The Canary Islands and

the Tsunami Threat

I.G.Kenyon

The Canary Islands comprise 7 volcanic islands that rise 6 to 8 km from the seafloor Eruptions occur on average every 30 years

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

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

Landslide History of the Canary Islands 2

Submarine avalanche deposits around the Western Canary Islands

Most Recent

Most likely to collapse next

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 15–20,000 years ago

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

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

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

Basic Geographical Facts

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

Geological Structure 1La 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)

Geological Structure 2A 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

Recent EruptionsCumbre 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

Present day profile of Cumbre Vieja

Steep 1949 fault

Cross section showing the internal structure of Cumbre Vieja

Lower part of block saturated by seawater will lower frictional

cohesion here

Sea level

Cumbre Vieja sequence lies unconformably on older

avalanche deposits from the earlier Cumbre Nueva collapse

Deformation zone of many small faults

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

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

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

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

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

3D Block diagram of La Palma showing the major structural features

Older avalanche deposits

Next likely collapse

La Palma is very steep sided with slopes of >15-20º

Atlantic Ocean floor 4,000

metres deep

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 1990’s suggest that the landslide is moving seawards

at a rate of between 0.5 – 1.0 cm per year

The fault scarp exposed near the summit of Cumbre Vieja

Fault scarp here is approximately 2 metres in height

Downthrown side

Upthrown side

Dip of fault plane

A normal fault

Surprisingly the 1971 eruption did not cause the faulted block to reactivate movement westwards

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

1971 eruption by day 1971 eruption at night

1971 eruption Teneguia

Likely position of head scar following

slope failure

La Palma – birds eye view showing likely position of the next landslide

1949 eruptions that produced major faulting on the flank of Cumbre Vieja

(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

Tsunami Generation 1

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

Tsunami Generation 2

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

Tsunami Generation 3

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

Towards the northeast, Spain and England are likely to experience waves of 5 to 7 metres in height

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

The End

Tsunami Generator!