Lecture 14 Glaciers

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Lecture 18 Glaciers
 What
is A Glacier?
 Movement of A Glacier
 Type of Glaciers
 Landforms Created by Glaciers
 Glacial Deposits
 Glacier Stages in North America
Glaciers cover 10% land surface, but as much as 30% in the
recent geologic past. Many regions including much of the
Midwest bear the mark of glaciers.
 What
is A Glacier?
A glacier is a mass of ice on land,
formed from compaction and
recrystallization of snow, that has
moved or is moving under the influence
of its gravity.

South Cascade Glacier (about 3 km long) in
the N. Cascade Range, Washington. (A. Post)

Conversion of fresh snow into dense,
crystalline glacial ice. (Tarbuck and Lutgens)

Mass budget of a glacier. The snowline separates the zone of
accumulation and the zone of wastage (ablation). Above the
snowline, snow remains even after summer melting. Below the
snowline, the snow from the previous winter completely melts as
does some of the underlying ice. (Tarbuck and Lutgens)
Movement
of A Glacier

Near the surface of a glacier, ice is brittle. When the glacier
moves over irregular terrain, the uppermost zone (called zone
of fracture) is subjected to tension, resulting in cracks called
crevasses.

Below about 50 meters, great pressure causes ice to flow like
plastic materials. A second important mechanism basal slip. In
this process, meltwater at the bottom of the glacier acts as a
hydraulic jack and a lubricant.

The flow of a glacier is greatest in the center away from walls
and bed rocks where friction drag is weakest.
Crevasses form in the top brittle zone of glaciers. (W.W. Norton)

A deep crevasse on
Mt. McKinley,
Alaska. Crevasses
could pose great
danger to climbers.
(by R. Kaufman Photographers)
Movement of glacier by basal slip. Meltwater at the
bottom of the glacier acts as a hydraulic jack and a
lubricant.
At depth, glaciers flow in the solid state (plasticly). (W.W. Norton)
Different parts of a glacier flow at different velocities. (W.W. Norton)
Glacial advance or retreat is determined by the balance between
the accumulation of snow and the removal of ice by sublimation,
melting, and calving (ablation). (W.W. Norton)
Glacial Advance and Retreat
Glacial advance and retreat is determined by the balance between the
accumulation of snow and the removal of ice by sublimation, melting,
and calving (ablation). When the rate of ablation below the snowline
equals the rate of accumulation above it, the glacier is stationary, as in
View 1. During glacial retreat, View 2, the rate of ablation exceeds the
rate of accumulation, and the position of the toe retreats toward the
origin of the glacier. Glacial advance, View 3, occurs when the rate of
accumulation exceeds the rate of ablation. For all views, pay attention
to the motion of the stones. Note that in all cases, ice flows downhill.
[by Stephen Marshak]
Play Animation Windows version >>
Play Animation Macintosh version >>
Type
of Glaciers
 Valley
(alpine) glaciers: Valley glaciers
are bounded by valleys.
 Ice
sheets (continental glaciers): Ice
sheets exist on a much larger scale.
Current ice sheets are Greenland and
Antarctica.

The only present-day continental ice sheets are those covering
Greenland and Antarctica, which represent 10% of Earth's land
area. The Antarctic Ice Sheet has a maximum thickness of 4.3
km and an area of 14 million square kms. (Tarbuck and Lutgents)
 Landforms
created by glaciers
 Glaciers
erode land by plucking (lifting
pieces of bedrock) and abrasion
(grinding and scraping).
 Glacial
abrasion sometimes is evident in
the long scratches and grooves in the
bedrock (called glacial striations).

Glacial polish,
striations, and
grooves on bedrock,
Glacier Bay National
Park, Alaska. (Carr
Clifton)
 Valley
glaciers sharpen the
topography by erosive processes
concentrated on valleys.

