Glacier Basics

An introduction to where, how, and why glaciers form

What is a glacier?

• Large mass of moving ice that originated on land from accumulation, compaction, and recrystallization of snow

• When snow is added to a glacier faster than ice and snow melt, the glacier gets larger

• Largest fresh-water reservoir on Earth (75% of world’s fresh water)

• Two main types– Continental glaciers /ice sheets– Alpine/valley glaciers

Continental Glaciers

• Massive sheets of ice that may cover millions of square kilometers, that may be thousands of meters thick, and that are not confined by surrounding topography

• Are also called ice sheets

Alpine Glacier

• Narrow, wedge-shaped mass of ice that forms in a mountainous region that that is confined to a small area by surrounding topography

Where are glaciers found?

Hugo Ahlenius, UNEP/GRID-Arendal, overview of world glaciers and ice caps, UNEP/GRID-Arendal Maps and Graphics Library, (Accessed 22 June 2009)

•Glaciers are found on every continent except Australia•Found in polar regions and mountain ranges (and other planets)•Usually glaciers start at higher elevations and flow downward and outward

Glacier formation•Glaciers form wherever more snow accumulates in the winter than melts in the summer.•Glaciers form above the snowline or lower limit of perennial snow in the zone of accumulation.•Glaciers move downhill in response to gravity•Glaciers melt or sublimate in the zone of wastage/ablation•Crevasses are large cracks which reach the glacier surface and form by a number of ways

Glacial Movement

• Glaciers move in 2 ways:• Basal Slip: simple response due to gravity

whereby the ice moves downhill over bedrock and slipping over a thin layer of water.

• Internal Plastic Flow: If you stack a LOT of ice, the pressure increases and the ice behaves plastically, meaning that the lower layers will start to deform. Flow in a glacier is smooth at depth, but in the top ~50 m ice is brittle and cracks

Glacial erosion

• Rate of glacial erosion depends on– Rate of glacial movement– Ice thickness– Underlying rock characteristics

Glacial erosion

• Plucking– Deep depression in rock when moving glacier

loosens and dislodges rock from bedrock at base or side of glacier

– Sediment carried and deposited by glaciers is called glacial drift

• Abrasion– Caused by glacial flow– Striations form

How do glaciers change a landscape?

Grosser Aletschgletscher (Bernese Alps), Dirk Beyer

Mainglacier

Medialmoraines

Horn Arête CirqueLateral

moraines

How do glaciers change a landscape?Erosional Features

Tarnlake

Paternoster lakes

U-shaped valley

Hangingvalley

Truncated spur

Cirque

HornsArête

www.wiredantarctica.com

Erosional features of a glaciated valley, DK

Glacier anatomy: Alpine Glaciers

www.wiredantarctica.com

Yosemite Valley

Hangingvalley U-shaped valley

Hanging Valley: Form where a higher elevation glacier flows into a larger glacier that has a deeper canyon

Crevasses

• Form by:– Two glaciers colliding– Accelerations in

glacier speed– Changes in

underlying topography

Glacial Erosion - Landforms

• Cirque- Moving glacier pulls blocks of rock from the floor of the valley and create a bowl shaped depression

Glacial Erosion - Landforms

• Arête- Sharp, jagged ridge forms between cirques

Glacial Erosion - Landforms

• Horn- When several arêtes join, they form a sharp, pyramid-like peak

Glacial Erosion - Landforms

• Fjord- forms where a glacier has eroded a valley into the sea. When glacier retreats, the sea fills in.

Glacial Deposition - Landforms

• Erratic- Large boulders carried a long distance from their original location

Glacial Deposition - Landforms

• Moraines- The debris that glacier grind up, carry, and push out in front of it

Glacial Deposition - Landforms

• Drumlins- long, tear shaped mound

Glacial Deposition - Landforms

• Braided Streams- multiple small, shallow channels

Glacial Deposition - Landforms• Kettles- occurring as the result of blocks of

ice caving in