suggested
Spring break reading…
Surface processes:
The Control of Nature
Glaciers and deserts
by John McPhee
Reading:
This week: 8 and 9
Next week: 9 and 10
describes our efforts to control
three natural hazards:
1. The Mississippi Floods
2. The Heimaey Eruption, Iceland
3. Debris flows into the LA Basin
Environmental Geology – Glaciers and deserts
Glacial formation
The snow is compacted by
overlying layers until it is
solid ice
The snow accumulation in the winter
must exceed melting in the summer
Environmental Geology – Glaciers and deserts
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
Environmental Geology – Glaciers and deserts
Environmental Geology – Glaciers and deserts
1
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 Î erosion
Environmental Geology – Glaciers and deserts
Environmental Geology – Glaciers and deserts
Antarctic Peninsula
Advance or ablation
Larsen B ice sheet breakup
January 31st to March 5th 2002
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
iceberg calving
Environmental Geology – Glaciers and deserts
Environmental Geology – Glaciers and deserts
2
Glacial erosion
Glacial erosion
Flow characteristics
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
Environmental Geology – Glaciers and deserts
Glacial erosion
in the valley
After the glacier
has melted:
Environmental Geology – Glaciers and deserts
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
Environmental Geology – Glaciers and deserts
Environmental Geology – Glaciers and deserts
3
Deserts
Deserts
Global air flow
Global air flow
Once we add the Earth’s rotation
…is controlled by
1. heating of the Earth’s surface by the Sun
2. Rotation of the Earth
3. Local topographic variations
Air circulation:
Small scale variations in
topography modify this
larger scale picture
…again plate tectonics
Environmental Geology – Glaciers and deserts
Environmental Geology – Glaciers and deserts
February 26, 2000
February 28, 2000
Saharan dust storm
Saharan dust storm
30°N
Environmental Geology – Glaciers and deserts
30°N
Environmental Geology – Glaciers and deserts
4
February 26, 2000
Saharan dust storm
Wind erosion
Canary Islands show the effect of local tomography
Topographic high generates
clouds, precipitation removed
sand from atmosphere
Abrasion
sandblasting
Deflation
• Removal of fine sediment
• Generates dessert pavement
Î
Sand shadow
around volcano
wind
direction
Environmental Geology – Glaciers and deserts
Vegetation prevents erosion:
The 1930’s dust bowel
Environmental Geology – Glaciers and deserts
Wind deposition
• 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…
Sand dunes
Migrate across non-dessert environments
Environmental Geology – Glaciers and deserts
Environmental Geology – Glaciers and deserts
5
Desertification
Desertification
Desert distribution
Rain shadows
50°
Deserts are concentrated around 30°N and 30°S
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
Environmental Geology – Glaciers and deserts
Desertification,
Environmental Geology – Glaciers and deserts
Desertification,
Vegetation and humans
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
Environmental Geology – Glaciers and deserts
Environmental Geology – Glaciers and deserts
6