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A glacier (; ) is a persistent body of dense
ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less bluish-white color. In the , ice is abundant and occurs naturally from as close to the Sun as to ...

ice
that is constantly moving under its own weight. A glacier forms where the accumulation of
snow Snow comprises individual ice Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the mai ...

snow
exceeds its
ablation Ablation is removal or destruction of material from an object by vaporization, chipping, or other erosive processes. Examples of ablative materials are described below, and include spacecraft 275px, The US Space Shuttle flew 135 times from ...
over many years, often
centuries A century is a period of 100 year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the season ...

centuries
. Glaciers slowly deform and flow under stresses induced by their weight, creating
crevasse A crevasse in Tangra Mountains, Antarctica A crevasse is a deep crack, Fracture (geology), crevice or fissure found in an ice sheet or glacier, or earth. Crevasses form as a result of the movement and resulting stress associated with the shear ...

crevasse
s,
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 whe ...

serac
s, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as
cirque A cirque (; from the Latin word ''circus'') is an amphitheatre An amphitheatre ( British English) or amphitheater ( American English; both ) is an open-air venue used for entertainment, performances, and sports. The term derives from the anc ...
s,
moraine A moraine is any accumulation of unconsolidated debris (regolith and Rock (geology), 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 glac ...

moraine
s, or
fjord In physical geography Physical geography (also known as physiography) is one of the two fields of geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the ...

fjord
s. Glaciers form only on land and are 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 ...

sea ice
and lake ice that forms on the surface of bodies of water. On Earth, 99% of glacial ice is contained within vast
ice sheet In , an ice sheet, also known as a continental glacier, is a mass of that covers surrounding terrain and is greater than . The only current ice sheets are in and ; during the at (LGM) the covered much of , the ice sheet covered and the c ...

ice sheet
s (also known as "continental glaciers") in the
polar region Northern Hemisphere permafrost (permanently frozen ground) in purple The Polar Regions, also called the frigid zones Zone or The Zone may refer to: Places Climate and altitude zones * Death zone (originally the lethal zone), altitudes above a ...
s, but glaciers may be found in
mountain range A mountain range is a series of mountains ranged 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 arisen from the same cause, us ...

mountain range
s on every continent other than the Australian mainland, including Oceania's high-latitude
oceanic island image:MODIS - Great Britain and Ireland - 2012-06-04 during heat wave.jpg, upright=1.15, Ireland (left) and Great Britain (right), are large islands of north-west Europe image:Small Island in Lower Saranac Lake.jpg, A small island in Lower Sara ...
countries such as New Zealand. Between latitudes 35°N and 35°S, glaciers occur only in the
Himalayas The Himalayas, or Himalaya (; Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Ind ...

Himalayas
,
Andes The Andes, Andes Mountains or Andean Mountains ( es, Cordillera de los Andes) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of Sout ...

Andes
, and a few high mountains in
East Africa East Africa or Eastern Africa is the eastern subregion of the Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including ...
,
Mexico Mexico, officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the south and west by the Pacific Ocean; to ...

Mexico
,
New Guinea New Guinea (; : ''Niu Gini''; id, Papua, historically ) is the , and with an area of , the largest island in the . Located in in the southwestern , it is separated by the wide from . Numerous smaller islands are located to the west and east ...

New Guinea
and on
Zard Kuh Zard-Kuh (meaning "Yellow Mountain", also spelled Zardkuh, Zarduh Kuh or Zard Kuh-e Bakhtiari; Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, Persian people, the ...
in Iran. With more than 7,000 known glaciers,
Pakistan Pakistan, . Pronounced variably in English as , , , and . officially the Islamic Republic of Pakistan, is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a popul ...

Pakistan
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's , with an average thickness of . Greenland and
Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America South America is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convent ...

