GEOL: CHAPTER 14 Glaciers and Glaciation

Ice Age: 1.8 million to 10,000 years ago Ice Age: 1.8 million to 10,000 years ago Warming: Holocene Maximum 6,000 years ago Warming: Holocene Maximum 6,000 years ago Medieval Warm Period: A.D. Medieval Warm Period: A.D. Little Ice Age: 1500 to mid-1800s Little Ice Age: 1500 to mid-1800s Glacier: a mass of ice that moves by plastic flow and basal slip Glacier: a mass of ice that moves by plastic flow and basal slip Ice Ages

Confined to mountain valleys Confined to mountain valleys Flow from higher to lower elevations Flow from higher to lower elevations Also called “alpine glaciers” and “mountain glaciers” Also called “alpine glaciers” and “mountain glaciers” Tidewater glaciers flow into the ocean Tidewater glaciers flow into the ocean Can be several kilometers across, 200 km long, and hundreds of meters thick Can be several kilometers across, 200 km long, and hundreds of meters thick Valley Glaciers

A valley glacier in Alaska. Notice the tributaries that unite to form a larger glacier.

Cover at least 50,000 km 2 Cover at least 50,000 km 2 Also called ice sheets Also called ice sheets Not confined by topography Not confined by topography Flow outward in all directions from central area Flow outward in all directions from central area 3000m thick; Greenland and Antarctica 3000m thick; Greenland and Antarctica Ice cap: less than 50,000 km 2 Ice cap: less than 50,000 km 2 Continental Glaciers

The West and East Antarctic ice sheets merge to form a nearly continuous ice cover that averages 2,130m thick.

View of part of the Antarctic ice sheet. Notice the nunatak, which is a peak extending out of the glacial ice.

Can flow into oceans and calve icebergs Can flow into oceans and calve icebergs Remote from oceans: glaciers flow to lower elevations and melt into streams or groundwater Remote from oceans: glaciers flow to lower elevations and melt into streams or groundwater Sublimation: ice changes to water vapor Sublimation: ice changes to water vapor Glaciers and the Hydrologic Cycle

Glaciers form where more snow falls than melts in the warm season: net accumulation of snow Glaciers form where more snow falls than melts in the warm season: net accumulation of snow Snow converts to firn: granular type of snow Snow converts to firn: granular type of snow Buried firn is compacted and recrystallized to glacial ice Buried firn is compacted and recrystallized to glacial ice Origin of Glacial Ice

Fig. 14-3, p. 286 Stepped Art Glacial ice Granular snow Firn Snowflakes

Plastic flow: stress and strain induce permanent deformation and movement Plastic flow: stress and strain induce permanent deformation and movement Occurs continuously Occurs continuously Basal slip: sliding over the underlying surface Basal slip: sliding over the underlying surface Occurs occasionally Occurs occasionally Crevasses: upper 40 m behaves like a solid and fractures Crevasses: upper 40 m behaves like a solid and fractures How Glaciers Move

Need adequate snowfall Need adequate snowfall Need cold enough temperatures Need cold enough temperatures Valley glaciers on highest mountains in western United States Valley glaciers on highest mountains in western United States Common in Canada and other northern countries Common in Canada and other northern countries Distribution of Glaciers

Glacial budget: balance between expansion and contraction of glacier in response to accumulation and wastage Glacial budget: balance between expansion and contraction of glacier in response to accumulation and wastage Zone of accumulation: where additions exceed losses and surface always covered with snow Zone of accumulation: where additions exceed losses and surface always covered with snow Zone of wastage: losses exceed additions, from melting, sublimation, calving Zone of wastage: losses exceed additions, from melting, sublimation, calving Glacial Budget

Firn limit: separates the zone of accumulation from the zone of wastage Firn limit: separates the zone of accumulation from the zone of wastage If moves up the glacier, the glacier is receding If moves up the glacier, the glacier is receding If moves down the glacier, the glacier is advancing If moves down the glacier, the glacier is advancing Glacial Budget, cont.

Winter Accumulation (B) and summer snow and ice melt (A) are equal. That is, additions and losses are equal so the glacier’s terminus remains stationary. The terminal moraine is deposited at the terminus of a glacier.

Summer snow and ice melt (A) are much greater than winter accumulation (B), and the glacier’s terminus retreats, although the glacier continues to move by plastic flow and basal slip. The recessional moraine is deposited at the glacier’s new terminus.

Winter accumulation (B) is much greater than summer snow and ice melt (A), so the glacier’s terminus advances. As it does so, it overrides and modifies its previously deposited moraines.

