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Australia: The Land Where Time Began |
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Landslides, Tsunamis
During the transition from glacial to interglacial conditions episodes
of intense landslide activity have been identified in the latest
Pleistocene and Holocene in places such as the UK, the Pyrenees,
European Alps, Carpathian Mountains, the peaks of the Canadian Rockies,
the Andes and The Southern Alps of New Zealand.
Of all the factors that may have had a role water was involved
with all of them. The
author1 suggests that the increased development of
instability and slope failure that occurred in post-glacial times may
have been contributed to by such factors as the glaciers and ice caps
retreating leaving expanses of bare rock and debris, rising temperatures
leading to the thawing of mountain permafrost, and the increase of
precipitation rates as weather patterns changed. In some places other
factors such as rapid uplift and increased earthquake activity resulting
from the unloading of ice may also have contributed by deforming slopes
that were already destabilised and shaking them.
There are a number of options for slopes to be destabilised and
landslides triggered in a world where so much water is around as the
climate transitioned from cold to warm. In high mountain ranges where
ice physically binds together a rock face or slope rising temperatures
melting the ice can destabilise the slope, and the possible expansion of
the warming rock may also have contributed slightly. Persistent heavy
precipitation can saturate and mobilise loose debris that has been
exposed by retreating ice. Rock masses can be destabilised, especially
where it is strongly fractured, by rainwater or the melting of snow or
ice. In an unstable rock mass a failure surface needs to form before it
can be the site of a landslide. This can happen over periods of days or
weeks, or in the case of the greatest volcanic landslides, it can take
decades, centuries or even longer. Some idea of the operation of the
process can be gained from analysis of a notorious landslide that
occurred in Italy in the 1960s.
A small watercourse in the mountainous north of Italy that flowed into
the larger Piave River through a spectacular narrow defile about 300 m
high was chosen for the site of a dam. In 1956 work began on the dam
that would be the tallest in the world at that time. By 1959 the dam was
262 m high. New cracks were found in the northern face of Mt Toc that
formed the southern margin of the Vajont valley as a road was being
constructed along the planned reservoir. Serious concerns about the
stability of Mt Toc were expressed by a number of scientists, but
construction continued and the dam was completed in February 1960. Later
that year almost 1 million cubic metres of
rock collapsed into the reservoir, as well as a number of smaller
landslides, but they
continued with the finishing of the dam. When the water reached 170 m
deep the north face of the mountain began slowly moving towards the
reservoir, an M-shaped crack in the face of the mountain opened that was
2 km long and half a kilometre above the floor of the valley,
marking the detachment of an enormous mass of rock from the mountain
face, though the work continued, in spite of the crack widening by a
millimetres per day for the next 3 years. It was noticed that the
opening of the crack slowed every the water level was lowered, which it
was periodically. It was assumed that the widening of the crack could be
controlled by controlling the water level of the reservoir. By the time
the water depth reached 245 m the opening rate of the crack increased to 3 cm per
day. This caused the decision to be made to drain the dam to slow the
widening. At 10:30 PM on 9 October 1963 250 million tonnes of rock was
carried by a landslide into the reservoir, completely filling it. The
115 million m3 of water in the reservoir was pushed over the
dam wall with a splash wave that was 150 m high that struck the town of
Longarone that was close to the foot of the dam in seconds almost
completely destroying it and all its residents before carrying on along
the valley, the wall of water and the cushion of air in front of it
rushing down the valley destroying the villages of Pirago, Villanova,
Rivalta and Fae. In all it killed 2,500 people.
The engineers had assumed that the pore water pressure in the rock of Mt
Toc increased as the water level in the reservoir rose and decreased as
it was lowered,. They believed they could control the cracking by
manipulating the level of water in the reservoir. 40 years later Chris
Kilburn, a volcanologist and landslide expert of University College
London and Dave Petley of University of Durham determined what actually
happened. They found that as the water level rose water had been
percolating down to a clay layer 200 m below the surface. Water is able
to corrode the rock at the pressures encountered at these depths. The
tips of existing cracks had been weakened by the water, which encouraged
growth of these cracks, a process called slow cracking. Over time larger
discontinuities will be formed by the coalescence of these cracks.
Deformation of the destabilised rock mass will occur slowly as long as
these cracks are largely isolated from one another. The fractures begin
to join up as the coalescence continues, movement accelerating until
there is only a single discontinuity, a failure plane is formed that
completely detaches the deforming mass of rock above from the stable
rock below. From this point the process cannot be stopped and the
sliding that follows is catastrophic.
According to the author1 the Vajont Dam, water entered the clay,
which is hard and brittle at these depths, as the water level in the dam rose. New cracks opened as the clay
slowly expanded, these cracks joining up progressively. Cracking stopped
or slowed when the water levels were lowered but didn’t close, the
growth and coalescence of the cracks continued. Lowering the water level
did nothing to halt the process of development of instability, slowing
the cracking rate but not stopping it. Slope failure was certain once
the water level reached 245 m as by then the fractures within the clay
had joined up to a sufficient extent.
