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{{Short description|Persistent body of ice that moves downhill under its own weight}}
{{About|the geological formation}}
{{Redirect|Ice river|the Chinese ski course|Ice River (ski course)}}
{{Use American English|date=September 2024}}{{Use dmy dates|date=January 2025}}
[[File:Geikie Plateau Glacier.JPG|thumb|325x325px|Glacier of the Geikie Plateau in [[Greenland]]]]
[[File:Wildspitze seen from Hinterer Brunnkogel, with visible ascent track of ski mountaineer.jpg|thumb|325px|The Taschachferner in the [[Ötztal Alps]] in [[Austria]]. The mountain to the left is the [[Wildspitze]] (3.768 m), second highest in Austria]]
[[File: Baltoro glacier from air.jpg|thumb|325px|With 7,253 known glaciers, [[Pakistan]] contains more glaciers than any other country on earth outside the polar regions.<ref name="Craig" /> At {{convert|62|km|mi|0}} in length, the pictured [[Baltoro Glacier]] is the [[List of glaciers#List of longest glaciers on Earth in non-polar regions|fifth longest alpine glacier]] in the world.]]
A '''glacier''' ({{IPAc-en|US|pron|ˈ|ɡ|l|eɪ|ʃ|ər}}; {{IPAc-en|UK|pron|ˈ|ɡ|l|æ|s|i|ə}} or {{IPAc-en|pron|'|g|l|eɪ|s|i|ə}}) is a persistent body of dense ice, a form of rock,<ref name=usgs_rock>{{Cite web |title=Is glacier ice a type of rock? |date=26 April 2019 |publisher=[[United States Geological Survey]] |access-date=2025-02-06 |url=https://www.usgs.gov/faqs/glacier-ice-a-type-rock}}</ref> that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its [[Ablation#Glaciology|ablation]] over many years, often centuries. It acquires distinguishing features, such as [[crevasse]]s and [[serac]]s, 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]]s, [[moraine]]s, or [[fjord]]s. Although a glacier may flow into a body of water, it forms only on land<ref name=oed>“Glacier, N., Pronunciation.” Oxford English Dictionary, [[Oxford University Press|Oxford UP]], June 2024, https://doi.org/10.1093/OED/7553486115. Accessed 25 Jan. 2025.</ref><ref name=nps>{{cite web|title=Glacier Features - Mount Rainier National Park (U.S. National Park Service)|website=National Park Service|url=https://www.nps.gov/mora/learn/nature/glacier-features.htm?fullweb=1|access-date=18 January 2025}}</ref><ref name=dkp>{{cite book|title=The Ultimate Family Visual Dictionary|publisher=[[DK (publisher)|DK Pub]]|chapter=Geology, Geography, and Meteorology|year=2012|pages=286–287|language=en|isbn=978-0-1434-1954-9}}</ref> and is distinct from the much thinner [[sea ice]] and lake ice that form on the surface of bodies of water.

