Slumps caused by thawing ground on Mars and Earth

The Planetary Geomorphology Working Group of the International Association of Geomorphologists has a rather nice article online comparing the landforms caused by slumping during thawing of the ground with similar features that have been seen on Mars. The article is available here:

http://www.psi.edu/pgwg/images/dec09image.html

On Earth, thaw slumps occur in permafrost areas like Alaska. This is an oblique aerial image of these features, taken from the site above:

The Natural Resources Canada has quite a nice (although I suspect somewhat old) cartoon to illustrate how these landslides work:


Essentially, the thaw of permafrost (ground that is usually frozen) allows the weak materials to fail and flow. This typically exposes a new face of frozen ground that, if the temperatures are high enough again, thaws and flows. Thus, over time, the back scarp of the landslide moves back into the hill - i.e. it retrogresses. This is what one looks like in a vertical view on Google Earth:


The features observed (by satellite obviously) on Mars have much the same morphology as retrogressive thaw slides on Earth:


The arrowed features are interpreted as pingos, which are only found in permafrost areas on Earth. Thus, the arcuate features upslope of the pingos are interpreted as the arcuate backscarp of the landslides. The existence of these features of course implies that at some point the ground thawed and there was liquid water present at some point in the past.

The Naches / Nile River landslide and the role of the quarry

Spectacular image of the Naches / Nile River landslide, from the Washington State DNR Flickr site

Hat tip to Andrew Giles for pointing this out to me.

The Seattle Times have run an article today about the role of the quarry in the triggering of the Naches landslide. They quite rightly point out that there is no firm evidence either way as to whether the quarry played a role or not, but the do make some interesting comments about the possible role:

"Washington's Department of Natural Resources warned a Naches, Yakima County, gravel pit four years ago that its operations might be destabilizing a portion of the slope that collapsed onto Highway 410 this week. Records from 2005 show a department geologist noticed a 10-foot-wide fissure between the towering basalt cliffs and a broad talus slope below. The gravel mine appeared to have removed deposits that were buttressing the slope, documents say. "Your surface mining activity may be exacerbating slope instability and, therefore, may be creating a potential hazard to adjacent property and danger to the public health, safety (and) welfare," says a notice issued to the mine owners in September 2005."

The article quotes Prof. David Montgomery from the University of Washington, who is undeniably a world class academic geologist:

"It's definitely premature to rule out the gravel pit as a contributing factor...They were definitely digging at the toe of an active landslide, and that's a recipe for a slide."

The article then goes on to note that the Department of Natural Resources (DNR) wrote a letter dated July 2008 that noted that the agency still had not received a monitoring plan.

The image above clearly shows that this is primarily a slump/rotational failure as outlined in my previous post. This is rather beautifully illustrated by the image below, also from Washington State DNR Flickr site. The back-tilting of the trees are a sure-fire sign that the slope has rotated:

There is undoubtedly some translational movement too, as would be expected in such a large failure.