This same slow cracking process is likely, according to the author1,
to have a role in triggering large landslides in episodes of rapid
climate change in the past, especial in the post glacial period when
meltwater from retreating ice sheets, increased precipitation, and
surface runoff at higher levels were contributing to the formation of
many new water bodies around the margins of the melting ice sheets and
glaciers in mountainous areas. The author1 suggests perfect
conditions for the destabilising of adjacent slopes and rock faces
resulted from water levels that were rising and around the margins of
the ocean basins that had been depleted during the glacial phase may
have promoted increased instability levels. He also suggests that the
draining of some of the extremely large pro-glacial lakes such as Lake
Agassiz may have resulted, at least in part, from the overtopping and
erosion of the barriers retaining the meltwater by the triggering of
waves by landslides.
In the oceans
According to the author1 submarine slope collapses are among
the largest of the landslides that have been identified up to the
present, their possible triggers being one or more a number of
possibilities. There is hard evidence that submarine landslides do not
occur at a constant rate, being triggered at times of abrupt climate
change. The author1 suggests this should not be surprising as
at times when the climate swings from glacial to interglacial and back
again there are very large changes in the global sea level, especially
at high latitudes, together with a variety of disturbances in the crust
along the margins of the oceans as the ice sheets expand and contract.
Off the coast of Norway, in the North Atlantic, something happened in a
huge mass of glacial sediment in the Early Holocene a bit over 8,000
years ago. A chunk of sediment with a volume of 3,500 km3
separated from the submarine continental shelf and slid downslope to
spread out on the floor of the deep sea, extending more than half way
to Iceland. The scar remaining from the collapse was nearly 300 km wide.
This Storegga Slide triggered a tsunami that has been estimated to have
been 25 m high, the evidence of which as been found in the form of
distinctive sand layers in peat on the Shetland Islands, the
northernmost part of the UK. The sand was recognised in 1990 as the
evidence of the Storegga Slide by David Smith, a Quaternary scientist in
the UK, and others. The author1 says this is similar to the
maximum run-up of the Indian Ocean tsunami that occurred in 2004 on
Sumatra near the epicentre of the earthquake. Since then evidence of the
Storegga Slide has been found at several places along the coast of
Norway, where the tsunami run-up appears to have been 10-12 m, and along
the east coast of Scotland and northeast England were there were run-ups
of 3-6 m. The author1 suggest that no other evidence has been
found because either they are yet to be recognised or that conditions
were unsuitable for the preservation of such evidence.
He also says there is no doubt that the tsunami would not have
been limited to the vicinity of Norway and the northernmost part of the
UK, and it had probably inundated most coastal areas in the region of
the North Atlantic.
Surveys carried out since the Shetland findings have identified more
sandy deposits that suggest the islands were struck by 2 other, though
smaller, tsunamis, one of which occurred about 5,000 years ago and the 2nd
during Roman or early Medieval times, both almost certainly being the
result of submarine slides somewhere in the North Atlantic.
The author1suggsts that as there are 3 tsunamis recorded in
the Shetlands it indicates that there are many more tsunamis such as
that triggered by the Storegga Slide. There are many deposits on the
ocean floor around the world that originated in island and coastal
volcanoes, though most originated as submarine slides around the margins
of the continents. Regions showing evidence of particular instability
can often be recognised where there have been a number of landslides, a
case in point being the continental shelf off Norway that has evidence
of 3 landslides besides the Storegga Slide. According to the author1
a number of features are shared by the places in the submarine
environment displaying evidence of repeated landslides. There are
typically thick sediment deposits, a ready source of landslide material,
the instability is increased by the sloping seafloor, and once a slide
commences, the slope being steep enough gives added impetus. Some
features usually found in submarine environments prone to slope collapse
are unusually large sediment deposits as are found near the deltas of
rivers, or by earthquake activity, a characteristic of deep ocean
trenches at the subduction zones around the world. The large changes in
global sea level, rises and falls, and ancient landmasses that are
subject to uplift and subsidence as the ice sheets expanded and
retreated, would have added dynamism to the marine endowments that were
prone to landslides.
Building up to a slide
According to the author1 there is not much doubt that the
occurrence of the Storegga Slide resulted directly from the changing
climate; the change from a glacial phase to an interglacial phase that
occurred rapidly at the start of the Holocene, producing perfect
conditions for landslides. During the last glacial phase massive ice sheets
covered the Scandinavian countries, as well as much of Northern Europe,
in places reaching depths of 2.5 km. According to the author1
at this time the 2 things which prepared the conditions for the Storegga Slide
when the glacial phase finally ended were the subsidence of the
lithosphere that was covered by ice by several hundred metres and the
exposure of the continental shelf in the area as a result of lowered sea
levels. The fast-moving glaciers that carried large amounts of sediment
and debris across the continental shelf to the sea where it was dumped
on the continental slope that continued on to the deep ocean floor. The
main body of the Scandinavian Ice Sheet had gone by 8,000 years ago and
sea levels had risen to almost modern levels, resulting in the
continental shelf adjacent to Norway was again submerged. Following the
unloading as the ice melted the lithosphere rose, releasing the pressure
that had kept the underlying faults inactive during the glacial phase,
resulting in a marked increase of seismic activity. In the underlying
clays the pressure of the pore-water increased, weakening them and
instability was promoted by the addition to the top of the clay-rich
continental shelf of large amounts of glacial sediment. Seismic activity
opened a failure surface within the clays and started the slide.
