Geology Reference
In-Depth Information
ative budget will
retreat,
or move upslope, as it melts and shrinks. After many years of
warm temperatures and minimal snowfall, a glacier may completely disappear.
Glacial retreat and disappearance are signals scientists have observed to document
modern climate warming. Glaciers in many parts of the northern hemisphere are retreat-
ing, and some have disappeared altogether.
Flowing solidly down the mountain
Regardless of whether a glacier is advancing or retreating, the ice of the glacier is always
flowing downhill — from the zone of accumulation toward the zone of ablation.
It may seem strange to think of a solid moving or flowing. The flow of a solid is different
from the flow of a liquid. In a solid, the atoms (see Chapter 5) are closely packed togeth-
er and don't have any room to move freely like they do in a liquid or gas. Glacier ice
moves in two ways:
Plastic flow
or
internal deformation:
In order for a solid to experience
plastic flow,
the interlocked molecules (in this case, ice crystals) shift together un-
der intense pressure. In a glacier, the pressure comes from the weight of the ice
piled above; it forces the ice crystals to slip past one another.
The rate of plastic flow in a glacier increases as the temperature of the ice ap-
proaches the melting point — the warmer the ice, the more quickly it flows.
Basal sliding:
Glaciers may experience some melting of ice along the bottom of the
glacier where it contacts the ground surface. Friction of the ice against the bed-
rock produces a small amount of heat and results in a thin layer of meltwater. The
water from the melting helps the glacier slip-slide along the ground, a movement
called
basal sliding.
As the glacier moves, the upper surface of the glacier is not subjected to the great
amounts of pressure that cause the internal ice to flow. The surface ice is brittle and re-
sponds to the movement of the glacier by cracking and breaking — a response that you
more typically expect from a solid. As the internal ice flows, cracks or
crevasses
are cre-
ated in the surface of the glacier.
