A
glacier is a large, slow moving river of ice, formed
from compacted layers of snow, that slowly deforms and flows in
response to gravity.
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Glacier ice is the largest reservoir of
fresh water on Earth, and second only to oceans as the largest
reservoir of total water. Glaciers cover vast areas of polar
regions but are restricted to the highest mountains in the
tropics. Elsewhere in the solar system, the vast polar ice caps
of Mars rival those of the Earth.
Geologic features created by
glaciers include end, lateral, ground and medial moraines that
form from glacially transported rocks and debris; U-shaped
valleys and cirques at their heads, and the glacier fringe,
which is the area where the glacier has recently melted into
water.
The word glacier comes from French via the Vulgar
Latin glacia, and ultimately from Latin glacies
meaning ice.[1]
Types
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Comment "My students need to understand glacier movements for their EOG. The video is marvellous as is the info - grade 5 North Carolina. V. Elaine S"
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There are two main types of glaciers: alpine glaciers, which
are found in mountain terrains, and continental glaciers, which
can cover larger areas. Most of the concepts in this article
apply equally to alpine glaciers and continental glaciers.
A temperate glacier is at the melting point throughout
the year from the surface to the base of the glacier. The ice of
polar glaciers is always below the freezing point with
most mass loss due to sublimation. Sub-polar glaciers
have a seasonal zone of melting near the surface and have some
internal drainage, but little to no basal melt.
Thermal classifications of surface conditions vary so glacier
zones are often used to identify melt conditions. The dry snow
zone is a region where no melt occurs, even in the summer. The
percolation zone is an area with some surface melt, and
meltwater percolating into the snowpack, often this zone is
marked by refrozen ice lenses, glands, and layers. The wet snow
zone is the region where all of the snow deposited since the end
of the previous summer has been raised to 0 °C. The superimposed
ice zone is a zone where meltwater refreezes at a cold layer in
the glacier forming a continuous mass of ice.
The smallest alpine glaciers form in mountain valleys and are
referred to as valley glaciers. Larger glaciers can cover
an entire mountain, mountain chain or even a volcano; this type
is known as an ice cap. Ice caps feed outlet glaciers,
tongues of ice that extend into valleys below, far from the
margins of those larger ice masses. Outlet glaciers are formed
by the movement of ice from a polar ice cap, or an ice cap from
mountainous regions, to the sea.
The largest glaciers are continental ice sheets, enormous
masses of ice that not visibly affected by the landscape and
covering the entire surface beneath them, except possibly on the
margins where they are thinnest. Antarctica and Greenland are
the only places where continental ice sheets currently exist.
These regions contain vast quantities of fresh water. The volume
of ice is so large that if the Greenland ice sheet melted, it
would cause sea levels to rise some six meters (20 feet) all
around the world. If the Antarctic ice sheet melted, sea levels
would rise up to 65 meters (210 feet).
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Flying over a huge glacier on the way to McMurdo Station |
Plateau glaciers resemble ice sheets, but on a smaller
scale. They cover some plateaus and high-altitude areas. This
type of glacier appears in many places, especially in Iceland
and some of the large islands in the Arctic Ocean, and
throughout the northern Pacific Cordillera from southern British
Columbia to western Alaska.
Tidewater glaciers are glaciers that flow into the
sea. As the ice reaches the sea pieces break off, or calve,
forming icebergs. Most tidewater glaciers calve above sea level,
which often results in a tremendous splash as the iceberg
strikes the water. If the water is deep, glaciers can calve
underwater, causing the iceberg to suddenly explode up out of
the water. The Hubbard Glacier is the longest tidewater glacier
in Alaska and has a calving face over ten kilometers long.
Yakutat Bay and Glacier Bay are both popular with cruise ship
passengers because of the huge glaciers descending hundreds of
feet to the water. This glacier type undergoes centuries-long
cycles of advance and retreat that are much less affected by
climate currently causing the retreat of most other glaciers.
Formation
The snow which forms temperate glaciers is subject to
repeated freezing and thawing, which changes it into a form of
granular ice called névé. Under the pressure of the layers of
ice and snow above it, this granular ice fuses into denser firn.
