ÜDS-Fen 2001-1 Soru=66

The writer points out that most true avalanches ----.

Strictly speaking the term "avalanche" should be
restricted to falls of snow and ice in mountainous
regions but popular usage has extended its meaning
to cover rock fails and landslips in all environments.
The period of greatest danger from avalanches proper
is during a thaw, when melt-water makes a good
lubricant for the snow and ice banked steeply against
rock faces. The rising cloud of white dust, the vertical
grooves and patches of bare rock formed by the
scouring action, and the dull roar of the avalanche are
all common features of mountains above the
permanent snow line. Rock fragments may also be
carried down, for the recurrent freezing and thawing of
water lodged in joints and crevices of the rock forms a
powerful agent of disintegration. The action is the
same as that which leads to burst pipes. Freezing
causes expansion of the water in the spaces of a joint
and produces a pressure sufficient to break the rock.