On Earth, 99% of glacial ice is contained within vast [[ice sheet]]s (also known as "continental glaciers") in the [[polar region]]s, but glaciers may be found in [[mountain range]]s on every continent other than the Australian mainland, including Oceania's high-latitude [[oceanic island]] countries such as [[New Zealand]]. Between latitudes 35°N and 35°S, glaciers occur only in the [[Himalayas]], [[Andes]], and a few high mountains in East Africa, Mexico, [[New Guinea]] and on [[Zard-Kuh]] in Iran.<ref name="Post 2000">{{cite book|last1=Post|first1=Austin|last2=LaChapelle|first2=Edward R|title=Glacier ice|publisher=University of Washington Press|year=2000|location=Seattle|isbn=978-0-295-97910-6}}</ref> With more than 7,000 known glaciers, [[Pakistan]] has more glacial ice than any other country outside the polar regions.<ref>{{Cite news|last=Staff |date=June 9, 2020 |title=Millions at risk as melting Pakistan glaciers raise flood fears |url=https://www.aljazeera.com/news/2020/06/millions-risk-melting-pakistan-glaciers-raise-flood-fears-200609033202702.html |access-date=2020-06-09 |work=[[Al Jazeera Media Network|Al Jazeera]]}}</ref><ref name="Craig">{{Cite news |last=Craig |first=Tim |date=2016-08-12 |title=Pakistan has more glaciers than almost anywhere on Earth. But they are at risk. |language=en-US |newspaper=[[The Washington Post]] |url=https://www.washingtonpost.com/world/asia_pacific/pakistan-has-more-glaciers-than-almost-anywhere-on-earth-but-they-are-at-risk/2016/08/11/7a6b4cd4-4882-11e6-8dac-0c6e4accc5b1_story.html |access-date=2020-09-04 |issn=0190-8286 |quote=With 7,253 known glaciers, including 543 in the Chitral Valley, there is more glacial ice in Pakistan than anywhere on Earth outside the polar regions, according to various studies.}}</ref> Glaciers cover about 10% of Earth's land surface. Continental glaciers cover nearly {{convert|5|e6sqmi|e6km2|order=flip|abbr=unit}} or about 98% of [[Antarctica]]'s {{convert|5.1|e6sqmi|e6km2|order=flip|abbr=unit|sigfig=3}}, with an average thickness of ice {{convert|7,000|ft|m|order=flip|abbr=on}}. Greenland and [[Patagonia]] also have huge expanses of continental glaciers.<ref>National Geographic Almanac of Geography, 2005, {{ISBN|0-7922-3877-X}}, p. 149.</ref> The volume of glaciers, not including the ice sheets of Antarctica and Greenland, has been estimated at 170,000&nbsp;km<sup>3</sup>.<ref>{{cite web |title=170'000 km cube d'eau dans les glaciers du monde |url=http://www.arcinfo.ch/fr/monde/170-000-km-cube-d-eau-dans-les-glaciers-du-monde-577-1052031 |archive-url=http://archive.wikiwix.com/cache/20170817140736/http://www.arcinfo.ch/fr/monde/170-000-km-cube-d-eau-dans-les-glaciers-du-monde-577-1052031 |url-status=dead |archive-date=August 17, 2017 |work=[[ArcInfo (newspaper)|ArcInfo]] |date=Aug 6, 2015}}</ref>

Glacial ice is the largest reservoir of [[fresh water]] on Earth, holding with ice sheets about 69 percent of the world's freshwater.<ref>{{Cite web|title=Ice, Snow, and Glaciers and the Water Cycle|url=https://www.usgs.gov/special-topic/water-science-school/science/ice-snow-and-glaciers-and-water-cycle?qt-science_center_objects=0#qt-science_center_objects|access-date=2021-05-25|website=www.usgs.gov| date=8 September 2019 }}</ref><ref name=IMS>{{cite journal|author1=Brown, Molly Elizabeth |author2=Ouyang, Hua |author3=Habib, Shahid |author4=Shrestha, Basanta |author5=Shrestha, Mandira |author6=Panday, Prajjwal |author7=Tzortziou, Maria |author8=Policelli, Frederick |author9=Artan, Guleid |author10=Giriraj, Amarnath |author11=Bajracharya, Sagar R. |author12=Racoviteanu, Adina |title=HIMALA: Climate Impacts on Glaciers, Snow, and Hydrology in the Himalayan Region|journal=Mountain Research and Development|date=November 2010 |volume=30 |issue=4 |pages=401–404 |publisher=International Mountain Society|doi=10.1659/MRD-JOURNAL-D-10-00071.1 |hdl=2060/20110015312 |s2cid=129545865 |doi-access=free |bibcode=2010MRDev..30..401B |hdl-access=free }}</ref> Many glaciers from [[Temperate climate|temperate]], [[Alpine climate|alpine]] and seasonal [[polar climate]]s store water as ice during the colder seasons and release it later in the form of [[meltwater]] as warmer summer temperatures cause the glacier to melt, creating a [[Water resources|water source]] 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 (meteorology)|precipitation]], [[temperature|mean temperature]], and [[cloud cover]], [[Retreat of glaciers since 1850|glacial mass changes]] are considered among the most sensitive indicators of [[climate change]] and are a major source of variations in [[Current sea level rise|sea level]].

