Geology Reference
In-Depth Information
A well-developed pingo can be a striking geomorphic form (Figure 6.12). However,
pingos are not common features and their existence is usually the result of a number of
distinct and limiting geomorphic and hydrologic conditions. Pingos vary from a few meters
to over 60 m in height and up to 300 m in diameter. They range in form from symmetric,
to asymmetric, to elongate. Not all pingos have a typical conical form. Their one common
characteristic, however, usually concealed by 1.0-10.0 m of overburden, is a core of massive
ice or icy sediments. The ice may be remarkably pure, sometimes with seasonal bubble-
rich and bubble-poor banding, or it may consist of layers of icy sediments. Fractures and
faults are sometimes seen within the pingo ice core. Frequently, pingos portray dilation
cracks and are ruptured near the summit. This is the fi rst stage in decay, melt of its ice
core, collapse of the mound, and ultimate formation of a shallow-rimmed depression
(Figure 6.13).
Pingos were fi rst described from northern Canada by Dr John Richardson (1851). They
were later described in more detail from northern Alaska (Leffi ngwell, 1919) and from
Siberia (Tsytovich and Sumgin, 1937). A. Leffi ngwell (1919) was the fi rst to suggest
hydraulic pressure as the cause while the botanist, A. E. Porsild (1938) was the fi rst to
suggest freezing in a closed system. Although the terms “open-system” and “closed-
system” explain pingo growth, the words “hydraulic” and “hydrostatic” better identify the
source of the water pressure that initiates and sustains pingo growth (Mackay, 1979a).
These terms are used here. Put simply, hydraulic-system pingos derive their water pressure
from a topographic gradient and hydrostatic-system pingos derive their water pressure
from pore-water expulsion beneath aggrading permafrost in saturated sand.
Figure 6.12.
Large hydraulic (open) system pingo (“Inner Pingo”) in Adventdalen, Svalbard.
Water issuing from the summit crater has formed an icing which extends down the fl ank of the hill.
The photo, taken April 2006, is supplied courtesy of Professor O. Humlum.