Over a period of years, layers of firn undergo further
compaction and become glacial ice. In addition, a few hours
after deposition, snow will begin to undergo metamorphism
because of the presence of temperature gradients and/or convex
and concave surfaces within individual crystals (causing
differential vapour pressure). This causes the sublimation of
ice from smaller crystals and the deposition of water vapour
onto larger crystals, so many crystals become progressively more
rounded over time. Depending on the type of metamorphism, the
snowpack may become stronger or weaker as a result.
The distinctive blue tint of glacial ice is often wrongly
attributed to Rayleigh scattering which is supposedly due to
bubbles in the ice. The blue color is actually created for the
same reason that water is blue, that is, its slight absorption
of red light due to an overtone of the infrared OH stretching
mode of the water molecule [1].
The lower layers of glacial ice flow and deform plastically
under the pressure, allowing the glacier as a whole to move
slowly like a viscous fluid. Glaciers usually flow downslope,
although they do not need a surface slope to flow, as they can
be driven by the continuing accumulation of new snow at their
source, creating thicker ice and a surface slope. The upper
layers of glaciers are more brittle, and often form deep cracks
known as crevasses or bergschrunds as they move.
Crevasses form due to internal differences in glacier
velocity between two quasi-rigid parts above the deeper more
plastic substrate far below. As the parts move at different
speeds and directions, shear forces cause the two sections to
break apart opening the crack of a crevasse all along the
disconnecting faces. Projected in effect over three dimensions,
one may settle and tip, the other upthrust or twist, or all such
combinations due to the effects of each floating on the plastic
layers below and any contact with rock and such. Hence the
distance between the two separated parts while touching and
rubbing deep down, frequently widens significantly towards the
surface layers, many times creating a wide chasm.
These crevasses make travel over glaciers hazardous.
Subsequent heavy snow may form a fragile snow bridge, increasing
the danger by hiding their presence at the surface. Glacial
meltwaters flow throughout and underneath glaciers, carving
channels in the ice (called moulins) similar to cave
formation through rock and also helping to lubricate the
glacier's movement.
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A river of melt-water running down the slope of
a toe of the Athabasca Glacier.
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Anatomy
The upper part of a glacier that receives most of the
snowfall is called the accumulation zone. In general, the
accumulation zone accounts for 60-70% of the glacier's surface
area. The depth of ice in the accumulation zone exerts a
downward force sufficient to cause deep erosion of the rock in
this area. After the glacier is gone, this often leaves a bowl
or amphitheater-shaped isostatic depression called a cirque.
On the opposite end of the glacier, at its foot or terminal,
is the deposition or ablation zone, where more ice
is lost through melting than gained from snowfall and sediment
is deposited. The place where the glacier thins to nothing is
called the ice front.
The altitude where the two zones meet is called the
equilibrium line, also called the snow line. At this
altitude, the amount of new snow gained by accumulation is equal
to the amount of ice lost through ablation. Due to erosive
forces at the edges of the moving ice, glaciers turn V-shaped
river-carved valleys into U-shaped glacial valleys.
The "health" of a glacier is defined by the area of the
accumulation zone compared to the ablation zone. When directly
measured this is glacier mass balance. Healthy glaciers have
large accumulation zones. Several non-linear relationships
define the relation between accumulation and ablation.
In the aftermath of the Little Ice Age, around 1850, the
glaciers of the Earth have retreated substantially. Glacier
retreat has accelerated since about 1980 and is correlated with
global warming. [2]
Occurrence
Permanent snow cover is affected by factors such as the
degree of slope on the land, amount of snowfall and the force
and nature of the winds. As temperature decreases with altitude,
high mountains — even those near the Equator — have permanent
snow cover on their upper portions, above the snow line.
Examples include Mount Kilimanjaro in Tanzania and the Tropical
Andes in South America; however, the only snow to occur exactly
on the Equator is at 4,690 m (15,387 ft) on the southern slope
of Volcán Cayambe in Ecuador.
