Dictionary Definition
glacier n : a slowly moving mass of ice
User Contributed Dictionary
English
Pronunciation
- /ˈglæsiə/, /ˈgle(ɪ)siə/, /ˈgle(ɪ)sjɚ/, /ˈgle(ɪ)ʃɚ/
- /"gl
Extensive Definition
about the
geological formation
- "Glacial" and "Glaciation" redirect here. For the geological periods, see glacial period. For the story by Alastair Reynolds, see Glacial (short story).
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.
The processes and landforms caused by glaciers and related to them
are glacial (adjective); this term should
not be confounded with glacial (noun), a cold period in ice ages (see
glacial
period). The process of glacier growth and establishment is
called glaciation.
The word glacier comes from
French
via the Vulgar Latin
glacia, and ultimately from Latin glacies meaning
ice.
Overview
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.Many geologic processes are
interrupted or modified significantly by glaciers. 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. Much precipitation
becomes trapped in the glaciers instead of flowing immediately back
to the oceans, causing sea level drops and greatly modifying the
hydrology of streams. The Earth's
crust is pushed down by the weight of the ice, and meltwater
commonly collects and forms lakes along the ice
margins.
Glacial epochs have come and
gone repeatedly over the last million years. Presently, Earth is in
a relatively warm period, called an interglacial, exacerbated
by global
warming with the resulting
retreat of the glaciers. The Earth has been cyclically plunged
into cold episodes, however, called glacials, in which the extent
of glaciers is expanded, colloquially referred to as ice
ages.
Types of glaciers
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 http://webexhibits.org/causesofcolor/5C.html.
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.
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 increased since the 1980s, the coldest
decade since 1900. http://www.grida.no/climate/ipcc_tar/wg1/064.htm
Occurrence
Glaciers occur on every continent and in approximately 47 of the world's countries. Extensive glaciers are found in Antarctica, Patagonia, Canada, Greenland and Iceland. Mountain glaciers are widespread e.g. in the Andes, the Himalaya, the Rocky Mountains, the Caucasus, the Alps, in Norway, Japan, Turkey and the Iran. On mainland Australia no glaciers exist today, although a small glacier on Mount Kosciuszko was present in the last glacial period, and Tasmania was widely glaciated. On New Zealand's South Island the West Coast bears the Fox and Franz Josef Glaciers. In New Guinea small glaciers are located on its highest summit massif of Puncak Jaya. Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya and in the Ruwenzori Range.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 and the Tropical Andes in
South
America; however, the only snow to occur exactly on the Equator
is at 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 ( - ) 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 on Mars
Elsewhere in the solar system, the vast polar ice caps of Mars rival those of the Earth and show glacial features. Especially the south polar cap is compared to glaciers on earth. Other glacial features on Mars are glacial debris aprons and the lineated valley fills of the fretted terrain in northern Arabia Terra. Topographical features and computer models indicate the existence of more glaciers in Mars' past.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.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 meters
per day, as in the case of Antarctica's Byrd
Glacier, which moves 750-800 meters per year.
Many glaciers have periods of
very rapid advancement called surges.
These glaciers exhibit normal movement until suddenly they
accelerate, then return to their previous state. During these
surges, the glacier may reach velocities far greater than normal
speed. These surges may be caused by failure of the underlying
bedrock, the ponding of meltwater at the base of the glacier -
perhaps delivered from a supraglacial
lake - or the simple accumulation of mass beyond a critical
"tipping point".
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.
Ogives
Ogives are alternating dark
and light bands of ice occurring as ridges and valleys on glacier
surfaces. They only occur below icefalls but not all icefalls have
ogives below them. Once formed, they bend progressively downglacier
due to the increased velocity toward the glacier's centerline.
Ogives are likely linked to seasonal motion of the glacier as the
width of one dark and one light band generally equals the annual
movement of the glacier. The ridges and valleys are formed because
ice from an icefall is severely broken up thereby increasing
ablation surface area during the summertime creating a swale and
creating space for snow accumulation in the winter creating a
ridge. Sometimes ogives are described as either wave ogives or band
ogives in which they are solely undulations or varying color bands
respectively.
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
Before glaciation, mountain
valleys have a characteristic "V"
shape, produced by downward erosion by water. However, during
glaciation, these valleys widen and deepen, forming 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. Once
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.
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.
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 to 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 is 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.
