CANARY ISLANDS LANDSLIDES AND TSUNAMI GENERATION: CAN WE
USE TURBIDITE DEPOSITS TO INTERPRET LANDSLIDE PROCESSES?
R. B. WYNN and D. G. MASSON
Challenger Division, Southampton Oceanography Centre, European Way,
Southampton, SO14 3ZH, U.K.
Abstract
The Cumbre Vieja volcano, on La Palma in the western Canary Islands, is an unstable
area that may develop into a future landslide, generating a tsunami that could cause
damage far from the source. However, volcaniclastic turbidites that are directly
correlated with the two most recent Canary Islands landslides, show stacked sub-units
within a single turbidite bed. This may indicate multiple stages of landslide failure.
Similar findings have previously been reported from volcaniclastic turbidites linked to
Hawaiian landslides. Consequently the potential tsunami hazard from such failures
may be lower than previously predicted.
Keywords: Canary Islands, landslides, turbidites, tsunamis.
1. Introduction
Giant landslides, normally in the form of debris avalanches, have affected all of the
Canary Islands at some time in their evolution, although in the last 1 Ma landslide
activity has been concentrated on the younger and more volcanically active islands of
El Hierro, La Palma and Tenerife (Fig. 1) (Krastel et al., 2001; Masson et al., 2002).
Recent studies (Carracedo et al., 1999a; Day et al., 1999; Ward and Day, 2001) have
highlighted the Cumbre Vieja volcano on La Palma as a potentially unstable area,
which may form the site of a future landslide. It has been suggested that this landslide
could generate a ‘mega-tsunami’ with sufficient energy to cross the Atlantic Ocean and
inundate the eastern US coast with 20-25 m high waves (Ward and Day, 2001).
However, this assumes a ‘worst case’ scenario in terms of the volume and velocity of
any displaced block, with suggested values of 500 km3 and 100 m/s-1, respectively.
Simply halving these values would reduce the predicted tsunami wave height to about
5-10 m (Ward and Day, 2001).
Unfortunately, our knowledge of the initial stages of landslide formation, which largely
determines the characteristics of any related tsunami, is limited (Keating and McGuire,
2000; Waythomas, 2000; Ward, 2001). Although numerical modelling of tsunami
propagation is a relatively advanced science, models of landslide-related tsunami
remain hugely dependent on poorly defined geological parameters, such as slide
velocity and mobility. Most models of tsunami generation treat landslides as single,
rapidly moving blocks (Harbitz, 1992; Satake, 2001; Tappin et al., 2001; Ward, 2001;
Ward and Day, 2001), although there is only limited geological evidence to
substantiate this assumption, especially when applied to giant volcanic island slides
(e.g. Moore et al., 1994). Understanding the way in which any future landslide might
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develop is therefore crucial to establishing its hazard potential, including its ability to
generate a tsunami.
This understanding may be improved in part by the study of turbidite deposits that are
directly linked to volcanic island landslides. In this paper, turbidites associated with the
two most recent volcanigenic landslides to have affected the Canary Islands (the El
Golfo landslide on El Hierro and the Icod landslide on Tenerife) are investigated, and
compared to previously documented examples from the Hawaiian Islands.
Figure 1: (a) 3D image of the NW African margin showing location of the Canary Islands, Agadir
Basin and Madeira Abyssal Plain. White arrows show landslide directions from Tenerife (T), La
Palma (P) and El Hierro (H). Black arrows show schematic turbidity current pathways related to
emplacement of turbidites b and g in the Agadir Basin and Madeira Abyssal Plain. Stars mark
locations of sediment cores in the Agadir Basin (see Fig. 3). (b) Map of the western Canary Islands
(black shading) showing areas affected by landslides. Variations in grey shading serve only to
distinguish landslides of different ages. EG = El Golfo, CN = Cumbre Nueva, I = Icod.