A large piece of compressed ice, or a glacier, [[Blue ice (glacial)|appears blue]], as large quantities of [[Color of water|water appear blue]], 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 Terminology ==
The word ''glacier'' is a [[loanword]] from French and goes back, via [[Franco-Provençal language|Franco-Provençal]], to the [[Vulgar Latin]] ''{{lang|la|glaciārium}}'', derived from the [[Late Latin]] ''{{lang|la|glacia}}'', and ultimately [[Latin]] ''{{lang|la|glaciēs}}'', meaning "ice".<ref>{{cite book |last=Simpson |first=D.P. |url=https://archive.org/details/cassellslatindic00simp |title=Cassell's Latin Dictionary |publisher=Cassell Ltd. |year=1979 |isbn=978-0-304-52257-6 |edition=5 |location=London |page=883}}</ref><ref name=oed2>“Glacier, N., Etymology.” Oxford English Dictionary, [[Oxford University Press|Oxford UP]], June 2024, https://doi.org/10.1093/OED/8661871366. Accessed 25 Jan. 2025.</ref> 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 [[glacial period|glaciation]]. The corresponding area of study is called [[glaciology]]. Glaciers are important components of the global [[cryosphere]].

== Types ==

=== Classification by size, shape and behavior ===
{{Further|Glacier morphology}}
[[File:Quelccaya Glacier.jpg|left|thumb|The [[Quelccaya Ice Cap]] in Peru is the second-largest glaciated area in the tropics]]
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''.<ref>{{cite web |url=https://pubs.usgs.gov/of/2004/1216/text.html |title=Glossary of Glacier Terminology |publisher=USGS |access-date=2017-03-13}}</ref> A large body of glacial ice astride a mountain, mountain range, or [[volcano]] is termed an ''[[ice cap]]'' or ''[[ice field]]''.<ref>{{cite web |url=http://www.nichols.edu/departments/glacier/juneau%20icefield.htm |title=Retreat of Alaskan glacier Juneau icefield |publisher=Nichols.edu |access-date=2009-01-05 |archive-date=2017-10-23 |archive-url=https://web.archive.org/web/20171023193102/http://www.nichols.edu/departments/glacier/juneau%20icefield.htm |url-status=dead }}</ref> Ice caps have an area less than {{convert|50,000|km2|sqmi|abbr=on}} by definition.

Glacial bodies larger than {{convert|50,000|km2|sqmi|abbr=on}} are called ''[[ice sheet]]s'' or ''continental glaciers''.<ref>{{cite web |url=http://amsglossary.allenpress.com/glossary/search?id=ice-sheet1 |publisher=American Meteorological Society |title=Glossary of Meteorology |access-date=2013-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20120623093132/http://amsglossary.allenpress.com/glossary/search?id=ice-sheet1 |archive-date=2012-06-23}}</ref> Several kilometers deep, they obscure the underlying topography. Only [[nunatak]]s protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland.<ref>{{Cite web |url=https://www4.uwsp.edu/geo/faculty/lemke/geol370/activities/02_Morphological_Classification_of_Glaciers.pdf |archive-url=https://web.archive.org/web/20170812133941/https://www4.uwsp.edu/geo/faculty/lemke/geol370/activities/02_Morphological_Classification_of_Glaciers.pdf |archive-date=2017-08-12 |url-status=live |title=Morphological Classification of Glaciers |author=[[University of Wisconsin]], Department of Geography and Geology |date=2015 |website=www.uwsp.edu/Pages/default.aspx}}</ref> They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over {{convert|70|m|ft|abbr=on}}.<ref>{{cite web |url=http://pubs.usgs.gov/fs/fs2-00/ |title=Sea Level and Climate |work=USGS FS 002-00 |publisher=[[USGS]] |date=2000-01-31 |access-date=2009-01-05}}</ref> Portions of an ice sheet or cap that extend into water are called [[ice shelves]]; they tend to be thin with limited slopes and reduced velocities.<ref name="NSIDC">{{cite web|publisher=[[National Snow and Ice Data Center]] |website=nsidc.org |title=Types of Glaciers |url=http://www.nsidc.org/glaciers/questions/types.html |url-status=dead |archive-url=https://web.archive.org/web/20100417222017/http://nsidc.org/glaciers/questions/types.html |archive-date=2010-04-17}}</ref> Narrow, fast-moving sections of an ice sheet are called ''[[ice streams]]''.<ref>{{Cite journal |last1=Bindschadler |first1=R.A. |first2=T.A. |last2=Scambos |s2cid=17336434 |title=Satellite-image-derived velocity field of an Antarctic ice stream |journal=[[Science (journal)|Science]] |volume=252 |issue=5003 |pages=242–46 |year=1991 |doi=10.1126/science.252.5003.242 |pmid=17769268|bibcode=1991Sci...252..242B}}</ref><ref name=BAS2009>{{cite web |title=Description of Ice Streams |url=http://www.antarctica.ac.uk/about_antarctica/geography/ice/streams.php |publisher=[[British Antarctic Survey]] |access-date=2009-01-26 |url-status=dead |archive-url=https://web.archive.org/web/20090211004629/http://www.antarctica.ac.uk/about_antarctica/geography/ice/streams.php |archive-date=2009-02-11}}</ref> In Antarctica, many ice streams drain into large [[ice shelf|ice shelves]]. Some drain directly into the sea, often with an [[ice tongue]], like [[Mertz Glacier]].