The most widely supported mechanism for the Storegga Slide is increased
earthquake activity resulting from uplift of Scandinavia that occurred
after the close of the glacial phase, and the author1
suggests this was not the only time this occurred. The site of the most
recent landslide has been found to be the source of a number of other
slides, the timing of which matches that of the glacial-interglacial
cycle in a similar way to the Storegga Slide in the Holocene.
At least 1 link between climate change and the occurrence of submarine
landslides appear to be now well established. At times of low sea level
during glacial phases glaciers carry large volumes of sediment to the
ice sheet margins. When ice sheets melt and contract in interglacial
periods seismic activity associated with uplift trigger landslides in
the thick deposits of sediment.
Evidence of other large landslides has been found off the coast of
Ireland and near Rockall. 24 submarine landslides have been found
adjacent to the area formerly covered by the American Ice Sheet. Most of
these slides occurred during the last ice age and the Holocene. More
than 50 landslides have been recognised further south off the Atlantic
margin of the US, and most are believed to have formed in a similar
manner at some time in the glacial period or the Holocene. As with other
submarine landslides the trigger is believed to have been the seismic
activity associated with the post-glacial rebound following the melting
of the North American Ice Sheet. The author1 suggests that
the Storegga model can’t be used to explain all submarine landslides. In
the middle and low latitudes that were not covered by ice sheets in the
last ice age the situation is more complex where destabilisation can
occur by a range of different factors, though earthquakes are believed
to be the trigger, but not as a result of lithospheric rebound, rather
to local tectonic circumstances. Halite (a salty mineral) domes that
rise into the sediments, and eventually punch through the sediments on
the continental margin resulting from its low density, have been
suggested to have a role in destabilising slopes and encouraging
sliding. Repeated loading caused by storm waves, or even tides, are
other possible triggers that have been suggested.
Many of the slides that are further from the poles also seem to have
occurred at the transition from the glacial to the post-glacial
conditions, as occurred at the end of the last ice age. The athor1
suggests that in this context a major role may have been played by
rising sea levels by submerging and destabilising continental shelves
that had previously been exposed, and in the process maximising the
opportunities for slope failure. Rapid, temporary sea level rises that
occurred within glacial times, that were associated with interstadials,
short warm episodes, appear to have promoted instability and failure At
the margins of the ocean. A similar effect is apparent in post-glacial
times. A number of major slides occurred off the coast of Mauritania in
northwest Africa that appear to have been caused by destabilising sand
dunes that had formed on the continental shelf when the sea level was
lower. The author1 suggests it is clear that most submarine
landslides occur at times of rising sea level, or at least stable.
Submarine landslides - gas hydrates
Gas hydrates are solid mixtures of gas and water, usually methane, that are
ice-like and that are present in marine sediments and beneath the
permafrost in the Arctic where there are very large quantities. There
are extremely large quantities of carbon sequestered in these gas
hydrates on the sea floor, in total 2.5 times that in the atmosphere.
They have been implicated in past episodes of sudden onset of
exceptional warm spikes in the climate, such as in the Palaeocene-Eocene
Thermal Maximum (PETM) that began a bit less than 56 Ma. Gas hydrates
remain in solid form indefinitely in the thick sediment deposits on the
ocean floor along the margins of ocean basins as long as they are
subject to very low temperature and high pressure, the hydrate breaking
down
if the temperature rises only slightly or the pressure is decreased. If
they break down they release huge quantities of methane into the
atmosphere.
According to the author1 some aspects of gas hydrates are not
well understood. The constituents of gas hydrates, mostly methane and
water, but water with other compounds such as CO2, hydrogen
sulphide, and other compounds. They are not bound tightly enough to form
a chemical compound, being loosely attached to one another that allows
the transition from a solid to a gas and vice versa easy, the transition
being accomplished simply by changing the ambient pressure and
temperature. A sufficient supply of water and gas is required to be
present together, with pressures that are high enough and temperatures
are low enough, or in some cases where the 2 meet to provide the
perfect, ‘Goldilocks’, environment. The temperatures of the ocean were
significantly lower than at present during the last ice age and that
would presumably have allowed gas hydrates to form at shallower depths
than they can at the present. Such gas hydrate deposits in relatively
shallow water would have been particularly susceptible to changing
conditions of temperature and pressure in oceans that were
warming rapidly characteristic of post-glacial times.
A number of processes operating in the ocean sediments form methane, the
key component in most gas hydrates, mostly by the decomposition of
organic debris such as the remains of dead plankton, and by conversion
of CO2 in the sediment of the ocean floor. A sufficiently
large change in either the ambient temperature or pressure on the ocean
floor can promote the sudden transition from a solid to a gas, as the
stability of gas hydrates is controlled by both the temperature and
pressure. The risk of the destabilisation of gas hydrates increases as
the oceans warm in a warming climate, in which case they would suddenly
dump huge quantities of methane into the atmosphere. Countering this, as
the temperature of the Earth increase more ice melts at high latitudes
and altitudes, this increases the pressure on the gas hydrates tending
to stabilise them. Because it is not likely that the 2 effects will
exactly balance each other it will depend on which factor
will outweigh the other. If the water temperatures in the deep ocean
rise faster than the sea level rises this will favour the decomposition
of gas hydrates with the release of methane into the atmosphere that
would increase the atmospheric temperature even faster. If, on the other
hand, the sea level rises faster than the atmospheric temperatures then
the gas hydrates would remain stable stabilised, at least for a time.
