Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 245

is more likely to develop in fine-grained soils, whereas pore ice will develop in coarse-
grained soils and will not produce heaving. Under natural conditions, frost pull and frost
push operate simultaneously and are difficult to separate, but each contributes to rocks
moving toward the surface.
It was long believed that much of the frost-stirring evident at the surface in mountain
environments was caused by frequent freeze-thaw cycles. However, Fahey (1973) and
Thorn (1979) proved that diurnal (day to night) freeze and thaw generally penetrates no
more than about 10 cm, even on bare and exposed surfaces. Frost heave and thrust de-
pend on relatively deep freezing, so their operation is essentially limited to once a year,
on the annual freeze-thaw cycle. The prolonged and intense cold of the alpine winter is
more responsible for the major features of frost heave. Needle ice, however, operates at
the surface in response to diurnal freeze and thaw.
Needle Ice
Needle ice consists of small individual columns or filaments of ice 1-3 cm high, project-
ing from the soil surface (Fig. 5.6). Each ice needle originates directly from the soil,
and is usually capped by a thin ice layer on which dirt and small rocks can settle. The
needles may be densely packed like brush bristles or in scattered columns, depending
on variations in soil moisture and the extent of freezing. Needle ice typically forms at
night and melts during the day. If it does not melt during the day, and new growth accu-
mulates the following night, a tiered or storied effect may develop. Rarely, three to four
layers may develop, each representing a separate needle-ice event.
Needle ice requires a calm, clear night with freezing temperatures and a fine soil
with high moisture content. Under these conditions, the ice begins to segregate and
grow as the water migrates to the freezing plane (Outcalt 1971). The needles grow from
their bases and push upward at a rate that may reach several centimeters per hour. One
requirement for good needle ice growth is a fine soil that can hold ample water and will
allow rapid migration of the water to the freezing plane. A soil that is too coarse may
not hold enough moisture, while tight clays may inhibit the migration of water. A silty
loam seems to be the ideal texture.