Conversely, many regions of the Arctic and Antarctic receive
very little precipitation and therefore experience little
snowfall despite the bitter cold (cold air, unlike warm air,
cannot take away much water vapor from the sea). In Antarctica,
the snow does not melt even at sea level. In addition to the
dry, unglaciated regions of the Arctic, there are some mountains
and volcanoes in Bolivia, Chile and Argentina that are high
(4,500 metres (14,800 ft) - 6,900 m (22,600 ft)) and cold, but
the relative lack of precipitation prevents snow from
accumulating into glaciers. This is because these peaks are
located near or in the hyperarid Atacama desert. Further
examples of these temperate unglaciated mountains is the Kunlun
Mountains, Tibet and the Pamir Range to the north of the
Himalayas in Central Asia. Here, just like the Andes, mountains
in Central Asia can reach above 6,000 m (20,000 ft) and be
barren of snow and ice due to the rain shadow effect caused by
the taller Himalaya Range.
During glacial periods of the Quaternary, most of Siberia,
central and northern Alaska and all of Manchuria, were similarly
too dry to support glaciers, though temperatures were as low as
or lower than in glaciated areas of Europe and North America.
This was because dry westerly winds from ice sheets in Europe
and the coastal ranges in North America reduced precipitation to
such an extent that glaciers could never develop except on a few
high mountains like the Verkhoyansk Range (which still supports
glaciers today).
Glaciers occur on every continent and in approximately 47 of
the world's countries. Though Australia has no glaciers, New
Guinea is considered to be part of the Australian continent and
small glaciers are located on its highest summit massif of
Puncak Jaya. Africa has glaciers on Mount Kilimanjaro, Mount
Kenya and in the Ruwenzori Range.[2]
Motion
Ice behaves like an easily breaking solid until its thickness
exceeds about 50 meters (160 ft). The pressure on ice deeper
than that depth causes plastic flow. The glacial ice is made up
of layers of molecules stacked on top of each other, with
relatively weak bonds between the layers. When the stress of the
layer above exceeds the inter-layer binding strength, it moves
faster than the layer below.
Another type of movement is basal sliding. In this process,
the whole glacier moves over the terrain on which it sits,
lubricated by meltwater. As the pressure increases toward the
base of the glacier, the melting point of water decreases, and
the ice melts. Friction between ice and rock and geothermal heat
from the Earth's interior also contribute to thawing. This type
of movement is dominant in temperate glaciers. The geothermal
heat flux becomes more important the thicker a glacier becomes.
Fracture zone and cracks
The top 50 meters of the glacier are more rigid. In this
section, known as the fracture zone, the ice mostly moves
as a single unit. Ice in the fracture zone moves over the top of
the lower section. When the glacier moves through irregular
terrain, cracks form in the fracture zone. These cracks can be
up to 50 meters deep, at which point they meet the plastic like
flow underneath that seals them.
Cracks make glaciers a dangerous place to visit, because they
are not always easy to spot. They are not easy to spot because
they are often covered in snow.
Speed
The speed of glacial displacement is partly determined by
friction. Friction makes the ice at the bottom of the glacier
move slower than the upper portion. In alpine glaciers, friction
is also generated at the valley's side walls, which slows the
edges relative to the center. This was confirmed by experiments
in the 19th century, in which stakes were planted in a line
across an alpine glacier, and as time passed, those in the
center moved farther.
Mean speeds vary; some have speeds so slow that trees can
establish themselves among the deposited scourings. In other
cases they can move as fast as many meters per day, as is the
case of Byrd Glacier, an outlet glacier in Antarctica which
moves 750-800 meters per year (some 2 meters or 6 feet per day),
according to studies using satellites.
Many glaciers have periods of very rapid advancement called
surges.[3] These glaciers exhibit normal movement until suddenly
they accelerate, then return to their previous state. During
these surges, the glacier may reach velocities up to 100 times
greater than normal.
Moraines
Glacial moraines are formed by the deposition of material
from a glacier and are exposed after the glacier has retreated.