Exactly how the gas hydrate is released is relevant both in the past and
in the future is an area of interest. According to the author1
the question is how it reaches the atmosphere, is it by bubbling through
that water to the atmosphere or by some other method? A suggestion that
has attracted some attention is that proposes a role for submarine
landslides. According to this proposal a submarine landslide that was
sufficiently large would be capable of triggering the breakdown of the
gas hydrates beneath them by removing the overlying sediments that would
virtually instantly reduce the pressure acting on them, the released gas
contributing to a self-feeding runaway process in which the methane acts
to increase the area of the slope failure, leading to the release of
more methane from more gas hydrate as the pressure acting on the new
hydrates is lowered. Mark Maslin et
al., a Quaternary scientist,
University College London, is one of the scientists that has driven the
idea of a link between marine gas hydrate breakdown in the past and
marine landslides. They proposed a number of short-lived carbon isotope
excursions in the marine sediments that have occurred at various times
in the history of the Earth which they propose resulted from large
volumes of C12, which is lighter and considered to be a
significant carbon that is sourced from methane in gas hydrates. One
such event is the excursion that occurred coincident with the Chicxulub
impact that defines the KT boundary 65 Ma, that is also coincident with
a mass extinction that finally killed off most of the dinosaurs.
According to Maslin and Simon Day the cataclysmic event was so violent
that it triggered submarine landslides on a global scale. The removal of large volumes of sediment in these landslides
around the world is suggested by Maslin and Day to have caused sudden
drops in pressure acting on gas hydrates within the sediments and
causing them to transform from solid to gas almost instantaneously with
the immediate release of 300-1300 billion tonnes of methane to the
atmosphere. According to the author1 this proposal is
attractive but speculative, though it could account for the large rise
in atmospheric carbon dioxide that occurred following the impact. The
author1 suggests the evidence is strong for a contribution
from marine gas hydrates towards the warming event, that was sudden and
severe, that characterised the PETM.
The Maslin supporters point to evidence of slope failures on a large
scale in connection with gas hydrate destabilisation at this time along
the Atlantic Basin’s western margin during the PETM. It has also been
speculated that in Quaternary the switchback climate of that time may
have involved the release of marine gas hydrates, the resul being the
climate switched back and forth between extremely cold to balmy and back
again over the last 2 million years. According to the author1
submarine landslides have been suggested as the key, the suggestion
being that more occur at times of rapid climate change, which leads to
the destabilisation of more gas hydrate and a dramatic rise in the
atmospheric methane, which would accelerate warming even further. As for
the PETM and earlier periods the proposed link will probably remain no
more than speculative for some time to come. This results, at least in
part, from uncertainties in the atmospheric methane and partly because
of the difficulty with the dating of Submarine landslides. It has also
been suggested, though again with a lack of hard evidence, that at times
of rapid climate change destabilisation of marine gas hydrate, due
either to falling sea levels or ocean temperatures that are rising, may
actually trigger the marine landslides.
The author1 suggests it appears reasonable that marine
landslides and gas hydrate in marine sediments breakdown are linked in
some way, and plenty of circumstantial evidence links them, though it
could be some time before a robust cause and effect connection becomes
apparent. According to the author1 it is clear that a
connection exists between the incidence of both marine landslides and
methane gas release and periods in the past when the climate was being
disrupted as it was transitioning to a new relatively stable state. The
author1 suggests we would be wise to take note given the
projections for our future climate.
According to the author1 at the time of the megaflip when the
Late Pleistocene cold climate transitioned to the mild climate of the
Holocene with melting ice, increased rainfall, filling ocean basins and
glacial lakes that were expanding acted together to greatly increase the
erosion rate, mobilise slopes of loose debris, detach rock faces and
raise the pressure of pore water and promote failure surface growth.
There are already signs that avalanche and landslide activity in
mountainous areas may be increasing, probably as a result of climate
warming.
Valley of the Oxen (Velle del Bove)
This is an amphitheatre in the side of Mt Etna that has been dated to
about 7,500 years ago, in the Early Holocene, the period of time when
the crust in many places on the Earth was rebounding after the loss of
their ice cover and there was a burst of volcanic and seismic activity.
It was at this time when the cold climate of the ice age had given way
to wet and warm conditions that rainfall that was heavy and persistent
eroded large valleys on the sides of Mt Etna, as well as raising water
tables in the region, and saturating the sides of the volcano.