Cirque
 tarn
 arete: (French for knife-edge)
 horn
 hanging valley
 U-shaped valley
 fjord

Erosional landforms
created by valley
glaciers.

Cirques in Utah’s Uinta Range. These bowl-shaped
depressions are found in the heads of glacial valleys.
(J.S. Shelton)

Bridalveil Falls, Yosemite, cascades from a hanging
valley into the glacial trough. (E.J. Tarbuck)

Horns are sharp, pyramid-like peaks fashioned by
alpine glaciers. Matterhorn, Swiss Alps.

A U-shaped glacial trough in Glacier National Park,
Montana. The string of lakes along glacial trough are
called pater noster lakes. (J. Montagne)

A fjord -- a glacial valley flooded with seawater, Kenai
Fjords National Park, Alaska (P. Kresan)
 Continental
glaciers completely
override the terrain and tend to smooth
the land surface rather than sharpen it
as valley glaciers tend to.
 Roche
Moutonnee: an asymmetrical
knob of bedrock created by abrasion of
the side facing the oncoming ice sheet
and plucking on the back side.

Roche Moutonnee, Yosemite. (E.J. Tarbuck)

Glacial Deposits
Any sediment of glacial origin is called glacial drift.
Two distinct types of drift are:
(1)
till, material deposited directly by the ice;
(2)
stratified drift, sediment laid down by meltwater
from a glacier.
 Glacial
Till

Morains: Layers or ridges of till. Different names are
given to moraines depending on their position relative
to the glacier: lateral (sides of valley glaciers), end
(front), and ground (beneath ice) moraines.

Drumlins: streamlined, elongated, parallel hills of till

Erratics: dropped boulders that were transported
great distances

Glacial till is an unsorted mixture of different
sediment sizes. Scratched cobbles indicates
they were dragged along the glacier. (E.J.
Tarbuck)

Lateral
moraines in
Canada’s
Jasper
National Park.
(D. Barnes)

A drumlin field, Palmyra, New York. The drumlins are
steepest on the north side (top), indicating that the
ice advanced from this direction.

A drumlin in upstate New York. (Ward’s Natural
Science Establishment, Inc.)

A large glacially
deposited boulder (an
erratic) near Hensler,
North Dakota (by T.
Bean).
 Depositional
features associated
with stratified drift include
 kames
(steep hills of sand and gravel),
 eskers (ridges of under-ice river
deposits),
 kettle lakes (from melting of buried ice
blocks),
 outwash plains.

Common depositional landforms.
 Glacier
stages in North America

Four major glaciations occurred during the
Pleistocene epoch:
Nebraskan, Kansan, Illinoian, and Wisconsin.

Each advance persisted about 100,000 years with
each interglacial episode of warming lasting several
hundred thousand years.
Distribution of
major ice
sheets during
the Pleistocene
epoch. (W.W.
Norton)

End moraines in the Great Lakes region.
 These
glaciers had profound
effects on landscape.

A dramatic effect was the fall and rise of sea level
accompanied the advance and retreat of the glaciers.

Missouri river once flowed north to Hudson Bay
before glaciation.

The Great Lakes were formed in late Wisconsin time
about 2500 to 14000 years ago by glacial scour of
weak lowlands and stream valleys.

Current coastline compared to that of the last ice age
maximum (18,000 years ago) and the the would-be
coastline if present ice sheets are melted. (W.W. Norton)
A land bridge existed across the Bering Strait between Asia
and North America during the last ice age. (W.W. Norton)
Change of the course of a river by
a glacier. (W.W. Norton)
The major river systems of the North America flowed north
before the last ice age. (W.W. Norton)

Orbital changes in (A) eccentricity (in a cycle of
100,000 years), (B) obliquity (41,000 years), and (C)
precession (23,000 years).
The Earth’s orbital changes affect the amount of insolation
(exposure to the Sun’s rays) at high latitudes, causing distinct warm
and cold periods (the Milankovitch cycles). (W.W. Norton)
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