Ploughing Boulders on Periglacial Slopes

Just a short post of my thoughts on ploughing boulders.

I read a paper by Ballantyne (2001) recently which concentrates on the movement downslope of boulders in periglacial regions. A ploughing boulder is a boulder which moves downslope faster than the surrounding soil in periglacial regions. They are typically partially buried and have a trough upslope of them due to their preferential movement relative to the surrounding soil.

The paper beautifully illustrates two key findings:

1. Ploughing boulders are only found on slopes affected by solifluction.
2. The rate of ploughing boulder movement relates exponentially to slope gradient.
3. Boulder movement occurs only once per year, due to the thaw of seasonal ice.

While the findings in this paper are commendable, it's worth me explaining the mechanism to which Ballantyne (2001) refers as the reason why boulders move downslope faster than the surrounding soil.

I will attempt to explain the mechanism in a few short steps:

1. Because the boulder conducts heat better than the surrounding soil, an ice lens develops at the base of the boulder during winter.
2. Pore water in the surrounding soil also freezes during winter.
3. At the start of the thaw season, the ice lens melts before the surrounding pore ice, leaving a pocket of water trapped below the boulder.
4. This water pocket provides above-hydrostatic water pressure beneath the boulder, providing lubrication.
5. The surrounding pore ice melts and allows the boulder to slip downslope as a result of this lubrication.

The problem I have with this mechanism is the maintenance of such high water pressures beneath a boulder. The concept suggests that the surrounding soil is strong enough to hold in water pressurised by several tonnes of rock. To me, this seems counter-intuitive.

I would suggest a set of laboratory experiments to test the strength of frozen soil against pressurised water to see what air temperature the frozen soil "walls" fail at, and what air temperature and sunlight intensity would be required to accurately simulate the natural conditions.

References


Ballantyne, C. K., (2001). Measurement and theory of ploughing boulder movement. Permafrost and Periglacial Processes. 12, (267-288).