Transcript Document

The power of a glacier to move material is a function of its
thickness and its speed
The rate of erosion is greatest near the margins of glaciers, and
is greater in temperate glaciers than in polar glaciers.
Cold-based glaciers, however, often have longer lifespans
Erosional processes
1.
2.
3.
Abrasion
Plucking and Quarrying
Moving meltwater: abrasion and dissolution
Abrasion
Glacier ice cannot abrade most rock due to softness
(even cold glaciers).
Rock fragments act as abrasive elements
Ice is simply a power source and the matrix within which
rock abrades
Where do the rocks come from ?
Free rocks, subglacial freezing and thawing, quarrying or
valley walls
Significant abrasion may only occur when large clasts or a large
number of particles exist at the base
Quarrying and Bulldozing
Glaciers exert compressive forces on obstructing rock and tensile
forces when parts of the glacier freeze to the bottom
Glaciers are capable of removing fractured segments of rock
Loose or fractured substrate can be bulldozed
Thrust-faulting can move basal material to the surface
Repeated advancing and retreating or changes in applied force
load and unload the substrate, causing bending and fracturing.
This is exacerbated by freeze-thaw weathering.
Pressure melting point varies with snow accumulation, surface
melting and crevassing (freeze-thaw zones change).
If glacier is frozen to surface and rock is fractured, it may be plucked
by the glacier above and incorporated into the ice.
Plucking
Mechanism
A. 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 ice
D. After several cycles
Subglacial Meltwater Erosion
Large amount of water generated at base of temperate glaciers
Meltwater 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 features
Small-scale features
Large-scale features
Striae
Grooves
P-forms
Channels
Potholes
Roches moutonnées
Crag and tail
Drumlins
Flutes
Cirque
Snow hollows
Glaciated valley features
Striae
Scratches produced by abrasion
Preserved best in fine-grained, brittle rock (eg. limestone, quartzite)
Form parallel to flow
direction as rocks
within the ice matrix
abrade the underlying
substrate
The form of striae
provide a clue to the
size, concentration
and hardness of clasts
A. Multiple sets
(deeper ones survive)
B. Wedge-shaped
C. Nailhead
D. Rat-tail
E. Polished Surface
Simple striae:
Scratches of various length
Wedge-shaped
Clasts abrade bedrock progressively deeply
and nailhead striae: until they are retracted back into the ice
(triangular or ellipsoidal)
Rat tail striae:
Ridges formed downstream from an obstruction
due to abrasion
Polished surfaces
or fine scratches:
Moving mass of silt or sand finely abrades
underlying substrate
Crescentic marks:
Presence of moving clast under pressure
causes tensional stresses upstream and
compressional forces downstream. Gouges
or fractures form if bedrock strength exceeded.
Semilunate scours, concave upstream formed
after a rock fragment is removed from between
fractures
Crescentic gouges:
Rat-tail
Crescentic
Gouges
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 in
ice or subglacial water
Multiple grooves, Sperry Glacier, Montana
Potholes
Potholes:
Round (often deep) bedrock scours formed when
small cavities are enlarged and deepened by rock
clasts caught in turbulent vortices. The original
clast is often still in the (now dry) pothole.
Large-scale Erosional Features
Formed by glacial plucking, often accompanied by abrasion and
flowing water.
Roche moutonnée
Streamlined forms with a
smooth, gentle upslope
portion and a steep,
jagged downslope portion.
Formed by both ice sheets
and valley glaciers
Formation of Roche Moutonnée
1.
Pre-existing morphological irregularity of some sort (eg. small
outcrop 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 of
fragmented rocks downslope.
Roche moutonnée, Yosemite national Park
Roche moutonnée,
north of Ottawa,
Ontario
Crag and tail
Consists of a resistant bedrock knob and a streamlined remnant
of bedrock or sediments on the tail (lee side).
Crag and tail,
Princess Mary Lake,
Nunavut
Flutes
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.
Fluted terrain,
Peterborough,
Ontario
Cirque
How are they formed ?
Small, thin glaciers near the snowline respond to rapidly changing
climatic conditions.
Rotational mass movements of the glacier carry ice and sediments
toward the lip of the hollow
Erosion is efficient because of frequent freeze-thaw weathering
Sculpts mountains into steep arêtes (ridges) and horns (pyramidal
mountains). The same process may sculpt nunataks
Nivation 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
Pyramidal
form (horn)
caused by
cirque erosion
(Matterhorn,
Swiss Alps)
Nivation Hollows, Ellef Ringnes Island, Nunavut
Glaciated Valleys
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.
