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Here
there are hyperlinks to other sites - be thankful to all contributors...There
are three main types of glacial
erosion - plucking, abrasion and freeze thawThe
little diagram opposite shows a corrie
(cirque or cwm) hollow where a glacier starts to form and they
contributor erosion processes. |
| Plucking:involves
direct removal of material from the rock basement.
The more fractured, or broken, that the rock basement is, the easier it is
for the glacier to directly remove and incorporate the rock material. The
particles can be quite large, boulders the size of houses have been found
in debris fields. This material is then incorporated into the glacier
base, and can be used as a tool for abrasion
of the bed. |
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Abrasion
involves erosion of the substrate
by material stuck in the glacier base. Think of sandpaper moving over a
board (gritty ice base moving over substrate).
As the ice moves over rock basement, it freezes particles into its base
(small and large-scale plucking) or carries them along as it moves. These
particles give the ice base a gritty texture. The particles can be quite
large and they scrape, or abrade, the underlying ground further. As ice
passes over bedrock, abrasion can give it a polished look. Alternatively,
large particles in the ice can cut long scours in the basement, called striations.
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| The slow scraping and
grinding produces a very fine-grained material called rock
flour. The rock flour is similar to the shavings produced when
sanding a board. It is from bedrock that has been ground to almost a fine
powder by the action of the glaciers. Rock flour can be carried away from
the glacier by meltwater.
Where this meltwater reaches a lake or the ocean, the water turns very
milky with the suspended sedimentPhoto:
The turquoise blue waters of Cracker Lake, Canada - from rock flour |
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So what happens to all
of this material that is abraded
and plucked
from the land underlying the ice? Where does it go?
Ice sheets and glaciers act like conveyor belts. Most of the material
eroded from under the ice sheet is carried toward the ice nose. As the
material is eroded,
it can be frozen into the lower part of the ice sheet and moved along, or
it can be carried along just below the ice sheet, or it can be carried to
the ice front by meltwater
The photo shows: Ice as a conveyor belt. The
material eroded from under the glacier is carried to the glacier edges.
There it can be piled up in moraines
and other glacial features
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