Glaciers Of North Slope Borough, Alaska
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A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its
ablation Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosion, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft materi ...
over many years, often
centuries A century is a period of 100 years. Centuries are numbered ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is sometimes abbreviated as c. A centennial or ...
. It acquires distinguishing features, such as
crevasses A crevasse is a deep crack, that forms in a glacier or ice sheet that can be a few inches across to over 40 feet. Crevasses form as a result of the movement and resulting stress associated with the shear stress generated when two semi-rigid pie ...
and seracs, as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as
cirque A (; from the Latin word ') is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic , meaning a pot or cauldron) and (; ). A cirque may also be a similarly shaped landform ...
s,
moraine A moraine is any accumulation of unconsolidated debris (regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice shee ...
s, or
fjord In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Germany, Greenland, the Faroe Islands, Ice ...
s. Although a glacier may flow into a body of water, it forms only on land and is distinct from the much thinner
sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...
and lake ice that form on the surface of bodies of water. On Earth, 99% of glacial ice is contained within vast
ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at Las ...
s (also known as "continental glaciers") in the
polar region The polar regions, also called the frigid geographical zone, zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North Pole, North and South Poles), lying within the polar circles. These high l ...
s, but glaciers may be found in
mountain range A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arise ...
s on every continent other than the Australian mainland, including Oceania's high-latitude
oceanic island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island ...
countries such as
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
. Between latitudes 35°N and 35°S, glaciers occur only in the
Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
,
Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
, and a few high mountains in East Africa, Mexico,
New Guinea New Guinea (; Hiri Motu Hiri Motu, also known as Police Motu, Pidgin Motu, or just Hiri, is a language of Papua New Guinea, which is spoken in surrounding areas of Port Moresby (Capital of Papua New Guinea). It is a simplified version of ...
and on
Zard-Kuh Zard-Kuh (meaning "Yellow Mountain", also spelled Zardkuh, Zarduh Kuh or Zard Kuh-e Bakhtiari; Persian: زردکوه بختیاری) is a sub-range in the central Zagros Range, Iran. With an elevation of 4221 metres, the Zard-Kuh is located ...
in Iran. With more than 7,000 known glaciers,
Pakistan Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...
has more glacial ice than any other country outside the polar regions. Glaciers cover about 10% of Earth's land surface. Continental glaciers cover nearly or about 98% of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
's , with an average thickness of . Greenland and
Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and gl ...
also have huge expanses of continental glaciers. The volume of glaciers, not including the ice sheets of Antarctica and Greenland, has been estimated at 170,000 km3. Glacial ice is the largest reservoir of
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
on Earth, holding with ice sheets about 69 percent of the world's freshwater. Many glaciers from
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
,
alpine Alpine may refer to any mountainous region. It may also refer to: Places Europe * Alps, a European mountain range ** Alpine states, which overlap with the European range Australia * Alpine, New South Wales, a Northern Village * Alpine National Pa ...
and seasonal polar climates store water as ice during the colder seasons and release it later in the form of
meltwater Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be ...
as warmer summer temperatures cause the glacier to melt, creating a
water source Water supply is the provision of water by public utilities, commercial organisations, community endeavors or by individuals, usually via a system of pumps and pipes. Public water supply systems are crucial to properly functioning societies. Thes ...
that is especially important for plants, animals and human uses when other sources may be scant. However, within high-altitude and Antarctic environments, the seasonal temperature difference is often not sufficient to release meltwater. Since glacial mass is affected by long-term climatic changes, e.g.,
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, mean temperature, and
cloud cover Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud co ...
, glacial mass changes are considered among the most sensitive indicators of
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
and are a major source of variations in
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
. A large piece of compressed ice, or a glacier, appears blue, as large quantities of water appear blue. This is because water molecules absorb other colors more efficiently than blue. The other reason for the blue color of glaciers is the lack of air bubbles. Air bubbles, which give a white color to ice, are squeezed out by pressure increasing the created ice's density.


