Chapter 19 Geology: Glaciers and Glaciation

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glacier

a large, long lasting mass of ice, formed on land that moves under its own weight.

two types of glaciers

alpine glaciation and continental glaciation

valley glacier

a glacier that is confined to a valley and flows from a higher to a lower elevation.

ice sheet

mass of ice that is not restricted to a valley but covers a large area of land.

ice cap

found in a few mountain highlands in Iceland and on islands in the Artic Ocean, off Canada, Russia and Scandinavia. smaller version of ice sheets.

ablated

when a glacier ice moves downward and is eventually lost.

icebergs

these form when a moving glacier reaches a body of water, blocks of ice break off and float free.

advancing glaciers

glaciers with positive budgets push outward and downward at their edges.

receding glaciers

glaciers with negative budgets that grow smaller and their edges melt back.

zone of accumulation

the upper part of a glacier with perennial snow cover.

zone of ablation

lower part of a glacier where ice is lost/ablated by melting, evaporation and calving.

equilibrium line

the boundary between these two altitudinal zones of a glacier is an irregular line which marks the highest point at which the glacier's winter snow cover is lost during melt season. the position of this line on the glacier is an indication of whether a glacier has a positive or negative budget.

terminus

lower edge of a glacier. its position reflects the glacier's budget.

basal sliding

refers to the sliding of the glacier as a single body over the underlying rock. a thin film of meltwater that develops along the base from the pressure of the overlying glacier facilitates basal sliding.

plastic flow

movement that occurs within the glacier due to the plastic or "deformable" nature of the ice itself.

rigid zone

upper part of the glacier, typically pipes will move downglacier but remain unbent.

crevasses

open fissures in the glacier. occurs when the the glacier passes over a steep part of the valley floor. because the glacier moves faster when this happens, the upper rigid zone of ice cannot stretch to move as rapidly as the underlying plastic flowing of ice, cracks appear.

rock flour

a powder that is created by the grinding of rock across rock. composed of largely very fine particles of unaltered minerals.

u shaped valley

glacially carved valleys that are characteristic of glacial erosion just as v shaped valley is characteristic of stream erosion.

truncated spurs

ridges that have triangular facets produced by glacial erosion at their lower ends. in the process of caving the sidese of its valley, a glacier erodes or "truncates" the lower ends of ridges that extended to the valley.

cirque

a steep sided, half bowl shaped recess carved into a mountain at the head of valley carved by a glacier. this is not entirely carved by the glacier itself but is also shaped by the weathering and erosion of the rock walls above the surface of the ice.

horn

the sharp peak that remains all cirques have cut back into a mountain on several sides.

till

unsorted and unlayered rock debris carried or deposited by a glacier.

erratic

an ice transported boulder that has not been derived from underlying bedrock.

moraines

when till occurs as a body of unsorted debris either on a glacier or left behind by a glacier, the body is regarded as one of several types of this.

lateral moraines

elongated, low mounds of till that form along the sides of a valley glacier.

medial moraine

when tributary glaciers come together, the adjacent lateral moraines join and are carried downglacier as a single long ridge of till.

end moraine

if the terminus remains stationary for a few years or advances, a distinct ridge of till, piles up along the front edge of the ice.

ground moraine

as ice melts, debris that has been carried by a glacier is deposited to form this, a fairly thin, extensive layer of blanket of till.

varve

two layers of sediment representing one year's deposition in a lake. the light colored layer consists of slightly coarser sediment deposited during the warmer part of the year when the nearby glacier is melting and sediment is transported to the lake. the dark layer is finer sediment material that sinks down more slowly during the winter after the lake surface freezes and the supply of fresh, coarser sediment stops due to the lack of meltwater. the dark color is attributed to fine organic matter mixed with the clay.

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.

fjord

a coastal inlet that is a drowned glacially carved valley.

tillite

lithified till where unsorted rock particles, including angular, striated, and faceted boulders, have been consolidated into a sedimentary rock.

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