2. Turbidite deposits associated with Canary Islands landslides
A sequence of turbidites, interbedded with pelagic/hemipelagic sediments, was
recovered in giant piston cores from the Agadir Basin (about 300 km north of the
western Canary Islands) at a water depth of 4500 m (Fig. 1a). Several of the turbidites
can be correlated across the entire Agadir Basin, and are also present in the Madeira
Abyssal Plain sequence, 600 km west of the Canary Islands (Weaver et al., 1992; Wynn
et al., 2002). Turbidites are dated using micropaleontological data correlated to oxygen
isotope stage boundaries, giving dates that are accurate to within a few thousand years
(Weaver et al., 1992; Wynn et al., 2002). In the Agadir Basin sequence, the two
youngest turbidites rich in volcaniclastic debris, indicating a Canary Island source, can
be confidently correlated with the two most recent landslides on the islands. These
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correlations are based upon landslide/turbidite age relationships (Masson, 1996; Ablay
and Hurlimann, 2000), geochemical data (Pearce and Jarvis, 1995), turbidite sand
mineralogy, and analysis of seafloor morphology, which defines likely turbidity current
pathways (Wynn et al., 2002).
Figure 2: 3D image of El Hierro viewed from the NW showing the El Golfo landslide scar and
deposit. The shoreline is shown as a black line, below which is a narrow zone of no data. Note the
contrast between the smooth landslide chute and the more rugged un-failed flanks to either side.
Large blocks forming the bulk of the landslide deposit can be seen beyond the base of the chute.
Location is shown in Fig. 1b.
The youngest turbidite (Turbidite b) is dated offshore at about 15 Ka, and correlates
with the El Golfo landslide on El Hierro (Fig. 2) (Masson, 1996; Gee et al., 2001).
Onshore studies, based on dating within the subaerial collapse scar, have suggested that
the main El Golfo failure (El Golfo I) occurred earlier than 100 ka (Carracedo et al.,
1999b), and that it was only a more recent, smaller event (El Golfo II) that occurred at
15 ka. However, there is no evidence for an older debris avalanche offshore of the
collapse scar, and no turbidite deposits in the surrounding basins that can be correlated
with an older event. The next youngest volcaniclastic turbidite to be investigated
(Turbidite g) is dated at about 170 Ka and correlates with the Icod landslide on the north
flank of Tenerife (Watts and Masson, 1995; 2001; Ablay and Hurlimann, 2000).
The basal sections of both turbidites b and g show distinctive sequences of stacked subunits, with the laminated sand/silt-rich bases of each fining-upwards sub-unit separated
by up to 10 cm of turbidite mud (Fig. 3). Each sub-unit displays similar colour and
composition to those above and below, but generally it is the sub-units towards the base
of each turbidite that show the coarsest grain size and greatest thickness (Fig. 3).
Turbidites b and g are separated from other turbidites higher and lower in the sequence
by well-defined pelagic/hemipelagic sediment units, indicating substantial time intervals
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(> ~1000 yrs) between deposition of individual turbidites (Wynn et al., 2002). In
contrast, no pelagic/hemipelagic intervals occur between sub-units within b and g, and
bioturbation appears to originate from the top of each turbidite and decreases
downwards (Fig. 3). This indicates that the individual sub-units are not separated by
substantial time intervals and are therefore all linked to one event.
Figure 3: Core photographs showing multiple, stacked sub-units (marked with numbered
italics) in turbidites b and g derived from the Canary Islands. The tops and bases of the
turbidites are bounded by pelagic/hemipelagic sediments (boundaries marked with dotted
line), while the downward decrease in bioturbation from the top of each turbidite indicates
that the sub-units are not separated by substantial time intervals, and may therefore represent
multiple stages of a single landslide event. Magnetic susceptibility curves highlight the
concentrations of volcaniclastic debris, which displays high magnetic susceptibility, in the
silty bases of each sub-unit. A similar trend can be observed in P-wave velocity and grain size
curves, although in this example magnetic susceptibility is used as a proxy for these
parameters as it has greater accuracy at high resolution (sub-cm scale).
