Spring break reading. Glacial formation. Surface processes: Glaciers and deserts. The Control of Nature

Transcription

1 suggested Spring break reading The Control of Nature by John McPhee Surface processes: Glaciers and deserts describes our efforts to control three natural hazards: 1. The Mississippi Floods 2. The Heimaey Eruption, Iceland 3. Debris flows into the LA Basin Reading: This week: 8 and 9 Next week: 9 and 10 Glacial formation The snow accumulation in the winter must exceed melting in the summer The snow is compacted by overlying layers until it is solid ice Types of glacier Alpine (or valley) glaciers 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 The elevation of the snowline decreases with latitude ice crystals

2 Types of glacier Continental glaciers Cover significant areas of landmass, not just the valleys Mass balance and flow Driving force: gravity Flow downhill or spreading out Plastic flow: greatest velocities at center of glacier Edges scrape along rock surfaces erosion Advance or ablation 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 Antarctic Peninsula Larsen B ice sheet breakup January 31 st to March 5 th 2002 iceberg calving

3 Glacial erosion Flow characteristics Glacial erosion near the source Glaciers cause rapid erosion because they are solid Plucking of rock fragments Abrasion of rock surfaces Cirque: plucking produces an eroded bowel Horn: when glaciers form around a peak e.g. Matterhorn Areta: steep ridge between parallel glaciers Glacial erosion in the valley After the glacier has melted: Glacial deposition at its limit While the ice is in place: fjord: flooded U-shaped valley Terminal (or end) morains: debris dumped at end of glacier when glacier retreats the morain remains U-shaped valley striations Terminal morains of Illinois Terminal morain left as glacier retreats

4 Deserts Global air flow is controlled by 1. heating of the Earth s surface by the Sun 2. Rotation of the Earth 3. Local topographic variations Deserts Global air flow Once we add the Earth s rotation Air circulation: Small scale variations in topography modify this larger scale picture again plate tectonics February 26, 2000 Saharan dust storm February 28, 2000 Saharan dust storm 30 N 30 N

5 February 26, 2000 Saharan dust storm Canary Islands show the effect of local tomography Wind erosion Topographic high generates clouds, precipitation removed sand from atmosphere Abrasion sandblasting Deflation Removal of fine sediment Generates dessert pavement Sand shadow around volcano wind direction Vegetation prevents erosion: The 1930 s dust bowel 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 no vegetation the wind started to remove the sediment Wind deposition Sand dunes Migrate across non-dessert environments

6 Desertification Desert distribution Deserts are concentrated around 30 N and 30 S Desertification Rain shadows 50 Mountains force air currents higher into the atmosphere air cool and pressures are lower both cause precipitation U.S. deserts due to rain shadows 30 Canary islands Desertification, Vegetation and humans Desertification, Vegetation and humans 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. The 1930 s Dust Bowel Semiarid environments have little vegetation but it is crucial to sustaining life Gray: already dessert Red/orange: high vulnerability