''[[Tidewater glacier cycle|Tidewater glaciers]]'' are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, [[Baffin Island|Baffin]], [[Devon Island|Devon]], and [[Ellesmere Island]]s in Canada, [[Southeast Alaska]], and the [[Northern Patagonian Ice Field|Northern]] and [[Southern Patagonian Ice Field]]s. As the ice reaches the sea, pieces break off or calve, forming [[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 [[tidewater glacier cycle|cycles of advance and retreat]] that are much less affected by climate change than other glaciers.<ref>{{Cite web |url=https://nsidc.org/cryosphere/glaciers/questions/types.html |title=What types of glaciers are there? |publisher=[[National Snow and Ice Data Center]] |website=nsidc.org |access-date=2017-08-12}}</ref>

=== Classification by thermal state ===
[[File:Ellesmere Island 06.jpg|thumb| Webber Glacier on [[Grant Land]] is an advancing polar glacier]]
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.<ref name="ColdBased">{{cite book|title=Encyclopedia of Snow, Ice and Glaciers |first1=Reginald D. |last1=Lorrain|first2=Sean J. |last2=Fitzsimons |editor1-first=Vijay P. |editor1-last=Singh |editor2-first=Pratap |editor2-last=Singh |editor3-first=Umesh K. |editor3-last=Haritashya |publisher=Springer Netherlands |pages=157–161 |doi=10.1007/978-90-481-2642-2_72 |chapter=Cold-Based Glaciers |series=Encyclopedia of Earth Sciences Series |date=2011 |isbn=978-90-481-2641-5}}</ref> This contrast is thought to a large extent to govern the ability of a glacier to effectively [[Glacial erosion|erode its bed]], as sliding ice promotes [[Plucking (glaciation)|plucking]] at rock from the surface below.<ref>[[Geoffrey Boulton|Boulton, G.S.]] [1974] "Processes and patterns of glacial erosion", (In Coates, D.R. ed., ''Glacial Geomorphology''. A Proceedings Volume of the Fifth Annual Geomorphology Symposia Series, held at Binghamton, New York, September 26–28, 1974. Binghamton, NY, State University of New York, pp. 41–87. (Publications in Geomorphology))</ref> Glaciers which are partly cold-based and partly warm-based are known as ''polythermal''.<ref name="ColdBased" />