Patagonia
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 Total dissolved solids (TDS) is a measure of the dissolved combined content of all ...

fresh water
on Earth, holding with ice sheets about 69 percent of the world's freshwater. Many glaciers from temperate, alpine 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 glaciers, glacial ice, tabular icebergs and ice shelf, ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reduci ...
as warmer summer temperatures cause the glacier to melt, creating a
water source Water supply is the provision of water by public utility, public utilities, commercial organisations, community endeavors or by individuals, usually via a system of pumps and Water pipe, pipes. Public water supply systems are crucial to properly ...
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 Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
, , and
cloud cover Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky The sky (also sometimes called celestial dome) is everything that lies above the surface of the Earth Earth is the third planet from th ...
, glacial mass changes are considered among the most sensitive indicators of
climate change Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known event ...
and are a major source of variations in
sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloquial, ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in th ...
. A large piece of compressed ice, or a glacier,
appears blue "Appears" is a song recorded by Japanese recording artist Ayumi Hamasaki. It was released by Avex Trax on November 10, 1999 as the sixth single from her second studio album ''Loveppears'' (1999), which was released on the same day. Alongside this, ...
, 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 In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (lin ...
from and goes back, via
Franco-Provençal Franco-Provençal (also Francoprovençal, Patois, Gaga, Savoyard, Arpitan or Romand) is a dialect group within Gallo-Romance languages, Gallo-Romance originally spoken in east-central France, western Switzerland and northwestern Italy. Franco ...
, to the
Vulgar Latin Vulgar Latin, also known as Popular or Colloquial Latin, is non-literary Literature broadly is any collection of written work, but it is also used more narrowly for writings specifically considered to be an art form, especially prose fiction, ...
', derived from the
Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, kn ...
', and ultimately
Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an appa ...

Latin
', 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 An ice age is a long period of reduction in the temperature of Earth Earth is the third planet from the Sun and the on ...
. The corresponding area of study is called
glaciology Lateral moraine on a glacier joining the Gorner Glacier, Zermatt">Gorner_Glacier.html" ;"title="moraine on a glacier joining the Gorner Glacier">moraine on a glacier joining the Gorner Glacier, Zermatt, Swiss Alps. The moraine is the high bank of ...
. Glaciers are important components of the global
cryosphere The cryosphere (from the Greek ''kryos'', "cold", "frost" or "ice" and ''sphaira'', "globe, ball") is an all-encompassing term for those portions of Earth Earth is the third planet from the Sun and the only astronomical object known to ...
. Perito Moreno Glacier Patagonia Argentina Luca Galuzzi 2005.JPG,
Ice calving in Lago Argentino Lago Argentino is a lake in the Patagonian provinces of Argentina, province of Santa Cruz Province (Argentina), Santa Cruz, Argentina, at . It is the biggest freshwater lake in Argentina, with a surface area of (maximum width: ). ...
from the
terminus Terminus may refer to: Places *Terminus, the unofficial original name of Atlanta, Georgia, United States **Terminus (office complex), an office complex in Atlanta *Lagos Terminus railway station, the main railway station of Lagos, Nigeria Art, en ...
of the
Perito Moreno Glacier The Perito Moreno Glacier () is a glacier located in the Los Glaciares National Park in southwest Santa Cruz Province, Argentina. It is one of the most important tourist attractions in the Argentinian Patagonia. The ice formation, in length, is ...

Perito Moreno Glacier
in western
Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America South America is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convent ...

Patagonia
, Argentina. Grosser Aletschgletscher 3178.JPG, The
Aletsch Glacier The Aletsch Glacier (german: Aletschgletscher) or Great Aletsch Glacier () is the largest glacier in the Alps. It has a length of about (2014), has about a volume of (2011), and covers about (2011) in the eastern Bernese Alps in the Switzerland, ...

Aletsch Glacier
, the largest glacier of the
Alps The Alps ; german: Alpen ; it, Alpi ; rm, Alps; sl, Alpe ) are the highest and most extensive mountain range A mountain range is a series of mountains ranged in a line and connected by high ground. A mountain system or mountain belt i ...

Alps
, in
Switzerland , french: Suisse(sse), it, svizzero/svizzera or , rm, Svizzer/Svizra , government_type = Federalism, Federal semi-direct democracy under an assembly-independent Directorial system, directorial republic , leader_title1 = Fe ...

Switzerland
. Quelccaya Glacier.jpg, The Quelccaya Ice Cap is the second-largest glaciated area in the
tropics The tropics are the region of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% i ...

tropics
, in Peru.


