Lecture OutlinesPhysical Geology, 14/eCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Plummer, Carlson & Hammersley

Glaciers & Glaciation Physical Geology 14/e, Chapter 12Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Glacier a large, long-lasting mass of ice, formed on land, that moves downhill under its own weightpart of Earths hydrospherealong with sea ice, glaciers are known as the cryosphere ~ 75% of the worlds supply of fresh water is locked up in glacial iceCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Glaciers & Earths Systems

Develop as snow is compacted and recrystallized, first into firn and then glacial iceCan only form where more snow accumulates during the winter than melts away during the spring and summeralpine glaciation occurs in mountainous regions as valley glacierscontinental glaciation covers large land masses in Earths polar regions in the form of ice sheetsglaciation occurs in areas cold enough to allow accumulated snow to persist from year to year

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Formation of Glaciers

Advancing glacier gains more snow than it loses, has a positive budgetReceding glacier loses more snow than it gains, has a negative budgetzone of accumulation snow addedzone of ablation melting and calving of icebergsequilibrium line separates accumulation and ablation zones, will advance or retreat depending on climate

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Anatomy of a Glacier

Valley glaciers and ice sheets move downslope under the force of gravity Movement occurs by basal sliding and plastic flow of the lower part of the glacier, and passive riding along of an overlying rigid zone crevasses fractures formed in the upper rigid zone during glacier flowDue to friction, glacier flow is fastest at the top center of a glacier and slowest along its marginsCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Movement of Glaciers

Glaciers erode underlying rock by plucking of rock fragments and abrasion as they are dragged alongbasal abrasion polishes and striates the underlying rock surface and produces abundant fine rock powder known as rock flourCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Glacial Erosion

U-shaped valleysHanging valleys smaller tributary glacial valleys left stranded above more quickly eroded central valleys

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Erosional Landscapes

Cirques steep-sided, half-bowl-shaped recesses carved into mountains at the heads of glacial valleysCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Erosional Landscapes

Artes sharp ridges separating glacial valleysHorns sharp peaks remaining after cirques have cut back into a mountain on 3+ sides

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Erosional Landscapes

Till general name for unsorted, unlayered glacial sedimentdeposits of till left behind at the sides and end of a glacier are called lateral, medial and end moraines, respectively

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Erosional Landscapes

Lateral moraines elongate, low mounds of till along sides of valley glaciers

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Glacial Deposition

Medial moraines lateral moraines trapped between adjacent ice streams

End moraines ridges of till piled up along the front end of a glacier

Recessional moraines successive end moraines left behind by a retreating glacierCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Glacial Deposition

Glacial outwash sediment deposited by large amounts of liquid water flowing over, beneath and away from the ice at the end of a glaciersediment-laden streams emerging from ends of glaciers have braided channel drainage patterns

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Glacial Deposition

Outwash landforms include drumlins, eskers, kettles and kamesannual sediment deposition in glacial lakes produces varves, which can be counted like tree ringsCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Glacial Deposition

In the early 1800s, past extensive glaciation of Europe was first hypothesizedhypothesis initially considered outrageous, but further observations by Louis Agassiz (initially a major opponent of the hypothesis) in Swiss Alps found much supporting evidence Agassiz traveled widely in Europe and North America, finding more and more supporting evidence, eventually leading to the theory of glacial agesTheory of glacial ages states that at times in the past, colder climates prevailed during which much more of the land surface of Earth was glaciated than at presentmost recent glacial age was at its peak only ~18,000 years ago

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Glacial Ages

Large-scale glaciation of North America during the most recent ice age produced the following effects:most of the soil and sedimentary rocks were scraped off underlying crystalline rock in northern and eastern Canada, and lake basins were gouged out of the bedrockextensive sets of recessional moraines were left behind by retreating ice sheets in the upper midwestern U.S. and Canada

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Direct Effects of Past Glaciation

Large pluvial lakes (formed in a period of abundant rainfall) existed in closed basins in Utah, Nevada and eastern CaliforniaLake BonnevilleLake MissoulaSea level was significantly lowered by large amounts of water locked up into ice sheets, allowing stream channels and glaciers to erode valleys below present-day sea levelfiords are coastal inlets formed by drowning of glacially carved valleys by rising sea level

Giant gravel ripples formed during draining of Lake MissoulaCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Indirect Effects of Past Glaciation

Tillites lithified glacial till, have distinctive textures that suggest emplacement of sediments by glaciersunsorted rock particles including angular, faceted and striated bouldersin some areas, old tillites directly overlie polished and striated crystalline rocksformed during late Paleozoic era in portions of the southern continents indicate that these landmasses were once joinedstrong evidence supporting theory of plate tectonics

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Evidence for older Glaciation

Meteorites - extraterrestrial rocks, fragments of material from space that have landed on Earths surfacedifficult to distinguish from terrestrial rockssmall number of meteorites appear to have come from the Moon and Marsa large number of meteorites have been concentrated where the Antarctic ice sheet ablates up against the Transantarctic Mtns. several of these appear to have come from Mars, including one that might bear signs of past life on MarsCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Mars on a Glacier

End of Chapter 12Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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