1

Environmental Geology – Glaciers and deserts

Spring break reading…

The Control of Natureby John McPhee

describes our efforts to control three natural hazards:

1. The Mississippi Floods

2. The Heimaey Eruption, Iceland

3. Debris flows into the LA Basin

suggested

Environmental Geology – Glaciers and deserts

Glaciers and deserts

Reading: This week: 8 and 9Next week: 9 and 10

Surface processes:

Environmental Geology – Glaciers and deserts

Glacial formation

The elevation of the snowline decreases with latitude

The snow accumulation in the winter must exceed melting in the summer

The snow is compacted by overlying layers until it is solid ice

ice crystals

Environmental Geology – Glaciers and deserts

Alpine (or valley) glaciersTypes of glacier

Result from snow accumulation at high elevations

Once significant ice volume has formed it starts to flow downhill, down already existing valleys

Denali national park

2

Environmental Geology – Glaciers and deserts

Continental glaciersTypes of glacier

Cover significant areas of landmass, not just the valleys

Environmental Geology – Glaciers and deserts

Mass balance and flow

Plastic flow:• greatest velocities at

center of glacier• Edges scrape along rock

surfaces erosion

Driving force: gravity• Flow downhill or

spreading out

Environmental Geology – Glaciers and deserts

Advance or ablation

iceberg calving

Depends on the mass balance: • rate of flow vs. removal (calving or

melting)• variations in climate on a decade

timescale cause glaciers to advance or retreat

Environmental Geology – Glaciers and deserts

Larsen B ice sheet breakupJanuary 31st to March 5th 2002

Antarctic Peninsula

3

Environmental Geology – Glaciers and deserts

Flow characteristicsGlacial erosion

Glaciers cause rapid erosion because they are solid

• Plucking of rock fragments

• Abrasion of rock surfaces

Environmental Geology – Glaciers and deserts

near the sourceGlacial erosion

Cirque: plucking produces an eroded bowel Horn: when glaciers

form around a peak e.g. Matterhorn

Areta: steep ridge

between parallel glaciers

Environmental Geology – Glaciers and deserts

in the valleyGlacial erosion

While the ice is in place:

striations

U-shaped valley

fjord: flooded U-shaped valley

After the glacier has melted:

Environmental Geology – Glaciers and deserts

at its limitGlacial deposition

Terminal (or end) morains:• debris dumped at end of

glacier • when glacier retreats the

morain remains

Terminal morainsof Illinois

Terminal morain left as glacier retreats

4

Environmental Geology – Glaciers and deserts

Global air flow…is controlled by

1. heating of the Earth’s surface by the Sun2. Rotation of the Earth3. Local topographic variations

Air circulation:

Deserts

Environmental Geology – Glaciers and deserts

Global air flowOnce we add the Earth’s rotation

Small scale variations in topography modify this larger scale picture

…again plate tectonics

Deserts

Environmental Geology – Glaciers and deserts

Saharan dust stormFebruary 26, 2000

30°N

Environmental Geology – Glaciers and deserts

Saharan dust stormFebruary 28, 2000

30°N

5

Environmental Geology – Glaciers and deserts

Saharan dust stormFebruary 26, 2000

Canary Islands show the effect of local tomography

Sand shadow around volcano

wind direction

Topographic high generates clouds, precipitation removed

sand from atmosphere

Environmental Geology – Glaciers and deserts

Wind erosion

Abrasion sandblasting

Deflation• Removal of fine sediment• Generates dessert pavement

Environmental Geology – Glaciers and deserts

The 1930’s dust bowelVegetation prevents erosion:

• Farmers migrated to the Great Plains after the Civil War

• Plowed fields, grazed land – both removing vegetation

• In the 1930’s several years of drought killed the crops, with novegetation the wind started to remove the sediment…

Environmental Geology – Glaciers and deserts

Wind deposition

Sand dunes

Migrate across non-dessert environments

6

Environmental Geology – Glaciers and deserts

Desert distributionDesertification

Deserts are concentrated around 30°N and 30°S

Environmental Geology – Glaciers and deserts

Rain shadowsDesertification

Mountains force air currents higher into the atmosphere

•air cool and pressures are lower•both cause precipitation

U.S. deserts due to rain shadows

Canary islands

30°

50°

Environmental Geology – Glaciers and deserts

Vegetation and humansDesertification,

Desertification: the conversion of land to dessert i.e. unable to support plant or animal life

The removal of vegetation is the number one cause. Humans are the main cause.

Semiarid environments have little vegetation but it is crucial to sustaining life

The 1930’s Dust Bowel

Environmental Geology – Glaciers and deserts

Vegetation and humansDesertification,

Gray: already dessertRed/orange: high vulnerability