Valley glaciers typically move faster than continental glaciers Valley glaciers typically move faster than continental glaciers Rates from centimeters to tens of meters per day Rates from centimeters to tens of meters per day More rapid flow on steeper slopes More rapid flow on steeper slopes Valley glaciers move faster in warmer months because melt increases basal slip Valley glaciers move faster in warmer months because melt increases basal slip Glacial Movement Rates

Friction from slow ice flow near the sides and bottom of glacier Friction from slow ice flow near the sides and bottom of glacier Rates are fastest in center and near top Rates are fastest in center and near top Glacial surge: greatly accelerated flow, usually from a valley glacier Glacial surge: greatly accelerated flow, usually from a valley glacier Tens of meters per day, for months Tens of meters per day, for months Glacial Movement Rates, cont.

Flow velocity in a valley glacier varies both horizontally and vertically. Velocity is greatest at the top center of the glacier, because friction with the walls and floor of the trough slows the flow adjacent to these boundaries. The lengths of the arrows in the figure are proportional to velocity.

Glaciers push unconsolidated materials Glaciers push unconsolidated materials Plucking: glacial ice pulls rock from bedrock Plucking: glacial ice pulls rock from bedrock Bedrock eroded through abrasion: Bedrock eroded through abrasion: Glacial polish: smooth surface Glacial polish: smooth surface Glacial striations: scratches Glacial striations: scratches Glacial flour: pulverized rocks Glacial flour: pulverized rocks Erosion and Transport

Glacial Striations and Polish Abrasion

Continental glacier sediments come mostly from abrasion of bedrock Continental glacier sediments come mostly from abrasion of bedrock Valley glaciers get sediments from: Valley glaciers get sediments from: Bedrock Bedrock Mass wasting Mass wasting Glacial Sediments

Sediment Transport by Valley Glaciers Debris on the surface of the Mendenhall Glacier in Alaska. The largest boulder is about 2 m across. Notice the icefall in the background. The person left of center provides scale.

U-shaped glacial troughs U-shaped glacial troughs Hanging valleys Hanging valleys Cirques Cirques Aretes Aretes Horns Horns Valley Glacier Erosional Features

Landforms Produced by Valley Glacier Erosion: U-shaped glacial troughs, cirques, and arêtes are visible in this view of the Chugach Mountains in Alaska.

Valley with steep walls and broad, flat floor formed by glacier movement through a stream valley Valley with steep walls and broad, flat floor formed by glacier movement through a stream valley Fiords: drowned glacial troughs Fiords: drowned glacial troughs High latitudes where glaciers exist near sea level High latitudes where glaciers exist near sea level As glaciers melt, sea level rises As glaciers melt, sea level rises U-Shaped Glacial Troughs

Classic U-Shaped Valley at Milford Sound, New Zealand

Tributary valley whose floor is above the main valley Tributary valley whose floor is above the main valley Often have spectacular waterfalls, like Nevada Falls in Yosemite National Park Often have spectacular waterfalls, like Nevada Falls in Yosemite National Park Often formed by glaciers Often formed by glaciers Hanging Valleys

Nevada Falls and Brideveil Falls from hanging valleys in Yosemite National Park

At upper ends of glacial troughs and at ridges that separate them At upper ends of glacial troughs and at ridges that separate them Cirque: steep-walled, bowl-shaped depression Cirque: steep-walled, bowl-shaped depression From frost wedging, glacial plucking, and glacial erosion From frost wedging, glacial plucking, and glacial erosion Tarn lakes Tarn lakes Cirques, Aretes, Horns

Arête: a narrow serrated ridge between two glacial valleys or cirques Arête: a narrow serrated ridge between two glacial valleys or cirques From headward erosion From headward erosion Horn: Steep-walled pyramid-shaped peak formed by headward erosion of at least three cirques Horn: Steep-walled pyramid-shaped peak formed by headward erosion of at least three cirques Cirques, Arêtes, Horns, cont.

The Matterhorn in Switzerland

Areas eroded by continental glaciers are often smooth and rounded Areas eroded by continental glaciers are often smooth and rounded Ice-scoured plains: large areas where continental glaciers removed soil and sediment Ice-scoured plains: large areas where continental glaciers removed soil and sediment Deranged drainage Deranged drainage Many lakes and swamps Many lakes and swamps Low relief Low relief Erosional Landforms of Continental Glaciers

This low-relief surface is an ice-scoured plain in the Northwest Territories of Canada. Numerous lakes, little or no soil, and extensive bedrock exposures are typical of these areas eroded by continental glaciers.