Etymology and related terms

The word ''glacier'' is a
loanword A loanword (also loan word or loan-word) is a word at least partly assimilated from one language (the donor language) into another language. This is in contrast to cognates, which are words in two or more languages that are similar because th ...
from French and goes back, via
Franco-Provençal Franco-Provençal (also Francoprovençal, Patois or Arpitan) is a language within Gallo-Romance originally spoken in east-central France, western Switzerland and northwestern Italy. Franco-Provençal has several distinct dialects and is separ ...
, to the
Vulgar Latin Vulgar Latin, also known as Popular or Colloquial Latin, is the range of non-formal Register (sociolinguistics), registers of Latin spoken from the Crisis of the Roman Republic, Late Roman Republic onward. Through time, Vulgar Latin would evolve ...
', derived from the
Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the form of Literary Latin of late antiquity.Roberts (1996), p. 537. English dictionary definitions of Late Latin date this period from the , and continuing into the 7th century in t ...
', and ultimately
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
', meaning "ice". The processes and features caused by or related to glaciers are referred to as glacial. The process of glacier establishment, growth and flow is called
glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
. The corresponding area of study is called
glaciology Glaciology (; ) is the scientific study of glaciers, or more generally ice and natural phenomena that involve ice. Glaciology is an interdisciplinary Earth science that integrates geophysics, geology, physical geography, geomorphology, climato ...
. Glaciers are important components of the global
cryosphere ] The cryosphere (from the Ancient Greek, Greek ''kryos'', "cold", "frost" or "ice" and ''sphaira'', "globe, ball") is an all-encompassing term for those portions of Earth's surface where water is in solid form, including sea ice, lake ice, ri ...
.


Types


Classification by size, shape and behavior

Glaciers are categorized by their morphology, thermal characteristics, and behavior. '' Alps, Alpine glaciers'' form on the crests and slopes of mountains. A glacier that fills a valley is called a ''valley glacier'', or alternatively, an ''alpine glacier'' or ''mountain glacier''. A large body of glacial ice astride a mountain, mountain range, or
volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...
is termed an ''
ice cap In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description Ice caps are not constrained by topographical features ...
'' or ''
ice field An ice field (also spelled icefield) is a mass of interconnected valley glaciers (also called mountain glaciers or alpine glaciers) on a mountain mass with protruding rock ridges or summits. They are often found in the colder climates and highe ...
''. Ice caps have an area less than by definition. Glacial bodies larger than are called ''
ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at Las ...
s'' or ''continental glaciers''. Several kilometers deep, they obscure the underlying topography. Only
nunatak A nunatak (from Inuit ''nunataq'') is the summit or ridge of a mountain that protrudes from an ice field or glacier that otherwise covers most of the mountain or ridge. They are also called glacial islands. Examples are natural pyramidal peaks. ...
s protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland. They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over . Portions of an ice sheet or cap that extend into water are called
ice shelves An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The b ...
; they tend to be thin with limited slopes and reduced velocities. Narrow, fast-moving sections of an ice sheet are called ''
ice streams An ice stream is a region of fast-moving ice within an ice sheet. It is a type of glacier, a body of ice that moves under its own weight. They can move upwards of a year, and can be up to in width, and hundreds of kilometers in length. They t ...
''. In Antarctica, many ice streams drain into large
ice shelves An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The b ...
. Some drain directly into the sea, often with an
ice tongue An ice tongue is a long and narrow sheet of ice projecting out from the coastline. An ice tongue forms when a valley glacier moves very rapidly (relative to surrounding ice) out into the ocean or a lake. They can gain mass from water freezing at t ...
, like
Mertz Glacier Mertz Glacier () is a heavily crevassed glacier in George V Coast of East Antarctica. It is the source of a glacial prominence that historically has extended northward into the Southern Ocean, the ''Mertz Glacial Tongue''. It is named in honor ...
. '' Tidewater glaciers'' are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, Baffin,
Devon Devon ( , historically known as Devonshire , ) is a ceremonial and non-metropolitan county in South West England. The most populous settlement in Devon is the city of Plymouth, followed by Devon's county town, the city of Exeter. Devon is ...
, and
Ellesmere Island Ellesmere Island ( iu, script=Latn, Umingmak Nuna, lit=land of muskoxen; french: île d'Ellesmere) is Canada's northernmost and List of Canadian islands by area, third largest island, and the List of islands by area, tenth largest in the world. ...
s in Canada,
Southeast Alaska Southeast Alaska, colloquially referred to as the Alaska(n) Panhandle, is the southeastern portion of the U.S. state of Alaska, bordered to the east and north by the northern half of the Canadian province of British Columbia (and a small part ...
, and the Northern and
Southern Patagonian Ice Field The Southern Patagonian Ice Field ( es, Hielo Continental or '), located at the Southern Patagonic Andes between Chile and Argentina, is the world's second largest contiguous extrapolar ice field. It is the bigger of two remnant parts of the Pat ...
s. As the ice reaches the sea, pieces break off or calve, forming
iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...
s. Most tidewater glaciers calve above sea level, which often results in a tremendous impact as the iceberg strikes the water. Tidewater glaciers undergo centuries-long cycles of advance and retreat that are much less affected by climate change than other glaciers.


