Glaciers and Glaciations

Introduction Definition Location Formation Movement Features

Definition A thick mass of ice that originates on land from the accumulation, compaction, and recrystallization of snow

Location Occupy 10% of Earth’s surface Primarily located in polar regions (Antarctica & Greenland) But found on every continent Form above the snow line

Formation New layers form each year Weigh of overlying layers compresses buried layers Snow recrystallizes – looks like sugar Snow begins to grow, air pockets decrease compacts & becomes very dense After 2 winters => FIRN

Formation Firn Generally 16x the size of a snow crystal ½ as dense as water Increase in size as the overburden increases Over time, grows to form even larger crystals Forms glacial ice

Formation

Movement When ice sheet thickness > 18 meters, the ice sheet: Deforms Flows Movement slower at base than at top Advance and retreat Surge

Movement Two basic types of movement Plastic flow Occurs within the ice Under pressure, ice behaves as a plastic material

Movement Two basic types of movement Basal slip Entire ice mass slipping along the ground Most glaciers are thought to move this way by this process

Movement Rates of movement Average velocities vary considerably Rates of up to several meters per day Some glaciers exhibit extremely rapid movements called surges

Movement

Budget of a glacier Accumulation + loss = glacial budget

If accumulation exceeds loss (called ablation), the glacial front advances

If ablation increases and/or accumulation decreases, the ice front will retreat

Retreat of the Franz Josef Glacier: The following photographs are selected from a series that show the retreat of the Franz Josef Glacier in New Zealand over a period of 14 years. Respectively, they are from the years 1951, 1957, and (World Data Center for Glaciology, Boulder)

Features Crevasses Cracks in the surface of the glacier Caused by movement

Features Moraines Long, dark bands of debris Visible on the top of the glacier Medial Moraines Lateral Moraines Barnard Glacier shows several medial moraines. In this case, the thickest medial moraines occur where additional glaciers flow into Barnard Glacier. Source: omponents.html

Types of Glaciers Ice Ice Sheets, Ice Shelves, Ice Caps, Ice Streams/Outlet Glaciers, and Ice fields Glaciers Mountain Glaciers, Valley Glaciers, Piedmont Glaciers, Cirque Glaciers, Hanging Glaciers, and Tidewater Glaciers.

Ice Sheets Greenland and Antarctica 50,000 square kilometers Antarctica 4200 meters thick in some areas Covers nearly all of the land features except the Transantarctic Mountains Source of Ice Core Data & Paleoclimate Research

Ice Shelves Occur when ice sheets extend over the sea, and float on the water Thicknesses: few 100 m to 1000s of meters Retreating ice shelves may provide indications of climate change

18 February 2003

25 March 2003

Ice caps Mini ice sheets form primarily in polar and sub-polar regions that are relatively flat and high in elevation

Ice Streams & Outlet Glaciers Ice streams are channelized glaciers Flow more rapidly than the surrounding body of ice

Ice Fields Similar to ice caps Flow is influenced by the underlying topography Typically smaller than ice caps Kalstenius Ice Field, located on Ellesmere Island, Canada, shows vast stretches of ice.

Mountain Glaciers Develop in high mountainous regions Often flow out of icefields The largest mountain glaciers are found Arctic Canada & Alaska the Andes in South America the Himalayas in Asia Antarctica.

Types of Glaciers

Valley (alpine) glaciers Commonly originate from mountain glaciers or ice fields Flows down a valley from an accumulation center at its head Look like giant tongues May be very long Can reach sea level.

Peidmont Occur when steep valley glaciers spill into relatively flat plains Spread out into bulb-like lobes. The massive lobe of Malaspina Glacier is clearly visible in this photograph taken from a Space Shuttle flight in 1989.

Cirque Glaciers Named for the bowl-like hollows they occupy (cirques) Found high on mountainsides Tend to be wide rather than long.

Hanging Glaciers Also called ice aprons Cling to steep mountainsides Wider than they are long Common in the Alps This hanging glacier above Lyman Lake in Washington State may look simply like a mass of snow, but the crevasses are evidence that it really is a glacier.

Tidewater Glaciers Flow far enough to reach out into the sea Responsible for calving numerous small icebergs Lamplugh Glacier, in Glacier Bay, Alaska, shows the snout of a typical tidewater glacier.

Glacial Landforms Glacial Erosion Glacial Deposits

Glacial Erosion Glaciers erode the land in two ways Plucking – lifting of rocks Abrasion Rock flour (pulverized rock) Glacial striations (grooves in the bedrock)

Glacial Landforms Glacial Valleys Fjords Pater noster lakes Cirques Tarns Arêtes Horns

Glacial Landforms

Glacial Deposits Glacial drift – refers to all sediments of glacial origin Types of glacial drift Till – material that is deposited directly by the ice Stratified drift – sediments laid down by glacial meltwater

Glacial Deposits Landforms made of till Moraines - layers or ridges of till Lateral moraine Medial moraine End moraine – terminal or recessional Ground moraine

Glacial deposits Depositional features Outwash plain, or valley train Kettles Drumlins Eskers Kames

Landforms made of stratified drift Outwash plains (with ice sheets) Valley trains (when in a valley) Broad ramp-like surface composed of stratified drift deposited by meltwater leaving a glacier Located adjacent to the downstream edge of most end moraines Often pockmarked with depressions called kettles

Landforms made of stratified drift Ice-contact deposits Deposited by meltwater flowing over, within, and at the base of motionless ice Features include Kames Kame terraces Eskers

Ice Ages Have occurred throughout Earth’s history

Ice Ages Began 2 to 3 million years ago Division of geological time is called the Pleistocene epoch Ice covered 30% of Earth's land area

Ice Ages Four major stages recognized in North America Nebraskan Kansan Illinoian Wisconsinan

Effects of the last Ice Age Forces migration of animals and plants

Effects of the last Ice Age Changes in stream courses

River drainage patterns today River drainage patterns pre- Ice Age

Effects of the last Ice Age Rebounding upward of the crust in former centers of ice accumulation

Effects of the last Ice Age Worldwide change in sea level

Sea Level at LGM~12,000 ybp ~10,000 ybpPresent Effects of Changes in Sea Level Across Beringia

Effects of the last Ice Age Erosion and deposition End moraines of the Wisconsinan and Illinoian stages

Effects of the last Ice Age Climatic changes

Some possible causes of glaciation Changes in oceanic circulation Thermohaline Current (AKA deep ocean circulation patterns)

Variations in Earth’s orbit The Milankovitch hypothesis Shape (eccentricity) of Earth’s orbit varies Angle of Earth’s axis (obliquity) changes Earth’s axis wobbles (precession) Changes in climate over the past several hundred thousand years are closely associated with variations in the geometry of Earth’s orbit

Changes in oceanic circulation Disruptive factors Earth heats up Ice at polar caps melt Increases amount of fresh water in oceans Decreases density of sea water Thermohaline /deep ocean currents can’t form Thermohaline circulation belt slows, stops, or moves towards the equator Climate not moderated Poles freeze, start of new ice age?

~ End ~