Chapter 15 - Glaciers & Glaciation

Created by

Kirsty Camba

Cards (52)

Glaciers are a part of 2 fundamental cycles in the Earth system, the hydrologic and rock cycle.
Glacier
thick ice mass that forms over hundreds or thousands of years.
originates on land from the accumulation, compaction, and recrystallization of snow.
Valley (Alpine) Glaciers 
small glaciers exist in lofty mountain areas, where they follow valleys that were occupied by streams.
the glaciers advance slowly, perhaps a few cm per day.
because of their setting, these moving ices are termed valley glaciers/alpine glaciers.
like rivers, valley glaciers can be longer than they are wide.
some extend for a fraction of a kilometer, whereas others go on for many tens of kilometer.
e.g. west branch of Hubbard Glacier runs thru 112 km of mountainous terrain in Alaska & Yukon Territory.
Ice Sheets 
exist on a much larger scale
the low total annual solar radiation reaching the poles makes these regions hospitable to great ice accumulations.
each of polar region supports an ice sheet
on Greenland in the Northern Hemisphere & Antartica in the Southern Hemisphere
Ice Sheets
Ice Sheet 
Ice Age Ice Sheets
about 18 000 yrs ago, glacial ice covered not only Greenland and Antartica but also large portions of North America, Europe and Siberia.
that period is known as the Last Glacial Maximum.
implies that there were other glacial maximums.
throughout the Quaternary which began about 2.6 million yrs ago, ice sheets have formed, advanced over broad areas and then walked away.
Ice Sheet
2. Greenland and Antartica
Sea ice - ice that covers the Artic Ocean
floats because ice is less dense than water
although sea ice never completely disappears from Arctic, the area covered with sea ice expands & contracts with the seasons.
thickness ranges from a few cm for new ice to 4 meters for sea ice that has survived for years.
Ice Sheet
2. Greenland & Antartica
Greenland supports an ice sheet
extends between 60° to 80° north latitude.
this largest island on Earth is covered by imposing ice sheet that occupes 1.7 million sq km (about 80% of the island)
averaging nearly 1500 meters thick, the ice extends above the island's bedrock in some places.
Ice Sheet 
2. Greenland & Antartica
Antartice Ice Sheet
thickness of about 4300 meters and covers the entire continent, an area of more than 13.9 million sq km.
often called continental ice sheets because of their huge features.
combined areas of present day continental ice sheets represent almost 10% of Earth's land area.
Ice Sheet 
3. Ice Shelves
along portions of Antartic coast, glacial iceflows flows into the adjacent ocean creating these features.
these large relatively flat masses of glacial ice extend seaward from the coast but remain attached to the land along one or more sides.
about 80% of the ice lies below the surface of the ocean so in shallow water, the ice shelf "touches" bottom and is said to be grounded.
are thickest on their landward sides and become thinner seaward.
Ice Sheet
3. Ice Shelves
Antartica's ice shelves extend over approximately 1.4 million sq km.
Ross and Ronne-Fichner Ice Shelves
largest with the Ross Ice Sheet covering an area approximately the size of Texas.
Other Types of Glaciers
Ice caps
masses of glacial ice that cover uplands and plateaus.
occur in many places, including Iceland and several of the large islands in the Arctic ocean.
ice caps and ice sheets feed outlet glaciers that flow down valleys around the margin of the cap or sheet.
Piedmont glaciers
occupy broad lowlands at the bases of steep mountains.
form when one or more alpine glaciers emerge from the confining walls of mountain valleys.
Formation and Movement of Glacial Ice
Glacial Ice Formation
when temperatures remain below freezing following a snowfall, the fluffy accumulation of delicate hexagonal crystal soon changes.
this process of recrystallization makes the snowflakes smaller. thicker and more spherical.
Formation and Movement of Glacial Ice
Glacial Ice Formation
the snow also compacts, reducing the pore spaces between grains.
the resulting granular recrystallized snow is called firn and is commonly found making up old snow banks near the end of winter.
as more snow is added, the pressure on the lower layers gradually increases, compacting the ice grains at depth.
once the thickness of ice and snow exceeds 50 meters, the weight is sufficient to fuse firm into a solid mass of interlocking ice crystals and glacial ice has now been formed.
Formation and Movement of Glacial Ice 
Glaciers Move
the way in which ice moves is complex and has 2 basic types.
plastic flow
involves movement within the ice
ice behaves as brittle solid until the pressure on it is equivalent to the weight of about 50 meters.
once the load is surpassed, ice behaves as a plastic material and flow begins
such flow occurs because of molecular structure of ice.
Formation and Movement of Glacial Ice
Glaciers Move
Plastic Flow
glacial ice consists of layers of molecules stacked one upon the other.
the bonds between layers are weaker than those within each layer.
therefore, when a stress exceeds the strength of the bonds between the layers, the layers remain intact and slide over one another.
Formation and Movement of Glacial Ice
Glaciers Move
Basal Slip
occurs when an entire ice slips along the ground