Classification by thermal state

Thermally, a ''temperate glacier'' is at a melting point throughout the year, from its surface to its base. The ice of a ''polar glacier'' is always below the freezing threshold from the surface to its base, although the surface
snowpack Snowpack forms from layers of snow that accumulate in geographic regions and high elevations where the climate includes cold weather for extended periods during the year. Snowpacks are an important water resource that feed streams and rivers as th ...
may experience seasonal melting. A ''subpolar glacier'' includes both temperate and polar ice, depending on the depth beneath the surface and position along the length of the glacier. In a similar way, the thermal regime of a glacier is often described by its basal temperature. A ''cold-based glacier'' is below freezing at the ice-ground interface and is thus frozen to the underlying substrate. A ''warm-based glacier'' is above or at freezing at the interface and is able to slide at this contact. This contrast is thought to a large extent to govern the ability of a glacier to effectively erode its bed, as sliding ice promotes plucking at rock from the surface below. Glaciers which are partly cold-based and partly warm-based are known as ''polythermal''.


Formation

Glaciers form where the
accumulation Accumulation may refer to: Finance * Accumulation function, a mathematical function defined in terms of the ratio future value to present value * Capital accumulation, the gathering of objects of value Science and engineering * Accumulate (hi ...
of snow and ice exceeds
ablation Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosion, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft materi ...
. A glacier usually originates from a
cirque A (; from the Latin word ') is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic , meaning a pot or cauldron) and (; ). A cirque may also be a similarly shaped landform ...
landform (alternatively known as a corrie or as a ) – a typically armchair-shaped geological feature (such as a depression between mountains enclosed by
arête An arête ( ) is a narrow ridge of rock which separates two valleys. It is typically formed when two glaciers erode parallel U-shaped valleys. Arêtes can also form when two glacial cirques erode headwards towards one another, although frequen ...
s) – which collects and compresses through gravity the snow that falls into it. This snow accumulates and the weight of the snow falling above compacts it, forming
névé Névé is a young, granular type of snow which has been partially melted, refrozen and compacted, yet precedes the form of ice. This type of snow is associated with glacier formation through the process of ''nivation''. Névé that survives a ...
(granular snow). Further crushing of the individual snowflakes and squeezing the air from the snow turns it into "glacial ice". This glacial ice will fill the cirque until it "overflows" through a geological weakness or vacancy, such as a gap between two mountains. When the mass of snow and ice reaches sufficient thickness, it begins to move by a combination of surface slope, gravity, and pressure. On steeper slopes, this can occur with as little as of snow-ice. In temperate glaciers, snow repeatedly freezes and thaws, changing into granular ice called
firn __NOTOC__ Firn (; from Swiss German "last year's", cognate with ''before'') is partially compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé. It is ice that ...
. 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. Glacier ice is slightly more dense than ice formed from frozen water because glacier ice contains fewer trapped air bubbles. Glacial ice has a distinctive blue tint because it absorbs some red light due to an
overtone An overtone is any resonant frequency above the fundamental frequency of a sound. (An overtone may or may not be a harmonic) In other words, overtones are all pitches higher than the lowest pitch within an individual sound; the fundamental i ...
of the infrared OH stretching mode of the water molecule. (Liquid water appears blue for the same reason. The blue of glacier ice is sometimes misattributed to
Rayleigh scattering Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the ...
of bubbles in the ice.)


Structure

A glacier originates at a location called its glacier head and terminates at its glacier foot, snout, or
terminus Terminus may refer to: * Bus terminus, a bus station serving as an end destination * Terminal train station or terminus, a railway station serving as an end destination Geography *Terminus, the unofficial original name of Atlanta, Georgia, United ...
. Glaciers are broken into zones based on surface snowpack and melt conditions. The ablation zone is the region where there is a net loss in glacier mass. The upper part of a glacier, where accumulation exceeds ablation, is called the
accumulation zone On a glacier, the accumulation zone is the area above the firn line, where snowfall accumulates and exceeds the losses from ablation, (melting, evaporation, and sublimation). The annual equilibrium line separates the accumulation and ablation zo ...
. The equilibrium line separates the ablation zone and the accumulation zone; it is the contour where the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. In general, the accumulation zone accounts for 60–70% of the glacier's surface area, more if the glacier calves icebergs. Ice in the accumulation zone is deep enough to exert a downward force that erodes underlying rock. After a glacier melts, it often leaves behind a bowl- or amphitheater-shaped depression that ranges in size from large basins like the Great Lakes to smaller mountain depressions known as
cirque A (; from the Latin word ') is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic , meaning a pot or cauldron) and (; ). A cirque may also be a similarly shaped landform ...
s. The accumulation zone can be subdivided based on its melt conditions. # The dry snow zone is a region where no melt occurs, even in the summer, and the snowpack remains dry. # The percolation zone is an area with some surface melt, causing meltwater to percolate into the snowpack. This zone is often marked by refrozen ice lenses, glands, and layers. The snowpack also never reaches the melting point. # Near the equilibrium line on some glaciers, a superimposed ice zone develops. This zone is where meltwater refreezes as a cold layer in the glacier, forming a continuous mass of ice. # 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 health of a glacier is usually assessed by determining the
glacier mass balance Crucial to the survival of a glacier is its mass balance or surface mass balance (SMB), the difference between accumulation and ablation (sublimation and melting). Climate change may cause variations in both temperature and snowfall, causing c ...
or observing terminus behavior. Healthy glaciers have large accumulation zones, more than 60% of their area is snow-covered at the end of the melt season, and they have a terminus with a vigorous flow. Following the
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Ma ...
's end around 1850, glaciers around the Earth have retreated substantially. A slight cooling led to the advance of many alpine glaciers between 1950 and 1985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous.