These features usually appear as linear mounds of till, a
non-sorted mixture of rock, gravel and boulders within a matrix
of a fine powdery material. Terminal or end moraines are formed
at the foot or terminal end of a glacier. Lateral moraines are
formed on the sides of the glacier. Medial moraines are formed
when two different glaciers, flowing in the same direction,
coalesce and the lateral moraines of each combine to form a
moraine in the middle of the merged glacier. Less apparent is
the ground moraine, also called glacial drift, which
often blankets the surface underneath much of the glacier
downslope from the equilibrium line. Glacial meltwaters contain
rock flour, an extremely fine powder ground from the underlying
rock by the glacier's movement. Other features formed by glacial
deposition include long snake-like ridges formed by streambeds
under glaciers, known as eskers, and distinctive
streamlined hills, known as drumlins.
Stoss-and-lee erosional features are formed by
glaciers and show the direction of their movement. Long linear
rock scratches (that follow the glacier's direction of movement)
are called glacial striations, and divots in the rock are
called chatter marks. Both of these features are left on
the surfaces of stationary rock that were once under a glacier
and were formed when loose rocks and boulders in the ice were
transported over the rock surface. Transport of fine-grained
material within a glacier can smooth or polish the surface of
rocks, leading to glacial polish. Glacial erratics are rounded
boulders that were left by a melting glacier and are often seen
perched precariously on exposed rock faces after glacial
retreat.
The term moraine is of French origin, and it was
coined by peasants to describe alluvial embankments and rims
found near the margins of glaciers in the French Alps. In modern
geology, the term is used more broadly, and is applied to a
series of formations, all of which are composed of till.
Drumlins
Drumlins are asymmetrical, canoe shaped hills with
aerodynamic profiles made mainly of till. Their heights vary
from 15 to 50 meters and they can reach a kilometer in length.
The tilted side of the hill looks toward the direction from
which the ice advanced (stoss), while the longer slope
follows the ice's direction of movement (lee).
Drumlins are found in groups called drumlin fields or
drumlin camps. An example of these fields is found east
of Rochester, New York, and it is estimated that it contains
about 10,000 drumlins.
Although the process that forms drumlins is not fully
understood, it can be inferred from their shape that they are
products of the plastic deformation zone of ancient glaciers. It
is believed that many drumlins were formed when glaciers
advanced over and altered the deposits of earlier glaciers.
Erosion
Rocks and sediments are added to glaciers through various
processes. Glaciers erode the terrain principally through two
methods: abrasion and plucking.
As the glacier flows over the bedrock's fractured surface, it
softens and lifts blocks of rock that are brought into the ice.
This process is known as plucking, and it is produced when
subglacial water penetrates the fractures and the subsequent
freezing expansion separates them from the bedrock. When the
water expands, it acts as a lever that loosens the rock by
lifting it. This way, sediments of all sizes become part of the
glacier's load.
Abrasion occurs when the ice and the load of rock fragments
slide over the bedrock and function as sandpaper that smooths
and polishes the surface situated below. This pulverized rock is
called rock flour. This flour is formed by rock grains of a size
between 0.002 and 0.00625 mm. Sometimes the amount of rock flour
produced is so high that currents of meltwaters acquire a
grayish color.
Another of the visible characteristics of glacial erosion are
glacial striations. These are produced when the bottom's ice
contains large chunks of rock that mark trenches in the bedrock.
By mapping the direction of the flutes the direction of the
glacier's movement can be determined. Chatter marks are seen as
lines of roughly crescent shape depressions in the rock
underlying a glacier caused by the abrasion where a boulder in
the ice catches and is then released repetitively as the glacier
drags it over the underlying basal rock.
A glacier may also erode its environment through katabatic
winds.
The rate of glacier erosion is variable. The differential
erosion undertaken by the ice is controlled by six important
factors:
- Velocity of glacial movement
- Thickness of the ice
- Shape, abundance and hardness of rock fragments
contained in the ice at the bottom of the glacier
- Relative ease of erosion of the surface under the
glacier.
- Thermal conditions at the glacier base.