Torrential rain caused the ashy sides of the volcano to be dislodged as
huge mudflows as an explosive combination of water and magma blasted out
rock columns, debris and dust. As magma was forcing its way to the
surface from beneath the summit the pore water of the sodden rocks was
heated to boiling point forcing it to expand. As a result of the boiling
groundwater the pressure broke the volcano in two. The immediate result
was this was the detachment of a huge chunk of the eastern flank and
rushed down the side of the volcano. When this mass of volcanic debris
landed in the Ionian Sea it caused a tsunami that raced to the east.
Based on a computer model by Maria Pareschi et
al. the researchers estimated
that all of the eastern Mediterranean would have been affected by the
tsunami, Greece, the Levant and North Africa, and according to the
author1, if the model proves to be accurate, the run up at
Calabria in southern Italy may have been as high as 40 m, in Greece and
Libya about 8-13 m, and on the coasts of Egypt and the Levant, about 2-4
m. It has been suggested by the Italian researchers that it may have
been this tsunami that destroyed Atlit-Yarn, Neolithic village in
coastal Israel, the village having been suggested by archaeological
excavations to have been destroyed by a tsunami at about this time.
Natural rubble piles
Most volcanoes are not solid, but formed a giant mound of volcanic and
lava rubble that can be easily destabilised and the hearts of many
volcanoes have also been weakened by hydrothermal weathering caused by
the hot fluids that are constantly being circulated, the result being
interiors that are a clayey mush that destabilises much easier than
solid rock. There are also many other reasons for volcanoes to be
destabilised rapidly and making them prone to lateral collapse. When the
substratum beneath volcanoes is uplifted, subsides or is tilted it may
promote instability over a long time period. Environmental factors such
as rising or falling sea levels, especially when the climate is changing
abruptly. The location of most volcanoes on or close to the margins of
tectonic plates, sites that are subject to many earthquakes, there are
many episodes of seismic activity that can trigger collapse of most, if
not all, of a volcano’s flank. Volcanoes often are either snow-capped or
have a wet local climate, and water in either solid or liquid form often
have a role in the triggering of earthquakes, and earthquakes are good
at destabilising volcano flanks, making them more prone to collapse. The
promotion of landslides can also be achieved by heavy rain, and the
melting of snow or ice, especially through their effect of pressurising
pore water in the structure of the volcano. According to the author1
this is a key reason for the instability and collapse of a volcano
apparently being linked to climate change.
According to the author1 in general the largest volcanoes
which have been active for thousands or even millions of years are
subject to the largest collapses. The largest volcanoes, such as the
Canary Island and the Hawaiian Archipelago volcanoes, both of which are
ocean island volcanoes, periodically have collapses of large chunks of
their margins into the sea, leaving large amphitheatres of solid rock.
El Hierro, one of the Canary Islands, is a good example of this; huge
collapses that occurred in prehistoric times have sculpted the island
into the shape of a tricorne hat. The have been at least 12 large
collapses on the 7 islands of the archipelago that are known either by
the scars they left on the land or by the trains of disrupted debris
they left on the ocean floor. A massive amount of rock collapsed into
the sea from the island of Tenerife about 500,000 years ago, the
catastrophic event leaving 2 spectacular valleys, the Orotava Valley and
the Icod Valley, and on Gran Canaria, a neighbouring island, an 80 m
high remnant of a giant collapse of much of the southern flank of a
volcano about 3 Ma, the iconic monolith called Roque Nublo. The
Taburiente Caldera on the nearby island of La Palma is a vast hole that
was gouged out by a landslide more than 500,000 years ago dominates the
islands northern part, and is bounded by vertical cliffs more than 1 km
and 2 days are required to walk I to it. The Cumbre Vieja volcano to the
south of the Taburiente Caldera is the most active in the archipelago in
recent times is of great interest to geologists and geophysicists. The
main cause for this interest is that a large chunk of the western flank
sunk towards the sea by a few metres, a worrying indication that is
becoming unstable. There has been a lively debate among scientists about
if and when it drops into the sea and if it does how big with the
resulting tsunami be.
In the Canary Islands the largest of the volcano collapses have volumes
of a few hundred km3 while that St Helens volcano that
appeared to so large had a volume of about 2.5 km3. The
collapses of Hawaiian volcanoes put even the collapses in the Canary
Islands in the shade, the largest of the 70 known collapses having
volumes of more than 1,000 km3. The Nuuanu landslide, the
largest of the Hawaiian landslides, spread across the submarine flank of
the Koolau volcano that is believed to probably be extinct, and forms
the eastern half of Oahu, the island on which Honolulu is situated. When
it collapsed a few million years ago a huge mass of its flank dropped
into the sea spreading across 23,000 km2 of the adjacent sea
bed, an area about as large of the state of Vermont. This landslide had
a volume of 5,000 km3 or more. One of the individual blocks
that was part of this landslide is the Tuscaloosa Seamount that is 30 by
17 km and nearly 2 km above the floor of the Pacific. As with other
giant landslides the Nuuanu landslide travelled at high speed and came
to a stop at about 230 km from the volcano. It has been estimated that
the debris flow reached a speed of about 270 km per hour or more, and
the huge amount of energy generated by this collapse was so great that
after crossing the 5 km deep seafloor around the Hawaiian Islands, that
resulted from the downward pressure of the archipelago, the debris flow
ran about 150 km up the far side of the deep trough. The tsunami this
caused must have been truly enormous.
