The rate of erosion is greatest near the margins of glaciers, andis greater in temperate glaciers than in polar glaciers.Cold-based glaciers, however, often have longer lifespansErosional processes
1.Abrasion2.Plucking and Quarrying3.Moving meltwater: abrasion and dissolutionThe power of a glacier to move material is a function of its thickness and its speed
Glacier ice cannot abrade most rock due to softness (even cold glaciers).Rock fragments act as abrasive elementsIce is simply a power source and the matrix within whichrock abrades Where do the rocks come from ? Free rocks, subglacial freezing and thawing, quarrying orvalley wallsSignificant abrasion may only occur when large clasts or a large number of particles exist at the base
Quarrying and BulldozingGlaciers exert compressive forces on obstructing rock and tensileforces when parts of the glacier freeze to the bottomGlaciers are capable of removing fractured segments of rockLoose or fractured substrate can be bulldozedThrust-faulting can move basal material to the surfaceRepeated advancing and retreating or changes in applied forceload and unload the substrate, causing bending and fracturing.This is exacerbated by freeze-thaw weathering.Pressure melting point varies with snow accumulation, surfacemelting and crevassing (freeze-thaw zones change).
If glacier is frozen to surface and rock is fractured, it may be pluckedby the glacier above and incorporated into the ice.
Plucking MechanismA. Glacier frozen to bed where PMP below surface
B. Frozen bed may expand(eg. due to thinning)C. Glacier advances, plucking some of the substrate frozen to the iceD. After several cycles
Subglacial Meltwater ErosionLarge amount of water generated at base of temperate glaciersMeltwater may flow through fractures, tunnels and thin sheets.Subglacial lakes form under thick polar glaciers.Sudden release generates powerful subglacial floods.Water flows abrade the substrate because they carry sediment.The water itself may dissolve carbonates.
Erosional featuresLarge-scale featuresRoches moutonnesCrag and tailDrumlinsFlutesCirqueSnow hollowsGlaciated valley featuresSmall-scale featuresStriaeGroovesP-formsChannelsPotholes
StriaeScratches produced by abrasionPreserved best in fine-grained, brittle rock (eg. limestone, quartzite)Form parallel to flow direction as rocks within the ice matrix abrade the underlying substrateThe form of striae provide a clue to the size, concentration and hardness of clasts
A. Multiple sets(deeper ones survive)
E. Polished Surface
Simple striae:Scratches of various lengthWedge-shapedClasts abrade bedrock progressively deeplyand nailhead striae:until they are retracted back into the ice(triangular or ellipsoidal)Rat tail striae:Ridges formed downstream from an obstructiondue to abrasionPolished surfacesMoving mass of silt or sand finely abradesor fine scratches:underlying substrateCrescentic gouges:Semilunate scours, concave upstream formedafter a rock fragment is removed from betweenfracturesCrescentic marks:Presence of moving clast under pressure causes tensional stresses upstream and compressional forces downstream. Gouges or fractures form if bedrock strength exceeded.
Grooves Linear erosional features formed in solid bedrock: Less than 2m deep and about 50-100 m long.
Striae are visible inside.
Likely formation mechanism: Large boulders or bands of debris gouge the substrate. Followed by further abrasion by sediments inice or subglacial water
Multiple grooves, Sperry Glacier, Montana
Potholes: Round (often deep) bedrock scours formed when small cavities are enlarged and deepened by rockclasts caught in turbulent vortices. The originalclast is often still in the (now dry) pothole.
Large-scale Erosional FeaturesFormed by glacial plucking, often accompanied by abrasion andflowing water.Roche moutonneStreamlined forms with asmooth, gentle upslopeportion and a steep,jagged downslope portion.Formed by both ice sheetsand valley glaciers
Formation of Roche Moutonne1.Pre-existing morphological irregularity of some sort (eg. smalloutcrop of relatively hard, especially igneous rock)
2.High stresses form upstream causing basal melting and the glacier slides
3.Embedded clasts abrade the bedrock upslope
4.Downslope, there is a pressure drop, so the pressure melting point rises. The glacier freezes to the base.
5.As glacier pulls away, tension causes quarrying or plucking offragmented rocks downslope.