- Permeability and water pressure at the glacier base.
Material that becomes incorporated in a glacier are typically
carried as far as the zone of ablation before being deposited.
Glacial deposits are of two distinct types:
- Glacial till: material directly deposited from glacial
ice. Till includes a mixture of undifferentiated material
ranging from clay size to boulders, the usual composition of
a moraine.
- Fluvial and outwash: sediments deposited by water. These
deposits are stratified through various processes, such as
boulders being separated from finer particles.
The larger pieces of rock which are encrusted in till or
deposited on the surface are called glacial erratics.
They may range in size from pebbles to boulders, but as they may
be moved great distances they may be of drastically different
type than the material upon which they are found. Patterns of
glacial erratics provide clues of past glacial motions.
Glacial valleys
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A
glaciated valley in the Mount Hood Wilderness showing the characteristic U-shape
and flat bottom. |
Before glaciation, mountain valleys have a characteristic "V"
shape, produced by downward erosion by water. However, during
glaciation, these valleys widen and deepen, which creates a
"U"-shaped glacial valley. Besides the deepening and widening of
the valley, the glacier also smooths the valley due to erosion.
In this way, it eliminates the spurs of earth that extend across
the valley. Because of this interaction, triangular cliffs
called truncated spurs are formed.
Many glaciers deepen their valleys more than their smaller
tributaries. Therefore, when the glaciers recede from the
region, the valleys of the tributary glaciers remain above the
main glacier's depression, and these are called hanging valleys.
In parts of the soil that were affected by abrasion and
plucking, the depressions left can be filled by lakes, called
paternoster lakes.
At the 'start' of a classic valley glacier is the cirque,
which has a bowl shape with escarped walls on three sides, but
open on the side that descends into the valley. In the cirque,
an accumulation of ice is formed. These begin as irregularities
on the side of the mountain, which are later augmented in size
by the coining of the ice. After the glacier melts, these
corries are usually occupied by small mountain lakes called
tarns.
There may be two glacial cirques 'back to back' which erode
deep into their backwalls until only a narrow ridge, called an
arête is left. This structure may result in a mountain pass.
Glaciers are also responsible for the creation of fjords
(deep coves or inlets) and escarpments that are found at high
latitudes.
Arêtes and horns (pyramid peak)
An arête is a narrow crest with a sharp edge. The meeting of
three or more arêtes creates pointed pyramidal peaks and in
extremely steep-sided forms these are called horns.
Both features may have the same process behind their
formation: the enlargement of cirques from glacial plucking and
the action of the ice. Horns are formed by cirques that encircle
a single mountain.
Arêtes emerge in a similar manner; the only difference is
that the cirques are not located in a circle, but rather on
opposite sides along a divide. Arêtes can also be produced by
the collision of two parallel glaciers. In this case, the
glacial tongues cut the divides down to size through erosion,
and polish the adjacent valleys.
Sheepback rock
Some rock formations in the path of a glacier are sculpted
into small hills with a shape known as roche moutonnée or
sheepback. An elongated, rounded, asymmetrical, bedrock knob
can be produced by glacier erosion. It has a gentle slope on its
up-glacier side and a steep to vertical face on the down-glacier
side. The glacier abrades the smooth slope that it flows along,
while rock is torn loose from the downstream side and carried
away in ice, a process known as 'plucking'. Rock on this side is
fractured by combinations of forces due to water, ice in rock
cracks, and structural stresses.
Alluvial stratification
The water that rises from the ablation zone moves away from
the glacier and carries with it fine eroded sediments. As the
speed of the water decreases, so does its capacity to carry
objects in suspension. The water then gradually deposits the
sediment as it runs, creating an alluvial plain. When this
phenomenon occurs in a valley, it is called a valley train.
When the deposition is to an estuary, the sediments are known as
"bay mud".
Alluvial plains and valley trains are usually accompanied by
basins known as kettles. Glacial depressions are also produced
in till deposits. These depressions are formed when large ice
blocks are stuck in the glacial alluvium and after melting, they
leave holes in the sediment.
