slide 4.ppt
- 1. Tim Horner CSUS Geology Department Glaciers and Glaciation Physical Geology, Chapter 12
- 2. Glaciers and Earth’s Systems • A glacier is a large, long-lasting mass of ice, formed on land, that moves downhill under its own weight • Glaciers are part of Earth’s hydrosphere • Along with sea ice, glaciers are known as the cryosphere • About 75% of the world’s supply of fresh water is locked up in glacial ice
- 3. a long, narrow inlet with steep sides or cliffs, created by a glacier.Fjords ridge of a mountain that protrudes from an ice field or glacier that otherwise covers most of the mountain ...Nunatak Periglacial' describes a landscape that undergoes seasonal freezing and thawing, typically on the fringes of past and present glaciated regions.
- 4. Formation of Glaciers • Glaciers develop as snow is compacted and recrystallized, first into firn and then glacial ice • A glacier can only form where more snow accumulates during the winter than melts away during the spring and summer • Two types of glaciated terrains on Earth: – Alpine glaciation occurs in mountainous regions in the form of 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
- 5. Anatomy of a Glacier • An advancing glacier gains more snow than it loses, has a positive budget – End or terminus of glacier advances downslope • A receding glacier has a negative budget – Terminus of glacier shrinks back upslope • Snow is added in the zone of accumulation of glaciers, whereas melting (and calving of icebergs) occurs in the zone of ablation • The equilibrium line, which separates accumulation and ablation zones, will advance or retreat depending on climate
- 6. Movement of Glaciers • 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 are 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
- 7. plastic flow (plural plastic flows) (physics) Any fluid flow in which movement is proportional to the applied force (above the yield value). (geology) Any deformation caused by a sustained force. Basal sliding is the act of a glacier sliding over the bed due to meltwater under the ice acting as a lubricant. This movement very much depends on the temperature of the area, the slope of the glacier, the bed roughness, the amount of meltwater from the glacier, and the glacier's size.
- 8. Glacial Erosion • 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
- 9. Erosional Landscapes • Erosional landforms produced by valley glaciers include: – U-shaped valleys – Hanging valleys • Smaller tributary glacial valleys left stranded above more quickly eroded central valleys – Cirques • Steep-sided, half-bowl-shaped recesses carved into mountains at the heads of glacial valleys – Arêtes • Sharp ridges separating glacial valleys – Horns • Sharp peaks remaining after cirques have cut back into a mountain on 3+ sides
- 10. Erosional Landscapes • Erosional landforms produced by valley glaciers include: – U-shaped valleys – Hanging valleys • Smaller tributary glacial valleys left stranded above more quickly eroded central valleys
- 11. Erosional Landscapes • Erosional landforms produced by valley glaciers include: – Cirques • Steep-sided, half-bowl-shaped recesses carved into mountains at the heads of glacial valley
- 12. Erosional Landscapes • Erosional landforms produced by valley glaciers include: – Arêtes • Sharp ridges separating glacial valleys – Horns • Sharp peaks remaining after cirques have cut back into a mountain on 3+ sides
- 13. Glacial Deposition • General name for unsorted, unlayered glacial sediment is till – Deposits of till left behind at the sides and end of a glacier are called lateral, medial and end moraines, respectively • Lateral moraines are elongate, low mounds of till along sides of valley glaciers
- 14. Glacial Deposition • Medial moraines are lateral moraines trapped between adjacent ice streams • End moraines are ridges of till piled up along the front end of a glacier • Successive end moraines left behind by a retreating glacier are called recessional moraines
- 15. Glacial Deposition • Large amounts of liquid water flow over, beneath and away from the ice at the end of a glacier • Sediment deposited by this water is known as glacial outwash • Sediment-laden streams emerging from ends of glaciers have braided channel drainage patterns • Outwash landforms include drumlins, eskers, kettles and kames • Annual sediment deposition in glacial lakes produces varves, which can be counted like tree rings
- 16. Glacial Deposition • Large amounts of liquid water flow over, beneath and away from the ice at the end of a glacier • Sediment deposited by this water is known as glacial outwash • Sediment-laden streams emerging from ends of glaciers have braided channel drainage patterns • Outwash landforms include drumlins, eskers, kettles and kames • Annual sediment deposition in glacial lakes produces varves, which can be counted like tree rings
- 17. Direct Effects of Past Glaciation • 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
- 18. Indirect Effects of Past Glaciation • Large pluvial lakes (formed in a period of abundant rainfall) existed in closed basins in Utah, Nevada and eastern California – Great Salt Lake is remnant of much larger pluvial Lake Bonneville – Huge floods emanated as ice-dammed lakes (e.g., Lake Missoula) drained catastrophically • 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
- 19. Evidence for Older Glaciation • Rocks called 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 • Tillites 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