Motion

Glaciers move, or flow, downhill by the force of
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
and the internal deformation of ice. Ice behaves like a brittle solid until its thickness exceeds about 50 m (160 ft). The pressure on ice deeper than 50 m causes plastic flow. At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers. When the stress on the layer above exceeds the inter-layer binding strength, it moves faster than the layer below. Glaciers also move through
basal sliding Basal sliding is the act of a glacier sliding over the bed due to meltwater under the ice acting as a lubricant. This movement very much depends on the temperature of the area, the slope of the glacier, the bed roughness, the amount of meltwater f ...
. In this process, a glacier slides over the terrain on which it sits, lubricated by the presence of liquid water. The water is created from ice that melts under high pressure from frictional heating. Basal sliding is dominant in temperate or warm-based glaciers. Although evidence in favor of glacial flow was known by the early 19th century, other theories of glacial motion were advanced, such as the idea that meltwater, refreezing inside glaciers, caused the glacier to dilate and extend its length. As it became clear that glaciers behaved to some degree as if the ice were a viscous fluid, it was argued that "regelation", or the melting and refreezing of ice at a temperature lowered by the pressure on the ice inside the glacier, was what allowed the ice to deform and flow. James Forbes came up with the essentially correct explanation in the 1840s, although it was several decades before it was fully accepted.


Fracture zone and cracks

The top of a glacier are rigid because they are under low
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
. This upper section is known as the ''fracture zone'' and moves mostly as a single unit over the plastic-flowing lower section. When a glacier moves through irregular terrain, cracks called
crevasse A crevasse is a deep crack, that forms in a glacier or ice sheet that can be a few inches across to over 40 feet. Crevasses form as a result of the movement and resulting stress associated with the shear stress generated when two semi-rigid pie ...
s develop in the fracture zone. Crevasses form because of differences in glacier velocity. If two rigid sections of a glacier move at different speeds or directions,
shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...
forces cause them to break apart, opening a crevasse. Crevasses are seldom more than deep but, in some cases, can be at least deep. Beneath this point, the plasticity of the ice prevents the formation of cracks. Intersecting crevasses can create isolated peaks in the ice, called
serac A serac (from Swiss French ''sérac'') is a block or column of glacial ice, often formed by intersecting crevasses on a glacier. Commonly house-sized or larger, they are dangerous to mountaineers, since they may topple with little warning. Even ...
s. Crevasses can form in several different ways. Transverse crevasses are transverse to flow and form where steeper slopes cause a glacier to accelerate. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form near the edge of the glacier, caused by the reduction in speed caused by friction of the valley walls. Marginal crevasses are largely transverse to flow. Moving glacier ice can sometimes separate from the stagnant ice above, forming a
bergschrund A bergschrund (from the German for ''mountain cleft'') or rimaye (from French; ) is a crevasse that forms where moving glacier ice separates from the stagnant ice or firn above. It is often a serious obstacle for mountaineers, who sometimes abb ...
. Bergschrunds resemble crevasses but are singular features at a glacier's margins. Crevasses make travel over glaciers hazardous, especially when they are hidden by fragile
snow bridge Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...
s. Below the equilibrium line, glacial meltwater is concentrated in stream channels. Meltwater can pool in proglacial lakes on top of a glacier or descend into the depths of a glacier via moulins. Streams within or beneath a glacier flow in englacial or sub-glacial tunnels. These tunnels sometimes reemerge at the glacier's surface.