Generally, the diameter of these depressions does not exceed
2 km, except in Minnesota, where some depressions reach up to 50
km in diameter, with depths varying between 10 and 50 meters.[citation
needed]
Deposits in contact with ice
When a glacier reduces in size to a critical point, its flow
stops, and the ice becomes stationary. Meanwhile, meltwater
flows over, within, and beneath the ice leave stratified
alluvial deposits. Because of this, as the ice melts, it leaves
stratified deposits in the form of columns, terraces and
clusters. These types of deposits are known as deposits in
contact with ice.
When those deposits take the form of columns of tipped sides
or mounds, which are called kames. Some kames form
when meltwater deposits sediments through openings in the
interior of the ice. In other cases, they are just the result of
fans or deltas towards the exterior of the ice produced by
meltwater.
When the glacial ice occupies a valley it can form terraces
or kame along the sides of the valley.
A third type of deposit formed in contact with the ice is
characterized by long, narrow sinuous crests composed
fundamentally of sand and gravel deposited by streams of
meltwater flowing within, beneath or on the glacier ice. After
the ice has melted these linear ridges or eskers remain as
landscape features. Some of these crests have heights exceeding
100 meters and their lengths surpass 100 km.
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The
Perito Moreno Glacier in Argentina |
Loess deposits
Very fine glacial sediments or rock flour is often picked up
by wind blowing over the bare surface and may be deposited great
distances from the original fluvial deposition site. These
eolian loess deposits may be very deep, even hundreds of meters,
as in areas of China and the Midwestern United States.
Transportation
Entrainment is the picking up of loose material by the
glacier from along the bed and valley sides. Entrainment can
happen by regelation or by the ice simply picking up the debris.
Basal Ice Freezing is thought to be to be made by
glaciohydraulic supercooling, though some studies show that even
where physical conditions allow it to occur, the process may not
be responsible for observed sequences of basal ice. Plucking
is the process involves the glacier freezing onto the valley
sides and subsequent ice movement pulling away masses of rock.
As the bedrock is greater in strength than the glacier, only
previously loosened material can be removed. It can be loosened
by local pressure and temperature, water and pressure release of
the rock itself. Supraglacial debris is carried on the
surface of the glacier as lateral and medial moraines. In summer
ablation, surface melt water carries a small load and this often
disappears down crevasses. Englacial debris is moraine
carried within the body of the glacier. Subglacial debris
is moved along the floor of the valley either by the ice as
ground moraine or by meltwater streams formed by pressure
melting.
Deposition
Lodgement till is identical with ground moraine, it is
material that is smeared on to the valley floor when its weight
becomes too great to be moved by the glacier. Ablation
Till is a combination of englacial and supraglacial moraine,
it’s released as a stationary glacier begins to melt and
material is dropped in situ. Dumping is when a glacier
moves material to its outermost or lowermost end and dumps it.
Deformation Flow is the change of shape of the rock and
land due to the glacier.
Isostatic rebound
This rise of a part of the crust is due to an isostatic
adjustment. A large mass, such as an ice sheet/glacier,
depresses the crust of the Earth and displaces the mantle below.
The depression is about a third the thickness of the ice sheet.
After the glacier melts the mantle begins to flow back to its
original position pushing the crust back to its original
position. This post-glacial rebound, which lags melting of the
ice sheet/glacier, is currently occurring in measurable amounts
in Scandinavia and the Great Lakes region of North America.
An interesting geomorphological feature created by the same
process, but on a smaller scale, is known as dilation-faulting.
It occurs within rock where previously compressed rock is
allowed to return to its original shape, but more rapidly than
can be maintained without faulting, leading to an effect similar
to that which would be seen if the rock were hit by a large
hammer. This can be observed in recently de-glaciated parts of
Iceland.