Types


Classification by size, shape and behavior

Glaciers are categorized by their morphology, thermal characteristics, and behavior. ''
Alpine Alpine may refer to: Places * Alps, a European mountain range * Alpine states, associated with the mountain range, or relating to any lofty mountain areas * Mountainous or alpine; the mountains. Australia * Alpine, New South Wales, a Northern Vill ...

Alpine
glaciers'' form on the crests and slopes of
mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at least ...

mountain
s. 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 of a , such as , that allows hot , , and to escape from a below the surface. On Earth, volcanoes are most often found where are or , and most are found underwater. For example, a , such as the , has volcanoe ...

volcano
is termed an ''
ice cap upright=1.35, Vatnajökull, Iceland ">Iceland.html" ;"title="Vatnajökull, Iceland">Vatnajökull, Iceland 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 coveri ...
'' or ''
ice field An ice field (also spelled icefield) is a mass of interconnected valley glaciers A glacier (; ) is a persistent body of dense ice Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, trans ...
''. Ice caps have an area less than by definition. Glacial bodies larger than are called ''
ice sheet In , an ice sheet, also known as a continental glacier, is a mass of that covers surrounding terrain and is greater than . The only current ice sheets are in and ; during the at (LGM) the covered much of , the ice sheet covered and the c ...

ice sheet
s'' or ''continental glaciers''. Several kilometers deep, they obscure the underlying topography. Only
nunatak A nunatak (from Inuit language, 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 p ...
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 A glacier ( or ) 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 Abl ...
; they tend to be thin with limited slopes and reduced velocities. Narrow, fast-moving sections of an ice sheet are called '' ice streams''. 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 A glacier ( or ) 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 Abl ...
. Some drain directly into the sea, often with an ice tongue, 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 o ...
. '' Tidewater glaciers'' are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, , , and
Ellesmere Island Ellesmere Island (Inuktitut: ''Umingmak Nuna'', meaning "land of muskoxen"; french: Île d'Ellesmere) is Canada's northernmost and third largest island, and the tenth largest in the world. It comprises an area of , slightly smaller than Great ...

Ellesmere Island
s in Canada,
Southeast Alaska Southeast Alaska, colloquially referred to as the Alaska Panhandle or Alaskan Panhandle, is the southeastern portion of the of , bordered to the east by the northern half of the of . The majority of Southeast Alaska's area is part of the , the U ...

Southeast Alaska
, and the Northern and
Southern Patagonian Ice Field The Southern Patagonian Ice Field ( es, Hielo Continental or '), located at the Southern Patagonia, Patagonic Andes between Chile and Argentina, is the world's second largest contiguous extrapolar ice field. It is the bigger of two remnant parts o ...

Southern Patagonian Ice Field
s. As the ice reaches the sea, pieces break off or calve, forming
iceberg An iceberg is a piece of freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color ...

iceberg
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. Glacier mouth.jpg, Mouth of the Glacier near Innergschlöß, Austria. GrottaGelo.jpg, The '' Grotta del Gelo'' is a cave of
volcano A volcano is a rupture in the of a , such as , that allows hot , , and to escape from a below the surface. On Earth, volcanoes are most often found where are or , and most are found underwater. For example, a , such as the , has volcanoe ...

volcano
, the southernmost glacier in
Europe Europe is a continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of scienc ...

Europe
. Fjordsglacier.jpg, Sightseeing boat in front of a tidewater glacier,
Kenai Fjords National Park Kenai Fjords National Park is an American national park A national park is a park in use for Conservation (ethic), conservation purposes, created and protected by national governments. Often it is a reserve of natural, semi-natural, or dev ...

Kenai Fjords National Park
, Alaska.


