I. Glacial Processes

A. Introduction

• 77% of all fresh water is tied up as ice

• the vast majority of this ice is found on Antarctica & Greenland - the remainder covers mountains & fills valleys

• defn - "glacier" - a large mass of perennial ice - it rests on the land or floats shelflike on the sea adjacent to the land

• glaciers are formed from the accumulation & recrystallization of snow - they are not frozen lakes or groundwater ice

• the snow then flows slowly under the pressure of its on weight & the pull of gravity

• glaciers form in areas of permanent snow - i.o.w., above the snowline

• "snowline" - the lowest elevation where snow remains year-round - thus, snow accumulation persists throughout the year

• glaciers form at high latitiudes & high elevations - above the snowline

• even on high mountains along the equator - e.g., Andes in S.A.

B. Glacier Types

(1) Alpine Glacier (mountain glacier)

• a glacier found in a mountain range - three subtypes:

(a) "valley glacier" - a glacier confined within a valley

(b) "piedmont glacier" - glaciers that extend beyond their valleys and flow freely over nearby lowlands

(c) "tidal glacier" - glaciers that end in the sea & break off ("calving") to form "icebergs"

(2) Continental Glacier

defn - a continuous mass of unconfined ice, covering at least 50,000 km2

• most extensive as "ice sheets" covering Greenland (80%) & Antarctic (90%)

• Greenland & Antartica are isostatically depressed by their respective ice sheets

• "outlet glacier" - "is said to drain a continental glacier" - flows outward from a continental glacier - forms around the periphery of a continental glacier

• "ice caps" - cover uplands and plateaus and resemble ice sheets but are much smaller than ice sheets found in Iceland and large islands in the Arctic Ocean

C. Formation of Glacial Ice

• glaciers are composed of dense ice formed from snow and water

• glaciers are dynamic open systems with inputs of snow & water, & outputs of melting ice, evaporation, & sublimation

• net gains or losses of glacial ice determine whether it shrinks or expands

• glaciers modify the landscape as they move along

• glaciers form in "snowfields"

• snowfields are found at the highest elevations of icesheets; or at the heads of valley glaciers

• orographic processes can enhance the development of snowfields

• the snow in the snowfield forms layers as it accumulates - analagous to sedimentary deposits

• as new layers of snow are added, the thickness of the overlying mass of snow increase

• resulting in increased weight & pressure on the bottom most layers

• rain & summer snowmelt trickle down into the snowfield & refreezes

• the crystals are compacted (air spaces between crystals are eliminated); & the snow becomes more dense

• under pressure, the ice crystals recrystallize: regrowth & consolidation - forming "firn" - a transitional form - in between snow & ice - has a compact & granular texture

• as the above mentioned prosesses continue over time, eventually glacial ice is formed

• this process is analagous to the formation of metamorphic rocks from sediment (snow & firn)

• the formation of glacial ice takes longer in dry (Anarctica) climates than moist climates

• glaciers lose mass - "ablation" - from: surface melting, sublimation, wind removal by deflation, and the calving of ice blocks

• a glacier's area (zone) of accumulation is at higher elevations - because of colder temperatures

• "glacial budget" - the balance between accumulation & ablation
• if ablation & accumulation balance, the terminus (end of the glacier) is stationary
• if ablation exceeds accumulation, the terminus retreats
• if accumulation exceeds ablation, the terminus advances
• whether the terminus is advancing, stationary, or retreating, the glacial ice always flows forward

D. Glacial Movement

• glacial ice behaves as a plastic

• it distorts & flows in response to the the weight and pressure of the overlying ice; & in response to the slope angle

• flow per year: 0 to 1-2 km

• the internal glacial ice moves downslope most quickly, faster than the ice near the surface or the ice near the base of the glacier (basal ice)

• friction between the sides of the glacier & the valley walls; in addition to tension & compression lead to the formation of "crevasses" - vertical cracks in the glacier

• glaciers transport tremendous amounts of rock debris

• the glacier mechanically picks up material and carries it away

• the debris may freeze to the basal layers of the glacier - "glacial plucking"

• this embedded material then scours the landscape as the glacier moves along - "abrasion" - produces a smooth (polished) surface on exposed bedrock; also produces "striations"

E. Erosional Landforms Created by Alpine Glaciation

• U-Shaped Glacial Valley (trough) - glaciers take path of least resistance; prior to glaciation, mountain valleys are narrow and v-shaped; during glaciation the valleys are widened, deepened, and straightened

• truncated spurs - triangular-shaped cliffs resulting form the widening of the former stream valley by the glacier

• hanging valleys - result when tributary glaciers erode less deeply than the main glacier - often produce waterfalls (e.g., Bridalveil Falls in Yosemite)

• cirque - a bow-shaped depression at the head of the glacier - are enlarged through frost wedging and plucking - it is the main area of snow accumulation and ice formation