Ice ages
Divisions
A quadruple division of the Quaternary glacial period has
been established for North America and Europe. These divisions
are based principally on the study of glacial deposits. In North
America, each of these four stages was named for the state in
which the deposits of these stages were well exposed. In order
of appearance, they are the following: Nebraskan, Kansan,
Illinoisan, and Wisconsinan. This classification was refined
thanks to the detailed study of the sediments of the ocean
floor. Because the sediments of the ocean floor are less
affected by stratigraphic discontinuities than those on land,
they are useful to determine the climatic cycles of the planet.
In this matter, geologists have come to identify over twenty
divisions, each of them lasting approximately 100,000 years. All
these cycles fall within the Quaternary glacial period.
During its peak, the ice left its mark over almost 30% of
Earth's surface, covering approximately 10 million km² in North
America, 5 million km² in Europe and 4 million km² in Asia. The
glacial ice in the Northern hemisphere was double that found in
the Southern hemisphere. This is because southern polar ice
cannot advance beyond the Antarctic landmass. It is now believed
that the most recent glacial period began between two and three
million years ago, in the Pleistocene era.
The last major glacial period began about 2,000,000 years B.P.
and is commonly known as the Pleistocene or Ice Age. During this
glacial period, large glacial ice sheets covered much of North
America, Europe, and Asia for long periods of time. The extent
of the glacier ice during the Pleistocene, however, was not
static. The Pleistocene had periods when the glaciers retreated
(interglacial) because of mild temperatures, and advanced
because of colder temperatures (glacial). Average global
temperatures were probably 4 to 5° Celsius colder than they are
today at the peak of the Pleistocene. The most recent glacial
retreat began about 14,000 years B.P. and is still going on. We
call this period the Holocene epoch.
Causes
Generalized glaciations have been rare in the history of
Earth. However, the Ice Age of the Pleistocene was not the only
glacial event, since tillite deposits have been identified.
Tillite is a sedimentary rock formed when glacial till is
lithified.
These deposits found in strata of differing age present
similar characteristics as fragments of fluted rock, and some
are superposed over bedrock surfaces of channeled and polished
rock or associated with sandstone and conglomerates that have
features of alluvial plain deposits.
Two Precambrian glacial episodes have been identified, the
first approximately 2 billion years ago, and the second
(Snowball Earth) about 650 million years ago. Also, a well
documented record of glaciation exists in rocks of the late
Paleozoic (the Carboniferous and Permian).
Although there are several scientific hypotheses about the
determining factors of glaciations, the two most important ideas
are plate tectonics and variations in Earth's orbit (Milankovitch
cycles).
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Jostedalsbreen glacier in Norway |
Plate tectonics
Because glaciers can form only on dry land, plate tectonics
suggest that the evidence of previous glaciations seen in
tropical latitudes is due to the drift of tectonic plates from
tropical latitudes to circumpolar regions. Evidence of glacial
structures in South America, Africa, Australia, and India
support this idea, because it is known that they experienced a
glacial period near the end of the Paleozoic Era, some 250
million years ago.
The idea that the evidence of middle-latitude glaciations is
closely related to the displacement of tectonic plates was
confirmed by the absence of glacial traces in the same period
for the higher latitudes of North America and Eurasia, which
indicates that their locations were very different from today.
Climatic changes are also related to the positions of the
continents, which has made them vary in conjunction with the
displacement of plates. That also affected ocean current
patterns, which caused changes in heat transmission and
humidity. Since continents drift very slowly (about 2 cm per
year), similar changes occur in periods of millions of years.
A study of marine sediment that contained climatically
sensitive microorganisms until about half a million years ago
were compared with studies of the geometry of Earth's orbit, and
the result was clear: climatic changes are closely related to
periods of obliquity, precession, and eccentricity of the
Earth's orbit.
In general it can be affirmed that plate tectonics applies to
long time periods, while Milankovitch's proposal, backed up by
the work of others, adjusts to the periodic alterations of
glacial periods of the Pleistocene. In both mechanisms the
radiation imbalance of the earth is thought to play a large role
in the build-up and melt of glaciers.
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Comments |
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My students need to understand
glacier movements for their EOG. The video is
marvellous as is the info - grade 5 North
Carolina. V. Elaine S |
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this is the best website yet
about glaciers! thank you! |