Speed

The speed of glacial displacement is partly determined by
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
. Friction makes the ice at the bottom of the glacier move more slowly than ice at the top. In alpine glaciers, friction is also generated at the valley's sidewalls, which slows the edges relative to the center. Mean glacial speed varies greatly but is typically around per day. There may be no motion in stagnant areas; for example, in parts of Alaska, trees can establish themselves on surface sediment deposits. In other cases, glaciers can move as fast as per day, such as in Greenland's
Jakobshavn Isbræ Jakobshavn Glacier ( da, Jakobshavn Isbræ), also known as Ilulissat Glacier ( kl, Sermeq Kujalleq), is a large outlet glacier in West Greenland. It is located near the Greenlandic town of Ilulissat (colonial name in da, Jakobshavn) and ends a ...
. Glacial speed is affected by factors such as slope, ice thickness, snowfall, longitudinal confinement, basal temperature, meltwater production, and bed hardness. A few glaciers have periods of very rapid advancement called surges. These glaciers exhibit normal movement until suddenly they accelerate, then return to their previous movement state. These surges may be caused by the failure of the underlying bedrock, the pooling of meltwater at the base of the glacier — perhaps delivered from a
supraglacial lake A supraglacial lake is any pond of liquid water on the top of a glacier. Although these pools are ephemeral, they may reach kilometers in diameter and be several meters deep. They may last for months or even decades at a time, but can empty in t ...
 — or the simple accumulation of mass beyond a critical "tipping point". Temporary rates up to per day have occurred when increased temperature or overlying pressure caused bottom ice to melt and water to accumulate beneath a glacier. In glaciated areas where the glacier moves faster than one km per year,
glacial earthquake Glacial earthquakes are earthquakes as large as magnitude 5.1 that occur in glaciated areas where the glacier moves faster than one kilometer per year. The number of glacial earthquakes in Greenland shows a peak every year in July, August and Se ...
s occur. These are large scale earthquakes that have seismic magnitudes as high as 6.1."Seasonality and Increasing Frequency of Greenland Glacial Earthquakes"
, Ekström, G., M. Nettles, and V.C. Tsai (2006) ''Science'', 311, 5768, 1756–1758,
"Analysis of Glacial Earthquakes"
Tsai, V. C. and G. Ekström (2007). J. Geophys. Res., 112, F03S22,
The number of glacial earthquakes in Greenland peaks every year in July, August, and September and increased rapidly in the 1990s and 2000s. In a study using data from January 1993 through October 2005, more events were detected every year since 2002, and twice as many events were recorded in 2005 as there were in any other year.


Ogives

Ogives or Forbes bands are alternating wave crests and valleys that appear as dark and light bands of ice on glacier surfaces. They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals the annual movement of the glacier. Ogives are formed when ice from an icefall is severely broken up, increasing ablation surface area during summer. This creates a
swale Swale or Swales may refer to: Topography * Swale (landform), a low tract of land ** Bioswale, landform designed to remove silt and pollution ** Swales, found in the formation of Hummocky cross-stratification Geography * River Swale, in North ...
and space for snow accumulation in the winter, which in turn creates a ridge. Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives.