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 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 of snow and ice exceeds
ablation Ablation is removal or destruction of material from an object by vaporization, chipping, or other erosion, erosive processes. Examples of ablative materials are described below, and include spacecraft material for ascent and atmospheric reentry, ...
. A glacier usually originates from a
cirque A cirque (; from the Latin word ''circus'') is an amphitheatre An amphitheatre ( British English) or amphitheater ( American English; both ) is an open-air venue used for entertainment, performances, and sports. The term derives from the anc ...
landform (alternatively known as a "corrie" or as a "cwm") – a typically armchair-shaped geological feature (such as a depression between mountains enclosed by
arête An arête is a narrow ridge of Rock (geology), rock which separates two valleys. It is typically formed when two glaciers erode parallel U-shaped valleys. Arêtes can also form when two glacial cirque (landform), cirques erode headwards towards ...
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 Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all know ...
(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 15 m (50  ft) of snow-ice. In temperate glaciers, snow repeatedly freezes and thaws, changing into granular ice called
firn Firn (; from Swiss German Swiss German (Standard German Standard High German (SHG), less precisely Standard German or High German (not to be confused with High German The High German languages or High German dialects (german: hochde ...
. 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 frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosoph ...

overtone
of the infrared OH stretching mode of the water molecule. (Liquid
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
appears blue for the same reason. The blue of glacier ice is sometimes misattributed to
Rayleigh scattering Rayleigh scattering ( ), named after the nineteenth-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 o ...

Rayleigh scattering
of bubbles in the ice.) GornerGlacier 002.jpg,
Gorner Glacier The Gorner Glacier (german: Gornergletscher) is a valley glacier A glacier ( or ) 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#Glaciology ...
in Switzerland. Aerial Photo of Monte Rosa Massif - Wallis - Switzerland (cropped).jpg, An
aerial photograph Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other flying object. Platforms for aerial photography include fixed-wing aircraft An aircraft is a vehicle that is able to flight, fly by gaining sup ...

aerial photograph
of the Gorner Glacier (left side of image) together with the
Grenzgletscher The Gorner Glacier (german: Gornergletscher) is a valley glacier A glacier ( or ) 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#Glaciology ...

Grenzgletscher
(r.) flowing into it, both framing the
Monte Rosa : , other_name = Monte Rosa massif , translation = Mount Glacier , photo = Dufourspitze (Monte Rosa) and Monte Rosa Glacier as seen from Gornergrat, Wallis, Switzerland, 2012 August.jpg , photo_caption = Central ...

Monte Rosa
massif in the middle Packrafting at Spencer Glacier. Chugach National Forest, Alaska.jpg , A
packraft Packraft and trail boat are colloquial terms for a small, portable inflatable boat An inflatable boat is a lightweight boat constructed with its sides and bow made of flexible tubes containing pressurised gas. For smaller boats, the floor and hull ...

packraft
er passes a wall of freshly exposed blue ice on Spencer Glacier, in Alaska. Glacial ice acts like a filter on light, and the more time light spends traveling through the ice, the bluer it becomes. 153 - Glacier Perito Moreno - Grotte glaciaire - Janvier 2010.jpg , A
glacier cave Ice formations in the Titlis glacier cave A glacier cave is a cave formed within the ice of a glacier A glacier ( or ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulat ...
located on the
Perito Moreno Glacier The Perito Moreno Glacier () is a glacier located in the Los Glaciares National Park in southwest Santa Cruz Province, Argentina. It is one of the most important tourist attractions in the Argentinian Patagonia. The ice formation, in length, is ...

Perito Moreno Glacier
in Argentina.


Structure

A glacier originates at a location called its glacier head and terminates at its glacier foot, snout, or
terminus Terminus may refer to: Places *Terminus, the unofficial original name of Atlanta, Georgia, United States **Terminus (office complex), an office complex in Atlanta *Lagos Terminus railway station, the main railway station of Lagos, Nigeria Art, en ...
. 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 Accumulation may refer to: Finance * Accumulation function, a mathematical function defined in terms of the ratio future value to present value * Capital accumulation Capital accumulation (also termed the accumulation of capital) is the dynamic ...
. 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 cirque (; from the Latin word ''circus'') is an amphitheatre An amphitheatre ( British English) or amphitheater ( American English; both ) is an open-air venue used for entertainment, performances, and sports. The term derives from the anc ...
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 A glacier (; ) is a persistent body of dense ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less ...

glacier mass balance
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 cooling that occurred after the Medieval Warm Period The Medieval Warm Period (MWP) also known as the Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate Climate is the ...
'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 Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
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 A glacier ( or ) 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#Glaciology, ablation over many years, ...
. 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. 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 In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

pressure
. 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 in Tangra Mountains, Antarctica A crevasse is a deep crack, Fracture (geology), crevice or fissure found in an ice sheet or glacier, or earth. Crevasses form as a result of the movement and resulting stress associated with the shear ...

