreline at the time of the Crescent River
debris avalanche is not known, but may have been a considerable
distance up-valley from the modern shoreline. Bathymetric data show
383
an area of elevated relief extending another three kilometers into
Cook Inlet beyond the current shoreline (Fig. 9).
The Crescent River deposit is exposed in wave-eroded cliffs 4 to
6 m high across most of the 15 km wide Crescent River valley. The total
thickness of the deposit is not known and its volume is poorly
constrained but was at least several times larger than the 0.5–1.0 km3
debris avalanches produced by Augustine volcano. The maximum
velocities attained by debris avalanches tend to increase with the
height of the source volcano and the deposit volume, and can be as
high as 50 to 80 km/hr (Ui et al., 2000). Because of its much greater
volume and run-out distance, the Crescent River debris avalanche
from Redoubt Volcano probably traveled at a higher rate of speed and
displaced more seawater than any of the Augustine Volcano debris
avalanches discussed above.
At least two paleotsunami deposits are preserved in terrestrial peat
deposits near the Millers Landing area of Homer, Alaska, one of which
appears to record a tsunami correlative in age with an eruption of
Redoubt Volcano at the time of the Crescent River debris avalanche
(Davis, 2006). The paleotsunami deposit consists of a laterally
extensive sand layer as thick as 7.5 cm, containing clasts of gravel
very similar in appearance and lithology to beach gravel present today
along the shoreline of Kachemak Bay. The sand layer extends along the
peat exposure for more than 10 m laterally, and also extends at least
several meters back into the face of the exposure.
Peat accumulates in low energy, still-water environments. In
contrast, the transport of sand and gravel requires waves or currents
that are competent to transport coarse sediment. Radiocarbon dating
indicates the Miller's Landing peat bog began forming about
8000 years ago (Davis, 2006). The layer of sand and beach gravel in
the peat therefore records a significant interruption in a very long-term
pattern of peat deposition in an area of extensive inland peat bogs.
Several lines of evidence suggest the sand and gravel layer records a
tsunami that transported coarse material from the beach zone into
peat bogs. The clasts of gravel found together with the sand measure
up to 8 cm in diameter, are rounded and smooth, and have a distinctive,
Fig. 9. Generalized map of the Redoubt Volcano area showing the 33 km long Crescent River debris avalanche, and the much older Harriet Creek debris avalanche (Beget and Nye,
1994). Also shown is the extent of floods produced by eruptions in 1989–90 in the Drift River Valley (dark pattern). The Crescent River debris avalanche flowed into Cook Inlet ca.
3600 yr BP.
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J. Begét et al. / Journal of Volcanology and Geothermal Research 176 (2008) 377–386
highly flattened prolate shape identical to modern beach gravel in this
area. Scanning electron microscope (SEM) images of sand grains from
the sand layer in the peat deposit show the sand grains are identical to
modern beach sand found on the shore of Kachemak Bay. They display
conchoidal fractures, breakage planes and angular edges characteristic
of glacially derived sediment transported into Kachemak Bay by
outwash streams draining the Harding Ice Field and other glaciated
mountains around Kachemak Bay. In contrast, sand grains in local
streams are mainly derived from Tertiary marine sediment exposed
north of Homer that do not have glacial surface textures (Davis, 2006).
The granulometry of the sand and gravel horizon also indicates
these sediments were most likely transported into the peat bog from
the shore, rather than from a fluvial source. Grain-size analysis shows
that the sand in the peat is significantly finer-grained and better
sorted than local stream sediments but are virtually identical to beach
sand (Davis, 2006). Local geography also argues against a fluvial
source, as the nearest stream competent to transport sand and gravel
occurs about 7 km to the east. Finally, similar sand horizons found in
coastal peat in other areas of the world are the results of tsunamis
(Bondevik et al., 1997; Bondevik et al., 1998; Dawson and Shi, 2000;
Bondevik et al., 2003).