Geography

Glaciers are present on every continent and in approximately fifty countries, excluding those (Australia, South Africa) that have glaciers only on distant
subantarctic The sub-Antarctic zone is a region in the Southern Hemisphere, located immediately north of the Antarctic region. This translates roughly to a latitude of between 46° and 60° south of the Equator. The subantarctic region includes many islands ...
island territories. Extensive glaciers are found in Antarctica, Argentina, Chile, Canada, Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in the
Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
, the
Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
, the
Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...
, the
Caucasus The Caucasus () or Caucasia (), is a region between the Black Sea and the Caspian Sea, mainly comprising Armenia, Azerbaijan, Georgia, and parts of Southern Russia. The Caucasus Mountains, including the Greater Caucasus range, have historically ...
,
Scandinavian mountains The Scandinavian Mountains or the Scandes is a mountain range that runs through the Scandinavian Peninsula. The western sides of the mountains drop precipitously into the North Sea and Norwegian Sea, forming the fjords of Norway, whereas to the ...
, and the
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
.
Snezhnika Snezhnika ( bg, Снежника 'the snow patch') is a glacieret in the Pirin Mountains of Bulgaria, a remnant of the former Vihren Glacier.Grunewald, p. 129. The glacieret lies at an elevation between and in the deep Golemiya Kazan cirque at th ...
glacier in
Pirin , photo=Pirin-mountains-Bansko.jpg , photo_caption=Pirin scenery in winter , country= Bulgaria, , parent= , geology= granite, gneiss, marble, limestone , area_km2=2585 , range_coordinates = , length_km=80 , length_orientation= north-s ...
Mountain,
Bulgaria Bulgaria (; bg, България, Bǎlgariya), officially the Republic of Bulgaria,, ) is a country in Southeast Europe. It is situated on the eastern flank of the Balkans, and is bordered by Romania to the north, Serbia and North Macedon ...
with a
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
of 41°46′09″ N is the southernmost glacial mass in Europe.Grunewald, p. 129. Mainland Australia currently contains no glaciers, although a small glacier on
Mount Kosciuszko Mount Kosciuszko ( ; Ngarigo: , ), previously spelled Mount Kosciusko, is mainland Australia's tallest mountain, at 2,228 metres (7,310 ft) above sea level. It is located on the Main Range of the Snowy Mountains in Kosciuszko National ...
was present in the last glacial period. In New Guinea, small, rapidly diminishing, glaciers are located on
Puncak Jaya Puncak Jaya (; literally "Glorious Peak") or Carstensz Pyramid, Mount Jayawijaya or Mount Carstensz () on the island of New Guinea, with an elevation of , is the list of islands by highest point, highest mountain peak of an island on Earth. Th ...
. Africa has glaciers on
Mount Kilimanjaro Mount Kilimanjaro () is a dormant volcano in Tanzania. It has three volcanic cones: Kibo, Mawenzi, and Shira. It is the highest mountain in Africa and the highest free-standing mountain above sea level in the world: above sea level and ab ...
in Tanzania, on
Mount Kenya Mount Kenya (Kikuyu: ''Kĩrĩnyaga'', Kamba, ''Ki Nyaa'') is the highest mountain in Kenya and the second-highest in Africa, after Kilimanjaro. The highest peaks of the mountain are Batian (), Nelion () and Point Lenana (). Mount Kenya is locat ...
, and in the
Rwenzori Mountains The Ruwenzori, also spelled Rwenzori and Rwenjura, are a range of mountains in eastern equatorial Africa, located on the border between Uganda and the Democratic Republic of the Congo. The highest peak of the Ruwenzori reaches , and the range' ...
. Oceanic islands with glaciers include Iceland, several of the islands off the coast of Norway including Svalbard and Jan Mayen to the far north, New Zealand and the subantarctic islands of Marion Island, Marion, Heard Island, Heard, Kerguelen Islands#Grande Terre, Grande Terre (Kerguelen) and Bouvet Island, Bouvet. During glacial periods of the Quaternary, Taiwan, Hawaii (island), Hawaii on Mauna Kea and Tenerife also had large alpine glaciers, while the Faroe Islands, Faroe and Crozet Islands were completely glaciated. The permanent snow cover necessary for glacier formation is affected by factors such as the degree of slope on the land, amount of snowfall and the winds. Glaciers can be found in all
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
s except from 20° to 27° north and south of the equator where the presence of the descending limb of the Hadley circulation lowers precipitation so much that with high insolation snow lines reach above . Between 19˚N and 19˚S, however, precipitation is higher, and the mountains above usually have permanent snow. Even at high latitudes, glacier formation is not inevitable. Areas of the Arctic, such as Banks Island, and the McMurdo Dry Valleys in Antarctica are considered polar deserts where glaciers cannot form because they receive little snowfall despite the bitter cold. Cold air, unlike warm air, is unable to transport much water vapor. Even during glacial periods of the Quaternary, Manchuria, lowland Siberia, and Alaska Interior, central and northern Alaska, though extraordinarily cold, had such light snowfall that glaciers could not form. In addition to the dry, unglaciated polar regions, some mountains and volcanoes in Bolivia, Chile and Argentina 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.


Glacial geology

Glaciers erode terrain through two principal processes: abrasion (geology), abrasion and plucking. As glaciers flow over bedrock, they soften and lift blocks of rock into the ice. This process, called plucking, is caused by subglacial water that penetrates fractures in the bedrock and subsequently freezes and expands. This expansion causes the ice to act as a lever that loosens the rock by lifting it. Thus, sediments of all sizes become part of the glacier's load. If a retreating glacier gains enough debris, it may become a rock glacier, like the Timpanogos Glacier in Utah. Abrasion occurs when the ice and its load of rock fragments slide over bedrock and function as sandpaper, smoothing and polishing the bedrock below. The pulverized rock this process produces is called rock flour and is made up of rock grains between 0.002 and 0.00625 mm in size. Abrasion leads to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides, which add even more material to the glacier. Glacial abrasion is commonly characterized by glacial striations. Glaciers produce these when they contain large boulders that carve long scratches in the bedrock. By mapping the direction of the striations, researchers can determine the direction of the glacier's movement. Similar to striations are chatter marks, lines of crescent-shape depressions in the rock underlying a glacier. They are formed by abrasion when boulders in the glacier are repeatedly caught and released as they are dragged along the bedrock.The rate of glacier erosion varies. Six factors control erosion rate: * 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 When the bedrock has frequent fractures on the surface, glacial erosion rates tend to increase as plucking is the main erosive force on the surface; when the bedrock has wide gaps between sporadic fractures, however, abrasion tends to be the dominant erosive form and glacial erosion rates become slow. Glaciers in lower latitudes tend to be much more erosive than glaciers in higher latitudes, because they have more meltwater reaching the glacial base and facilitate sediment production and transport under the same moving speed and amount of ice. Material that becomes incorporated in a glacier is 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'': sediments deposited by water. These deposits are stratified by size. Larger pieces of rock that are encrusted in till or deposited on the surface are called "glacial erratics". They range in size from pebbles to boulders, but as they are often moved great distances, they may be drastically different from the material upon which they are found. Patterns of glacial erratics hint at past glacial motions.