3. Turbidite deposits associated with Hawaiian landslides
The stacked sub-units shown by turbidites linked to Canary Islands landslides are
unique within the Agadir Basin, being very different in character to turbidites derived
from the adjacent Moroccan slope and shelf (Wynn et al., 2002). However, they are not
unique in a global context, as studies of turbidites linked to volcanic island landslides
around the Hawaiian Islands have shown that these turbidites also show evidence of
Canary Islands landslides and tsunami generation
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stacked sub-units (Garcia, 1996). They are described as showing multiple, closely
spaced sandy horizons with sharp bases and gradational tops. One of the turbidites
contains four sub-units and is 11 cm thick, with each sub-unit consisting of a 0.5-2 cm
thick sandy base overlain by a 1-3 cm thick muddy top. The petrology of the turbidite
sands indicates that they are sourced from the Hawaiian Islands, and assessment of
turbidite ages allows them to be roughly correlated with individual Hawaiian landslides
(Garcia and Hull, 1994; Garcia, 1996). The structure and grain size of the turbidite subunits indicates that they were deposited in quick succession by a series of turbidity
currents, and there are no obvious seafloor complexities (e.g. channels) separating the
source area and the site of deposition. Garcia (1996) therefore suggests that the origin
of the sub-units is related to multiple stages of failure within a single catastrophic
landslide event, with each stage of failure initiating a single turbidity current.
4. Origin of the sub-units in Canary Island turbidites
We tentatively suggest that the stacked sub-units shown in turbidites linked to Canary
Island landslides also represent multiple stages of failure within single landslide events.
In the Agadir Basin, turbidite g has nine sub-units, potentially corresponding to nine
stages of failure, and turbidite b contains three sub-units, suggesting three stages of
failure (Fig. 3). Alternative interpretations for the origin of the sub-units, such as flow
reflection from basin margins or seamounts (Rothwell et al., 1992), flow surging or
eddying (Kneller and McCaffrey, 1999) or transport through multiple flow pathways
related to complex channel systems (Masson, 1994) can be ruled out in this area. Flow
reflection is eliminated because the core sites in the Agadir Basin are >100 km away
from any significant topography (Fig. 1a). Flow surging or eddying in the low-density
part of stratified turbidity currents was briefly discussed by Kneller and McCaffrey
(1999), but these effects probably involved interaction of flow with a basin margin
slope, which is not applicable in this case. In addition, the grain size and thickness of
individual sub-units indicates that each would have taken at least two days to deposit,
based upon settling velocities of fine-grained sediment (Stow and Bowen, 1980). This
time interval is probably far outside the time-scale of flow surging or eddying within a
single flow, although data to substantiate this are lacking. Finally, the general absence
of submarine channels between the Canary Islands and the Agadir Basin constrains any
flow originating on the islands to a single broad pathway (Fig. 1a). This means that
multiple sub-units in the Agadir Basin turbidites cannot easily be explained by flow
through a number of separate pathways. Further west, in the Madeira Abyssal Plain,
turbidite b is much thicker than in the Agadir Basin, and shows up to 8 basal sub-units
(Rothwell et al., 1992). However, the plain is fed by a complex channel system
(Masson, 1994), and it is therefore impossible to determine whether the sub-units in
this particular basin represent multiple flow pathways or multiple stages of failure.
It is difficult to assess the exact trigger mechanism for individual turbidity currents
responsible for depositing the sub-units. Garcia (1996) suggested that Hawaiian
landslides may have ‘evolved’ into turbidity currents at their distal margins. Another
possibility is that individual landslides occurred as retrogressive failures. Under this
interpretation, the initial failure would have left a steep, unstable scar on the island
flank, while emplacement of the landslide deposit upon unconsolidated seafloor
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sediments caused them to fail, initiating a turbidity current. Every successive failure of
the over-steepened landslide scar would then see more material flow downslope,
displacing the previous deposits and pushing them outwards onto undisturbed seafloor.
This would lead to destabilisation of a new section of seafloor and triggering of a
further turbidity current. The whole event could last several days in this scenario, with
each failure stage separated by periods of a day or more as discussed above (assuming
the velocity of individual turbidity currents was similar between the source and the
basin floor). Another alternative is that individual turbidity currents were initiated by
mixing at the upper interface of each landslide sub-event on its passage downslope,
with a muddy suspension cloud on the top of each slide generating a low-density
turbidity current.