The paleotsunami deposit at Homer is directly overlain by a
volcanic ash deposit with geochemical characteristics that closely
resemble volcanic ash derived from Redoubt Volcano (Fig. 10). As
discussed earlier in this paper, the tsunami deposit from the 1883
eruption of Augustine Volcano is directly overlain by volcanic ash
produced from the same eruption (Beget and Kowalik, 2006). The
recognition of a tephra from Redoubt Volcano directly overlaying the
paleotsunami deposit at Homer is consistent with our interpretation
that this deposit records a tsunami triggered by a debris avalanche at
Redoubt Volcano.
Peat underlying the paleotsunami sand horizon has been radiocarbon dated at 3570 ± 70 yr BP and 3760 ± 70 yr BP (Davis, 2006), ages
virtually identical to that of the Crescent River debris avalanche (Beget
and Nye, 1994). Paleoseismic records do not record a major earthquake
at this time (Combellick, 1991). The paleotsunami deposit found in
coastal peat bluffs in Homer is therefore tentatively correlated with a
tsunami generated by the Crescent River debris avalanche, and the
volcanic ash overlying the paleotsunami sand is thought to record an
Fig. 10. Plot of electron microprobe analytical data from glass shards of major tephras
found in the Cook Inlet area, compared to the composition of selected major elements in
glass shards in tephra collected above a 3600 yr BP paleotsunami deposit collected in
terrestrial peat at Millers Landing near Homer. The 3600 yr BP tephra at Millers Landing
falls within the field of Redoubt Volcano tephra compositions, and apparently records
an explosive eruption associated with the contemporaneous Crescent River lahar.
Modified from Beget et al. (1994).
explosive eruption of Mount Redoubt that occurred about the same
time as the Crescent River debris avalanche.
The Miller's Landing peats are currently eroding at rates of 1–
50 cm/yr due to wave action (Davis, 2006). The paleotsunami deposits
exposed in the peat today were not originally deposited at the
shoreline, but instead at a site tens of meters or even several hundred
meters inland. The 3600 yr BP paleotsunami deposit currently lies
about 90 cm above the modern high tide line, but this area subsided
approximately one meter during the 1964 Good Friday earthquake,
and may also have been affected by other prehistoric earthquakes, so
the heights of prehistoric tsunamis cannot be accurately determined.
6. Relationship between volcanic tsunamis and the archeological
record in Cook Inlet
Much of the coastline around Cook Inlet consists of wave-cut cliffs
tens of meters high and broad and active braided stream systems that
usually do not preserve paleotsunami deposits (Waythomas, 2000).
The recent identification of tsunami deposits correlated with the 1883
eruption of Augustine Volcano, together with younger tsunami
deposits left by waves triggered by the1964 earthquake, provides a
sedimentological model for recognizing and understanding the
distribution and character of paleotsunami deposits in this area
(Beget and Kowalik, 2006; Keskinen and Beget, 2006). Although
paleotsunami deposits are not ubiquitous in Cook Inlet, the recognition of paleotsunami deposits in one part of southern Cook Inlet requires
that the same wave also affected other parts of Cook Inlet, although the
local magnitude of the waves may have varied significantly (Kienle et al.,
1986; Troshina, 1996; Beget and Kowalik, 2006).
The 1883 tsunami occurred close to low tide, so its effects on native
communities around Cook Inlet were minimal. In contrast, if this
tsunami had occurred just a few hours earlier at high tide, areas up to
6 m above the high tide line would have been affected at sites all
around southern Cook Inlet (Kienle et al., 1987; Troshina, 1996; Beget
and Kowalik, 2006). Furthermore, since volcanic eruptions capable of
triggering tsunamis probably occur at random times relative to the
local tidal cycle, only some of the eleven debris avalanches produced
by Augustine Volcano during the last 2000 yr likely occurred at or near
high tide (Beget and Kienle, 1992).
One of the largest debris avalanche in the last 2000 yr from
Augustine Volcano occurred about 450 calendar years ago, when the
West Island debris avalanche flowed into Cook Inlet on the west side
of Augustine Volcano (Beget and Kienle, 1992; Waitt and Beget, 1996,
in press). Despite the evidence of wave erosion and tsunami deposits
on West Island and Augustine Island at elevations of 16 m above high
tide, no correlatve distal tsunami deposits have yet been found and no
notable breaks or changes in the archeological record occur at this
time (De Laguna, 1975; Workman, 1980; Klein, 1997.). We suggest the
West Island debris avalanche occurred near low tide, and while it
generated a tsunami that affected West Island and Augustine Island,
the wave evidently had minimal effects on native people living in
areas around Cook Inlet, although they likely observed it and may
have recorded it (Fig. 7).