Moraines

Glacial
moraine A moraine is any accumulation of unconsolidated debris (regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice shee ...
s are formed by the deposition of material from a glacier and are exposed after the glacier has retreated. They usually appear as linear mounds of till, a non-sorted mixture of rock, gravel, and boulders within a matrix of 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 merge and the lateral moraines of each coalesce to form a moraine in the middle of the combined glacier. Less apparent are ground moraines, also called ''glacial drift'', which often blankets the surface underneath the glacier downslope from the equilibrium line. The term ''moraine'' is of French origin. It was coined by peasants to describe alluvial embankments and rims found near the margins of glaciers in the French
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
. In modern geology, the term is used more broadly and is applied to a series of formations, all of which are composed of till. Moraines can also create moraine-dammed lakes.


Drumlins

Drumlins are asymmetrical, canoe-shaped hills made mainly of till. Their heights vary from 15 to 50 meters, and they can reach a kilometer in length. The steepest side of the hill faces the direction from which the ice advanced (''stoss''), while a longer slope is left in the ice's direction of movement (''lee''). Drumlins are found in groups called ''drumlin fields'' or ''drumlin camps''. One of these fields is found east of Rochester, New York; it is estimated to contain about 10,000 drumlins. Although the process that forms drumlins is not fully understood, their shape implies 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 valleys, cirques, arêtes, and pyramidal peaks

Before glaciation, mountain valleys have a characteristic V-shaped valley, "V" shape, produced by eroding water. During glaciation, these valleys are often widened, deepened and smoothed to form a U-shaped valley, "U"-shaped glacial valley or glacial trough, as it is sometimes called. The erosion that creates glacial valleys truncates any spurs of rock or earth that may have earlier extended across the valley, creating broadly triangular-shaped cliffs called truncated spurs. Within glacial valleys, depressions created by plucking and abrasion can be filled by lakes, called paternoster lakes. If a glacial valley runs into a large body of water, it forms a
fjord In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Germany, Greenland, the Faroe Islands, Ice ...
. Typically glaciers deepen their valleys more than their smaller tributary, tributaries. Therefore, when glaciers recede, the valleys of the tributary glaciers remain above the main glacier's depression and are called hanging valleys. At the start of a classic valley glacier is a bowl-shaped cirque, which have escarped walls on three sides but is open on the side that descends into the valley. Cirques are where ice begins to accumulate in a glacier. Two glacial cirques may form back to back and erode their backwalls until only a narrow ridge, called an
arête An arête ( ) is a narrow ridge of rock which separates two valleys. It is typically formed when two glaciers erode parallel U-shaped valleys. Arêtes can also form when two glacial cirques erode headwards towards one another, although frequen ...
is left. This structure may result in a mountain pass. If multiple cirques encircle a single mountain, they create pointed pyramidal peaks; particularly steep examples are called Glacial horn, horns.


Roches moutonnées

Passage of glacial ice over an area of bedrock may cause the rock to be sculpted into a knoll called a ''roche moutonnée,'' or "sheepback" rock. Roches moutonnées may be elongated, rounded and asymmetrical in shape. They range in length from less than a meter to several hundred meters long. Roches moutonnées have a gentle slope on their up-glacier sides and a steep to vertical face on their down-glacier sides. The glacier abrades the smooth slope on the upstream side as it flows along, but tears rock fragments loose and carries them away from the downstream side via plucking.


Alluvial stratification

As the water that rises from the ablation zone moves away from the glacier, it carries fine eroded sediments with it. As the speed of the water decreases, so does its capacity to carry objects in suspension. The water thus 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 in an estuary, the sediments are known as bay mud. Outwash plains and valley trains are usually accompanied by basins known as "Kettle (landform), kettles". These are small lakes formed when large ice blocks that are trapped in alluvium melt and produce water-filled depressions. Kettle diameters range from 5 m to 13 km, with depths of up to 45 meters. Most are circular in shape because the blocks of ice that formed them were rounded as they melted.


