Laboratory frost sorting by needle ice: a pilot experiment on the effects of stone size and extent of surface stone cover

Sorted patterned ground is ubiquitous where gravelly fine soil experiences freeze-thaw cycles, but experimental studies have rarely been successful in reproducing such patterns. This article reports an attempt to reproduce miniature sorted patterns by repeating needle-ice formation, which simulates frost sorting in regions dominated by diurnal freeze-thaw cycles. Six full-scale laboratory models were tested. They consisted of near-saturated volcanic fine soil topped by small stones of uniform size; the models explored a range of stone size (similar to 6, similar to 12, similar to 17 and similar to 22mm) and surface abundance (20, 40 and 60% cover). The stones were placed in a grid on the surface. These models were subjected to 20-30 temperature excursions between 10 degrees C and -5 degrees C in 12hours. The evolution of surface patterns were visually traced by photogrammetry. A data logging system continuously monitored vertical soil displacements, soil temperatures and moistures at different depths. All experimental runs displayed needle-ice formation (2-3cm in height) and resulting displacement of stones. The soil domains tended to heave faster and higher than the stones, leading to outward movement of the former and concentration of the stones. In plan view, smaller stones showed relatively fast and long-lasting movements, while larger stones stabilized after the first five cycles. The 20% stone cover produced stone islands, whereas the 40% cover resulted in sorted labyrinths (a circle-island complex) that may represent incipient sorted circles. The average diameter or spacing of these forms are 12-13cm, being comparable to those in the field. The experiments imply that needle-ice activity promotes rapid formation of sorted patterns, although the formation of well-defined sorted circles may require hundreds of diurnal frost heave cycles. Copyright (c) 2014 John Wiley & Sons, Ltd.