Roche moutonne, Yosemite national ParkRoche moutonne, north of Ottawa, Ontario
Crag and tail
Consists of a resistant bedrock knob and a streamlined remnantof bedrock or sediments on the tail (lee side).Crag and tail, Princess Mary Lake, Nunavut
Sub-parallel grooves with ridges of variable size
They form in flat areas, parallel to the direction of glacier movement
Form on bedrock or sediment-covered terrain.
Mostly erosional, but also depositional as basal sediment is squeezed into fractures at the base of the glacier.
How are they formed ?
Small, thin glaciers near the snowline respond to rapidly changingclimatic conditions.
Rotational mass movements of the glacier carry ice and sedimentstoward the lip of the hollow
Erosion is efficient because of frequent freeze-thaw weathering
Sculpts mountains into steep artes (ridges) and horns (pyramidal mountains). The same process may sculpt nunataksNivation Hollows
Small niches cut into the sides of mountains through freeze-thaw cycles that break up local rocks and the movement of the resulting sediment downslope
Pyramidalform (horn) caused bycirque erosion(Matterhorn,Swiss Alps)
Nivation Hollows, Ellef Ringnes Island, Nunavut
Scoured by streams, then modified by glaciers
Traverse Shape: U-shape in cross section (glacial modification of V-shaped fluvial valley)
How does it change to a U-shape ?1.Velocity of glacier higher at mid-sections of V-shapedvalley walls than at base or upper sections of wall2.Velocity may reach zero at the base and upper sectionsof valley walls. A U-shape is most efficient for glacier flow.
Generally, glaciers help to straighten and deepen valleys.
1.Erodability usually varies along longitudinal profile as a result of lithological and structural characteristics: eg. shale eroded preferentially to granite. This leads to steps.
Hanging valleys form where small glaciers meet larger onesdue to their weaker erosive capability
Glaciated valleys can be carved and then flooded during and/orafter ice retreat, resulting in fjords.Sognefjord,Norway
DRUMLIN FIELDTERMINAL MORAINE
Types of Glacial Drift
Supraglacial DriftSubglacial (Basal) DriftEnglacial Drift
Sediment added to a glacier by
(a)plucking and abrasion of the substrate(b)falling from side or head walls of valleys and nunataks(c)wind transportation of material onto glacier surface
Ice sheets get most of their sediment load from the surfaceValley glaciers get their sediment from both the bed and side
Sediments are transported(a)above the glacier (supraglacial drift)(b)within the glacier (englacial drift)(c)at the base (subglacial or basal drift)
Particles tend to concentrate in patches called moraines
(a)lateral moraines are derived from the valley walls(b)medial moraines form from the joining of lateral moraines(c)basal moraines form from the material eroded at the base(d)internal moraines form when sediments fall into crevasses, where lateral moraines coalesce at the confluence of glaciers or when basal drift is thrust upward at the terminus (thrust-faulting)
Subglacial drift-composed of material derived from the local substrate (some clasts may be added from other parts of the glacier or from previously-deposited glacial sediments)-subglacial drift, where there is basal melting, forms a water-saturated moving carpet, facilitating basal sliding-clasts abrade against bedrock and may also be crushed-fine powder or silt can also develop as a by-product of abrasionSupraglacial drift-Important in valley glaciers in which the confining walls provide the material (largely angular particles)-In ice sheets, from nunataks, upward thrusting of basal material and windblown sediment
Glacial DepositionTill: Material deposited directly by a glacier
Oval, streamlined, hills, shaped like inverted spoons or tear-drops (blunt, rounded heads and long, pointed tails along a straight axis). Lemniscate loop shape. Simple or composite
Generally 1-2 km long, 400 to 600 m wide and 15 to 30 m in height(rock drumlins can be larger)
Vary in size and shape, especially in different fields
Often occur in staggered pattern associated with small end moraines, and eskers
Glacial Landforms Formed by Glacial Sediments
Drumlin Composition:Composed of till, sometimesstratified
Erosional HypothesesDepositional HypothesesMeltwater Hypothesis
Some drumlins have stratified material, always draped by till
Pre-existing sediments get waterlogged and are reworked easily by advancing temperate glaciers
Drumlins form around points wh