Glacial deposits

When a glacier's size shrinks below a critical point, its flow stops and it becomes stationary. Meanwhile, meltwater within and beneath the ice leaves Stratigraphy, stratified alluvial deposits. These deposits, in the forms of columns, Terrace (geology), terraces and clusters, remain after the glacier melts and are known as "glacial deposits". Glacial deposits that take the shape of hills or mounds are called ''kames''. Some kames form when meltwater deposits sediments through openings in the interior of the ice. Others are produced by fans or river delta, deltas created by meltwater. When the glacial ice occupies a valley, it can form terraces or kames along the sides of the valley. Long, sinuous glacial deposits are called ''eskers''. Eskers are composed of sand and gravel that was deposited by meltwater streams that flowed through ice tunnels within or beneath a glacier. They remain after the ice melts, with heights exceeding 100 meters and lengths of as long as 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 processes, eolian loess deposits may be very deep, even hundreds of meters, as in areas of China and the Midwestern United States. Katabatic winds can be important in this process.


Climate change

Glaciers are a valuable resource for tracking climate change over long periods of time because they can be hundreds of thousands of years old. To study the patterns over time through glaciers, ice cores are taken, providing continuous information including evidence for climate change, trapped in the ice for scientists to break down and study. Glaciers are studied to give information about the history of climate change due to natural or human causes. Human activity has caused an increase in greenhouse gases creating a global warming trend, causing these valuable glaciers to melt. Glaciers have an albedo effect and the melting of glaciers means less albedo. In the Alps the summer of 2003 was compared to the summer of 1988. Between 1998 and 2003 the albedo value is 0.2 lower in 2003. When glaciers begin to melt, they also cause a rise in sea level, "which in turn increases coastal erosion and elevates storm surge as warming air and ocean temperatures create more frequent and intense coastal storms like hurricanes and typhoons." Thus, human causes to climate change creates a positive Feedback, feedback loop with the glaciers: The rise in temperature causes more glacier melt, leading to less albedo, higher sea levels and many other climate issues to follow. From 1972 all the way up to 2019 NASA has used a Landsat program, Landsat satellite that has been used to record glaciers in Alaska, Greenland and
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
. This Landsat project has found that since around 2000, glacier retreat has increased substantially.


Isostatic rebound

Large masses, such as ice sheets or glaciers, can depress the crust of the Earth into the mantle. The depression usually totals a third of the ice sheet or glacier's thickness. After the ice sheet or glacier melts, the mantle begins to flow back to its original position, pushing the crust back up. This post-glacial rebound, which proceeds very slowly after the melting of the ice sheet or glacier, is currently occurring in measurable amounts in Scandinavia and the Great Lakes region of North America. A geomorphological feature created by the same process on a smaller scale is known as ''dilation-faulting''. It occurs where previously compressed rock is allowed to return to its original shape more rapidly than can be maintained without faulting. This leads to an effect similar to what would be seen if the rock were hit by a large hammer. Dilation faulting can be observed in recently de-glaciated parts of Iceland and Cumbria.


On Mars

The polar ice caps of Mars show geologic evidence of glacial deposits. The south polar cap is especially comparable to glaciers on Earth. Topographical features and computer models indicate the existence of more glaciers in Mars' past. At mid-latitudes, between 35° and 65° north or south, Martian glaciers are affected by the thin Martian atmosphere. Because of the low atmospheric pressure, ablation near the surface is solely caused by Sublimation (phase transition), sublimation, not melting. As on Earth, many glaciers are covered with a layer of rocks which insulates the ice. A radar instrument on board the Mars Reconnaissance Orbiter found ice under a thin layer of rocks in formations called lobate debris aprons (LDAs).Holt, J. et al. 2008. Radar Sounding Evidence for Ice within Lobate Debris Aprons near Hellas Basin, Mid-Southern Latitudes of Mars. Lunar and Planetary Science XXXIX. 2441.pdf


See also

* * * * *


References


Bibliography

*


General 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.


Further reading

* Twila Moon, Moon, Twila
Saying goodbye to glaciers
''Science,'' 12 May 2017, Vol. 356, Issue 6338, pp. 580–581,


External links

* , a report in the Global Environment Outlook (GEO) series.
Glacial structures – photo atlas



Photo project tracks changes in Himalayan glaciers since 1921
* Short radio episode

' from ''The Mountains of California'' by John Muir, 1894. California Legacy Project
Dynamics of Glaciers

GletscherVergleiche.ch
– Before/After Images by Simon Oberli {{Authority control Glaciology Glaciers, Bodies of ice Montane ecology