Geology Lecture Outline

Glaciers (Ch18)

 

I. Lecture Content

The Hydrologic Cycle

Origin and Nature of Glacial Ice

Types of Glaciers

Behavior of Glaciers & Glacial Budgets

Glacial Erosion and Transport

Glacial Deposition and Deposits

Causes of Glaciation and Global Climatic Cycles

Pleistocene Ice Age

Glaciers and Isostacy

 

II. Introduction

A. Glaciation, Ice Ages and Earth's Changing Climate

1. Defined: Glaciers are masses of ice, which move over

the land by plastic flow and basal slip

 

2. Glaciers and ice caps critical part of the hydrologic cycle

       Around 2% of hydrosphere

       Roughly 75% of Earth's freshwater

       "Inactivated" part of the short-term hydrologic cycle

 

3. Intimately related with global climatic changes

 

4. Profound effect on atmospheric and oceanic conditions

 

5. We are currently in what is called a relatively warm

interglacial period, which is part a much longer duration,

ongoing 1.6 million year-old Ice Age event

 

B. Glaciation and Earth's Changing Surface

1. A glacier is like a cross between a moving frozen river,

a bulldozer, and a gigantic piece of very rough sandpaper

 

2. Some of Earth's most spectacular landscapes are the

result of glacial action over tens of thousands of years

 

3. Humans started evolving on Earth just prior to the latest

Ice Age event, which started 1.6 million years ago.

 

III. Origin and Nature of Glacial Ice

A. Formation and Growth of Glaciers

1. Principle mechanism by which glaciers form and grow

 

       Winter snowfall exceeds summer snowmelt

 

       Snow begins to slowly accumulate year after year

 

2. Transformation of snow to glacial ice a multi-step process

v   Snowflakes Granular snow Firn Glacial ice

20% solid 80%air 90% solid 10% air

 

3. Thawing-freezing (firn) cycles combined with compaction

 

4. Glaciers typically form at both high elevations (mountain

valleys) and in polar regions

 

IV. Types of Glaciers

A. Valley Glaciers

1. Defined: Glaciers that are confined to mountain valleys

       High elevations

 

       High latitudes

 

2. Several smaller tributary glaciers typically merge to form

a much larger glacier

 

       Very similar to river systems

 

3. Valley glaciers flow from higher to lower elevations

 

4. Excellent examples of mountain valley glaciers are found

at some of the most scenic locations on Earth:

 

       Alaska and the Canadian Rockies

 

       New Zealand

 

       European Alps

 

       The South American Andes

 

5. Worldwide, many valley glaciers are actively retreating

 

        Consequence of global warming?

 

 

B. Continental Glaciers

1. Defined: Thick, continental-size sheets of ice presently

found in the northern and southern polar regions

       Greenland

       Antarctica

 

2. Take form as very thick and extensive sheet-like bodies

of glacial ice called ice sheets, ice caps and ice shelves

       Up to 3000 meters thick near the center of sheet

       Edges typically meet the ocean in Antarctica

 

       Ice shelves are ice sheets that float on the ocean

 

       "Calving" of icebergs occur along margins of ice

sheet where it meets the ocean

 

3. Volume of ice making up the polar ice sheets is HUGE!

        Antarctic ice sheet = ~ 30,000,000 cubic kilometers

 

        Greenland ice sheet = ~ 2,600,000 cubic kilometers

 

4. Unlike valley glaciers, continental ice sheets flow radially

outward in all directions from its thickest central region

outward toward its margins (thinnest)

 

5. Presently there is a scientific debate over whether the

polar ice sheets are shrinking, growing or stable

 

 

V. Behavior of Glaciers and Glacial Budgets

A. Movement of Glaciers

1. Valley glaciers start to move when accumulation of snow

& ice reaches about 40 meters in vertical thickness

 

2. Two primary mechanisms of glacial movement

 

        Plastic flow occurs inside the glacier

 

        Basal slip occurs at the bottom surface of glacier

 

        Primary moving force is gravity

 

        See Figure 17.3 for illustration

 

3. Velocity of a glacier depends on several factors

        Thickness of glacier

 

        Downhill slope or gradient

 

        Presence of water along basal surface

 

        Roughness and relief of ground surface

 

4. Velocity profile of a glacier is much like a regular river

 

       Swiftest near the top and middle line

 

       Slowest near the bottom and sides

 

5. Average speeds of glaciers have a considerable range

 

        Centimeters per day (slow ones)

 

        10's of meters per day (fast ones)

 

        Valley glaciers are generally faster than continental

ice sheets

 

B. Glacial Mass Budget - Accumulation and Wastage

1. Change in a glacier's size and shape is controlled by

the dynamic balance between the accumulation and

removal (wastage) of its snow and ice mass

 

2. Zone of Accumulation - Upper region of a glacier where

yearly snow accumulation (increase) occurs

 

3. Zone of Wastage - Lower region of a glacier where yearly

snow and ice wastage (decrease) occurs

       Melting and Sublimation

 

4. The annual snow line (firn limit) marks the boundary

between the two zones

 

5.Glacial growth -- accumulation > wastage

 

6. Glacial retreat -- wastage > accumulation

 

7. Stable glacier-- accumulation = wastage

 

VI. Erosional Processes and Features of Glaciers

A. Erosional Processes

1. Glaciers act like a gigantic grinding, scraping, milling,

Polishing, gouging, tearing, yanking, pushing machine

       Abrasion

 

       Plucking

 

       Bulldozing

2. Glaciers use their weight + momentum + abrasives to do

their erosional work on the underlying earth surface

 

3. Glacial erosion action generates huge amounts sediment

called glacial drift, having with a wide range of sizes

 

