Glaciers. A glacier is a persistent mass of ice: snow accumulation exceeds melting. generally occur in two areas: high latitudes, or high elevations

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1 Page 1 of7 Glaciers A glacier is a persistent mass of ice: snow accumulation exceeds melting generally occur in two areas: high latitudes, or high elevations hence CONTINENTAL and ALPINE glaciation Glaciers expand and contract as a result of changes in their MASS BALANCE: The ZONE OF ACCUMULAnON is the area where the yearly accumulation of snow exceeds melting. The ZONE OF ABLAnON is the area of the glacier where yearly melting exceeds local accumulation. -n Mll'lnc.~v ~,) moj I 1U ~ t- SOC() ~~~~S The boundary between the two is the EQUILIBRIUM LINE and this shifts with seasonal changes in precipitation and long term climatic change. Glacial ice is in constant slow motion.....jv,.) '15 ) Gravity drives it from the zone of accumulation out to the zone of ablation. This movement produces glacial erosion, transport and deposition. Movement occurs by two processes:

2 kirkby/ss 1O.html 4/24/2006 Page 2 of7 Internal Flow At high pressure (depth) ice crystals deform and become aligned. Brittle surficial ice is carried along and fractures to form crevasses. Basal Sliding meltwater can also collect at the glaciers' basal surfaces allowing them to slide quickly. The much greater viscosity of ice gives it a distinctive style of erosion, transportation and deposition. G\C\ u.,.. 1U1 e..u...\-- ( cl 'J_ 5 ~ ~ '\It,JIu0 & 15'1~{ \1 II vj.t V --j~~ ~( \I-th Glacial Erosion Glaciers primarily erode by abrasion and plucking. Solution and chemical weathering are minimal. Striations and grooves ar common features in glacial areas. Glacial valleys tend to have U-shaped cross sections, and do not necessarily meet at the same elevation (hanging valleys). Glacial Transportation Glaciers carry a poorly mixed sediment load, concentrated along the glaciers' bottoms and sides (the source ar as). I ebergs - parts of glaciers that were calved off provide a unique transport mechanism. Glacial Deposition Material deposited by glaciers is called DRIFT.

3 httn llt<l]r. p"p'o llmn p,tlll/r.olm:p,,,1l kirkhv/",,' 0 html 41?41?OOfi Page 3 of7 Drift includes: unsorted sediment (TILL and ERRATICS) that forms different land forms called MORAINES. Drift also includes: sorted and stratified sediment formed as outwash deposits by meltwater. In addition, glaciers are also responsible for loess deposits wind blown deposits of fine dust and silt that can form rich soil horizons. Glacial Ages Global changes in ice volume have characterized Earth's History for past 2-4 million years Widespread glaciations have also characterized other geologic periods. Glacial ages were first recogniz d years ago, but we are only beginning to understand their origin. At present 10% of land surface is covered by glaciers 30,000 years ago the figure was nearly 30% Most of this increase was due to expansion of the northem continental ice sheets, but alpine glaciers also expanded. We ar still in the Pleistocene (the "Ice Age") climatic change will occur!

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5 http;//ta1c.geo.umn.edu/courses/1001/1001 kirkby/sslo.html 4/24/2006 Page 4 of7 Effects: land forms and glacial deposits; drainage patterns (including subsequent influences on landfonns, and ocean circulation); and crustal deformation (density of ice ~l/3 density of rock). sea level fluctuations (including all subsequent influences on flora and fauna migrations); climatic change (both positive and negative feedback loops); civilization? Evidence Long before a glacial origin was widely recognized, naturalists knew that many alpine valleys and widespread 'drift' deposits could not have been fonned by nvers. Ironically, locals recognized their glacial origins decades before the 'experts'. At first, the evidence for glacial peri ds was based solely on landfonns, till, and erratics. More recently, biological and chemical evidence from high latitude deep sea cores revised our understanding of the earth's recent glacial history. Fluctuations between wann and cold shallow-water communities occur in high latitude deep sea cores, recording limatic change. Changes in the amount of sand/silt found in these cores also suggests changes in ice berg frequency The oxygen isotope record of shells in the cores also varies.

6 001/1 001_kirkby/ss1O.html 4/24/2006 Page 5 of7 Although different oxygen isotopes behavior in a similar manner, they aren't identical. Water composed of 016 evaporates more easily than water composed of 018. Hence, as glaciers expand, 016 rich precipitation is stored as glacial ice, and the 018/016 ratio of the remaining sea water increases slightly. Lake pollen also provides an important record of glaciation -hj w4-h~4ctf.'~ (~Lt..:41:1 - :30-4--' - J Causes: hi~ l+i~ n- ~ The causes of widespread glaciation remain somewhat uncertain. This is a major concern since we are still in a glacial period and have the potential to significantly impact global climate. 8>- TVr-11 ~ o-l The 'best guess' is that glacial periods result from a combination of: 0C5~~ ov{i '\'h~. plate tectonics (palogeography) astronomical variations in the earth's orbit and complex changes in circulation patterns of the atmosphere and ocean systems. Large continental glaciers only occur when large continental areas are present in polar regions. Recent glaciations were concentrated in the northern, high latitude continents. But during the Upper Paleozoic, a large supercontinent (Gondwanaland) was present at the south pole, and Upper Paleozoic glaciations were concentrated in the southern contin nts. In contrast, during the Cretaceous, there was r lati ely littl land area in either polar region, and

7 httn'llt<llr O"P'C) llmn p,nn/rolm;p,<:/l OOl/l nnl kirkhv/<:<:l n html 41?41?nnf\ Page 6 of7 consequently little or no glaciation. But plate tectonics does not fully explain glaciation - esp cially glacial cycles. Glacial cycles appear to be directly or indirectly related to changes in the distribution of solar radiation as a result of variations in the earth's orbit. There are three major variations in the earth's orbit, each with its own period. Eccentricity is the change from a nearly circular to a slightly elliptical orbital path. Eccentricity changes occur on two periods of roughly 100,000 and 400,000 years. Although eccentricity has the smallest effect on solar radiation distribution, it is the only one that changes the total amount of solar radiation reaching the Earth. Obliquity is variation in the tilt of the earth's axis of rotation (up to 3/2 of degree). This change in tilt has a period of- 41,000 years. The earth's axis of rotation also 'wobbles' with time. I This change is called precession and has periods of21,000 to 23,000 years. Orbital changes only have a slight effect on the total amount of global radiation, but they have a pronounced effect on the spatial and temporal distribution of 3adiation in the higher latitudes. This area i the most sensitive to climatic change.

8 httn-llt::lk. P~() "mn~rlllk()"rs~s/1001/1001 kirkhv/sslo html 4/74/700(-\ Page 7 of7 The periods of orbital changes matches the climatic record observed in deep sea cores. Hence, orbital variations probably control the timing of glacial and interglacial cycles. However the changes in solar radiation at higher latitudes are not great enough to cause glaciation by themselves. Instead, they probably act as a trigger mechanism, to change atmosphere and oceanic circulation patterns that in tum produce glaciations. (example - the maximum variation in radiant energy over northern Europe due to orbital changes is about 10%) (In contrast, the heat brought in by the North Atlantic ocean current is three times this amount) NA ~r- OL~.../! /