Lecture Outlines Physical Geology, 14/e Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plummer, Carlson & Hammersley

Glaciers & Glaciation Physical Geology 14/e, Chapter 12 Copyright © 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 weight part of Earth’s hydrosphere along with sea ice, glaciers are known as the cryosphere ~ 75% of the world’s supply of fresh water is locked up in glacial ice Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glaciers & Earth’s Systems

Develop as snow is compacted and recrystallized, first into firn and then glacial ice Can only form where more snow accumulates during the winter than melts away during the spring and summer alpine glaciation occurs in mountainous regions as valley glaciers continental glaciation covers large land masses in Earth’s polar regions in the form of ice sheets glaciation 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 budget Receding glacier – loses more snow than it gains, has a negative budget zone of accumulation – snow added zone of ablation – melting and calving of icebergs equilibrium 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 flow Due to friction, glacier flow is fastest at the top center of a glacier and slowest along its margins Copyright © 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 along basal abrasion polishes and striates the underlying rock surface and produces abundant fine rock powder known as rock flour Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glacial Erosion

U-shaped valleys Hanging 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 valleys Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Erosional Landscapes

Arêtes – sharp ridges separating glacial valleys Horns – 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 sediment deposits 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 glacier Copyright © 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 glacier sediment-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 kames annual sediment deposition in glacial lakes produces varves, which can be counted like tree rings Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glacial Deposition

In the early 1800s, past extensive glaciation of Europe was first hypothesized hypothesis 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 ages Theory 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 present most 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 bedrock extensive 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 California Lake Bonneville Lake Missoula Sea 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 level fiords are coastal inlets formed by drowning of glacially carved valleys by rising sea level Giant gravel ripples formed during draining of Lake Missoula Copyright © 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 glaciers unsorted rock particles including angular, faceted and striated boulders in some areas, old tillites directly overlie polished and striated crystalline rocks formed during late Paleozoic era in portions of the southern continents indicate that these landmasses were once joined strong 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 Earth’s surface difficult to distinguish from terrestrial rocks small number of meteorites appear to have come from the Moon and Mars a 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 Mars Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mars on a Glacier

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