lowest portions of most glaciers move by this sliding process.
Formation and Movement of Glacial Ice
Glaciers Move
Zone of Fracture
uppermost 50 meters of a glacier
consist of brittle ice
when glacier moves over irregular terrain, the zone of fracture is subjected to tension resulting in cracks called crevasses.
these gaping cracks which often travel across glaciers, may extend to depths of 50 meters.
below this depth, plasic flow seals them off.
Formation and Movement of Glacial Ice
Budget of a Glacier: Accumulation vs. Wastage
Glacial Zones
Zone of accumulation
this is where snow accumulation and ice formation occurs.
its outer limits are defined by the snowline or equilibrium line.
the elevation at which the accumulation and wasting of glacial ice is equal.
elevation of this boundary varies from sea level in polar regions to altitudes approaching 5000 meters near the equator.
the addition of snow thickens the glacier and promotes movement in this layer.
Formation and Movement of Glacial Ice 
Budget of a Glacier: Accumulation vs. Wastage
Glacial Zones
Zone of Wastage
below the snowline
there is a net less to the glacier as all of the show from the previo winter melts, along which some of the glacial ice.
Formation and Movement of Glacial Ice
Ablation
loss of ice by a glacier
Calving
in addition to melting, this is the process wherein glaciers waste away as large pieces of ice break off the front of the glacier.
creates icebergs in places where the glacier has reached the sea or a lake.
primary means by which ice is lost.
relatively flat icebergs produced here can be several kilometer across and up to about 600 meters.
Formation and Movement of Glacial Ice
2. Glacial Budget
stationary depends on the budget of the glacier
glacial budget is the balance or lack of balance, between accumulation at the upper end of the glacier and ablation of the lower end.
if ice accumulation exceeds ablation, the glacial front advances until the 2 factors balance.
when this happens, the terminus of the glacier is stationary.
Formation and Movement of Glacial Ice
2. Glacial Budget
Unbalanced Glacial Budgets
valley glaciers around the world have been retreating at unpredecented rates over the past century.
many valley glaciers have disappeared altogether.
e.g. 150 yrs ago, there were 147 glaciers in Montana's Glacier National Park. Today, only 37 remain.
Greenland's ice sheet and portions of Antartica's ice are also shrinking.
Glacial Erosion 
How Glaciers Erode?
Due to plucking and abrasion
Plucking
as a glacier flows over a fractured bedrock surface, it loosens and lifts blocks of rock and incorporates them into the ice.
occurs when meltwater penetrates the cracks and joints of bedrock beneath a glacier and freezes.
because water expands when it freezes, it exerts tremendous leverage that pries the rock loose.
in this manner, sediment of all sizes becomes part of the glacier's load.
Glacial Erosion?
How Glaciers Erode?
Due to plucking and abrasion
Abrasion
as the ice and its load of rock fragments slide over bedrock, they function like sandpaper, smoothing and polishing over the surface below.
rock flour
the pulverized rock produced by the glacial "grist mill"
so much rock flows may be produced that meltwater streams flowing out of a glacier often have the cloudy appearance of skim milk.
Glacial Erosion 
How Glaciers Erode?
Due to plucking and abrasion
Abrasion
glacial striations
when the ice at the bottom of a glacier often contains large rock fragments, these long scratches and grooves may even be gauged into the bedrock.
provide clues to the direction of ice flow.
by mapping the striations over large areas, patterns of glacial flow can often be reconstructed.
Glacial Erosion 
Rate of erosion is also highly variable. This is largely controlled by:
speed of glacier movement
ice thickness
erodibility of the surface beneath the glacier
shape, abundance, and hardness of the rock fragments in the ice at the base of the glacier.
Glacial erosion
Landforms created by glacial erosion
glaciated mountain region has a sharp and angular topography
because alpine glaciers tend to accentuate the irregularities of the mountain landscape by creating steeper canyon walls & making bold peaks even more jagged.
by contrast, continental ice sheets override the terrain and subdue.
Glacial erosion
Landforms created by glacial erosion
Glaciated Valleys
unlike streams which create their own valleys, glacies take the path of least resistance and follow the paths of existing stream valleys.
during glaciation, a narrow valley is transformed as the glacier widens and deepens it, creating a U-shaped glacial trough.
the glacier also straightens the valley
as ice flows around sharp curves, its great erosional force removes the spurs of land that extend into the valley.
Glacial erosion
Landforms created by glacial erosion
Glaciated Valleys
prior to glauation, the mouths of tributary streams join the main valley (trunk valley) of the elevation of the stream in the valley.
consequently, the valley containing the main glacier (trunk glacier) is eroded deeper than the smaller valleys that feed it.
thus, after the ice has receded, the valleys of tributary glaciers are left standing above the main glacial trough and are called hanging valleys.
Glacial erosion
Landforms created by glacial erosion

