Transcript GSC 1530 Chapter 12 - Oakland Community College

GSC 1620 Chapter 9
Glaciers and Deserts
Background
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Glaciers (~10%) and deserts (~30%)
currently cover about 40% of the Earth’s land
surface and play a critical role in global
climate
We need to understand how these systems
form and change, especially how they may
be affected by global climate change
(chapter 10)
Question to ponder: how will changes in
these systems affect the rest of the world?
Definitions
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Glacier – a thick, land-based ice mass that
flows downslope
Glaciers currently cover about 10% of the
Earth’s land surface
We’ll recognize two broad categories of
glaciers: alpine (mountain, valley synonyms)
and continental (ice sheets) (see slide)
Glaciers
Glaciers
Alpine
~200,000
~5% glacial ice volume
Continental
2
~95% glacial ice volume
Antarctica: 1.4 times U.S.
land area; hosts ~66% of
Earth’s fresh water!
Glacier Water
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Glaciers constitute
the world’s largest
volume freshwater
resource although
most of this water
isn’t directly
accessible to us due
to its frozen state
(see figures)
Glaciers contain
~67%of world’s
fresh water
Glacier Formation
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We’ll first examine alpine glaciers
All glaciers form by the transformation of
snow above the snowline – the lowest
altitude at which snow exists perennially (see
slides)
Glacier Formation
Alpine Glaciation
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Alpine glaciers typically originate in bowlshaped depressions (known as cirques) near
the summit of a mountain face
When the glacier reaches a threshold
thickness, usually about 55 – 60 meters
(about 170 – 200 ft), it begins to move
downslope under the influence of gravity; the
highly erosive ice carves a valley in the
process (see slides)
Alpine Glacier Movement
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Alpine glaciers primarily move downslope by
a combination of internal flow (creep) and
basal sliding (slip) facilitated by internal melt
waters reducing friction between the glacial
bed and underlying surface (see slide)
The typical alpine glacier moves about 0.5 –
2 meters per day
Glacial Budget
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If the glacier’s snow and ice accumulation
exceeds the loss of ice by ablation the
glacier is said to be advancing (expanding in
volume)
If ablation exceeds accumulation the glacier
is said to be retreating (decreasing in
volume) (see slide)
Glacial Erosion
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Glaciers are very
effective agents of
erosion; they pluck and
grind (abrade) the rock
they pass over and
against
Glacial Erosion
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A variety of features, including cirques,
arêtes, horns, U-shaped valleys, glacially
grooved (striated) rocks and fiords are
evidence of prior alpine glacier erosion (see
slides); be familiar with the U-shaped valleys
Note
U-shaped
valley
Glacial Deposits
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Glaciers transport and deposit large amounts
of rock and sediment
Glacial drift – term used to describe
sediments of glacial origin; can be
subdivided into till and stratified drift (see
slide)
Glacial Deposits
Glacial Drift
Till
Directly deposited by glacial ice
Poorly sorted, variable rounding,
non-layered, mechanically abraded
Stratified Drift
Deposited by glacial meltwaters
Moderately sorted, layered
Glacial Deposits
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Landforms composed primarily of till are
called moraines
Lateral, medial, terminal, recessional and
ground moraines are recognized (see slides);
be familiar with the first three
Origin of a
moraine
Continental Glaciers
1.4 times U.S.
land area;
hosts ~66% of
Earth’s fresh
water!
Glacial Mass Balance Studies
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Glacial Mass Balance Study – measure of glacial ice
volume change with time
These studies are aided by satellite and aerial
photography and ground-based studies
Why are these studies important? (see slide)
Remember: glacial ice effectively reflects sunlight;
the world’s glaciers are often referred to as its “air
conditioning system”
Deserts
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Deserts (arid) and steppes (semiarid) lands
occupy about 30-35 percent of Earth’s surface
– more than any other ecosystem type
Therefore its important to understand these
environments and whether deserts are
expanding or contracting (see slide)
Don’t forget – there are polar deserts (Antarctica and most of
the Arctic)!
Deserts
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Desert – a sparsely vegetated, arid region
that typically receives less than ten inches of
rainfall per year and where the rate of
evaporation greatly exceeds the rate of
precipitation
Note no mention of temperature in my
definition (see slides)
Desertification
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Desertification – expansion, or spread, of
deserts largely as a consequence of human
activities
A worldwide problem, including U.S.
In the U.S. moderate to severe desertification
has occurred in parts of the southwest, plains
and western states in the last 150 years (see
slide)
black = desert
Desertification
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What are the major human activities that
could encourage desertification? Hint: any
process that would foster removal of the thin
topsoil layer in an arid or semiarid region
adjacent to a desert would encourage
desertification
Causes?
Desertification Causes
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Clearcutting of native vegetation (droughtresistant trees and grasses like the Tallgrass
Prairie) (see figure)
Desertification
Historic extent of N. American Prairie
(Illinois State University)
Big Bluestem Grass
Desertification Causes
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Poor farming choices (growth of waterhungry crops prone to failure during a
drought)
Overgrazing by livestock
Poor water management (excessive
diversions of surface waters/depletion of
aquifers)
Off-road vehicle use
Desertification
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Short- and long-term consequences of U.S.
desertification, regionally and nationally?
How could desertification be reduced by
actions at the local, state or federal level?
(see slides)
1930s Dust Bowl
Dust Storm
Crop Fields Buried By
Windblown Soil
Describe some short- and long-term consequences of these events.
Dust Bowl storm near Elkhart, Kansas in 1937