Glacial deposition takes place
in two forms: glaciofluvial deposition and till deposits.
- Glaciofluvial deposition comes from glacial meltwater. The water that is a result from melting glaciers carry material much like a river would and sorts it is it moves along. Examples of these landforms would include outwash plains and kettle holes.
- Till deposits are unsorted mounds of sand, gravel and rock that form around a glacier. Examples of these are moraines, kame terraces etc.
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).
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.
See also
- Aufeis
- Cryoseism
- Effects of global warming
- Erebus Ice Tongue
- Glacial motion
- Glacier Bay National Park and Preserve
- Global warming
- Icefall
- Ice cap
- Ice field
- Ice sheet
- Kenai Fjords National Park
- List of glaciers
- Misty Fjords National Monument
- Quaternary period
- Retreat of glaciers since 1850
- Irish Sea Glacier
Cited references
Uncited references
- An excellent less-technical treatment of all aspects, with superb photographs and firsthand accounts of glaciologists' experiences. All images of this book can be found online (see Weblinks: Glaciers-online)
- An undergraduate-level textbook.
- A textbook for undergraduates avoiding mathematical complexities
- A textbook devoted to explaining the geography of our planet.
- A comprehensive reference on the physical principles underlying formation and behavior.
External References
Glaciers shrinking at record rate Retrieved Mar 17 2008Commons media
commons Glacierglacier in Afrikaans:
Gletser
glacier in Tosk Albanian:
Gletscher
glacier in Arabic: نهر
جليدي
glacier in Aragonese:
Chelera
glacier in Asturian:
Glaciar
glacier in Belarusian:
Ледавік
glacier in Belarusian
(Tarashkevitsa): Ледавік
glacier in Bulgarian:
Ледник
glacier in Catalan:
Glacera
glacier in Czech:
Ledovec
glacier in Welsh:
Rhewlif
glacier in Danish:
Gletsjer
glacier in German:
Gletscher
glacier in Estonian:
Liustik
glacier in Modern Greek
(1453-): Παγετώνας
glacier in Spanish:
Glaciar
glacier in Esperanto:
Glaĉero
glacier in Basque:
Glaziar
glacier in Persian: یخچال
طبیعی
glacier in French:
Glacier
glacier in Hindi:
हिमनद
glacier in Croatian:
Ledenjak
glacier in Ido:
Glaciero
glacier in Indonesian:
Gletser
glacier in Icelandic:
Jökull
glacier in Italian:
Ghiacciaio
glacier in Hebrew: קרחון
יבשתי
glacier in Kazakh:
Мұздық
glacier in Swahili
(macrolanguage): Barafuto
glacier in Luxembourgish:
Gletscher
glacier in Lithuanian:
Ledynas
glacier in Limburgan:
Gletsjer
glacier in Hungarian:
Gleccser
glacier in Malay
(macrolanguage): Glasier
glacier in Dutch:
Gletsjer
glacier in Japanese:
氷河
glacier in Norwegian:
Isbre
glacier in Norwegian Nynorsk:
Isbre
glacier in Occitan (post
1500): Glacièr
glacier in Polish:
Lodowiec
glacier in Portuguese:
Geleira
glacier in Russian:
Ледник
glacier in Slovak:
Ľadovec
glacier in Slovenian:
Ledenik
glacier in Serbian:
Ледник
glacier in Finnish:
Jäätikkö
glacier in Swedish:
Glaciär
glacier in Thai:
ธารน้ำแข็ง
glacier in Vietnamese: Sông
băng
glacier in Turkish:
Buzul
glacier in Ukrainian:
Льодовик
glacier in Walloon:
Glaecî
glacier in Yiddish:
גלעטשער
glacier in Contenese:
冰川
glacier in Chinese:
冰川
Synonyms, Antonyms and Related Words
Dry Ice, berg, calf, cryosphere, firn, floe, frazil, frozen water, glaciation, glacieret, glaze, glazed frost, granular
snow, ground ice, growler, ice, ice banner, ice barrier, ice
belt, ice cave, ice cubes, ice dike, ice field, ice floe, ice foot,
ice front, ice island, ice needle, ice pack, ice pinnacle, ice
raft, ice sheet, iceberg, icefall, icequake, icicle, jokul, lolly, neve, nieve penitente, pack ice,
serac, shelf ice, sleet, slob, sludge, snow ice, snowberg