== Formation ==
[[File:153 - Glacier Perito Moreno - Grotte glaciaire - Janvier 2010.jpg|left|thumb|A [[glacier cave]] located on the [[Perito Moreno Glacier]] in Argentina]]
Glaciers form where the [[Glacier ice accumulation|accumulation]] of snow and ice exceeds [[ablation]]. A glacier usually originates from a [[cirque]] landform (alternatively known as a corrie or as a {{Lang|cy|cwm|italic=no}}) – a typically armchair-shaped geological feature (such as a depression between mountains enclosed by [[arête]]s) – which collects and compresses through gravity the snow that falls into it. This snow accumulates and refreezes, turning into [[névé]] (granular snow). Further crushing of the individual snowflakes and expelling the air from the snow turns it into [[firn]] and eventually "glacial ice". This glacial ice will fill the cirque until it "overflows" through a geological weakness or vacancy from the edge of the cirque called the "lip" or threshold. When the mass of snow and ice reaches sufficient thickness, it begins to move by a combination of surface slope, gravity, and pressure.<ref name=nps/><ref name=dkp/><ref name=nv>{{cite web | url=http://people.hofstra.edu/j_b_bennington/33notes/glaciers.html | title=Geol 33 Environmental Geomorphology | publisher=Hofstra University | access-date=25 January 2025 | archive-url=https://web.archive.org/web/20160303204533/http://people.hofstra.edu/j_b_bennington/33notes/glaciers.html | archive-date=2016-03-03 | url-status=dead }}</ref><ref name=vdb>{{cite journal |last1=van den Broeke |first1=Michiel |title=Depth and Density of the Antarctic Firn Layer |journal=Arctic, Antarctic, and Alpine Research |date=1 May 2008 |volume=40 |issue=2 |pages=432–438 |doi=10.1657/1523-0430(07-021)[BROEKE]2.0.CO;2 |s2cid=198156588 |url=https://www.tandfonline.com/doi/full/10.1657/1523-0430%2807-021%29%5BBROEKE%5D2.0.CO%3B2 |issn=1523-0430}}</ref> On steeper slopes, this can occur with as little as {{Convert|15|m|ft|abbr=on}} of snow-ice.

In temperate glaciers, snow repeatedly freezes and thaws, changing into granular ice or [[névé]]. Under the pressure of the layers of ice and snow above it, this granular snow fuses into denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice.<ref name=nv/><ref name=vdb/>{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|pp=260–262}} 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]] of the infrared [[Infrared spectroscopy|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]] of bubbles in the ice.)<ref>{{cite web|url=http://webexhibits.org/causesofcolor/5C.html |title=What causes the blue color that sometimes appears in snow and ice? |publisher=Webexhibits.org |access-date=2013-01-04}}</ref>

== Structure ==
[[File:Ellesmere Island 05.jpg|thumb|The overhanging icefront of the advancing Webber Glacier with waterfalls (Borup Fiord area, Northern Ellesmere Island) on July 20, 1978. Debris rich layers have been sheared and folded into the basal cold glacier ice. The glacier front is 6 km broad and up to 40 m high]]
A glacier originates at a location called its glacier head and terminates at its glacier foot, snout, or [[Glacier terminus|terminus]].

Glaciers are broken into zones based on surface snowpack and melt conditions.<ref>Benson, C.S., 1961, "Stratigraphic studies in the snow and firn of the Greenland Ice Sheet", ''Res. Rep. 70'', U.S. Army Snow, Ice and Permafrost Res Establ., Corps of Eng., 120 pp.</ref> 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]]. 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]]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 segregation|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&nbsp;°C.

The health of a glacier is usually assessed by determining the [[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]]'s end around 1850, [[Retreat of glaciers since 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.<ref>{{cite web |url=http://www.grid.unep.ch/activities/global_change/switzerland.php |title=Glacier change and related hazards in Switzerland |publisher=UNEP |access-date=2009-01-05 |archive-url=https://web.archive.org/web/20120925064555/http://www.grid.unep.ch/activities/global_change/switzerland.php |archive-date=2012-09-25 |url-status=dead}}</ref><ref>{{cite journal |url=http://folk.uio.no/kaeaeb/publications/grl04_paul.pdf |archive-url=https://web.archive.org/web/20070604183847/http://folk.uio.no/kaeaeb/publications/grl04_paul.pdf |archive-date=2007-06-04 |url-status=live |title=Rapid disintegration of Alpine glaciers observed with satellite data|doi=10.1029/2004GL020816 |year=2004 |bibcode=2004GeoRL..3121402P |volume=31 |issue=21 |pages=L21402 |journal=[[Geophysical Research Letters]] |last1=Paul |first1=Frank |last2=Kääb |first2=Andreas |last3=Maisch |first3=Max |last4=Kellenberger |first4=Tobias |last5=Haeberli |first5=Wilfried |doi-access=free}}</ref><ref>{{cite web |url=http://www.nichols.edu/departments/Glacier/glacier_retreat.htm |title=Recent Global Glacier Retreat Overview |format=PDF |access-date=2013-01-04}}</ref>