It could be argued that the homogeneous internal appearance of the landslide deposits,
as imaged in 2-D seismic profiles (Masson et al., 2002), indicates that they were
actually emplaced during a single event. However, field and experimental study of
large-scale debris flows has shown that in strongly surging (or even separate successive
flows), the final deposit can be homogeneous in character and lacking internal
boundaries (Major, 1997). Therefore, although not necessarily directly applicable to the
landslides discussed in this paper, which show a range of characteristics from classic
debris avalanche through to debris flow (Masson et al., 2002), these experimental
results show that at least some multiple-stage mass flows can produce a homogeneous
final deposit that contains no evidence of internal boundaries.
The interpretations presented above favour a setting where a landslide is emplaced on
the seafloor in multiple stages, although we recognise that different stages of failure
may have widely varying volumes. We also recognise the possibility that the landslide
could have been emplaced as a single unit, and that the stacked turbidite sub-units were
formed by a different mechanism to those proposed. Alternative interpretations could
include multiple failure of the landslide deposits and/or the slope sediments beyond the
toe of the landslide, shortly after the landslide was emplaced. Unfortunately, without
direct observation, it is difficult to see how confirmation of one or other of these
interpretations may be achieved with certainty.
5. Implications for the tsunami hazard potential
Previous studies, based upon direct observation, have shown that relatively small (<3
km3), high-velocity (100 m/s-1) landslides, formed by lateral collapse of small volcanic
islands, are capable of generating significant tsunamis upon entering the sea (e.g.
Satake 2001; Satake and Kato, 2001). However, the true relationship between giant
(>100 km3) volcanic island landslides and tsunamis is poorly understood, as direct
observations of landslide velocity and failure style in this particular setting are lacking.
In some cases, such as the submarine Storegga Slide offshore Norway, tsunami
deposits have been directly linked to the landslide, and can provide useful information
on tsunami wave height and run-up (Bondevik et al., 1997). However, in other areas
such as Hawaii, data linking tsunami deposits to source landslides has proved more
controversial (Felton et al., 2000; Keating and McGuire, 2000; Waythomas, 2000;
Ward and Day, 2001). In the Canary Islands, onshore evidence of past landslide-related
Canary Islands landslides and tsunami generation
331
tsunamis is completely lacking (Krastel et al., 2001). However, the preservation
potential of tsunami deposits is poor in the rugged island environment, and the lack of
identifiable deposits should be viewed as a function of low preservation potential,
rather than as evidence for the absence of tsunamis.
Our offshore geological data may therefore provide new insights into giant volcanic
island landslide mechanisms, and may help us to better assess the potential hazard from
any future landslide showing similar characteristics to those studied here. The new
dataset presented from the Canary Islands complements that already obtained from
offshore Hawaii, and reinforces the idea that landslides previously regarded as single
catastrophic events could actually result from multiple failures spread over periods of
several days. Separate subsidiary failures occurring over this time scale would each
generate a discrete tsunami, but these successive failures are too widely spaced in time
to have a cumulative effect on tsunami magnitude. The El Golfo and Icod landslides
have total volumes of 150-180 and 80-150 km3, respectively (Ablay and Hurlimann,
2000; Masson et al., 2002). As has already been demonstrated by Ward and Day
(2001), a failure of this size occurring as a single event and entering the sea at high
velocity could be capable of generating a catastrophic tsunami far from the source area,
although this has recently been disputed (Mader, 2001; Pararas-Carayannis, 2002).
However, if as suggested here, each landslide comprised up to nine stages of failure,
the average volume of each subsidiary failure could be as low as 10-25 km3. A failure
of this size is much less likely to generate tsunamis that are damaging far from the
source.
6. Acknowledgements
We would like to thank the ship staff of RRS Discovery and RRS Charles Darwin for
their assistance in data collection, and also John Roberts of GEOTEK Ltd for the use of
their Multi Sensor Core Logger. We are grateful for constructive reviews by Brian
Bornhold, Peter Talling and Homa Lee, which helped to improve the manuscript.
Michael Garcia, Steven Ward and Simon Day are also thanked for their comments on
an earlier version of this paper.
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