The Glacial Conveyor Belt
Figure 1. When a climate change occurs, some areas may receive more snow each winter than melts in the
summer. At the end of the melt season, there is still snow left from last winter. New snow falls, adding
to the old, year after year.
Figure 2. Once the ice becomes thick enough, pressure at the bottom causes the ice to become plastic, and it flows
like a gooey liquid away from the thicker center of the ice sheet. The motion is slow (a few meters per
day typically), but powerful.
Figure 3. Gravity is always pulling on the glacier, so the ice is always flowing away from its thicker center. The
thick cold "blob" of ice often covers all pre-existing hills in its path.
Figure 4. The position of the ice front depends on how much ice is moving forward, compared to the amount that
melts. IF more melts, the glacier shrinks, and the ice front appears to retreat (even though it is still
flowing). If less melts, the glacier grows, and the ice front advances. The position of the ice front is
very dependent on climate.
Figure 5. Advancing Glacier: Since the ice is always flowing forward, new moraine is constantly being formed at
the ice front. The flowing ice at the bottom of the glacier acts as a "conveyor belt" for sediments it picks
up along the way. If the climate allows the glacier to grow (flow being greater than melt), the ice front
will advance over the moraine. The sediment again gets scraped up and carried further from the place
where the glacier originally picked it up.
Figure 6. Retreating Glacier: If the climate causes the glacier to shrink (melting exceeds flow), the ice front
retreats. A ridge of unsorted sediment called a terminal moraine is often left at the position of the ice
front as the retreat begins. When the ice front is actively retreating, the unsorted moraine sediments are
deposited in a layer called a ground moraine.
Figure 7. Retreating Glacier: After a period of retreat, the forward ice flow will balance the melting, and the ice
front will remain in place for a period of time (all the while, continuing to flow outward from its
center). Since the moraine is being deposited by the "conveyor belt" in the same place, another ridge of
unsorted sediment is formed. This one is called an end moraine.
Figure 8. Retreating Glacier: IF the climate warms again and melting increases, the ice front resumes its retreat,
creating another area of ground moraine. Note that all during the melting, gravity continues to spread
the ice out and pull it forward in a flow. This flow causes constant erosion under the glacier, and
provides the ice front with a never-ending supply of unsorted sediment to pick up and move elsewhere.
Figure 9. Retreating Glacier: If the climate cools again, and flow balances melting, a new end moraine is formed.
This process of melt and flow allows glaciers to deposit huge amounts of unsorted moraine, and it
allows them to gouge out large amounts of surface material from the ground.
Figure
10.
Glacier Gone!: As a glacier retreats, there is ONE terminal moraine left behind. There can be, however,
MANY ground and end moraines. During the melting process, huge volumes of water (from the melted
ice) also transport sediment away from the glacier, forming outwash deposits. Because water is such a
good sorting agent, outwash deposits ARE SORTED into layers of different size and density sediments.
The water can also form lakes.