== Motion ==
{{Redirect|Ice flow|floating ice|Ice floe}}
[[Image:Stress-strain1.svg|thumb|upright=1.2|The stress–strain relationship of plastic flow (green section): a small increase in stress creates an exponentially greater increase in strain, which equates to deformation speed.]]
Glaciers move downhill by the force of [[gravity]] and the internal deformation of ice.<ref name="GreveBlatter2009">{{cite book|author1=Greve, R.|author2=Blatter, H. |s2cid=128734526 |year=2009|title=Dynamics of Ice Sheets and Glaciers|publisher=Springer|doi=10.1007/978-3-642-03415-2|isbn=978-3-642-03414-5}}</ref> At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers. When the amount of strain (deformation) is proportional to the stress being applied, ice will act as an elastic solid. Ice needs to be at least {{cvt|30|m|ft}} thick to even start flowing, but once its thickness exceeds about {{cvt|50|m|ft}}, stress on the layer above will exceeds the inter-layer binding strength, and then it will move faster than the layer below.<ref>W.S.B. Paterson, Physics of ice</ref> This means that small amounts of stress can result in a large amount of strain, causing the deformation to become a [[Plasticity (physics)|plastic flow]] rather than elastic. Then, the glacier will begin to deform under its own weight and flow across the landscape. According to the [[Glen–Nye flow law]], the relationship between stress and strain, and thus the rate of internal flow, can be modeled as follows:<ref name="Easterbrook">Easterbrook, Don J., Surface Processes and Landforms, 2nd Edition, Prentice-Hall Inc., 1999{{page needed|date=February 2014}}</ref><ref name="GreveBlatter2009" />

:<math>
\Sigma = k \tau^n,\,
</math>

where: 
:<math>\Sigma\,</math> = shear strain (flow) rate
:<math>\tau\,</math> = stress
:<math>n\,</math> = a constant between 2–4 (typically 3 for most glaciers)
:<math>k\,</math> = a temperature-dependent constant

[[File:Geirangerfjord (6-2007).jpg|thumb|upright|Differential erosion enhances relief, as clear in this incredibly steep-sided Norwegian [[fjord]].]]

The lowest velocities are near the base of the glacier and along valley sides where friction acts against flow, causing the most deformation. Velocity increases inward toward the center line and upward, as the amount of deformation decreases. The highest flow velocities are found at the surface, representing the sum of the velocities of all the layers below.<ref name="Easterbrook" /><ref name="GreveBlatter2009" />

Because ice can flow faster where it is thicker, the rate of glacier-induced erosion is directly proportional to the thickness of overlying ice.  Consequently, pre-glacial low hollows will be deepened and pre-existing topography will be amplified by glacial action, while [[nunatak]]s, which protrude above ice sheets, barely erode at all – erosion has been estimated as 5&nbsp;m per 1.2&nbsp;million years.<ref name=ngeo2008>{{cite journal | author =  Kessler, Mark A.| year =  2008| doi = 10.1038/ngeo201| title = Fjord insertion into continental margins driven by topographic steering of ice| journal = Nature Geoscience | volume =  1 | pages =  365 | last2 =  Anderson | first2 =  Robert S. | last3 =  Briner | first3 =  Jason P. | issue=6 | bibcode=2008NatGe...1..365K}} Non-technical summary: {{cite journal | author = Kleman, John | year = 2008 | doi = 10.1038/ngeo210 | title = Geomorphology: Where glaciers cut deep | journal = Nature Geoscience | volume = 1 | pages = 343 | issue=6|bibcode = 2008NatGe...1..343K }}</ref> This explains, for example, the deep profile of [[fjord]]s, which can reach a kilometer in depth as ice is topographically steered into them.  The extension of fjords inland increases the rate of ice sheet thinning since they are the principal conduits for draining ice sheets. It also makes the ice sheets more sensitive to changes in climate and the ocean.<ref name=ngeo2008/>

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 David Forbes|James Forbes]] came up with the essentially correct explanation in the 1840s, although it was several decades before it was fully accepted.<ref>{{cite journal| title= A short history of scientific investigations on glaciers|year= 1987 |volume=Special issue |issue= S1 |journal=Journal of Glaciology|pages= 4–5|author=Clarke, Garry K.C.|bibcode= 1987JGlac..33S...4C |doi= 10.3189/S0022143000215785 |doi-access= free }}</ref>

