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Processes of Glacial Erosion

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Processes of Glacial Erosion

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Topics
Erosional processes • conditions under which glaciers erode their bases • models and mechanisms of erosion • glacial quarrying • particle entrainment Erosional features • micro-scale erosional features (striae etc.) • meso-scale (e.g., roches mouton��e, whalebacks) • macro-scale (rock drumlins, cirques, U-valleys, ar��tes etc.) • rates of erosion (glaciers vs rivers)

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Conditions under which glaciers erode their bases effectively
• Glaciers with frozen bases do not generally slide along the ice-bed interface, and therefore do not erode their bases. • In general greatest erosion is associated with warm-based glaciers that have basal melting. • Transition from warm to cold (or water to ice) will contribute to quarrying. • Fast-flowing glaciers are likely to erode more effectively than slow glaciers. • Fluctuations in water pressure will promote quarrying • In areas glaciated several times most of the glacial erosion may be completed during the first phase of glaciation.

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After Bennett and Glasser, 2009

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Mechanisms and models of sub-ice erosion
Erosion of rock beneath a glacier is primarily achieved by rock and mineral fragments embedded in the ice

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Abrasion rate vs ice-debris concentration
Basal debris concentration Rate of abrasion
too little debris too much debris – ice slows maximum abrasion rate
Modified after Bennett and Glasser, 2009

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Boulton model
▪ ��effective normal pressure�� (weight of overlying ice minus basal water pressure) will affect the rate of abrasion ▪ increasing basal water pressure increases the rate of basal sliding but decreases the effective normal pressure ▪ ice with more rock fragments is more effective at abrading rock, but tends to move slower than ��clean�� ice ▪ abrasion and lodgement (deposition of till) are part of a continuum
After Bennett and Glasser, 2009

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Weight of overlying ice

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Weight of overlying ice
Water under pressure facilitates flow but reduces the effective normal pressure of the ice

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Boulton model
The key variable is how hard you push down on the sander.

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Hallet model
▪ ��effective normal pressure�� is not important because the rock clasts are ��floating�� in the ice and are not necessarily pushed down by the ice because of the ice is so much weaker than the rock fragments ▪ pressure of fragments against the bed is determined by the rate of flow towards the bed ▪ that rate is controlled by the rate of melting at the ice-bed interface ▪ basal melting is favoured by (i) rapid flow, (ii) thick ice and (iii) high surface T
After Bennett and Glasser, 2009

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The key variable is the rate at which the lower ice melts and new rock fragments are exposed to erode the rock below.
Hallett model

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Glacial Quarrying
• Removal of large pieces from bedrock • Starts with fracturing (may or may not be initiated by glacial processes) • Water pressure changes may be important • Freeze-thaw cycles are probably not important but freezing of water due to P changes are • The presence of basal cavities is important

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After Bennett and Glasser, 2009

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After Bennett and Glasser, 2009

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Variable glacial quarrying depending on rock strength

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Variable glacial quarrying depending on rock strength

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Glacial quarrying

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Rock and debris entrainment
• Freezing of water onto the base of the glacier, trapping debris (various mechanisms exist for water at the ice-bed contact to freeze onto the overlying ice and incorporate rock fragments into the glacier) • Thrust faults within the ice can push debris up into the glacier

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Glacial erosional features

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After Bennett and Glasser, 2009
U nconfined (and confined) flow
Co nfined flow
Rock drumlins (rocher profil��)

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Micro-scale erosional features
• striations • micro crag and tails • crescentic fractures and gouges • chatter marks

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Striae
• Type 1: become wider and deeper down-ice and then end suddenly as a deep gouge • Type 2: start thin, steadily widen, thin again • Type 3: start abruptly as deep gouges, then thin out

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Nick

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Micro crag-and-tails
Erosional features related to resistant areas (e.g., hard minerals) within a rock surface
Finlayson Pt., Victoria (http://www.geocaching.com)

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• Crescentic fractures: convex up-ice �� • Crescentic gouges • Chatter marks (irregular fractures)
Friction cracks

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http://dorothyspctblog.blogspot.ca
Crescentic fractures

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Crescentic gouges at Squamish

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Meso-scale erosional features
• whalebacks • roches mouton��es • grooves • melt-water channels

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Whalebacks are streamlined on all sides (including the up- and down-ice ends). They are formed beneath relatively thick, slow, warm- based ice with little meltwater and no basal cavities.

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Roches mouton��es are streamlined at their up-ice end and quarried at their down-ice end. They are formed beneath warm-based ice with abundant basal melt-water, with fluctuations in water pressure and with basal cavities. The existence of basal cavities may imply thin ice.

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Roche mouton��e Whaleback ice flow
ice-cavity with water

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Macro-scale erosional features
• U-shaped valleys • cirques, ar��tes & horns • rock drumlins • areal scouring

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Fitzsimmons Ck. valley, Whistler-Blackcomb

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Hanging Valleys at Woodfibre, Howe Sound

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After Bennett and Glasser, 2009

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Rock drumlin, Bowyer Island, Howe Sound
Many of the meso- to macro-scale erosional features of the Howe Sound area have shapes like this. The implications are a thick warm-based glacier without: basal cavities, abundant basal melt-water, or fluctuations in water pressure.

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Jean-Claude Dionne , G��ographie physique et Quaternaire, vol. 38, n�� 1, 1984, p. 69-74.
Ice flow direction

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Areal Scouring Hottah Lake area, NWT
(http://www.nwtgeoscience.ca)

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The relative efficacy of fluvial and glacial erosion over modern to orogenic timescales
Michele N. Koppes & David R. Montgomery
Abstract Since the late nineteenth century, it has been debated whether rivers or glaciers are more effective agents of erosion. The dramatic landscapes associated with glaciated terrain have often led to the argument that glaciers are more erosive than rivers, and recent studies have documented the topographic signature of an ice-controlled limit of mountain height known as the ��glacial buzz-saw��. Here we present a new global compilation of erosion rates, which questions the conventional view of glaciers and erosion. In regions of rapid tectonic uplift, erosion rates from rivers and glaciers both range from 1 to over 10 mm yr−1, indicating that both are capable of generating erosion rates matching or exceeding the highest rates of rock uplift. Moreover, a comparison of erosion rates over timescales ranging from 101 to 107 years indicates that glacial erosion tends to decrease by one to two orders of magnitude over glacial cycles, whereas fluvial erosion rates show no apparent dependence on time. We conclude that tectonics controls rates of both fluvial and glacial erosion over millennial and longer timescales and that the highest rates of erosion (>10 mm yr−1) generally result from a transient response to disturbance by volcanic eruptions, climate change and modern agriculture.
Nature Geoscience, v.2, 644 - 647 (2009)

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Review of Glacial Erosion
Erosional processes • conditions under which glaciers erode their bases • models and mechanisms of erosion • glacial quarrying • particle entrainment Erosional features • micro-scale erosional features (striae etc.) • meso-scale (e.g., roches mouton��e, whalebacks) • macro-scale (rock drumlins, cirques, U-valleys, ar��tes etc.) • rates of erosion (glaciers vs rivers)
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