Earth Science, 13e Tarbuck & Lutgens

© 2012 Pearson Education, Inc.

Earth Science, 13e

Tarbuck & Lutgens


Glaciers, Deserts, and WindEarth Science, 13e

Chapter 6Stanley C. Hatfield

Southwestern Illinois College


Glaciers: a part of two basic cycles in the Earth system

Glaciers are a part of both the hydrologic cycle and rock cycle

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



Types of glaciers • Valley, or alpine glaciers – form in

mountainous areas • Ice sheets, or continental glaciers

• Large scale • e.g., Over Greenland and Antarctica

• Other types • Ice caps and piedmont glaciers


Currently ice sheets cover Greenland and Antarctica



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



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 – the breaking off of pieces of ice,

usually dropping into the ocean


The glacial budget



Glaciers erode by • Plucking – lifting of rock blocks • Abrasion

• Rock flour (pulverized rock) • Striations (grooves in the bedrock)



Landforms created by glacial erosion • Glacial trough - a wide U-shaped valley • Hanging valley - a smaller U-shaped valley

leading into the trough • Cirque - round valley at top of a glacier• Arête – sharp crowned ridge between

troughs• Horn - sharp peak at top of glaciers• Fiord - glacial trough in sea, inlet to land


Erosional landforms created by alpine glaciers


The Matterhorn in the Swiss Alps



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


Glacial till is typically unstratified and unsorted


Glacial deposits

• Depositional features • Moraines – layers or ridges of till• Types of moraines

• Lateral - gravel deposits on side of valleys• Medial - gravel ridge in between valleys• End - large gravel ridge crosswise to valley• Terminal end – the furthest the glacier advanced• Recessional end – the glacier sometimes melted

then advanced leaving smaller End moraines• Ground - filling in depressions with glacial till


Glacial deposits

• Outwash plain, or valley train - sloping plains consisting of deposits from melting edge

• Kettles - depressions created when a block of ice stays in a hole while the glacier around it melts and leaves deposits

• Drumlins - streamlined, asymmetrical hills composed of glacial dirt

• Eskers - ridges composed largely of sand and gravel deposited by a stream flowing beneath a glacier near its terminus

• Kames - large round hills of glacial till


Glacial depositional features



Glaciers of the past • Ice Age

• Began 2 to 3 million years ago • Division of geological time is called the

Pleistocene epoch • Ice covered 30 percent of Earth’s land area


Maximum extent of ice during the Ice Age



Glaciers of the past • 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



Causes of glaciation • 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


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


Deserts

Geologic processes in arid climates • 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 stream

valleys including • Wash – American• Arroyo – Mexican Spanish• Wadi – Middle Eastern, biblical


Deserts

Geologic processes in arid climates • Role of water in arid climates

• 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


A dry stream channel in the desert


The same stream channel following heavy rainfall


Deserts

Basin and Range: the evolution of a desert landscape • Uplifted crustal blocks • Interior drainage into basins produces

• Alluvial fans and bajadas – piles of silt spread in a fan shape as the Wash becomes a valley

• Playas and playa lakes – usually dry lake beds with silt bottoms


Deserts

Basin and Range: the evolution of a desert landscape • 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


Landscape evolution in a mountainous desert – early


Landscape evolution in a mountainous desert – middle


Landscape evolution in a mountainous desert – late


Wind erosion

• Deflation• Lifting of loose material • Produces

• Blowouts – rocky ground where topsoil has blown away

• Desert pavement – rocky ground over bedrock with no soil

• Abrasion – eroding rock worn away by blowing sand


Formation of desert pavement


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


Formation of sand dunes



• Sand dunes• Types of sand dunes

• Barchan dunes – parabolic dunes pointing away from the prevailing wind

• Transverse dunes – long waves of sand cross-ways to the prevailing wind

• Longitudinal dunes – long waves of sand parallel to the prevailing wind

• Parabolic dunes – water formed dunes pointing toward the water and prevailing wind

• Star dunes – dunes with pointed tops where there is little prevailing wind


Sand dune types


End of Chapter 6

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Earth Science, 13e Tarbuck & Lutgens