Mount St Helens and other volcanoes on land
Volcanoes on land tend to be much smaller than their marine
counterparts, and they are easier to study as well as study their
eruptions as they happen. The sizes of the landslides that occur when
volcanoes on land breakup are about 1,000 times smaller than the largest
of the Hawaiian landslides, the largest being up to a few 10s of km3.
The author1 suggests the largest landslide may have been the one that
occurred at Mt Shasta near Mount St Helens about 330 Ma that had a
volume of about 45 km3, the debris moving about 50 km from
the volcano. About 18,000 years ago there was another large landslide
that moved about 253 km of debris from the flank of Nevado de
Colimo volcano in Mexico west to the Pacific Ocean 120 km away. Other
large landslides from volcanoes have been identified at Fuego and Pacaya
in Guatemala, Taranaki (aka Egmont) in New Zealand and Mt Etna in Italy.
In 1956 one of a group of volcanoes on the Kamchatka Peninsula in
Russia, Bezymianny, lost its eastern flank in a massive landslide. A
blast from the side of the volcano and an eruption that continued for 4
hours was triggered by the sudden reduction of pressure of the magma in
the volcano.
Mount St Helens, 1 of 20 volcanic peaks in the Cascade Range, followed
the same sequence of events 24 years later in 1980. It had been inactive
for more than 120 years and the vent at the top of the volcano had been
blocked. Then in March 1980 swarms of small earthquakes in its vicinity
indicated things were changing as new magma was building up beneath the
volcano. With the usual vent blocked the magma pushed on the northern
flank which bulged out more than 150 m. On 18 May a magnitude 5.1
earthquake directly under the bulging flank triggered a collapse that
dropped the mass of debris at up to 250 km/hour. The explosive release
of gas from the magma sent a shockwave, that the author1
suggests may have been travelling as fast as speed of sound, that
rapidly overtook the landslide and 600 km2 of mature forest
was obliterated, the destruction reaching almost 30 km from the volcano.
The 57 people killed by the eruption were all caught by the lateral
blast, including David Johnston the geologist monitoring the north side
of the volcano. Once the north flank and the summit of Mount St Helens
was gone it erupted sending dust and gas to 25 km into the atmosphere in
the first 15 minutes and spreading ash across 11 states. It was the
largest eruption in the US excluding Alaska since 1915 when Lassen Peak
in California erupted. The damage bill ultimately reached $1 billion.
The author1suggests the Mount St Helens collapse is the best
studied and was the beginning of acute scientific interest in volcano
stability, as well as the dangers resulting from the failure of volcanic
edifices and volcanic landslides. As a result of research carried out
partly as a result of the collapse of Mount St Helens it has been found
that such events occur much more often than was previously realised.
There have been 20,000 deaths caused by 8 volcanic landslides over the
last 350 years, though the average number of collapses over the last 500
years may be up to 20. 480 collapses at 316 volcanoes have been found by
a recent survey, based on the
knowledge of the frequency of such events from historical records, and
the author1 suggests this is much less than the actual
number. According to the author1 100 million people now live
near volcanoes that have collapsed previously, and as such collapses
appear to occur preferentially at times of the transitions of climate,
this suggests there could be increased danger faced by those people. Landslides
According to the author1 it is now clear there is a climate
dimension associated with geological hazards such as earthquakes and
volcanic eruptions. Another geological phenomenon that displays such an
association with times when the climate is in transition, as are
temperature, rainfall and other factors, is landslide, whether
associated with volcanoes or not. According to the author1 it is
difficult to prove irrefutably that the instability and collapse of
volcanoes are influenced or modulated by climate change, in part as a
result of the incomplete record, and partly as a result of dating errors
of some collapses, both of which make correlation much more difficult.
Largely as a consequence of the construction of a database of all known
collapses of volcanoes at University College London, there is the
beginning of a picture emerging. In the geological record there are 500
collapses that have been recognised, most of which occurred in the Late
Pleistocene of Holocene, having ages of less than 40,000 years. As the
present is approached there are increasing numbers of collapses
recognised, as might be expected, as younger collapses will have had
less time to erode or be covered by the products of later volcanic
eruptions. Nearly half of all known volcanic collapses occurred during
the last 10,000 years during the Holocene, as a consequence of more
recent collapses being less affected by erosion and later volcanic
activity. Researchers have tried to reduce this age bias in the record
by a number of methods, one of which is to ignore all recent collapses,
such as those that have occurred over the last 2,000 years and older
than 40,000 years. This just about marks the limits of the carbon-14
dating method. Ignoring all collapses having volumes of less than 1 km3
is used to reduce bias associated with the improved chances of survival
of smaller collapses of younger age. The author suggests that this
method increases the chances of being able to work out collapse rate
variations that may give a clue to a link with climate change, though
the record provided by the remaining collapses is still not close to
providing a complete record of the number of collapses in that period.