• tarn lakes - lakes that often form in cirques after the glacier melts away

• pater noster lakes - lakes that fill depressions in the floor of the u-shaped valley after the glacier melts

• fiords - steep-sided sea inlets - formed as the valley glacier carved-out the valley floor to below sea level near the shoreline

• aretes - sharp-edged ridges dividing two cirques on opposite sides of a divide - divide is eroded through frost wedging and plucking

• horns - a pyramid like peak - formed when a group of cirques surround a high mountain and weather and erode the peak from all sides

F. Depositional Processes and Landforms

• glaciers produce landforms when they melt and deposit their eroded and transported debris

• when the glacier melts, the debris accumulates to mark the former margins (sides and end) of the glacier

• "glacial drift" - general term for all glacial deposits - both sorted and unsorted

• "till" - material deposited directly by the ice - are unstratified and unsorted

• "stratified drift" - material deposited by glacial meltwater - are sorted and stratified

• rock fragments are carried on the ice surface or embedded in the base of the glacier

• "ablation till" - unsorted material deposited from the ice - upon melting the former surface rock fragments are lowered to the ground surface

• "lodgement till" - rock fragments embedded in the base of the glacier are deposited on the ground when the glacier melts - are poorly sorted & are difficult to cultivate for farming

• Moraines - the name for the specific landforms formed from the deposition of till - are thus poorly sorted and unstratified - lateral, medial, & terminal

• "till plains" ("ground moraine")- are flat plains of unsorted coarse till formed behind terminal moraines - are associated with low relief, and derranged stream drainage patterns

• "drumlin" - comprised of deposited till, streamlined in the direction of continental ice movement - blunt end upstream & tapered end downstream - rounded summit - (may originate when glaciers advance over previously deposited material and reshape the material)

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• "outwash plains" - lie in front of the end moraines - are formed from glacio-fluvial processes - and are comprised of stratified drift - is a broad, ramp-like surface"

• eskers - a sinuously curving, narrow deposit of sand and gravel (stratified drift) that forms along a meltwater stream channel, developing in a tunnel beneath the glacier. The retreating glacier leaves behind these narrow ridges - are parallel to the path of the glacier

• kame - a small hill comprised of stratified drift - represents a body of sediment deposited from meltwater running in, below, or on the glacier - when the ice melts, the material is left behind

• kettle - forms when an isolated block of ice slowly melts - as it melts, material accumulates around it, and the ice block becomes wholly or partly buried in stratified drift - after it totally melts, a steep-sided hole is left behind
• they frequently fill with water - called a "kettle lake"

G. The (Pleistocene) Ice Age

• the "Ice Age" began 2-3 million years ago

• most of the major glacial stages occured during the Pleistocene Epoch ... (began 1.6 million years ago) .....

• thus, Pleistocene is used as a synonym for the Ice Age ...

• what is an Ice Age ??

• not just a single glacial advance !!!

• but rather, - "the ice age" - a complex event - there were a number of ice advances, each separated by periods when climates were as warm or warmer than today

• evidence for this is based on layers of glacial drift and soil and plant deposits

• ice advances = "glacials"
• ice withdrawls = "interglacials"

• at peak of pleistocene, 30% of Earth's land area was covered with ice sheets glaciers

• traditionally, four major glacials & three interglacials were recognized(based on deposits in these areas): Nebraskan, Kansan, Illinoian, Wisconsinan
• many unconformities on land though

• now - from sea floor sediment cores we have a more complete/uninterrupted record:
• glacial/interglacial cycles occurred every 100,000 years - about 20 cycles of cooling and warming during the Ice Age

H. Effects of Ice-Age Glaciers

• decrease in sea-level - sea level was as much as 100 meters lower than today

• with advance and withdrawl of ice, plants and animals were forced to migrate - some could not adapt to changes and became extinct

• isostatic rebound - crust is rising now that ice is removed (Hudson Bay area has risen 300 meters)

• pluvial lakes formed in airid and semi-arid regions because the climate was cooler and wetter during glacial advances (e.g., Basin & Range region of Nevada and Utah - Lake Bonneville)

I. The Arctic Region

• the arctic ocean is covered by floating sea ice (frozen seawater) & glacier ice (frozen freshwater) in icebergs formed at the edge of the surrounding land

• this ice thins in the summer months

J. The Antarctic Region

• defined by the boundary between the warmer subantarctic water & the colder antarctic water

• this boundary is located near 60 degrees South latitude

• the "antarctic ice sheet":

• Antarctica is colder than the Arctic

• Antartica is underlain by a continent-sized land mass

• (the Arctic is underlain by a sea)

• Antartica can be thought of as a continent covered by an enormous glacier

• "ice shelves" - the farthest edge of an ice sheet that enters a bay along the coast - the farthest edge is marked by a sharp cliff (up to 98 ft.) - tabular islands are formed when sections of the shelf break off & move out to sea