GLACIATION - GLACIERS OF THE WORLD
Alpine Glaciers
-Glaciers that form in mountainous terrain
-Exist on every continent
-Their formation depends on both temperature and precipitation.
Continental Glaciers, also called an ice sheet
-Glaciers that form a continuous cover of ice over areas of 50,000 km2
or more
-Spreads out in all directions under its own weight
-Vast continental glaciers covered much of what are now North Americ
Europe, Asia, and parts of the southern continents at certain times in
the past. These periods are called ice ages.
Matterhorn in Switzerland..perhaps most famous glacial peak
THE MOVEMENT OF GLACIERS
A. Basal Slip:
1. One component of glacial movement
2. The entire mass of the glacier slides along the bedrock and is accelerated by the
presence of water between the ice and the bedrock.
3. Factors the increase the temperature at the bottom of a glacier.
-heat from the interior of the Earth
-friction from glacial movement
4. Ice can melt at the bottom of the glacier as a result of increased pressure.
B. Plastic Flow
1. At the surface of a glacier, pressure is low and the ice acts as a brittle solid.
2. At greater depths the pressure is sufficient to deform ice crystals in a plastic mann
3. The plastic portion of the glacier moves by plastic flow in addition to basal slip.
The ice deforms and flows without fracture.
4. Obstructions to glacial flow do not affect the lower plastic layer of the glacier but
cause the upper brittle zone to develop fractures or cracks called crevasses.
Plastic flow-internal deformation
within the ice.
Basal slip-sliding over
underlying surface.
Flow velocity in valley glacier
varies vertically and horizontally:
greatest at top center, friction at
edges and bottom slows it down
C. The Mass Balance of a Glacier
1.The buildup of snow, firn, and ice occurs in the accumulation area.
2. The firn line or snow line is the boundary between permanent snow and
seasonal snow. The firn line varies with conditions and may shift up and down the
glacier from year to year.
3. In the lower part of a glacier, called the ablation area or zone of wastage, more
snow is lost in the summer than accumulates in the winter. Glacial ice flows
downward form the accumulation area to the ablation area and continuously
replenishes it.
4. The end, or foot, of a glacier is called the terminus. usually located on land
a. Tidewater glaciers extend directly into the sea dropping off abruptly into the
sea.
-The terminus is often a steep ice cliff dropping off abruptly into the sea.
Giant chunks of ice break off, or calve, forming icebergs.
5. Glaciers grow and shrink
a. the lag time between a change in climate and the advance of a glacier
may range from a few years to several decades depends upon the size of the glacier,
its rate of motion, and the magnitude of the climate change
b. If the average annual snowfall decreases or the average yearly temperature rises,
the accumulation is shrinks both in size and thickness and the glacier retreats.
Greenland ice cap-in zone of accumulation-flows to ocean; icebergs
GLACIAL EROSION
Processes of Glacial Erosion
1. Glaciers scour huge areas of bedrock and erode landscapes
a. Meltwater seeps into cracks in bedrock at the base of
a glacier and then refreezes, prying loose particles or
rock in a process called plucking.
b. As a glacier flows over the loosened particles, it lifts them into the flowing
ice and carries them down slope.
2. Once rocks and the fine sediment are incorporated into a glacier, the entire mass
continues to slide over bedrock.
a. These particles embedded in the base of the glacier mark the bedrock by deep,
parallel grooves and scratches called glacial striations.
b. If sediment near the base of a glacier is very fine, the bedrock is not gouged or
scratched, but is instead polished to a smooth shiny finish creating glacial polish
c. The process of abrasion grinds rocks into silt-sized sediment called rock flour.
Glacial polish- ice abraded rock, imparts this sheen on rock
Glacial striations- straight scratches on rocks, basalt here..
Glacial flour-discolors this stream, v fine grained particles, by abrasion
Erosional Landforms Created by Alpine Glaciers.
1. A cirque is a horseshoe-shaped depression gouged out of the
mountainside.
A cirque is formed by the glacier movement and plucking rock leaving a depression’
Mechanical weathering by frost wedging and mass wasting increase the size of the depression.
2. Glaciers that form on opposite sides of a mountain ridge will result
in a sharp, narrow ridge, called an arête.
3. If three or more cirques form on different sides of a peak carving a ridge
4 . small glacial valleys, separated by arêtes, that lie high above the
floor of the main valley, is called a hanging valley.
Hanging valleys form when these are tributary glaciers that do not scour the valley as deeply as
the large central glacier. If the larger central glacier cuts off the lower portion of an arête, a
triangular-shaped rock face called a truncated spur forms.
