24 September 2012
This week’s field visits to large landslides in New Zealand has one again highlighted one of the strangest phenomena associated with very large landslides. Many (but not all) big rock avalanches display this behaviour, but it seems impossible. Let me explain.
This is the Round Top rock avalanche on the western side of New Zealand, not far from Hokitika. It is large – perhaps 40 million cubic metres – and comparatively old (several centuries at least).
The deposit is, as you can see, very large. It is spread over a wide area as a thin sheet, and now has farms, woodlands and roads across it. The point with the star is a small quarry – Google Earth estimates that this is about 3.5 km from the crown of the landslide, so the material here has moved a very large distance. The deposit has thinned and spread, but when we look at the debris in the quarry we see this (with Mauri McSaveney for scale):
The source rock here is schist. As you can see from the above it is highly fragmented (and indeed in quarry is breaking down into gravel). However, where it has not yet been disturbed by quarrying and erosion the debris retains its original structure. Intuitively you might expect that the deposit is chaotic given the dynamic processes to which it has been subject, and given that it is broken up into gravel sized pieces. However, this is not the case – it retains its original structure. Quite how and why this should be remains a source of some controversy. It implies that during movement the rock was not subject to turbulence or to shear (i.e. there was no movement of particles relative to each other), which seems really surprising.
Round Top is by no means the only landslide to do this – and indeed is not the most spectacular example. The best that I have seen is the Flims landslide, which moved about 12 cubic kilometres of rock about 10,000 years ago. This is the largest known landslide in the Alps. On the far side of the valley from the source, and some way up the valley wall, is this astonishing outcrop:
The lithology is limestone. The rock is extremely fragmented – indeed if you pick at it with your fingernail it easily breaks into sand and grit sized particles, indicating a huge amount of fragmentation as it was transported. And yet, as you can see above, the original geological structure is preserved to the extent that a tectonic fold and a fault can still be seen easily. And so, even though the rock suffered enough stress to fracture it almost entirely, none of the particles were able to move relative to each other by even a millimetre.
Not all rock avalanches show this behaviour by any means, but many do. It is not clear why.