2.
Velocity of glacier higher at mid-sections of V-shaped
valley walls than at base or upper sections of wall
Velocity may reach zero at the base and upper sections
of valley walls. A U-shape is most efficient for
glacier flow.
Longitudinal Profile:
Generally, glaciers help to straighten and deepen valleys.
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 ones
due to their weaker erosive capability
Glaciated valleys can be carved and then flooded during and/or
after ice retreat, resulting in fjords.
Sognefjord,
Norway
DRUMLIN FIELD
TERMINAL MORAINE
Glacial Transportation
Types of Glacial Drift
Supraglacial Drift
Subglacial (Basal) Drift
Englacial Drift
Sediment added to a glacier by
(a)
(b)
(c)
plucking and abrasion of the substrate
falling from side or head walls of valleys and nunataks
wind transportation of material onto glacier surface
Ice sheets get most of their sediment load from the surface
Valley 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)
(b)
(c)
(d)
lateral moraines are derived from the valley walls
medial moraines form from the joining of lateral moraines
basal moraines form from the material eroded at the base
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 (thrustfaulting)
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 watersaturated 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
abrasion
Supraglacial 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 Deposition
Till: Material deposited directly by a glacier
Glacial Landforms Formed by Glacial Sediments
Drumlin Shape:
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
Drumlin Composition:
Composed of till, sometimes
stratified
Drumlin Origin:
Erosional Hypotheses
Depositional Hypotheses
Meltwater Hypothesis
Esker:
A sinuous low ridge composed of sand and gravel formed by deposition
from meltwater running through a channel beneath or within glacier ice.
Moraines:
Accumulations of glacial sediment that form under moving
parts of glaciers and under stagnant ice at glacier margins
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Ground Moraines
Terminal Moraines
Recessional Moraine
Interlobate Moraines
Push Moraines
Ice-thrust Ridges
Lateral Moraines
Prairie Mounds
Moraine Plateaus
Till Ridges
Ground Moraines
Basal lodgement till,
often draped by
ablation till
Deposited by rapidly
retreating glaciers
Usually less than 3m
in thickness
Corrugated surface
with irregular ridges
transverse to flow
direction and fluting
parallel to ice flow
Terminal Moraines
One or more subparallel ridges of accumulated glacial drift at the
front of a glacier
Similar in shape to the glacier terminus
Formed because glacier terminus remains stationary while the rest
of the glacier continues to carry sediment to the landform
Often have a hummocky topography (knobs and kettles)
Knobs and kettles are the result of differential ice melting and
sediment release
Topographic map of hummocky topography of a terminal moraine
Cross-sectional diagram of ground and end moraines.
Recessional Moraines
Moraines formed in the same way as end moraines, during short-lived
interruptions in glacier retreat (upslope from the main end moraine
features)
Landforms left at the lower end of a valley, by a retreating glacier.
Quarrying and abrasion is more severe higher in the valley, while
drift thickens downvalley
Interlobate Moraines
Form when a large
volume of sedimentladen meltwater is
funneled between
receding glacier lobes
(eg. Oak Ridges Moraine,
Ontario)
Up to 50m high and 10
to 100’s of kilometres
long
Consist of stratified sand
and gravel
Push Moraine
Glacier bulldozes and
deforms glacial drift
Occurs at margin of
the glacier
Usually less than 10m
in height
Ice-thrust Ridges
Deformed bedrock with
folds (often over basal till)
and faults
Usually covered in ablation
till, especially in depressions
Fields of ridges up to 30m high
Spaced 200 to 300 metres
apart, traced for 100’s of km
Most common where bedrock
is of varying strength
Eg. Milk River Ridge
Ice-thrust moraine, Saskatchewan.
Ridges in foreground are composed of
deformed glacial sediments and
uplifted sandstone bedrock.
Depressions can be seen in the
background.
Ice-thrust ridges in Saskatchewan.
Ridges on horizon are cored by bedrock
masses uplifted by ice pushing. Note
also the spillway.
Lateral Moraines
Ridges of till along edges of glaciated valleys
Debris is from the glacier and rocks fallen from the valley walls
Deposits are often reworked by meltwater streams (terracing)
Since they are ice-cored, there is differential melting leading to
deformation as well as some slumping.