5. U-shaped valleys form due to deepening and widening effects of
the glacier.
6. A small lake, or tarn, forms at the base of the cirque if the glacier
melts.
7. Paternoster lakes form as glaciers pluck out a sequence of small
basins in a glacial valley. Lakes will occur in a series connected by fast-flowing
streams, rapids, and waterfalls.
Erosional features produced by glacial
erosion…..
Cirque-bowl shaped depression. Steep walls on 3 sides, opens
into glacial trough on 4th side…
Erosional Landforms Created by Continental Glaciers
1. Continental Glaciers erode the landscape in much the same way
as mountain glaciers do
2. If a glacier covers a knob of bedrock, the rock will be sculpted to
form an elongate, streamlined hill called a roche moutonnee
a. The upstream side is typically gently inclined, rounded, and striated
by abrasion
b. Formed both by alpine and continental glaciers
Origin of a
Roche moutonee:
as ice forms over
hill, smoothes
upstream side by
abrasion, shapes
downstream side
by ‘plucking’
LANDFORMS CREATED BY GLACIAL DEPOSITION
A. Landforms composed of till
1. Drift is all rock or sediment transported and deposited by a glacier and is subdivided into
two categories:
a. Till is deposited directly by glacial ice and is not transported and deposited by a stream
b. Stratified drift consists of sediment that was transported by a glacier and then reworked and
deposited by water
2. Erratics are boulders transported by glaciers that are different than those found in the
vicinity
3. Moraines form when a glacier deposits till
a. Types of moraines
-Terminal moraines are found at the end of the glacier before the glacier begins to retreat
-Ground moraines are relatively thin layers of till spread over a broad area
-Recessional moraines are new mounds that form when a glacier stabilizes during its retreat
and the terminus remains in the same place for a year or more.
b. End moraines and ground moraines are characteristic of both alpine and continental glaci
c. The extent of continental glaciers of the Pleistocene Epoch can be determined by locating the
end moraines
d. Alpine glaciers also deposit lateral moraines along the sides of the ice
e. Tributary glaciers that both have lateral moraines can combine placing the moraines in the
center of the glacier forming medial moraines.
4. Drumlins are elongate hills composed of stratified till or drift
-Some were probably deposited beneath a glacier, Others may be remains of old moraines
-Formed when a glacier flows over and reshapes a mound of sediment-The stream-lined shape is elongated in the same
direction as the glacial flow
Valley glacier in Alaska:
Tributaries
Crevasses
Coarse material transported
along top surface of glacier
a. Lateral and medial moraines on
glacier in Alaska.
b. 2 parallel ridges extending from
mountain valley are lateral moraines
Origin of an end moraine: a. glacier retreats and its terminus stabilizes at
new position. b. new moraine is deposited.
a. End moraine deposited by valley glacier. B. close up-note poorly
sorted, highly variable grain sizes…
Evidence that glaciers
transport large boulders, and
indeed all sizes of rocks.
The boulder is 2m high
Ice sculpted plain in Canada
Glacial erratics-transported far from origin, unlike
other rocks in their vicinity
Outwash plain deposits in
Michigan. Note they are
poorly sorted, they do show
some evidence of laminations
and cross bedding; fairly
coarse grained…
Typical glacial topographysharp peaks and rounded, or
U-shaped valleys
Crevasses on glacier in Alaska
Landforms composed of Stratified Drift
1.Outwash is material that has been eroded by the glacier and is transported and deposited
downstream
2.Valley train is the outwash deposited in a mountain valley by the streams flowing from an
alpine glacier
3.Outwash plains form when sediment spreads out from the confines of the valley into a large
valley or plain. Is characteristic of continental glaciers.
4.Kames are small stream-deposited mounds that form when sediment collects in a crevasse
or other depression in the ice or at the margin of a glacier
5.Eskers are long, snake-like ridges that form as the bed deposit of a stream that flowed on,
within, or beneath the stagnant glacial ice.
6. Kames and eskers can be distinguished from till because they are sorted.
7. Kettles are formed from large blocks of ice that were left behind as the glacier receded
that were surrounded by sediment eventually forming g depressions.
8. Lakes commonly form adjacent to glaciers because ice and moraines dam normal drainage
patterns
a. Sediment in the lakes is often layered into coarse and fine-grained, organic-rich layers.
b. Layering due to seasons
c. One set of layers is called a varve marking one year’s time.
