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Glacier

2007 Schools Wikipedia Selection. Related subjects: General Geography

   A glacier is a large, long-lasting river of ice that is formed on land
   and moves in response to gravity and undergoes internal deformation.
   Glacier ice is the largest reservoir of fresh water on Earth, and
   second only to oceans as the largest reservoir of total water. Glaciers
   can be found on every continent, including on the greater Australian
   continent. Glaciers are more or less permanent bodies of ice and
   compacted snow that have become deep enough and heavy enough to flow
   under their own weight.

   Geologic features created by glaciers include end, lateral, ground and
   medial moraines that form from glacially transported rocks and debris;
   U-shaped valleys and corries ( cirques) at their heads, and the glacier
   fringe, which is the area where the glacier has recently melted into
   water.
   Aletsch Glacier, Switzerland
   Enlarge
   Aletsch Glacier, Switzerland

Types of glaciers

   Mouth of the Schlatenkees Glacier near Innergschlöß, Austria.
   Enlarge
   Mouth of the Schlatenkees Glacier near Innergschlöß, Austria.

   There are two main types of glaciers: alpine glaciers, which are found
   in mountain terrains, and continental glaciers, which cover large areas
   of continents. 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.
   "Poly-thermal" or "sub-polar", 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 are not affected by the landscape and extend over the entire
   surface, except 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 all around the world. If the
   Antarctic ice sheet melted, sea levels would rise up to 65 meters.

   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 to them.

Formation of glaciers

   Low and high contrast images of the Byrd Glacier. The low-contrast
   version is similar to the level of detail the naked eye would see —
   smooth and almost featureless. The bottom image uses enhanced contrast
   to highlight flow lines on the ice sheet and bottom crevasses.
   Enlarge
   Low and high contrast images of the Byrd Glacier. The low-contrast
   version is similar to the level of detail the naked eye would see —
   smooth and almost featureless. The bottom image uses enhanced contrast
   to highlight flow lines on the ice sheet and bottom crevasses.
   Formation of glacial ice
   Enlarge
   Formation of glacial ice

   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. 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
   colour is actually created for the exact same reason that water is
   blue, that is, its slight absorption of far red light due to an
   overtone of the infrared OH stretching mode of the water molecule .

   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 though they do not need a 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 Bergshrunds as they move. Crevasses form due to
   increases in glacier velocity. These crevasses make unprotected travel
   over glaciers extremely hazardous. Glacial meltwaters flow throughout
   and underneath glaciers, carving channels in the ice similar to caves
   in rock and also helping to lubricate the glacier's movement.

Anatomy of a glacier

   The Upper Grindelwald Glacier and the Schreckhorn, in Switzerland,
   showing accumulation and ablation zones
   Enlarge
   The Upper Grindelwald Glacier and the Schreckhorn, in Switzerland,
   showing accumulation and ablation zones

   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 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.
   At this altitude, the amount of new snow gained by accumulation is
   equal to the amount of ice lost through ablation. The downward erosive
   forces of the accumulation zone and the tendency of the ablation zone
   to deposit sediment also cancel each other out. Erosive lateral forces
   are not canceled; therefore, 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.

   Even in very cold climates, there may be unglaciated areas, which
   receive too little precipitation to form permanent ice. This was the
   case in most of Siberia, central and northern Alaska and all of
   Manchuria during glacial periods of the Quaternary, and occurs today in
   Antarctica's Dry Valleys and in that part of the Andes between 19°S and
   27°S above the hyperarid Atacama Desert where, although the mountains
   reach 6700 metres above sea level, the cold Humboldt Current completely
   suppresses precipitation.

Glacial motion

   The Perito-Moreno Glacier, showing cracks in brittle upper layer
   Enlarge
   The Perito-Moreno Glacier, showing cracks in brittle upper layer

   Ice behaves like an easily breaking solid until its thickness exceeds
   about 50 meters (160 ft). The increased pressure on ice deeper than
   that depth causes the ice to become plastic and 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

   Ice cracks in the Titlis Glacier
   Enlarge
   Ice cracks in the Titlis Glacier

   The top 50 meters of the glacier are more rigid. In this section, known
   as the fracture zone here 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 glacier a dangerous place to visit, because they are not
   always easy to spot.

Speed of glacial movement

   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 centre.
   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 centre moved further.

   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 ft per day), according to studies using satellites.

   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 up to 1,000 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 poorly-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 most common name for glacial sediment is moraine. The term 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. Currently, the term is used more broadly, and is applied to a
   series of formations, all of which are composed of till.

Drumlins

   A drumlin field forms after a glacier has modified the landscape. The
   teardrop-shaped formations denote the direction of the ice flow.
   A drumlin field forms after a glacier has modified the landscape. The
   teardrop-shaped formations denote the direction of the ice flow.

   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.

Glacial erosion

   Rocks and sediments are added to glaciers through various processes.
   Glaciers erode the terrain principally through two methods: scouring
   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 colour.

   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.

   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

   A glaciated valley in the Mount Hood Wilderness showing the
   characteristic U-shape and flat bottom.
   Enlarge
   A glaciated valley in the Mount Hood Wilderness showing the
   characteristic U-shape and flat bottom.
   This image shows the termini of the glaciers in the Bhutan Himalaya.
   Glacial lakes have been rapidly forming on the surface of the
   debris-covered glaciers in this region during the last few decades.
   Enlarge
   This image shows the termini of the glaciers in the Bhutan Himalaya.
   Glacial lakes have been rapidly forming on the surface of the
   debris-covered glaciers in this region during the last few decades.

   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 stop receding, 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 paternoster lakes, from the Latin for
   "Our Father", referring to a station of the rosary.

   At the head of a 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 glaciers separated by a dividing ridge. This, located
   between the corries, is eroded to create an arête. 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. With depths
   that can exceed 1,000 metres caused by the postglacial elevation of sea
   level and therefore, as it changed the glaciers changed their level of
   erosion.
   Features of a glacial landscape
   Features of a glacial landscape

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 produced 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 zone of ablation 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".
   Landscape produced by a receding glacier
   Landscape produced by a receding glacier

   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.

Isostatic rebound

   Isostatic pressure by a glacier on the Earth's crust
   Isostatic pressure by a glacier on the Earth's crust

   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 Earth's crust
   into the mantel displacing the mantel below, the depression is about a
   third the thickness of the ice sheet. After the glacier melts the
   mantel begins to flow back to its original position pushing the crust
   back to its original position, this process is slower than the melting
   of the ice sheet/glacier. This is post-glacial rebound and is currently
   occurring in measurable amounts in Scandinavia and the Great Lakes
   region of the United States.

Ice ages

Ice age 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,
   in contrast to that of the Earth's surface, are less affected by
   stratigraphic discontinuities, 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 Siberia. 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 of ice ages

   Little is known about the causes of glaciations.

   Generalized glaciations have been rare in the history of Earth.
   However, the Ice Age of the Pleistocene was not the only glaciative
   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 600 million years. Also, a well documented record of glaciation
   exists in rocks of the late Paleozoic (of 250 million years of age).

   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 is currently present in
   tropical latitudes 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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