An older tsunami occurred ca. 1600 yr ago when the Northeast
Point debris avalanche flowed into the sea on the east side of
Augustine Island. Paleotsunami deposits from this event occur at
several sites on the southern Kenai Peninsula, and are found at
elevations lying more than 7 m above high tide (Fig. 8). Given the
heights reached by the waves at sites near Nanwalek and Seldovia, ca.
80 km from Augustine Volcano, it seems likely this volcanic tsunami
happened at or near high tide and affected all of lower Cook Inlet.
The 1600 yr BP paleotsunami deposits indicate this event
inundated areas along the shoreline of southern Cook Inlet. The 7–
13 m wave heights retrodicted for the 1600 yr BP tsunami at Nanwalek
on the east side of lower Cook Inlet are much smaller than the
proximal tsunamis that caused tremendous destruction and loss of life
J. Begét et al. / Journal of Volcanology and Geothermal Research 176 (2008) 377–386
on the island of Sumatra in western Indonesia in 2004, but are similar
in height to the distal tsunamis that hit Thailand, Sri Lanka, India and
other areas around the margins of the Indian Ocean. A tsunami of this
magnitude would strongly affect people living near sea level on sand
spits or beaches on the lower Kenai Peninsula. The 1600 yr BP tsunami
may have killed people caught in lowland sites along the coast and
damaged villages, kayaks, and other artifacts at or just above sea level.
We speculate that the 1600 yr BP tsunami contributed to the
collapse of the Kachemak tradition and culture in Cook Inlet, which
occurred about the same time as the tsunami (Fig. 11). Archeological
studies show that “Kachemak tradition” people had lived successfully
in the Kachemak Bay region of Cook Inlet for almost 2000 yr,
producing distinctive artifacts with a high level of craftsmanship.
However, “by about 1500 yr ago the large sites….appear abandoned”
(Klein, 1997, p. 65). Klein goes on to speculate that natural disasters
such as earthquakes, volcanic eruptions or tsunamis may have caused
the demise of the Kachemak tradition people. Our discovery that a
volcanic tsunami from Augustine Volcano occurred at about the same
time as the collapse of the Kachemak culture in Kachemak Bay and
adjacent areas of southern Cook Inlet suggests a volcanic catastrophe
may be responsible for this cultural transition.
The 3600-year-old tsunami inferred to have been generated when
the Crescent River debris avalanche flowed into Cook Inlet on the
south side of Mt. Redoubt also occurred at about the same time as a
cultural change in the Cook Inlet area. Little is known about the height
of the wave produced at this time, although paleotsunami deposits at
Homer suggest a tsunami traveled 80 km across Cook Inlet and
affected areas along the Kenai Peninsula coast. The age of the 3600 yr
B.P. tsunami broadly corresponds to the end of the pre-Kachemak
cultural tradition in southern Cook Inlet, known as the “Basal Layer”
(Klein, 1997), and it is possible a volcanic tsunami produced during the
3600 yr BP eruption at Redoubt Volcano affected coastal populations
and played a role in this cultural transition (Fig. 11).
7. Summary and conclusions
Natural disasters like earthquakes, volcanic eruptions and tsunamis can result in destruction and loss of life over large areas. Such
events can strongly affect non-technological societies that subsist
through hunting and gathering, as these societies are wholly
dependent on local natural resources that may also be devastated by
Fig. 11. Timing of cultural changes in the Kachemak Bay and lower Cook Inlet area of
Alaska, compared to the timing of major volcanic debris avalanches and paleotsunami
deposits. The West Island debris avalanche and tsunami ca. 450 yr ago are not
associated with a cultural change, possibly because the event occurred at low tide. The
Northeast Point debris avalanche from Augustine Volcano produced a tsunami that
affected areas 6 m above the high tide line at ca. 1600 yr BP. This volcanic tsunami
occurred about the same time as the collapse of the Kachemak culture in the southern
Cook Inlet area. An earlier debris avalanche and lahar at Mt. Redoubt ca. 3600 yr BP
produced a tsunami at about the same time as the end of the “Basal Component“ phase
and the beginning of the Kachemak phase, but little is known about the size or extent of
this tsunami. Figure modified from Klein (1997).