4. Mass wasting also occurs on the mountainous flanks of

a glacier, which loads material on to the glacier's surface

 

5. Most of a glacier's eroded sediment is carried at its base

and along its margins

 

B. Erosional Features

1. Valley glacier erosion produce very distinctive landscape

features (below) - most notably in mountainous regions

 

       U-shaped valleys

       Hanging valleys

       Fiords

       Artes

       Horns

       Cirques

       Striated and polished bedrock

2. Continental glacier erosion also produces distinctive

landscape features

 

        Flat to rolling hill landscapes

        Extensive striated bedrock surfaces with little/no soil

        Deranged drainage patterns

        Numerous kettle lakes

VII. Depositional Processes and Features of Glaciers

A. Depositional Processes

 

1. Glaciers transport large quantities of eroded sediment

material, called drift, using several methods

       Dragged

 

       Carried

 

       Pushed

 

2. Glacial drift is concentrated into sheet- and ribbon-shaped

structures in and around a glacier, called moraines

 

3. There are several types of glacial moraines - each named

for their location in respect to the glacier

       Ground moraine

 

       Lateral moraine

 

       Medial moraine

 

       End moraine

 

4. Glaciers deposit their sediment load during the wastage

process which occurs primarily at it's leading end of

the glacier, called the terminus or snout.

 

       Wastage process = melting ice

 

5. Deposition of glacial sediment is done in two ways

 

       Directly from melted glacial ice

 

    Drops to the ground as an unsorted, mixed-up

mixture called till

 

        Indirectly from running glacial streams originating

from the glacier

 

    Streams carry sediment for some distance and

then deposits it as sorted and layered alluvium

called stratified drift

 

6. Stratified drift is a depositional product of braided

stream channel activity

 

       Mainly layers of poorly sorted sand and gravels

 

B. Depositional Features

1. Glaciers produce very distinctive depositional features

 

2. Glacial depositional features derive their uniqueness

from several glacial processes

 

       Bulldozing effect

 

       Melting in place (till drop) effect

 

       Intensive braided stream action

 

3. There are several types of glacial deposit features

 

       Ground moraines

       End moraines

    Terminal moraine

    Recessional moraine

 

       Drumlins

 

       Kames and Eskers

 

       Kettle lakes

 

       Outwash plains

 

       Glacial erratics and Dropstones

 

4. Ancient glaciation events can be recognized in Earth's

rock record by its unique erosion and deposition clues

 

        Glacial till deposits

 

        Extensive polished striations on bedrock surfaces

 

        Gondwanaland glaciation record

 

 

VIII. Global Climate Cycles and the Origin of Ice Ages

A. Earth Goes Through Cycles of Warming and Cooling

 

1. Back and forth shifts between the "Greenhouse Effect"

and the "Icehouse Effect"

 

2. Several terrestrial and extraterrestrial phenomena appear

to form a complex relationship that controls cyclic short-

and long -term changes in the Earth's climate

 

       Plate tectonics

 

    Volcanism

 

    Orogenies

 

    Plate configurations

 

       Ocean circulation and temperature patterns

 

       Atmospheric aerosols and circulation patterns

 

       Biosphere activity

 

       Earth's rotation and orbit patterns

 

       Solar energy flux

 

       Comet and meteor impacts

 

B. Initiator of the Major Ice Ages - The Milankovitch Theory

1. The Milankovitch Theory proposes that irregularities in

the Earth's orbit and rotation axis are sufficient to alter

the amount of solar energy that the Earth receives

        Change in orbital eccentricity - 100,000 year cycle

 

        Change in axial tilt - 40,000 year cycle

 

        Precession of the equinoxes - 22,000 year cycle

 

        Season changes versus aphelion and perihelion

 

2. Other theories that may explain glacial cycles

        Spikes in global volcanism

        Solar anomalies

        Meteor impact

 

IX. The Pleistocene Ice Age

A. Most Recent Global Ice Age Event

1. Very first signs of global refrigeration showed up nearly

40 million years ago in the Southern Hemisphere

        Cold ocean waters moving toward the equator

 

        Permanent ice sheet in Antarctica; est. by 15 MYA

 

2. Full-blown global ice age began about 1.6 MYA

        Termed the Pleistocene Ice Age

 

        Officially ended 10,000 years ago

 

3. The Pleistocene Ice Age includes several major ice sheet

advances and retreats

        Cold-warm cycle mechanism

 

        Major ice sheet retreat termed interglacial periods

 

        Last major advance climaxed around 18,000 years ago

and ended 10,000 years ago

B. Nature of the Pleistocene Ice Age

1. Maximum extent of ice sheets in North America reached

down into the U.S. 48 states

 

2. Pleistocene climate belts were somewhat like today's

but were shifted toward the equator

        Northern U.S. and Europe were very cold and dry

        Southern U.S. and Northern Africa were very wet

 

3. Abundance of large lakes to the south of the ice sheets

        Pluvial lakes

    Far from the glacial front

    Example is Lake Bonneville

 

        Proglacial lakes

    Near margin of the great ice sheets

    Some form behind giant ice dams

    Example is Lake Missoula

4. High precipitation and cool climate facilitated lake growth

 

X. Glacier Vocabulary - Ch 17

Abrasion

Arte

Basal slip

Cirque

Continental glacier

Drumlin

End moraine

Esker

Fiord

Firn

Glacial budget

Glacial drift

Glacial erratic

Glacial ice

Glacial polish

Glacial striation

Glacier

Ground moraine

Hanging valley

Horn

Ice age

Icehouse effect

Lateral moraine

Medial moraine

Milankovitch theory

Outwash plain

Plastic flow

Stratified drift

Terminal moraine

Till

U-shaped valleys

Valley glacier

Zone of accumulation

Zone of wastage