=== Fracture zone and cracks ===
[[File:TitlisIceCracksDeep.jpg|left|thumb|Ice cracks in the [[Titlis]] Glacier]]

The top {{convert|50|m|abbr=on}} of a glacier are rigid because they are under low [[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]]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 (geology)|shear]] forces cause them to break apart, opening a crevasse. Crevasses are seldom more than {{convert|150|ft|m|order=flip|abbr=on}} deep but, in some cases, can be at least {{convert|1000|ft|m|order=flip|abbr=on}} 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]]s.
[[File: Chevron Crevasses 00.JPG|thumb|Shear or herring-bone [[crevasse]]s on [[Emmons Glacier]] ([[Mount Rainier]]); such crevasses often form near the edge of a glacier where interactions with underlying or marginal rock impede flow. In this case, the impediment appears to be some distance from the near margin of the glacier.]]
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]]. 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]]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 [[Moulin (geomorphology)|moulins]]. Streams within or beneath a glacier flow in englacial or sub-glacial tunnels. These tunnels sometimes reemerge at the glacier's surface.<ref>{{cite news |url=http://www.nasa.gov/vision/earth/lookingatearth/moulin-20061211.html |title=Moulin 'Blanc': NASA Expedition Probes Deep Within a Greenland Glacier |publisher=[[NASA]] |date=2006-12-11 |access-date=2009-01-05 |archive-date=2012-11-04 |archive-url=https://web.archive.org/web/20121104182135/http://www.nasa.gov/vision/earth/lookingatearth/moulin-20061211.html |url-status=dead }}</ref>

===Subglacial processes===
[[File:Davies 2018 glacier sediment erosion rates.png|thumb|Erosion rates of subglacial sediment caused by the motion of different glaciers across the world <ref name="Davies2018">{{Cite journal |last1=Davies |first1=Damon |last2=Bingham |first2=Robert G. |last3=King |first3=Edward C. |last4=Smith |first4=Andrew M. |last5=Brisbourne |first5=Alex M. |last6=Spagnolo |first6=Matteo |last7=Graham |first7=Alastair G. C. |last8=Hogg |first8=Anna E. |last9=Vaughan |first9=David G. |date=4 May 2018 |title=How dynamic are ice-stream beds? |journal=The Cryosphere |volume=12 |issue=5 |pages=1615–1628 |doi=10.5194/tc-12-1615-2018 |doi-access=free |bibcode=2018TCry...12.1615D |hdl=2164/10495 |hdl-access=free }}</ref>]]

Most of the important processes controlling glacial motion occur in the ice-bed contact—even though it is only a few meters thick.<ref name=Clarke2005>{{cite journal | author = Clarke, G. K. C. | title = Subglacial processes | journal = Annual Review of Earth and Planetary Sciences | volume = 33 | issue = 1 | pages = 247–276 | year = 2005 | doi = 10.1146/annurev.earth.33.092203.122621| bibcode = 2005AREPS..33..247C }}</ref> The bed's temperature, roughness and softness define basal shear stress, which in turn defines whether movement of the glacier will be accommodated by motion in the sediments, or if it will be able to slide. A soft bed, with high porosity and low pore fluid pressure, allows the glacier to move by sediment sliding: the base of the glacier may even remain frozen to the bed, where the underlying sediment slips underneath it like a tube of toothpaste.  A hard bed cannot deform in this way; therefore the only way for hard-based glaciers to move is by basal sliding, where meltwater forms between the ice and the bed itself.<ref name=Boulton2006>{{cite book |doi=10.1002/9780470750636.ch2 |chapter=Glaciers and their Coupling with Hydraulic and Sedimentary Processes |title=Glacier Science and Environmental Change |year=2006 |last1=Boulton |first1=Geoffrey S. |pages=2–22 |isbn=978-0-470-75063-6 }}</ref> Whether a bed is hard or soft depends on the porosity and pore pressure; higher porosity decreases the sediment strength (thus increases the shear stress τ<sub>B</sub>).<ref name=Clarke2005/>