crevasse
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 Sheep shearing is the process by which the woollen fleece of a sheep Sheep (''Ovis aries'') are quadruped The zebr ...
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 whe ...

serac
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 File:Bergschrunds am Mont Dolent.JPG, alt=Rocky peaks protruding from undulating ice masses, Open bergschrunds at Mont Dolent A bergschrund (from the German language, German for ''mountain cleft'') or rimaye (from French language, French; ) is a ...
. 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 A snow bridge is an arc formed by snow across a crevasse Image:Glaciereaston.jpg, 200px, Crevasse in Easton Glacier, Mount Baker, in the North Cascades, Washington (state), Washington A crevasse is a deep crack, or fracture, found in an ice shee ...
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. TitlisIceCracks.jpg, Ice cracks in the Titlis Glacier. Glaciereaston.jpg, Crossing a
crevasse A crevasse in Tangra Mountains, Antarctica A crevasse is a deep crack, Fracture (geology), crevice or fissure found in an ice sheet or glacier, or earth. Crevasses form as a result of the movement and resulting stress associated with the shear ...

crevasse
on the Easton Glacier, Mount Baker, in the North Cascades, United States. 20171012-FS-Tongass-AD-001 (44831586714).jpg, An exposed glacier tube that once transported water down the interior of the glacier.


Speed

The speed of glacial displacement is partly determined by friction. 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. 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 Surge (glacier), 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 — 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 earthquakes 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 (landform), swale 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 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 ( es, Cordillera de los Andes) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of Sout ...

Andes
, the
Himalayas The Himalayas, or Himalaya (; Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Ind ...

Himalayas
, the Rocky Mountains, the Caucasus Mountains, Caucasus, Scandinavian mountains, and the
Alps The Alps ; german: Alpen ; it, Alpi ; rm, Alps; sl, Alpe ) are the highest and most extensive mountain range A mountain range is a series of mountains ranged in a line and connected by high ground. A mountain system or mountain belt i ...

Alps
. Snezhnika glacier in Pirin Mountain, Bulgaria with a latitude 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 was present in the last glacial period. In New Guinea, small, rapidly diminishing, glaciers are located on Puncak Jaya. Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya, and in the Rwenzori Mountains. 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 latitudes 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 (geology), 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 glac ...

moraine
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 A mountain range is a series of mountains ranged in a line and connected by high ground. A mountain system or mountain belt i ...

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. 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 Physical geography (also known as physiography) is one of the two fields of geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the ...

fjord
. 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 has 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 (geology), rock which separates two valleys. It is typically formed when two glaciers erode parallel U-shaped valleys. Arêtes can also form when two glacial cirque (landform), cirques erode headwards towards ...
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. 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 The pictures below illustrate how landscape features on Mars closely resemble those on the Earth. Wikielephantglacier.jpg, Romer Lake's Elephant Foot Glacier in the Earth's Arctic, as seen by Landsat 8. This picture shows several glaciers that have the same shape as many features on Mars that are believed to also be glaciers. The next three images from Mars show shapes similar to the Elephant Foot Glacier. File:Glacier as seen by ctx.JPG, Mesa in Ismenius Lacus quadrangle, as seen by CTX. Mesa has several glaciers eroding it. One of the glaciers is seen in greater detail in the next two images from HiRISE. Image from Ismenius Lacus quadrangle. File:Wide view of glacier showing image field.JPG, Glacier as seen by HiRISE under the HiWish program. Area in the rectangle is enlarged in the next photo. Zone of accumulation of snow at the top. Glacier is moving down valley, then spreading out on plain. Evidence for flow comes from the many lines on surface. Location is in Protonilus Mensae in Ismenius Lacus quadrangle. File:Glacier close up with hirise.JPG, Enlargement of area in rectangle of the previous image. On Earth, the ridge would be called the terminal moraine of an alpine glacier. Picture taken with HiRISE under the HiWish program. Image from Ismenius Lacus quadrangle.


See also

* * * * *


Notes


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

* 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