2. Cirques
these bowl shaped depressions have precipitous walls on 3 sides and are open on the downvalley side.
focal point of the glacial's growth because it is the area of snow accumulation and ice formation.
begins as irregularities in the mountainside that are subsequently enlarged by frost wedging and plucking along the sides and bottom of the glacier.
the glacier acts as a conveyor belt that carries away the debris. after glacier has melted away, the cirque basin is sometimes occupied by a small lake called tarn.
Glacial erosion
Landforms created by glacial erosion
3. Aretes & Horns
aretes: sinuous, knife-edged ridges
horns: pyramid-like peaks
both features can originate from the same basic processes
the enlargement of cirques pnduced by plucking and frost action.
several cirques around a single high mountain create the horns.
as the cirques enlarge and converge, an isolated horn is produced.
Glacial erosion
Landforms created by glacial erosion
3. Aretes & Horns
Aretes can form in a similar manner except that cirques are not clustered around a point, but exist on opposite sides of a divide.
as the cirques grow the divide separating them is reduced to a very narrow knife like partition.
An arete can also be created when glaciers that flow in parallel valleys narrow the intervening ridge as they scour and widen their valleys.
Glacial erosion
Landforms created by glacial erosion
4. Rouche Moutonee
asymmetrical knob of bedrock
formed when glacial abrasion smooths the gentle slope facing the oncoming ice and plucking steepens the opposite side as the ice rides over the knob.
indicate the direction of glacial flow because the gentler slope is generally on the side from which ice advanced.
Glacial erosion
Landforms created by glacial erosion
5. Fjords
deep, spectacular steep-sidded inlets of the sea that are present at high latitudes where mountains are adjacent to the ocean.
are drowned glacial troughs that became submerged as the ice left the valleys and sea level rose following the Ice Age.
depths of fjors may exceed 1000 meters.
sea level does not act as base level for glaciers
as a consequence, glaciers are capable of eroding their beds far below surface of the sea.
Glacial Deposits
Glacial Drift
an embraced term for sediments of glacial origin, no matter how, where, or in what shape they were deposited.
2 distinct types:
materials deposited directly by the glacier called tiff
sediments laid down by glacial meltwater called stratified tiff.
Glacial Deposits
Glacial till
as glacial ice melts and drops its load of rock fragments, till is deposited.
unlike moving water & wind, ice cannot sort the sediments it carries; therefore, deposits of till are unsorted mixtures of many particle sizes.
Glacial erratics
boulders found in the till or lying free on the surface if they are different from the bedrock below.
surce for most erratics are unknown but some boulders were transported as far as 500 km from their source area.
Glacial Deposits
2. Stratified Drift
sorted accdg. to the size and weight of the particles.
ice is not capable of sorting the way running water can, so these materials are not deposited directly but instead reflect the sorting action of glacial meltwater.
some of deposits are
made by streams issuing directly from glacier
involved with sediments that was originally laid down as till and was later picked up, transported, and redeposited by meltwater.
Glacial Deposits
2. Stratified Drift
accumulations of stratified drift often consist of sand and gravel because meltwater cannot move larger material & because finer rock flour remains suspended.
Moraines, Outwash plains and Kettles
Lateral and Medial Moraines
the sides of a valley glacier accumulate large quantities of debris from valley walls.
when the glacier wastes away, these materials are left as ridges called lateral moraines,
Medial Moraines - formed when 2 valley glaciers coalesce to form a single ice stream.
Moraines, Outwash plains and Kettles
2. End & Ground Moraines
no matter whether the front of a glacier/ice sheet is advancing retreating or stationary, the glacier or sheet is constantly moving sediment forward and dropping it at its terminus.
End moraine
a ridge of till that forms at the terminus of a glacier or ice sheet whenever the terminus is stationary.
forms when the ice is wasting away near the end of the glacier at a rate equal to the forward advance of the glacier.