A time of climate extremes encompassing the Last Glacial Maximum is
included in the period 40,000-2,000 years ago when the ice sheets had
reached their maximum extent, and the Holocene in which the climate was
transformed into the wetter, warmer state of the present. The author1
suggests that any correlation between volcanic collapses and changing
climate should become apparent over this time period, even with an
incomplete record. Research by Rachel Lowe, UCL, has found a
distribution of volcano collapse that is far from consistent, a number
of distinct troughs and peaks becoming apparent since the Last Glacial
Maximum, the troughs correlating with cold snaps and the peaks
correlating with warmer phases. A trough in the number of volcano
collapses occurs at the same time as the start of the Younger Dryas,
about 13,000 years ago, and another at the start of the 8.2 ka cooling
event. The succeeding warm phase is marked by a peak in the volcano
collapse record.
6,000-2,000 years ago there were a number of switches from cold and dry
to warm and wet and back again that apparently correlate with the
collapse record. The author1 suggests a changing sea level
could possibly be a possibility, rising sea levels being driven by a
warming climate promoting the collapse of the flanks of volcanoes that
face the sea and island volcanoes as the crust beneath them is bent
down. In the latest Pleistocene and Holocene there is not much evidence
that significant global sea level changes were associated with the rapid
switches from cold to warm that appear to influence collapse behaviour
of volcanoes. The author1 suggests a far more likely
explanation may be availability of water changes and loss of ice mass at
transitions from cold to warm climates. Loss of ice from a glaciated
volcano can be a very effective way of removing buttressing from the
volcano flanks leading the flank instability and the promotion of flank
collapse. When combined with a wetter climate increased runoff from
melting snow and ice can raise the pore-water pressure within the
edifice which can lead to a situation favouring sideways collapse, as in
the case of Valle del Bove on the flank of Mt Etna in Italy.
Simon Day, a British geologist, who has studied the possible future
collapse of Cumbre Vieja volcano on the island of La Palma, suggests
volcanoes are more likely to collapse when the climate is warm and wet
than when it is cold and dry. He reports that over the past couple of
hundred thousand years the greatest collapses of ocean island volcanoes
can be correlated with sea surface temperature record, in the Hawaiian
and Canary Islands, but also in other island archipelagos such as the
Cape Verde Islands off the African west coast and Réunion Island in the
Indian Ocean. Over this period major collapses appear to occur at times
of high sea surface temperatures, though not low temperatures, as far as
can be determined, being mindful of dating errors. It is suggested that
sea surface temperature reflects other environmental conditions that can
have the effect of promoting instability of volcanoes and edifice
failure, though there is no direct effect on volcanoes by the surface
temperatures of the ocean. The temperature of the ocean surface is a an
indicator of the general global climate, the oceans being cool during
glacial phases and considerably warmer during interglacials, and when
they are warm the sea level is high and the global conditions tend to be
generally clement and humid compared to the situation during glacials.
Day has noted that at low latitudes, where the largest of the oceanic
islands are situated, changes and peculiarities of Trade Winds, the
prevailing easterly winds in the tropics, resulted from the warmer seas.
The wind systems consequently dumped more rainfall on the upper flanks
of any oceanic island volcanoes in their path than in cooler times that
tended to be more arid prevailing during glacials. Day suggests this
would have raised water tables by several hundred metres thereby
increasing the opportunity for pore waters in the cores of volcanoes to
be heated and pressurised by rising magma that would prime the flank for
collapse.
These suggestions are supported by Gary McMurtry et
al., School of Ocean and
Earth Sciences and Technology (SOEST), University of Hawaii. In the
1990s their research was centred on the Alika-1 and Alika-2 landslides
that resulted from lateral collapse of the largest active volcano in the
world, Mount Loa, on the island of Hawaii, both of which occurred in
interglacial periods at times of warm, wet climates with high sea
levels, just as at present. According to the author1 there
seems little doubt that out present climate is conducive to promoting
the lateral collapse of volcanoes, and will probably become even more so
over time, though according to the author1 this doesn’t mean
that volcanoes are completely stable and unmoving under different
climate conditions. This is exemplified by some of the Canary Island
collapses that are believed to have occurred during times when the
climate was colder and drier and sea levels were rapidly falling, a
correlation that the author1 finds not entirely surprising as
the sudden reduction of the buttressing effect as huge volumes of water
were removed from the flanks of the volcano. The author1
suggests that in broad terms it is looking increasingly as if the
collapse of volcanoes occurs preferentially at time of warmth and
humidity.
It has been cautioned by McMurtry et
al. that there are many
regions where there are volcanoes that have collapsed in the past, and
if increased precipitation is the primary driver for volcano collapse
then as the world continues to warm, with the accompanying increased
humidity, these volcanoes may become increasingly likely to collapse
again, as it is expected that there will increasingly be a higher
frequency of extreme rainfall events. As there are many coastal and
island volcanoes that could be subject to increased rainfall there is
also the possibility that collapse of these island volcanoes and the
flanks of coastal volcanoes closest to the sea could generate tsunamis,
which could then occur more frequently than they have been in the recent
past. Tsunamis are often associated with undersea earthquakes such as
when 1,200 km of the Sunda megathrust ripped open in 2004 and the large
megathrust that occurred off the northeast coast of Japan in 2011. Huge
volumes of water can also be displaced by volcanoes with comparatively
catastrophic consequences, such as the cataclysmic eruption of Krakatoa
in 1883 that had appeared innocuous until it erupted between Sumatra and
Java in Indonesia that generated a large tsunami that killed 30,000
people as it destroyed many coastal villages. The majority of the 12
volcanic tsunamis that have occurred over the last 400 years have been
triggered by volcanic landslides. Included among these is a substantial
landslide on the Ritter Island volcano 100 km north of Papua New Guinea
in the Bismarck Sea. The largest volcanic island collapse in historical
times, that was twice as big as the Mount St Helens slide, generated a
20 m-high tsunami that killed 3,000 people on the adjacent islands.