Sediment deposited by glaciers Sediment deposited by glaciers Vast sheet covers northern U.S. Vast sheet covers northern U.S. Glacial erratics: rock fragments carried from source by glaciers Glacial erratics: rock fragments carried from source by glaciers Till: all sediment deposited by glacial ice Till: all sediment deposited by glacial ice Stratified drift: layered, with some sorting Stratified drift: layered, with some sorting Glacial Drift

A glacial erratic in the making. This boulder on the surface of the Mendenhall Glacier in Alaska will eventually be deposited far from its source.

House and Barn Rocks, glacial erratics found in Westford, MA

This glacial drift from the Matanuska Glacier, near Palmer, AK, is till, because it is unsorted and shows no stratification.

Till deposited at terminus of glacier that was stationary for a few years or decades Till deposited at terminus of glacier that was stationary for a few years or decades Flow continues in stationary glacier, which results in continued sediment deposits at terminus Flow continues in stationary glacier, which results in continued sediment deposits at terminus End Moraines

An end moraine (specifically—terminal moraine) deposited by a valley glacier.

Closeup of till found in an end moraine.

Ground moraine: layer of sediment released from melting ice as a glacier’s terminus retreats Ground moraine: layer of sediment released from melting ice as a glacier’s terminus retreats Recessional moraine: an end moraine that forms when glacier’s terminus retreats and then stabilizes Recessional moraine: an end moraine that forms when glacier’s terminus retreats and then stabilizes Ground and Recessional Moraines

Lateral moraine: ridge of sediment deposited along the margin of a valley glacier Lateral moraine: ridge of sediment deposited along the margin of a valley glacier Medial moraine: a moraine carried on the central surface of a glacier; formed when two lateral moraines merge Medial moraine: a moraine carried on the central surface of a glacier; formed when two lateral moraines merge Valley trains: long, narrow deposit of stratified drift from braided melt water streams Valley trains: long, narrow deposit of stratified drift from braided melt water streams Lateral and Medial Moraines

The types of medial and lateral moraines shown here are defined by their position. These moraines are on the Bernard Glacier in the St. Elias Mountains in Alaska.

Kettles: depressions from a retreating glacier that leaves a large block of ice that subsequently melts Kettles: depressions from a retreating glacier that leaves a large block of ice that subsequently melts Drumlins: elongated hills of till; often in extensive drumlin fields Drumlins: elongated hills of till; often in extensive drumlin fields Outwash plains: sediment deposited by melt water from glacial terminus Outwash plains: sediment deposited by melt water from glacial terminus Landforms Created by Continental Glaciers

Stages in the Development of Major Features Associated with Past Continental Glaciation 1

Stages in the Development of Major Features Associated with Past Continental Glaciation 2

Stages in the Development of Major Features Associated with Past Continental Glaciation 3

Kettle in a moraine in Alaska

A valley train in Alaska made up of stratified drift.

Kames: conical hills of stratified drift originally deposited in a depression on a glacier’s surface Kames: conical hills of stratified drift originally deposited in a depression on a glacier’s surface Eskers: long, sinuous ridge of stratified drift deposited by running water in a tunnel beneath stagnant ice Eskers: long, sinuous ridge of stratified drift deposited by running water in a tunnel beneath stagnant ice Landforms Created by Continental Glaciers, cont.

Glacial lakes are areas of deposition Glacial lakes are areas of deposition Varves: finely laminated mud deposits Varves: finely laminated mud deposits Dropstones: gravel in lake sediments Dropstones: gravel in lake sediments Carried and then dropped by icebergs from glacier Carried and then dropped by icebergs from glacier Deposits in Glacial Lakes

Varves and a Dropstone in Glacial Deposits These varves have a dropstone that was probably liberated from floating ice.

Explains cyclical variations in Earth’s climate as a result of variations in Earth’s rotation and orbit Explains cyclical variations in Earth’s climate as a result of variations in Earth’s rotation and orbit Alters the amount of solar radiation Earth receives Alters the amount of solar radiation Earth receives Milankovitch Theory

1. Orbital eccentricity: amount of change in Earth’s orbit around the sun 2. Axial tilt: shift of angle between Earth’s axis and line perpendicular to Earth’s orbital plane 3. Precession of the equinoxes: wobble in axial tilt Milankovitch Theory

There is a third Milankovitch Cycle: Axial Tilt that varies from 22.1  to 24.5  over a 41,000 year cycle. We are currently at a 23.5  tilt. This is not clear in the book.

Duration of several centuries Duration of several centuries Variation in solar energy: Variation in solar energy: Change in energy output of Sun Change in energy output of Sun Solar system moves through region of dust Solar system moves through region of dust Major volcanism: Major volcanism: Ash and gases reflect solar radiation and have a cooling effect Ash and gases reflect solar radiation and have a cooling effect Tambora, 1815 Tambora, 1815 Short-Term Climatic Events

Mt. Pinatubo Eruption 1991