Two stages in origin of glacial
Features: during and after glaciation
Photograph: streamlined hills are
drumlins.
Sinous ridge in S Dakota is an esker…
Glacial varves with
a drop stone. Each
varve, a dark-white
couplet is an annual
deposit.
Varves that formed in glacial lake above a till….
THE ICE AGES
A. Introduction
1. An ice age or glacial epoch is the time of extensive glacial activity,
when alpine glacier descent into lowland valleys and continental
glaciers spread over the higher latitude.
2. It is relatively easy to trace the extent of the most recent ice age
because landforms and other features created by the ice are well
preserved.
glacial evidence – when glacial till is lithified, it forms a sedimentary rock called tillite, a
conglomerate of unsorted glacial sediment
Geologic evidence shows that at least 8 major ice ages have occurred
Pleistocene Ice Age: the most recent ice age that occurred mainly during the
Pleistocene Epoch
During the most recent advance, glaciers reached their maximum extent about 18,000 years ago
B. Effects of Pleistocene Continental Glaciers
1. Terminal moraines lie south of the Great Lakes, and extend
westward into Montana, and eastward into southern New York.
2. The Great Lakes were also scoured and deepened by the continental ice
3. The outwash plains form the fertile farmland of the “breadbasket” of
North America.
4. Pluvial lakes formed in the Southwest
5. depression of the land
6. lowered sea level
7. Fjords are deep, marrow lakes that extend far into the land
a. Centers of ice accumulation and maximum extent of Pleistocene
glaciation in N America. B. same for Eurasia in Pleistocene
Pleistocene lakes in Western US:
a. Lake Missoula was pro glacial lake
formed in contact with glacier.
b. All others are pluvial lakes: formed
in depressions in front of glaciers, not
in contact with glaciers.
Horizontal lines on Sentinel Mtn. at Missoula, MT are wave-cut
shorelines of glacial Lake Missoula.
Gravel ridges are so called giant ripple marks that formed when
glacial Lake Missoula drained across this area in Montana
4 Stages in Evolution of the Great Lakes: as the glaciers retreated northwards, the
depressions began filling with water. Dashed lines are present day lake edges…
Fjord leading to ocean in Norway
Chp18: Glaciers and Glaciation
a. During the Little Ice Age, many
glaciers in Europe extended further
than they do today (1826).
b. today-terminus of glacier is behind
rock outcrop at base of valley.
C. Causes of Ice Ages
1. Major glacial epochs and plate tectonics
a. Ice ages occur when the continents are positioned close to the polar regions
b. Can be used to explain climate changes over periods of tens or hundreds of millions of years
2. The cycles of Pleistocene Ice Age and orbital variations
a. Explain the shorter cycles of glacial advances and retreats
b. variations in the earth’s orbit
-The shape of the Earth’s orbit around the sun changes on about a 100,000 cycle and is
known as eccentricity
- The tilt of the Earth’s axis changes on a 41,000 year cycle
-Precession, or the circling of the Earth’s axis, completes a full cycle every 23,000 years.
c. Orbital changes affect the distribution of sunlight with respect to latitude and season
d. Milutin Milankovitch plotted the cycles of the three different orbital variations
-calculated the combined effects of all three variations on the Earth’s climate
-showed that the three regular, periodic orbital variations should combine to generate
alternating cool and warm period in the higher latitudes
e. Climate studies or marine organisms make it possible to graph variations in global
temperature over he past several hundred thousand years. These data aggress with
Milankovitch calculations.
Milantkovich cycles-Earth rotation
variations in the earth’s orbit
-The shape of the Earth’s orbit around the sun changes
on about a 100,000 cycle and is
known as eccentricity
- The tilt of the Earth’s axis changes on a 41,000 year cycle
-Precession, or the circling of the Earth’s axis,
completes a full cycle every 23,000 years
a. W Antarctic and E Antarctic ice sheets
converge to cover most of Antarctica. 4000m
thick.
b. Greenland ice cap
c. Baffin Is, Canada ice cap
Chp18: Glaciers and Glaciation
Chp18: Glaciers and Glaciation
Chp18: Glaciers and Glaciation
Glacial surge: short-lived expansion of glacier. Here in Alaska,
glacier surged across bay, cutting off portion to right…
View from upper end of valley glacier, down valley to terminus
Chp18: Glaciers and Glaciation
a. Conversion of freshly fallen snow to glacial ice.
b. Typical blue color/tint to glacial ice.
Roche moutonee in Central Park, NY city