Lecture #14: Glaciers and Glaciation
A glacier is a large, long-lasting mass of ice found on land that mores because of gravity. The theory of glacial ages states that at times in the past, colder climates prevailed during which much more of the land surface of the earth was glaciated than at present.
Alpine glaciationis found in mountainous regions, while continental glaciation exists where large part of a continent is covered by glacial ice. A valley glacier is confined to a valley and flows from a higher to a lower elevation. An ice sheet is a mass of ice that is not restricted to a valley but covers a large area of land. Today ice sheets are only found in Greenland and in Antarctica. Recall that the polar regions of Mars are also covered by ice sheets. Ice caps are found in a few mountainous regions such as Norway and Chile and on islands in the Arctic Ocean.
Formation and growth of glaciers-- Snow converts to glacier ice in somewhat the same way that sediment turns into a sedimentary rock and then into metamorphic rock. New fallen snow is characterized by air trapped between six-pointed snow flakes. Snow flakes settle by compaction under their own weight with the shape points of new formed snow destroyed and some of the air driven out. The compacted mass of granular snow is a transition between snow and glacier ice, called firn. More air is gradually driven out and the snow recrystallizes to form glacier ice with the texture of a metamorphic rock. As ice builds up it moves downward under the influence of gravity. The front of the glacier is wasted, or lost, by melting or breaking off (calving) to form icebergs.
Movement of glaciers-- The upper part of a glacier is the zone of accumulation where the glacier is perennially covered with snow. The lower part of the glacier is the zone of wastage. The lower edge of the glacier is the terminus. Valley glaciers move downslope under the influence of gravity and their own weight at a rate up to 15 meters a day. During movement there are three parts of the glacier: The zone of basal sliding; the zone of plastic flow; and the rigid zone. The rigid zone is brittle and sometimes is broken into crevasses. Ice sheets move with these three zones but often spread laterally rather than flow downslope.
Erosional landscape associated with alpine glaciation--
U-shaped valleys - glacial erosion versus V-shaped valleys where are characteristic of stream erosion.
Truncated spurs- lower pares of ridges that have been carved into triangular facets by erosion.
Hanging valleys- tributary streams remain high on a valley wall.
Cirque- a steep-sided, rounded hollow carved into a mountain at the head of a glacial valley.
Horn- a sharp peak remaining after cirques have cut back into the mountain on several sides.
Erosional landscape associated with continental glaciation --
Rounded mountains - the Canadian Shield has been flattened by the most recent period of glaciation.
Grooved and striated bed rock- rocks carried by the glacier have carved tracks into bed rock.
Till- unsorted and unlayered rock debris carried and deposited by a glacier.
Moraine- a body of till carried within a glacier and left behind after the glacier has receded. New York's Long Island was built up by morainal debris, most of it probably scoured out of New England.
Lateral moraine- a ridgelike pile of till along the sides of a glacier.
End moraine- If the terminus of a glacier remains stationary for a few years a distinct ridge of till piles up along the front edge of the ice.
Terminal moraine- the end moraine marking the farthest advance of a glacier.
Ground moraine- a fairly thin, extensive layer or blanket of till dragged along by a glacier and deposited as the ice melts.
Drumlin- ground moraine reshaped into streamlined hills formed by an ice sheet overriding and reshaping a deposit of till left by an earlier glacial advance. Numerous drumlins are preserved in areas such as upstate New York.
Outwash- material deposited by the debris-laden meltwater coming from the zone of wastage where large quantities of meltwater usually run over, beneath, and away from the ice.
Esker - a long sinuous ridge of water-deposited sediment.
Kettle- a depression formed when an ice block incorporated in outwash finally melts. Kettle lakes dot the landscape in large sections of Wisconsin and Minnesota.
Loess- fine grained, wind-blown deposits of dust which formed as rock flour from grinding down of bed rock by glaciers and which was first deposited as outwash before being blown by the wind.
Tillite- lithified till. Late Paleozoic tillites in the southern continents (South Africa, Australia, Antarctica, South America) have been used as evidence that these land masses were once joined (see the lecture on Plate Tectonics).
Causes of Glacial Ages-
- Milankovitch, a Serbian astronomer, observed that variations in the earth's orbit, wobble of its axis, and inclination to the sun affects the amount of heat from solar radiation received by any particular portion of the earth. Eccentricity of the earth's orbit changes from a more circular to a more elliptical orbit on 100,000 year and 400,000 year cycles. Tilt of the earth's axis ranges from 21.5° to 24° on a cycle of every 41,000 years. The earth wobbles on its axis like a spinning top, making one revolution every 26,000 years.
- Changes in the atmosphere may affect its ability to filter solar radiation because much of the solar energy reaching our planet is either reflected back out to space or absorbed by the atmosphere. Air bubbles in ancient glacial ice suggests that CO2 and CH4 are much higher in the atmosphere during interglacial periods than during glacial periods.
- Changing of the positions of the continents during continental drift may place continental land masses in a position favorable for glaciation. The continents prevent warmer ocean water from circulating in polar regions and, hence, warming those regions. During the late Paleozoic a large continental land mass was located at the south pole.
- Changes in circulation of sea water so that the warm Atlantic Ocean water can not freely circulate into the Arctic Ocean to warm the Arctic Ocean. A warm Arctic Ocean would allow moisture to be picked up and precipitate heavy amounts of snow on continents and thus cause continental glaciation.