Lecture 18 GlaciersWhat is A Glacier?Movement of A GlacierType of GlaciersLandforms Created by GlaciersGlacial DepositsGlacier 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 mo

ves 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 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]

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Glacial Advance and Retreat

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 ea

ch interglacial episode of warming lasting several hun

dred 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)