The Canary Islands and the Tsunami Threat

1
The Canary Islands
and the Tsunami Threat
I.G.Kenyon
2
The Canary Islands comprise 7 volcanic islands
that rise 6 to 8 km from the seafloor Eruptions
occur on average every 30 years
3
Landslide History of the Canary Islands 1
At least 14 large landslides have been
mapped offshore from the Canary Islands Most of
these landslides have been
dated within the last 1 million years
4
Landslide History of the Canary Islands 2
Recurrence interval is 100,000 years for all
islands and around 300,000 years for
individual islands Landslides comprise 50 to 500
km³ of debris avalanches spread over
130 km from their source Around 25 of the mass
of the islands have been removed
by landslides
5
Submarine avalanche deposits around the Western
Canary Islands
Most likely to collapse next
Most Recent
6
Evidence of past Landslides and Tsunamis
Canary Island volcanoes have experienced
at least fourteen major collapses over several
million years Submarine landslides occur as fans
of debris on the sea floor and occur
once every 10,000 to 100,000years All
the islands except La Gomera have submarine
avalanche deposits around them The most recent
collapse was the El Golfo avalanche on El Hierro
1520,000 years ago
7
Evidence of past Landslides and Tsunamis
In The Bahamas, there are 1000 tonne blocks of
coral limestone found high above current sea
level. The only explanation for these is that
they were transported there by a large tsunami in
the recent geological past. The most recent
volcanic island landslide is thought to be
around 4,000 years old and occurred in the
Pacific Ocean from the flanks of Reunion
Island
8
Landslide deposits around La Palma
Cumbre Vieja volcano is likely to be the next
collapse
Last collapse on La Palma occurred between
175,000 and 536,000 years ago
9
Basic Geographical Facts
La Palma lies at the North West
edge of the Canarian chain Triangular in shape,
it covers an area
of 706km² Maximum height 2426m
(Roque de la Muchachos) Rests on
ocean floor 4000 metres in depth Considered the
steepest sided island in the world with
slopes averaging 15º to 20º
10
Geology map of La Palma showing the locations
and dates of historical eruptions from 1470
Major normal fault that has slipped 4 metres
towards the west
11
Geological Structure 1
La Palma is made up of two
distinct volcanic structures A circular
25km diameter shield volcano in the
north (Northern Shield) Extinct for last 400,000
years, the Northern Shield has a deeply
eroded radial network of barrancos Has a
6km diameter erosional depression on
its SW flank (Caldera de
Taburiente)
12
Geological Structure 2
A north-south elongated 20km long Cumbre
Vieja rift in the south Cumbre Vieja volcano
highly active for last
120,000 years Six eruptions in last 500 years,
two of them in the 20th century (1949 and
1971) Most active volcano in the Canary
Islands over the last 125,000 years
13
Recent Eruptions
Cumbre Vieja is the active volcanic centre
and comprises the southern third of the
island Has erupted 6 times
in the last 500 years Dormancy periods have
vary from 22 to 237 years
14
Cross section showing the internal
structure of Cumbre Vieja
Deformation zone of many small faults
Present day profile of Cumbre Vieja
Steep 1949 fault
Sea level
Lower part of block saturated by seawater will
lower frictional cohesion here
Cumbre Vieja sequence lies unconformably on older
avalanche deposits from the earlier Cumbre Nueva
collapse
15
Possible Trigger Factors 1
Rising magma may increase the pore-water pressure
within the volcano causing a reduction in
friction along the fault plane Dyke emplacement
may initiate collapse Rising magma may generate
small earthquakes which may also help further
destabilise the faulted block
16
Possible Trigger Factors 2
Climate change-rising sea levels and
wetter conditions may reduce friction
within the faulted block as more water
penetrates into the structure Warming of the
oceans destabilises gas hydrates stored in
marine sediments
17
Landslide Dimensions 1
The mass of rock likely to slide is
the same size as The Isle of Man The volume is
estimated between 150 and 500
km³ of rock Maximum dimensions are 25km
long x 20km wide x 1-2km thick Landslide
is like a half submerged wedge of
cheddar cheese lying on its side with
the thin half under water
18
(No Transcript)
19
Landslide Dimensions 2
Landslide estimated dimensions are based
on the average volume of observed avalanche
deposits around the Canary Islands If the block
that detaches is smaller, say only 250 km³
and moves at only 50 metres per second it
will generate a tsunami only 25-40 the size
of the worst case scenario Although this
landslide is half the magnitude and intensity,
the waves will still match the size and
destructive capacity of the 2004 Asian Tsunami
20
Landslide Dynamics
The landslide is likely to move
as a coherent block for at least
15km before breaking up It will cascade
down to a depth of 4000 metres to the ocean
floor at an average speed of 360 kilometres per
hour As the avalanche spreads out
on the ocean floor it will cover
an estimated area of 3,500km²
21
3D Block diagram of La Palma showing the
major structural features
La Palma is very steep sided with slopes of
gt15-20º
Next likely collapse
Atlantic Ocean floor 4,000 metres deep
Older avalanche deposits
22
Why is La Palma so unstable?
Instability initiated during the 1949 eruption
Fractures formed along the flanks of Cumbre
Vieja An entire flank separated from the rest of
the volcano and dropped 4 metres towards the
sea A north-south trending normal fault that dips
west extends for a distance of 4
kilometres The 1971 eruption further to the south
caused no further movement of the detached
block Monitoring during the 1990s suggest
that the landslide is moving
seawards at a rate of
between 0.5 1.0 cm per year
23
The fault scarp exposed near the summit of
Cumbre Vieja
Fault scarp here is approximately 2 metres in
height
Upthrown side
Dip of fault plane
Downthrown side
A normal fault
24
Surprisingly the 1971 eruption did not cause the
faulted block to reactivate movement westwards
1971 eruption by day
1971 eruption at night
Another eruption is likely in the next decade or
so The next eruption may cause the faulted block
to collapse or it might take another 5,
10 or even 20 further eruptions to fully
destabilise it
25
La Palma birds eye view showing likely position
of the next landslide
1949 eruptions that produced major faulting on
the flank of Cumbre Vieja
1971 eruption Teneguia
Likely position of head scar following slope
failure
26
Tsunami Generation 1
(A) Within 2 minutes of the initial
failure, a dome of water 900 metres
high may be generated (B) Within 5 minutes the
leading wave height will drop to 500
metres after 50km of travel (C)
Waves of over 200 metres high hit the
westernmost islands of the Canary
chain
27
Tsunami Generation 2
From 15 to 60 minutes waves sweep eastwards
through the rest of the Canary Islands and
50-100 metre waves make first landfall on the
African mainland D,E and F
28
Tsunami Generation 3
From 3-6 hours the tsunami expands across
the Atlantic with waves of 10 metres hitting
Newfoundland, whilst Florida and South America
can expect waves of 15-25 metres in height
29
Tsunami Generation 4
Tsunamis will reach the eastern seaboard of the
USA within 8-9 hours Even with maximum warning
time, it is unlikely that all areas at risk could
be sufficiently evacuated In some cases the best
escape may be vertical in high rise buildings in
Miami, New York, Boston and other port cities
30
Towards the northeast, Spain and England are
likely to experience waves of 5 to 7
metres in height
31
Could the faulted block be quarried out to
remove the hazard?
The block is 25km long x 20 km
wide and 1-2 km thick Assuming a truck
could remove 10
cubic metres of rock in a single journey Would
need 15 50 billion journeys to remove it If a
truck left every minute of every day
it would take 10 35
million years This does not include excavation
time nor that the lower
part of the block is under water
32
The End
Tsunami Generator!