18 November 2020
Argaga: the major rockfall on La Gomera on 14 November 2020
Videos were widely shared on Twitter over the weekend showing remarkable footage of a major rock slope collapse at Argaga on the Spanish island of La Gomera on 14 November 2020. This is a large, near vertical collapse event, with the debris falling onto a road and into the sea. Fortunately no-one was killed.
There are actually two videos of this event, both capturing slightly different elements. Together they provide a very useful documentation of the failure. The first starts about a minute before the collapse. When the video starts there is some dust close to the rock slope. Over time, isolated (though not trivial) falls occur across the slope:-
The person collecting this video is clearly concerned about the potential for a displacement wave (sensibly), and so evacuates when the collapse occurs.
The second catches less of the precursory activity, although the main collapse is well constrained:
But this one captures the aftermath of the landslide, including the displacement wave that is generated by the collapse, as well as the very impressive cloud of dust.
In this case failure has been controlled by near vertical joints in the volcanic rocks. Collapse would have been controlled by the development of new fractures that linked the existing joints together to create the release surface. As this proceeded the rock mass crept and deformed at an accelerating rate. It is this deformation that pops off the precursory rockfalls. These precursors are of course invaluable as they often warn people in the vicinity that failure is developing, and in this case it also meant that people started to video the slope.
El Pais reports that rockfalls are common on this stretch of cliff. The owners of this vehicle might consider themselves somewhat fortunate:
Quickslide 1: Hurricane Iota has made landfall in Central America
Hurricane Iota crossed the coast in Central America yesterday, bringing exceptional rainfall and very high winds. We await to see the level of damage that it has caused, but landslides are inevitable.