24 February 2013

An intrepretation of the Mount Dixon rock avalanche in New Zealand using satellite imagery

Posted by Dave Petley

Last month I posted on a couple of occasions about the Mount Dixon rock avalanche in New Zealand, which was caught on a memorable video by Neil Wiltshire.  In the aftermath of the landslide, NASA collected a satellite image of it using their EO-1 ALI instrument.  This image, and a comparator image of the site before the landslide, are now available online.

A NASA EO-1 ALI image of Mount Dixon before the landslide

A NASA EO-1 ALI image of the Mount Dixon rock avalanche

As a reminder, this is the video that Neil Wiltshire collected of the landslide:


The comparison between the video and the image is very revealing.  As I pointed out in my earlier post, the video shows that the landslide with through a rapid transition from a rock avalanche phase (basically a chaotic jumble of rock and ice moving at high-speed) into a sliding phase.  If you look carefully at the above image you can see this transition line really clearly.  I have annotated the image below to try to highlight the different phases of movement:


So my interpretation is as follows:

1. The landslide detached from the near vertical flank of Mount Dixon and fell about 500 metres;

2. In the impact zone it fragmented and transitioned into a rock avalanche

3. In the upper track the rock avalanche has left almost no debris and indeed appears to have eroded away a part of the glacier (in the centre of the track), which will have increased the volume of mobile mass through entrainment

4. In the lower track the rock avalanche starts to leave some small amount of material behind, suggesting the entrainment and deposition are starting to balance.  Note that it appears that there are two different erosion tracks in this phase, with the split occurring just above the point that I have marked as lower track.

5. At the transition line the rock avalanche appears to suddenly change into sliding mode, and large-scale deposition starts

6. As the video shows the main landslide motion ends with a transition to a creep phase.

A really interesting question is what this transition point represents?  Is this a point at which the slope gradient changes, which means that the landslide rapidly slowed? Or was the landslide losing velocity along track (imagine that velocity was initially high at the foot of the cliff, but the slope gradient was too low to allow that velocity to be maintained).  Thus, did the landslide reach a point at which the velocity was too low to maintain an avalanche state?  Or thirdly, intriguingly, was this change initiated by a change in the underlying glacier?  Take a look at the “before” satellite image – you’ll see that the lower track is characterised by crevasses.  At about the transition point this changes to smooth ice.  So, did this change in the basal conditions lead to a change in the landslide?

Many people will now be working on this landslide, so I will leave it to them to sort it out.  I will try to post again about the landslide when the research is published.  There is no doubt that this will provide a significant insight into the somewhat enigmatic behaviour of these large landslides.