385
the natural disasters (Torrence and Grattan, 2002). Such societies also
typically have very low population densities and can ill afford to lose
village sites or significant portions of their population. After major
natural disasters, people may choose to migrate away even if they
weren't directly affected by the event.
We have presented evidence for four volcanic tsunamis in the
southern Cook Inlet area. The 1883 tsunami was generated when
edifice collapse at Augustine Volcano produced a debris avalanche
that flowed into the sea on the north side of Augustine Island. The
1883 volcanic tsunami produced a wave ca. 6 m high in areas 80 km
from the volcano, but the tsunami occurred at low tide, minimizing its
effect on communities. This historical event provides a model for
understanding the processes and impacts associated with prehistoric
volcanic tsunamis in this area.
The large West Island debris avalanche on the west side of Augustine
Island occurred about 450 calendar years ago. Extensive areas of wave
erosion around much of the margin of West Island and tsunami deposits
and erosion on nearby parts of Augustine Island record a proximal wave
estimated at 21 ± 4 m high. No distal tsunami deposits and no impact on
archeological cultural traditions from this event have yet been
recognized, suggesting this tsunami occurred at or near low tide.
The Northeast Point debris avalanche on Augustine Island occurred
about 1600 yr BP, and is correlated with distal tsunami deposits on the
southern Kenai Peninsula lying as much as 7 m above the high tide line.
Archeologists have shown that the well-developed Kachemak cultural
tradition, which had existed for ca. 2000 years, came to an end in the
southern Cook Inlet area at this time (Klein, 1997). We speculate the
1600 yr BP tsunami caused significant damage in coastal areas around
southern Cook Inlet, and contributed to the end of the Kachemak culture.
The Crescent River debris avalanche was produced by edifice collapse
at Redoubt Volcano ca. 3600 yr BP, and is tentatively correlated with a
paleotsunami deposit of the same age underlying a Redoubt tephra layer
at a site near Homer on the Kenai Peninsula. Little is known about the
magnitude of this tsunami. Archeologists have shown that a cultural
transition also occurred in Cook Inlet about 3600 yr BP.
This report has significant implications for understanding the
volcanic tsunami hazard in coastal areas around Cook Inlet. The
uncertainties about the potential volcanic tsunami hazard in Cook
Inlet became problematical when Augustine Volcano began to erupt in
late December 2005, with eruptions continuing into 2006. It has been
known for some time that Augustine and Redoubt Volcanoes in the
Cook Inlet area have together produced more than a dozen major
debris avalanches that flowed into Cook Inlet during the last
4000 years (Beget and Kienle, 1992; Beget and Nye, 1994), but the
existence of a concomitant volcanic tsunami hazard has been
controversial. We demonstrate here that at least four volcanic
tsunamis have occurred in the Cook Inlet area during this time period.
Finally, we show that the large tidal range in Cook Inlet plays a
critical role in controlling tsunami run-up heights and modulating
tsunami hazards in southern Cook Inlet. Augustine Volcano has
produced debris avalanches large enough to travel several kilometers
into Cook Inlet every 150–200 years for the last two millenia (Beget and
Kienle, 1992), but apparently only a few of these generated tsunamis
large enough and occurred close enough to high tide to produce
significant damage to coastal areas around southern Cook Inlet. The
largest known volcanic tsunami in the southern Cook Inlet area
occurred at ca. 1600 yr BP, left deposits up to 7 m above the high tide
line along the southern Kenai Peninsula, and may have played a role in
the collapse of the “Kachemak tradition” culture. Based on its past
pattern of behavior, Augustine Volcano is likely to produce another
debris avalanche during a future eruption, but the degree of coeval
tsunami hazard will be dependent on the timing of that day's tides.
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