4 October 2011

Geological evidence for a large landslide in Tenerife

Posted by dr-dave

ResearchBlogging.org

Summary: This post provides a brief review of a new paper that describes a newly discovered catastrophic landslide deposit in Tenerife.

One of the most intriguing but poorly understood landslide types is that of the volcanic flank collapse.  This is one of those rare occasions in which the geological term actually does a pretty good job of describing the phenomenon.  In a volcanic flank collapse, the side of a volcano fails, usually catastrophically, generating a landslide (look at the comparatively small landslide at Casita to see just how damaging these landslides can be).  These slides can be really big – tens or even hundreds of cubic kilometres – and they can travel huge distances along the sea floor.  Such failures grabbed attention a few years ago due to the potential (overstated, in my opinion) for generating a catastrophic tsunami.

However, we understand such phenomena  really poorly.  There are a number of reasons for this, principally that: a. They occur rarely (globally about one in every 25 years on average), so actually recording one is a challenge; and b. the remains tend to lie in a very dispersed state on the floor of the deep ocean.  Fieldwork at 4 km water depth remains difficult, even if you are really good at holding your breath.

However, it is one particular aspect of these landslides that remains elusive, but is crucially important.  This is the trigger of the collapse event (i.e. of the landslide itself).  Numerous mechanisms have been proposed, including sea level change, climate change, hydrothermal pressure, intrusion of volcanic material, and various others.  It has proven very difficult to ascertain the importance of each of these.  This is an important question if we are to reliably estimate the hazard associated with future potential collapses.

In a paper published in Geology this month, Harris et al. 2011 report a very interesting find on the island of Tenerife, one of the Canary Islands. This is the remains of an ancient collapse event on the south-eastern part of Cañadas volcano.  The landslide deposit, which is up to 50 metres thick, has been mapped across a large area – 90 square kilometres – and this is just the onshore component of the mass, which may extend another 50 km offshore.  The deposit consists of a classic debris avalanche material, with large (typically up to 12 m long axis), shattered blocks in a highly disrupted, unsorted matrix.  This is typical of a highly energetic, very large collapse event. Intriguingly, in the upper part of the deposit some fluviolacustrine (water/lake) sediments are found in the remains of hollows, indicating that in the aftermath of the landslide shallow lakes formed on the surface, presumably as a result of blockages created by the landslide.  Associated with the landslide deposit are the remains of pyroclastic flows.

This is really interesting in itself, but the very well-preserved deposit allows both highly precise dating and a reconstruction of the events that occurred.  The dating yields a date of about 733,000 years ago, with an error of just 3,000 years.  So, the sequence of events is interpreted as being:

  1. An eruption event, termed the Helecho eruption started in the form of an explosive event that showered ash and then pyroclastic material across the local area;
  2. A dome grew on the volcano;
  3. This dome collapsed catastrophically, generating a landslide that travelled 17 km to the shoreline, and then probably much further in the ocean;
  4. Subsequent eruptions draped further pyroclastic, and then pumice, deposits on the surface of the landslide, and rainwater collected in hummocks to form small lakes;

So, in this case the volcanic flank collapse was triggered by a large, explosive volcanic eruption.  Interestingly, the authors note that the landslide collapse left a gap in the rim of the caldera which subsequently channeled pyroclastic deposits to the south-east.

Of course this paper does not solve the question of what triggers volcanic flank collapses, but it is an important data point that validates one of the most likely mechanisms.  It also provides a great opportunity to study these landslides in detail, which should give us a much greater insight into the dynamics of these immense mass movements.

Reference

Harris, P.D, Branney, M.J., & Storey, M. (2011). Large eruption-triggered ocean-island landslide at Tenerife: Onshore record and long-term effects on hazardous pyroclastic dispersal Geology, 39 (10), 951-954 : 10.1130/G31994.1