Glaciers
Maximum Extent of Pleistocene Glaciation - 1/3 of land surface Most recent glacial maximum peaked 18,000 years ago and is considered to have ended 10,000 B.P.
Current Extent of Glaciation - about 10% of land surface
Worldʼs s snowline
High latitude icecaps
Valley glaciers Aletsch
Valley glaciers mass balance results from interplay of: 1) accumulation of snow above equilibrium line and transformation of snow crystals to firn and glacial ice. 2) Ablation (mass loss) below equilibrium line. Accumulation> Ablation Valley glacier moves forward Ablation> Accumulation Valley glacier retreats
Transformation of snow to glacier ice Snow Glacier ice Firn Glacier ice
How do glaciers move? Glaciers must be thick enough so that they flow downslope. Flow is by basal slip or plastic flow. Plastic flow -- ice deforms internally and flows like plastic Basal slip -- ice moves over a thin layer of meltwater
Landforms produced by valley (alpine) glacier erosion U-shaped valleys - Horns - Cirques - Arêtes - Hanging valleys Before glaciation After glaciation
Landforms produced by erosion Cirques Horns Hanging valley U-shaped valley
Glacial deposits and associated landforms Glacial Deposits Drift -- general term applied to any deposit associated with glaciers Till -- sediment deposited directly from melting ice; till is unsorted and unstratified Outwash -- sediment deposited by meltwater streams These deposits are stratified and sorted. Landforms made of till Moraines -- ridges made of till that form at margins of a glacier Drumlins -- streamlined hills made (frequently, but not always) of till; steeper on the side from which ice came. Landforms made of stratified drift: Kames -- small, steep-sided hills or mounds made of sediments are deposited in meltwater lake or meltwater streams. Eskers are sinuous ridges made of stratified drift. The stratified drift is deposited in sub glacial meltwater streams. Kettle --
median End (or Terminal) moraine -- forms at bottom end of glacier Lateral moraine -- forms at side of glacier Ground moraine -- forms by deposition of till at the base of a glacier; not a ridge, just a lumpy plain. Medial Moraine when two glacial valleys merge
Moraine till is usually made of ill-sorted, matrix-abundant deposits with poorly rounded clasts
Permafrost, mean annual temperatures, and rock glaciers formation Permafrost: soil at or below the freezing point of water (0 C) for two or more years. - ice is not always present, but it frequently occurs - most permafrost is located in high latitudes, but alpine permafrost may exist at high altitudes in much lower latitudes.
Permafrost profile in equilibrium
Permafrost profile during adjustment to surface warming 20 Lugano T medie Davos T medie Milano T medie Basel T medie Temp medie*.kg 15 10 5 0 1860 1880 1900 1920 1940 1960 1980 2000
warming
Permafrost melting, rockfalls and rock glaciers formation
Glaciers, erosion, and isostasy. Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. Let us take for simplicity the example of ice floating in water. Z T Z U Z L A block of ice (density = 0.917 g/cm 3 ) is floating in the water (density = 1.00 g/cm 3 ). Let Z T = total thickness of ice block. Let Z U = the upper part (above water level). Let Z L = the lower part (below water level).
Glaciers, erosion, and isostasy. The pressure exerted by the block of ice on point A is P A =ρ ice g Z T while the pressure exerted by the adjacent column of water on point B is P B = ρ water g Z L Z T Z U Z L The block of ice is said to be in isostatic equilibrium when P A = P B and the dashed horizontal line along which pressure is the same is known as the depth of compensation. At P A = P B we have that: ρ ice g Z T = ρ water g Z L knowing that Z T = Z U + Z L Z L = Z T (ρ ice / ρ water ) Z U = Z T (ρ water ρ ice ) / ρ water For Z T = 200 m, Z U = 16.6 m For Z T = 150 m, Z U = 12.4 m For Z T = 100 m, Z U = 8.3 m For Z T = 50 m, Z U = 4.1 m
Imagine to take away from an ice block of thickness Z T1 a chunck of ice of thickness Z U1 emerging from sea level (panel 1). The new block (panel 2) will no more be in isostatic equilibrium because P B > P A and water will flow from B to A so that the block will rise (uplift) to a new compensation depth until P B = P A (panel 3) attaining a final elevation above sea level of Z U2 < Z U1 1) Z U1 2) 3) Z U2 Z T1 Z L1 Z T2 Z T2 Start in panel 1 with an iceberg of thickness Z T1 = 200 m; this iceberg emerges from sea level by Z U1 = 17 m and is submerged by Z L1 = 183 m. Now take away the emerging upper 17 m of the iceberg to obtain a new iceberg of thickness Z T2 = 183 m (panel 2). This new iceberg will uplift by 15 m to a new equilibrium position such that it will emerge from sea level by Z U2 = 15 m (panel 3). Erosion followed by isostatic compensation causes uplift and loss of elevation. Elevation loss = erosion - uplift = 17m - 15m = 2m
This leads to the rather couterintuitive conclusion that by eroding material from an elevated area, e.g., a mountain range, and allowing for isostatic compensation, the mountain range will tend to uplift and at the same time diminish its mean elevation (not to be confused with relief). This is why by erosion, old mountain ranges (m.r.) tend to have low elevations and their deepest roots exposed. This is valid for areas with no active horizontal (tectonic) deformation. Alps (young m.r.) Appalachians (old m.r.)
Fjeldskaar et al., QSR 2002
Fjeldskaar et al., QSR 2002
Scardia, Muttoni & Sciunnach GSA 2006 Beginning of major ice ages In the Alps at ~0.9 Ma (Muttoni et al., 2003; Scardia et al., 2006). Repeated waxing and waning of Alpine glaciers since then. Last major ice age ~18.000 yrs ago
Champagnac et al. Tectonophysics 2009
Champagnac et al. Tectonophysics 2009
End glaciers Champagnac et al. Tectonophysics 2009