The author1 suggests the tsunamis generated by volcanic
landslides should not be underrated, nor should their potential for
generating tsunamis, as they are extremely violent events with the
amounts of energy involved being staggering in the formation and
transport of even the smallest volcanic landslides. The comparatively
tiny Mount St Helens landslide generated almost as much energy as 2000
Hiroshima-sized bombs, would have been dwarfed by the energy generated
by the great Hawaiian landslides of the past, the energy of which would
have been on a par with the largest and most destructive earthquakes.
The tsunami results from the transfer of this enormous amount of energy
to the water into which the slide drops, giving it the capacity to cause
destruction of the opposite sides of oceans. The debris remaining from a
tsunami that was found more than 60 m above sea level on the flanks of
the Kohala Volcano in Hawaii was tracked down by Gary McMurtry and Dave
Tappin on the British Geological Survey, indicating it was a truly
awesome tsunami to reach that high up a volcano’s flanks. The height of
this enormous tsunami was actually much higher as for about the last
500,000 years the Kohala Volcano, along with the remainder of Hawaii’s
Big Island, has been subsiding under its own weight to the extent that
at the time of the tsunami about 120,000 years ago the volcano was
nearly 350 m higher than at present, thus changing the height reached by
the tsunami to more than 400 m. This incredible wave, or more probably a
series of waves, resulted from the sliding into the sea of about 500 km3
of the neighbouring Mauna Loa Volcano during the Alika-2 collapse. There
are other such deposits on a number of islands in the Hawaiian
Archipelago as well as in the Canary Islands. Layers of cobbles and
shell material have been found more than 180 m above sea level on the
island of Gran Canaria, the result of a huge volcanic collapse in
pre-historic times, the author1 suggesting it could possibly
have originated on the island of Mount Teide on the neighbouring island
of Tenerife, though the tsunami could have arrived from further away.
There may be other evidence of other giant tsunamis as there have been a
large number of collapses in the Canaries, the Hawaiian Archipelago,
Cape Verde and elsewhere. The author1 suggests there may be
some that have been mapped but misinterpreted as having resulted from
other geological phenomena, though it is likely that they have still to
be found.
The Cumbre Vieja volcano on La Palma in the Canary Islands a volcano is
presently unstable following an eruption in 1949 in which a large chunk
began to separate from the main body of the volcano and continuing
instability and movement has been found by monitoring. This may become a
problem in the future as evidence from ground deformation monitoring
indicates. In a paper published by Steve Ward, University of California,
Santa Cruz & Simon Day in which the presented the first computer model
forecasting the possible scale and extent of a tsunami that would be
generated by the flank falling into the sea. The prediction for a
worst-case scenario that involves a rock mass comparable in size to the
Alika-2 slide from Mauna Loa a tidal dome of water nearly 1 km high
would form within 2 minutes of the failure as the wast flank entered the
waters of the North Atlantic. After 10 minutes, according to the
prediction, a series of waves hundreds of metres high that are spread
out across a distance of about 250 km that inundate the shores of La
Palma, El Hierro and La Gomera, the 3 westernmost islands in the Canary
Archipelago. The coasts of the other islands of the archipelago,
Tenerife, Gran Canaria, Fuerteventura, and Lanzarote, would be swamped
over the about 1 hour. After about 6 hours the northwest coast of Africa
is struck by waves a bit short of 100 m high. Between 3 and 6 m waves
strike the shores of Spain and the UK and the coast of Brazil is struck
by waves of about 20 m high. About 12 hours after the landslide waves
strike the eastern Caribbean and the coast of Florida is struck by a
series of waves, about 12 or more, up to about 25 m high.
This predicted scenario depends on a number of factors, principally the
mass of the chunk that separates from the volcano, its speed of entry
and the persistence of the tsunami as it crosses the ocean. The collapse
is expected to move very rapidly, as no slow-moving collapses are known
of, either at the historical period or from the geological record. The
most heated part of the debate over this potential tsunami concerns the
how rapidly the energy of these moving waves will be dissipated and
quickly their height will diminish as they radiate from the source.
There is a difference between the models about the height of the waves
as they reach the US. Ward & Day favour a wave height of more than 20 m,
others say they wave heights are likely to be a few metres. The author1
says it is not known if a warmer, wetter climate would destabilise the
western flank of the Cumbre Vieja volcano sufficiently to drop it into
the North Atlantic, what can be predicted is that the chances of many
glaciated volcanoes around the world being destabilised under such a
climate is significantly increased.
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Climate Change |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |