MadSci Network: Earth Sciences

Re: Why doesn't someone remove the impermeable rock from La Palma?

Date: Tue Sep 18 17:04:13 2001
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 1000025710.Es

Dear William,

I imagine that your question was sparked by the recent media coverage of 
the risk of a huge landslide at La Palma, in which a giant slice of the 
island would slide into the sea creating a tsunami (tidal wave).  First of 
all, geological events of this magnitude have occurred throughout the 
history of the Earth, but they tend to be very infrequent by human 
standards.  The prediction that this event will happen does not mean it is 
going to happen soon, or perhaps even for thousands of years – if at all!  
The prediction that the mountain might collapse was made by geologists 
some time ago, but only picked up by the media this summer, probably to 
fill newspapers when there wasn’t much news!

That said, it would be impossible and probably unwise to try to prevent it 
by the means you suggest.  The prediction is that a volcanic eruption 
could inject water under high pressure into the rocks, and that the 
pressure and addition of fluid could overcome friction on the rift and 
trigger the collapse.  You must appreciate the size of the mountain to see 
why it is not practical to remove the area at risk.  The unstable 
mountainside sits on a large geological rift zone that runs through the 
mountain.  The unstable area is something like 30 km long and 10+ km wide, 
a huge area, and the rift will run through the mountain to a depth 
possibly of several kilometres.  To get a view of the scale, go to the 
following website and look up La Palma, where you will find a map of the 
island showing the rift zone:

To remove the mountainside by mechanical means would be an impossibly huge 
task, like taking Mount Everest apart with jackhammers and dynamite!  
Drilling explosives deep into the mountain would risk setting off the very 
landslide we want to prevent.

I think that we have to accept that large-scale geological catastrophes 
are part of the risk of living on planet Earth.  Thankfully the largest 
events occur only infrequently.  For example, there is a type of volcano 
called a collapsed caldera, of which there are many dozens on Earth.  On 
average one of these can be expected to erupt every few tens of thousands 
of years – when they occur they are titanic in scale, and have severe 
effects on global climate for decades, not to mention the destruction for 
hundreds or thousands of miles around them, but we will probably not see 
one in our lifetimes.  The possibility of the La Palma mountainside 
sliding into the sea should be ranked as one of these rare, though 
potentially devastating, events.  The best thing we can do is hope that it 
doesn’t happen in our lifetime, and that hope is probably well founded.

I hope this answers your question.

With best wishes,

David Scarboro

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