19. Glacial
Processes and Landforms
19.1 Basic Concepts
1.What is a
glacier?
A thick, long-lasting mass of ice
accumulated on land surfaces.
(1).Can be
up to two miles in thickness!
(2).Can last hundreds or thousands
of years.
2.What causes glaciers to form?
(1).Snowfall must exceed snowmelt, year
after year. This allows snowpack to become thicker.
Therefore, glaciers form in
climates that are either very snowy (lots of snowfall) or very cold (little
snowmelt), or both.
(2).As snowpack
gets thicker with new snow added on the surface each year,
compaction by gravity causes the buried snow to have air squeezed from it. In
addition, buried snow often melts and refreezes in summer. The net result is
that snow becomes dense ice, over many years.
1-yr old snow 50 % air 50 % ice
glacial ice 15
% air 85 % ice
3.what causes Glaciers
flow.
A
minimum of about 200 feet of ice accumulation is necessary to initiate movement
or flow within the glacier.
When
enough ice has accumulated, the ice begins to move downslope
under the weight and pressure of the overlying snow/ice.
Ice
moves out from a zone of accumulation under the force of gravity.
(1).Because the ice is so thick, the base is not restricted to flowing downhill. It
can engulf terrain features beneath it, and its base can be pushed upslope.
(2).With rare exceptions, glaciers
move exceedingly slowly. Rate of flow ranges from about an
inch to a foot a day.
(3).As ice flows outward from the zone
of accumulation, the lower or outer portions of the glacier move into warmer
environments where annual snowmelt exceeds annual snowfall.
This lower portion of a glacier is
called the zone of ablation. It is covered with snow in winter, but the surface
of the flowing ice is exposed in summer, with its many crevasses (flow cracks
in the ice).
4. mass balance of a glacier
The mass balance of a glacier is
defined by the balance between ice build up in the zone of accumulation and ice
melt in the zone of ablation.
-If ice build up exceeds ice melt,
then the glacier is growing in overall size (positive mass balance), and the
glacier will advance its terminus (or snout) outward.
-If ice melt exceeds ice build up,
then the glacier is getting smaller in overall size (negative mass balance) and
the glacier will "retreat".
This does not mean that its
direction of flow changes. It still flows downward and outward from the zone of
accumulation. The outward flow of ice at the terminus is not fast enough to
compensate for ice melt, so the snout shrinks upward and inward.
-If ice build up in the zone of
accumulation equals ice melt in the zone of ablation, then the location of the
glacier's terminus on the earth's surface remains stationary, even though ice
within the glacier continues to flow downward and outward toward the snout.
19.2
The effects of glaciation
The downward and outward flow of ice in a
glacier behaves like a conveyor belt. It serves as a denudational
agent. It can erode, transport, and deposit sediments,
just like winds, running water, and waves.
1.Erosion of
two types occurs:
(1).Plucking - basal ice thaws and
refreezes each summer. Surface rocks and soil get frozen onto the bottom of the
glacier and transported toward the terminus of the glacier.
(2).Abrasion - debris carried
along the base of the flowing ice can gouge out and physically wear down or
flatten the terrain it passes over.
2. Sediment transport
Clasts
of all sized, from boulders to clay particles, get plucked up and incorporated
into the glacier. The body of a glacier is a collection of sediments embedded
in a matrix of ice.
3.Deposition
(1).At the glacier's terminus, ice
melt liberates the clasts transported in the ice
mass, and they are deposited. Two types of glacial sediments:
a.Till
- dropped as a pile of glacial debris where the ice melts. This material is
poorly sorted, with a mix of many clast sizes.
b.Outwash
- fans of glacial sediments carried away from the glacial terminus by runoff of
glacial meltwater.
Since flowing water is
involved, it sorts sediments by clast size, like in a
delta or alluvial fan. The result is well sorted sediments
-coarser sand and gravel
deposited near the terminus
-finer silts and clays
washed down the outwash fan, and often carried away as sediment load in rivers
that drain the glacial meltwater.
19.3.Alpine
glaciation (mountain and valley glaciers, ice fields)
1.
general characters
Glaciers formed at high elevations in
the headwaters of stream valleys. Alpine glaciers and ice fields throughout the
world account for 4%
of glacial ice that covers continents.
Since highlands climates are both cold
and wet (with orographic precipitation), the setting
is ideal for glaciers to develop and grow.
Alpine glaciers modify preglacial
topography, but they don't create the mountains or valleys.
2.
Erosional and depositional features
- U-shaped valleys
Side slopes are oversteepened,
while valley bottoms are broadened. The result is that the valley cross-section
changes from V-shaped (typical of stream eroded valleys) to U-shaped (typical
of glacially modified valleys).
-
Cirques - the bowl-shaped basins in which the glacier forms, carved by erosion.
- Aretes (knife-edge in French) and Horns (pyramidal peak) -
knife-like mountain ridges and peaks formed by erosion by mountain glaciers on
two or more sides.
-Hanging valleys - unlike streams, glaciers
which join do not always erode to the same level. Small, side glaciers may not
erode as deeply as main valley glaciers. The result is a hanging valley, a
perched, U-shaped valley with a steep face where it enters the main valley.
-Moraine:
specific landforms produced by the deposition of glacial sediments. There are
several types of moraine:
(1)
lateral moraine: forms along each side of a glacier
(2)
medial moraine: if two glaciers with lateral morains join, a medial moraine may form.
(3)
Terminal moraine: eroded debris that is dropped at the glacier’s
farthest extent is called a terminal moraine
19.4 Continental glaciation
(ice sheets)
1. General character of continental glaciation
-several 1000s of feet thick
-overrides virtually all terrain
features, except high mountains.
Modern examples of continental ice
sheets cover
At the peak of the ice age. glacial
advances during the Pleistocene (beginning 1.65 million years ago to 10,000
years ago), continental ice sheets covered up to 30% of the earth's land
surface, especially over
2.Erosional
and depositional features of continental glaciation.
-
till plain: forms behind end moraines; it features unstratified coarse till, has low and rolling relief, and
has a deranged drainage patterns.
-
Outwash plains: beyond the morainal deposits
lie the outwash plains of stratified drift featuring stream channels that are meltwater-fed, braided, and overloaded with sorted and
deposited materials.
-
Esker: it forms along the channel of a meltwater
stream that flows beneath a glacier, in an ice tunnel.
-
Drumlin: it is a deposited till that has been streamlined in the
direction of continental ice movement, blunt end upstream and tapered end
downstream. It has lengths of 100-5000 m and heights up to 200 m.
-
Crevasses: cracks on the glaciers
-
Kettle. Sometimes an isolated block of ice, perhaps more than a
kilometer across, remains in a ground moraine, an outwash plain, or valley
floor after a glacier has retreated. As much as 20 or 30 years are required for
it to melt. When it finally melts, it leaves behind a steep-sided hole. Such a
feature then frequently fills with water. This is called a kettle.
-
Kame: in outwash plain,
small hills, knobs, or mounds of poorly sorted sand and gravel that is
directly deposited by water, by ice in crevasses;
-
Terminal moraines: same as alpine glaciers