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(18.1) : Glacial Advance
and Retreat
Glacial advance and retreat is determined
by the balance between the accumulation of snow and the removal
of ice by sublimation, melting, and calving (ablation). When
the rate of ablation below the snowline equals the rate of accumulation
above it, the glacier is stationary, as in View 1. During glacial
retreat, View 2, the rate of ablation exceeds the rate of accumulation,
and the position of the toe retreats toward the origin of the
glacier. Glacial advance, View 3, occurs when the rate of accumulation
exceeds the rate of ablation. For all views, pay attention to
the motion of the stones. Note that in all cases, ice flows downhill.
by Stephen Marshak
>>see: p.486
(18.2) : Milankovitch
Cycles
Why do glaciers advance and retreat periodically
during an ice age? In 1920, Mulutin Milankovitch showed that
regular variations in the shape of Earth's orbit and the orientation
of its axis create variations of solar intensity at high latitudes:
warm summers in which glaciers retreat, and cool summers when
they advance. These climate cycles, called Milankovitch Cycles,
are determined by three factors: orbital eccentricity, shown
in View 1, changes in the tilt of Earth's axis, View 2, and the
precession of Earth's axis, View 3.
