Transcript Glaciation - Crescent School

Glaciation
http://pubs.usgs.gov/factsheet/fs133-99/worldmap.gif
• about 15 million square kilometres of the
earth’s surface are covered with glaciers
• two types - alpine and continental
• Greenland and Antarctica
continental glaciation
both
have
• continental glaciation is found at high
latitudes; alpine is found at high altitudes
How glaciers form
• two basic conditions are necessary for
glaciation: (1) prolonged cold (2) ample
snow – An ice age! Not all of the snow melts
in the summer – so if it survives and when
the next winter comes an accumulation of
snow begins from year to year.
• over time the overlying layers of snow
cause the lower layers of snow to
recrystallize (refreeze) and the pore space
is reduced, ie., the snow becomes denser –
the air is squeezed out
• At the end of the winter, the snow that is
left is called neve (50% of the air has been
removed). If the snow survives the summer
it is called firn.
• it is called glacier ice when the pore space is
reduced to about 5% and it is impervious to
water
• after about 60m of ice has been
accumulated the pressure of the overlying
layers causes the ice to flow outwards from
the center of accumulation - this will happen
sooner (thinner) on a slope
• Remember
the Pizza Doe example.
http://pubs.usgs.gov/factsheet/fs133-99/worldmap.gif
How ice erodes
1. scraps or scours - like sandpaper rocks are embedded into the bottom of the
ice sheet and erode the bedrock as it flows
2. conveyor belt - embedded rocks are
carried in the ice and deposited in the front
as the glacier flows
3. plucking - ice freezes onto the rock
and pulls or plucks it away as it flows
4. bulldozer - material is pushed along
in front
Mass balance
• glaciers are in a state of mass balance
meaning that the rate of accumulation of ice is
balanced by the rate of ablation (melting)
• note that ice is transferred from the zone of
accumulation to the zone of ablation by flowage
• The Key is that no matter if the glacier is
stationary, retreating or advancing snow is
always being brought to the front or snout of the
glacier.
The End!