HOME	Cryogenic Processes: Frost Rock Cracking and Shattering The reduction of bedrock to gravel is critical for the production of new soils. Frost cracked rock [not "limestone"]. Rose Blanche. 2007. Photo: Susan Maunder. [CLICK image to enlarge.] Frost shattered "limestone" boulder field. Formerly an exposed ridge of bedrock. Table Head Ecological Reserve. 2005. Photo: Pat Montague. [CLICK image to enlarge.] Frost shattered "limestone" rock field. Formerly an exposed ridge of bedrock. Just west of Sandy Cove Airstrip. 2002. Photo: Dulcie House. [CLICK image to enlarge.] Frost shattered and patterned "limestone" gravel. Burnt Cape Ecological Reserve. 2005. Photo: Pat Montague. [CLICK image to enlarge.] Water expands 9% by volume as it freezes. Water freezing into ice can generate expansion pressures of up to 200 megapascals - about 1000 times the average air pressure in a car tire! Thus, when water freezes within porous rock, or within fissures in more solid rock, the expansion forces generated by the growing ice are more than enough to cause the rock to fracture and move apart. Normal winter freeze-thaw cycles repeatedly provide new moisture to feed the ice-buildup process. The resulting ratchet-like progression of ice growth is capable of shattering, cracking, or simply forcing rock apart, with considerable efficiency. Soil Heaving: When water freezes within damp, porous soil, ice grows in the direction of heat loss with enough force to displace, or "heave", the soil and rocks around it. Primary Heave The upward movement of soil and rocks, associated with the formation of "needle ice". Occurs mostly during the early part of the freezing season. Lifts small amounts of the upper portion of the substrate. Needle Ice. Note the stones being lifted on the needle tips. Photo: Michael Burzynski. [CLICK image to enlarge.] Secondary Heave The upward movement of soil and rocks, associated with the growth of subsurface "ice lenses". Occurs mostly during midwinter. Lifts the substrate in coherent masses. Responsible for the majority of vertical soil displacement. Soil heaving is greatest in places, or spots, where there is little or no insulating and sheltering vegetation. A "sorted circle" near Sandy Cove Airstrip. Note that, within the circle, several experimental wooden dowels have been thrown up out of the ground by "secondary heave". Photo: Nathalie Djan-Chékar. [CLICK image to enlarge.] Soil Sorting During soil heaving, larger objects (ie. rocks and stones) tend to be displaced upwards to a proportionately greater extent than are smaller objects. As these larger objects approach the surface, a sort of convection current, within the periodically unfrozen soil, circulates them toward the outside rim of the "active area", where they tend to remain and accumulate. The underground spaces once occupied by the rising larger objects are quickly infilled with smaller and smaller objects, which end up forming an increasingly "muddy" core. However, the exact mechanism involved in all of this is still not completely understood. The result of soil sorting is "patterned ground". Patterned Ground: Circles or "Frost Boils" Sorted circles, have a border of stones surrounding a centre of finer material. Unsorted circles, usually occurring in more "peaty" areas, have no such border of stones. A particularly striking example of "Sorted Circles" at Port au Choix National Historic Site. Note the active, "boiling", dark centres. Photo: Michael Burzynski. [CLICK image to enlarge.] A much more subtle example of "Sorted Circles". Photo: Michael Burzynski. [CLICK image to enlarge.] Laura Noel (2000) found a distinct "zonation" of vegetation across the breadth of sorted circles, on the Great Northern Peninsula. Vegetation was rare or non-existent in the centres of circles. Braya species, adapted to soil disturbance, were found between the centres of circles and their border zones. And, competing "limestone" heath vegetation was found in the border zones, and beyond. Nets Nets are extensive complexes of "circles". They may be sorted or unsorted. A "sorted net" complex, made up of many individual sorted circles. Port au Choix National Historic Site. Photo: Michael Burzynski. [CLICK image to enlarge.] A "sorted net" complex, made up of many individual sorted circles. Near Sandy Cove Airstrip. Photo: Dulcie House. [CLICK image to enlarge.] Polygons Polygons are more mature, often larger, "nets". They may be sorted or unsorted. A "sorted polygon". Burnt Cape. Photo: Gene Herzberg. [CLICK image to enlarge.] A "sorted polygon" complex. Burnt Cape. Photo: Michael Burzynski. [CLICK image to enlarge.] Stone Stripes Stone stripes are alternating bands of larger and smaller rocks, occurring on 3-7 degree slopes, on hillsides or inclines. Stone stripes. Port au Choix National Historic Site. Photo: Michael Burzynski. [CLICK image to enlarge.] Stone stripes. Port au Choix National Historic Site. 2003. Photo: Dulcie House. [CLICK image to enlarge.] [Page last updated: November 12, 2021]