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Geology Ch. 22
Gaciers and Ice Ages
| Term | Definition |
|---|---|
| Erratic | A boulder or cobble that was picked up by a glacier and deposited hundreds of kilometers away from the outcrop from which it detached. |
| Crevasse | A large crack that develops by brittle deformation in the top 60 m of a glacier. |
| Glacier | A river or sheet of ice that slowly flows across the land surface and lasts all year long. |
| Ice age | An interval of time in which the climate was colder than it is today, glaciers occasionally advanced to cover large areas of the continents, and mountain glaciers grew; an ice age can include many glacials and interglacials. |
| Pleistocene Ice Age | The period of time from about 2 Ma to 14,000 years ago, during which the Earth experienced an ice age. |
| Albedo | The reflectivity of a surface. |
| Cirque | A bowl-shaped depression carved by a glacier on the side of a mountain. |
| Mountain (or alpine) glacier | A glacier that exists in or adjacent to a mountainous region. |
| Firn | Compacted granular ice (derived from snow) that forms where snow is deeply buried; if buried more deeply, firn turns into glacial ice. |
| Polar glacier | See Dry-bottom glacier. |
| Sublimation | The evaporation of ice directly into vapor without first forming a liquid. |
| Plastic deformation | The deformational process in which mineral grains behave like plastic and, when compressed or sheared, become flattened or elongate without cracking or breaking. |
| Basal sliding | The phenomenon in which meltwater accumulates at the base of a glacier, so that the mass of the glacier slides on a layer of water or on a slurry of water and sediment. |
| Surge | A pulse of rapid flow in a glacier. |
| Zone of ablation | The area of a glacier in which ablation (melting, sublimation, calving) subtracts from the glacier. |
| Equilibrium line | The boundary between the zone of accumulation and the zone of ablation. |
| Glacial advance | The forward movement of a glacier's toe when the supply of snow exceeds the rate of ablation. |
| Toe | The leading edge or margin of a glacier. |
| Glacial retreat | The movement of a glacier's toe back toward the glacier's origin; glacial retreat occurs if the rate of ablation exceeds the rate of supply. |
| Ablation | The removal of ice at the toe of a glacier by melting, sublimation (the evaporation of ice into water vapor), and/or calving. |
| Zone of accumulation | (1) The layer of regolith in which new minerals precipitate out of water passing through, thus leaving behind a load of fine clay; (2) the area of a glacier in which snowfall adds to the glacier. |
| Ice shelf | A broad, flat region of ice along the edge of a continent formed where a continental glacier flowed into the sea. |
| Tidewater glacier | A glacier that has entered the sea along a coast. |
| Iceberg | A large block of ice that calves off the front of a glacier and drops into the sea. |
| Drop stone | A rock that drops to the sea floor once the iceberg that was carrying the rock melts. |
| Sea ice | Ice formed by the freezing of the surface of the sea. |
| Glacial abrasion | The process by which clasts embedded in the base of a glacier grind away at the substrate as the glacier flows. |
| Glacial striation | Grooves or scratches cut into bedrock when clasts embedded in the moving glacier act like the teeth of a giant rasp. |
| Hanging valley | A glacially carved tributary valley whose floor lies at a higher elevation than the floor of the trunk valley. |
| Horn | A pointed mountain peak surrounded by at least three cirques. |
| Arête | A residual knife-edge ridge of rock that separates two adjacent cirques. |
| U-shaped valley | A steep-walled valley shaped by glacial erosion into the form of a U. |
| Roche moutonnee | A glacially eroded hill that becomes elongate in the direction of flow and asymmetric; glacial rasping smoothes the upstream part of the hill into a gentle slope, while glacial plucking erodes the downstream edge into a steep slope. |
| Fjord | A deep, glacially carved, U-shaped valley flooded by rising sea level. |
| End moraine | A low, sinuous ridge of till that develops when the terminus (toe) of a glacier stalls in one position for a while. |
| Lateral moraine | A strip of debris along the side margins of a glacier. |
| Medial moraine | A strip of sediment in the interior of a glacier, parallel to the flow direction of the glacier, formed by the lateral moraines of two merging glaciers. |
| Glacial drift | Sediment deposited in glacial environments. |
| Varve | A pair of thin layers of glacial lake-bed sediment, one consisting of silt brought in during the spring floods and the other of clay deposited during the winter when the lake's surface freezes over and the water is still. |
| Glacial outwash | Coarse sediment deposited on a glacial outwash plain by meltwater streams. |
| Glacial till | Sediment transported by flowing ice and deposited beneath a glacier or at its toe. |
| Terminal moraine | The end moraine at the farthest limit of glaciation. |
| Recessional moraine | The end moraine that forms when a glacier stalls for a while as it recedes. |
| Loess | Layers of fine-grained sediments deposited from the wind; large deposits of loess formed from fine-grained glacial sediment blown off outwash plains. |
| Esker | A ridge of sorted sand and gravel that snakes across a ground moraine; the sediment of an esker was deposited in subglacial meltwater tunnels. |
| Drumlin | A streamlined, elongate hill formed when a glacier overrides glacial till. |
| Glacial subsidence | The sinking of the surface of a continent caused by the weight of an overlying glacial ice sheet. |
| Pluvial lake | A lake formed to the south of a continental glacier as a result of enhanced rainfall during an ice age. |
| Patterned ground | A polar landscape in which the ground splits into pentagonal or hexagonal shapes. |
| Permafrost | Permanently frozen ground. |
| Tundra | A cold, treeless region of land at high latitudes, supporting only species of shrubs, moss, and lichen capable of living on permafrost. |
| Interglacial | A period of time between two glaciations. |
| Glaciation | A portion of an ice age during which huge glaciers grew and covered substantial areas of the continents. |
| Tillite | A rock formed from hardened ancient glacial deposits and consisting of larger clasts distributed through a matrix of sandstone and mudstone. |
| Snowball Earth | A model proposing that, at times during Earth history, glaciers covered all land, and the entire ocean surface froze. |
| Milankovitch cycle | Climate cycles that occur over tens to hundreds of thousands of years because of changes in Earth's orbit and tilt. |
| Little Ice Age | A period of cooler temperatures, between 1500 and 1800 C.E., during which many glaciers advanced. |
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