GLACIERS, DESERTS AND WINDS
Glaciers
•Glacier – a thick mass of ice that forms over land from the compaction and recrystallization of snow
and shows evidence of past or present flow
•Slow moving (cm/day)
•95% are shrinking
Glaciers: a part of two basic cycles in the Earth system
•Glaciers of the past
–Ice Age
•Began 2.5 million years ago
•Division of geological time is called the Pleistocene epoch
•Ice covered 30 percent of Earth’s land area
–Indirect effects of Ice Age glaciers
•Migration of animals and plants
•Rebounding upward of the crust
•Worldwide change in sea level
•Climatic changes
•Causes of glaciation
–Successful theory must account for
•Cooling of Earth, as well as
•Short-term climatic changes
–Proposed possible causes
•Plate tectonics
–Continents were arranged differently
–Changes in oceanic circulation
–Proposed possible causes
•Variations in Earth’s orbit
–Milankovitch hypothesis
»Shape (eccentricity) of Earth’s orbit varies
»Angle of Earth’s axis (obliquity) changes
»Axis wobbles (precession)
–Changes in climate over the past several hundred thousand years are closely
associated with variations in Earth’s orbit
•Glaciers are a part of both the hydrologic cycle and rock cycle
–Hydrologic:
•Cycles freshwater from snow to ice to ocean
–Rock:
•Erosion of sediment
•Sediment transport
•Sediment deposition
•Types of glaciers
–Valley, or alpine glaciers – form in mountainous areas
•Flow like rivers
•Can have tributaries
–Ice sheets, or continental glaciers
•Large scale
•Greenland (1.7 million km2)
•Antarctica (13.9 million km2)
–Other types
•Ice caps and piedmont glaciers
•Ice shelves
–Antarctic features
–Ice flows on top of the ocean
–Current size 1.4 million km2
Global Sea Level Rise
•If all ice melted:
–About 70 m rise in sea level
–Inundation of low coastal areas
–Loss of most of Earth’s freshwater
Glaciers: a part of two basic cycles in the Earth system
•Movement of glacial ice
–Types of glacial movements
•Plastic flow
•Slipping along the ground
–Zone of fracture
•Uppermost 50 meters
•Crevasses form in brittle ice
Glacial Flow
•Solid ice can flow under pressure (plastic)
–Glacier’s own mass sufficient
•Basal sliding (slipping) also important
–In relatively warm environments
–Conditions are right for melting at base
–Liquid water provides lower coefficient of friction between glacier and land
Zone of Fracture
•Uppermost 50 m of glacier
–Insufficient pressure for plastic flow
–Ice behaves like brittle solid
–Crevasses form on surface
Glaciers: a part of two basic cycles in the Earth system
•Movement of glacial ice
–Zone of accumulation – the area where a glacier forms
–Zone of wastage – the area where there is a net loss due to melting
–Calving – creation of icebergs where a glacier meets the sea
Glacier Budget
•Examining inputs and outputs
–Accumulation and wastage
–Balance or imbalance
–Health of glacier
–Sensitive to precipitation and temperature change
–Indicator of climate change
Measuring Movement
•Similarities to stream flow
•Early efforts used stakes in the ice (e.g. Rhone Glacier, Switzerland)
•Time lapse photography
•Satellite Imaging
Glacial erosion
•Glaciers erode by
–Plucking – lifting of rock blocks
•Water fills cracks, freezes, pries out boulders
–Abrasion
•Rock flour (pulverized rock)
•Striations (grooves in the bedrock)
Glacial Erosion Rates
•Dependent Upon:
–Rate of glacial movement
–Thickness of ice
–Shape, abundance, hardness of rocks
–Durability of bottom surface
Glaciers are a part of both the hydrologic cycle and rock cycle
•Landforms created by glacial erosion
–Glacial trough
–Hanging valley
–Cirque
–Arête
–Horn
–Fjord
Glacial Deposition
•Glacial deposits
–Glacial drift
•All sediments of glacial origin
•Types of glacial drift
–Till – material that is deposited directly by ice
–Stratified drift – sediment deposited by meltwater
–Depositional features
•Moraines – layers or ridges of till
•Types of moraines
–Lateral
–Medial
–End
–Ground
End Moraines
•During successive periods of retreat and stability:
–Terminal end moraines mark the furthest movement of a glacier
–Recessional end moraines mark periods of stability as the glacier retreated
•Glacial deposits
–Depositional features
•Outwash plain, or valley train
•Kettles
•Drumlins
•Eskers
•Kames
Deserts
•Geologic processes in arid climates
–Weathering
•Not as effective as in humid regions
•Mechanical weathering forms unaltered rock and mineral fragments
•Some chemical weathering does occur
–Clay forms
–Thin soil forms
–Role of water in arid climates
•Streams are dry most of the time
•Desert streams are said to be ephemeral
–Flow only during periods of rainfall
–Different names are used for desert streams including wash, arroyo, wadi,
donga, and nullah
•Desert rainfall
–Rain often occurs as heavy showers
–Causes flash floods
•Poorly integrated drainage
•Most erosional work in a desert is done by running water
•Basin and Range: the evolution of a desert landscape
–Uplifted crustal blocks
–Interior drainage into basins produces
•Alluvial fans and bajadas
•Playas and playa lakes
–Erosion of mountain mass causes local relief to continually diminish
–Eventually mountains are reduced to a few large bedrock knobs called inselbergs projecting
above a sediment-filled basin
•Wind erosion
–Deflation
•Lifting of loose material
•Produces
–Blowouts
–Desert pavement
–Abrasion
•Types of wind deposits
–Loess
•Deposits of windblown silt
•Extensive blanket deposits
•Primary sources are deserts and glacial stratified drift
–Sand dunes
•Mounds and ridges of sand formed from the wind’s bed load
•Characteristic features
–Slip face – the leeward slope of the dune
–Cross beds – sloping layers of sand in the dune
•Types of sand dunes
–Barchan dunes
–Transverse dunes
–Longitudinal dunes
–Parabolic dunes
–Star dunes