Porosity may vary through a range of methods.  
*Movement of the overlying glacier may cause the bed to undergo [[wikt:dilatancy|dilatancy]]; the resulting shape change reorganizes blocks.  This reorganizes closely packed blocks (a little like neatly folded, tightly packed clothes in a suitcase) into a messy jumble (just as clothes never fit back in when thrown in <!--this sentence only makes sense if the word "in" is repeated--> in a disordered fashion).  This increases the porosity.  Unless water is added, this will necessarily reduce the pore pressure (as the pore fluids have more space to occupy).<ref name=Clarke2005/>
*Pressure may cause compaction and consolidation of underlying sediments.<ref name=Clarke2005/> Since water is relatively incompressible, this is easier when the pore space is filled with vapor; any water must be removed to permit compression. In soils, this is an irreversible process.<ref name=Clarke2005/>
*Sediment degradation by abrasion and fracture decreases the size of particles, which tends to decrease pore space. However, the motion of the particles may disorder the sediment, with the opposite effect. These processes also generate heat.<ref name=Clarke2005/>

Bed softness may vary in space or time, and changes dramatically from glacier to glacier. An important factor is the underlying  geology; glacial speeds tend to differ more when they change bedrock than when the gradient changes.<ref name=Boulton2006/> Further, bed roughness can also act to slow glacial motion. The roughness of the bed is a measure of how many boulders and obstacles protrude into the overlying ice.  Ice flows around these obstacles by melting under the high pressure on their [[stoss (geography)|stoss side]]; the resultant meltwater is then forced into the cavity arising in their [[lee side]], where it re-freezes.<ref name=Clarke2005/>

As well as affecting the sediment stress, fluid pressure (p<sub>w</sub>) can affect the friction between the glacier and the bed. High fluid pressure provides a buoyancy force upwards on the glacier, reducing the friction at its base.  The fluid pressure is compared to the ice overburden pressure, p<sub>i</sub>, given by ρgh.  Under fast-flowing ice streams, these two pressures will be approximately equal, with an effective pressure (p<sub>i</sub> – p<sub>w</sub>) of 30&nbsp;kPa; i.e. all of the weight of the ice is supported by the underlying water, and the glacier is afloat.<ref name=Clarke2005/>

====Basal melting and sliding ====
[[File:Glacier cross-section.jpg|thumb|upright|A cross-section through a glacier. The base of the glacier is more transparent as a result of melting.]]
Glaciers may also move by [[basal sliding]], where the base of the glacier is [[lubrication|lubricated]] by the presence of liquid water, reducing basal [[shear stress]] and allowing the glacier to slide over the terrain on which it sits. [[Meltwater]] may be produced by pressure-induced melting, friction or [[geothermal heat]]. The more variable the amount of melting at surface of the glacier, the faster the ice will flow. Basal sliding is dominant in temperate or warm-based glaciers.<ref name="Schoof2010">{{Cite journal | last1 = Schoof | first1 = C. | title = Ice-sheet acceleration driven by melt supply variability | journal = Nature | volume = 468 | pages = 803–806 | year = 2010 | pmid = 21150994 | doi = 10.1038/nature09618|bibcode = 2010Natur.468..803S | issue=7325| s2cid = 4353234 }}</ref>

:&tau;<sub>D</sub> = &rho;gh&nbsp;sin&nbsp;&alpha;
:where &tau;<sub>D</sub> is the driving stress, and &alpha; the ice surface slope in radians.<ref name=Clarke2005/>

:&tau;<sub>B</sub> is the basal shear stress, a function of bed temperature and softness.<ref name=Clarke2005/>

:&tau;<sub>F</sub>, the shear stress, is the lower of &tau;<sub>B</sub> and &tau;<sub>D</sub>.  It controls the rate of plastic flow.

The presence of basal meltwater depends on both bed temperature and other factors. For instance, the melting point of water decreases under pressure, meaning that water melts at a lower temperature under thicker glaciers.<ref name=Clarke2005/> This acts as a "double whammy", because thicker glaciers have a lower heat conductance, meaning that the basal temperature is also likely to be higher.<ref name=Boulton2006/>  Bed temperature tends to vary in a cyclic fashion. A cool bed has a high strength, reducing the speed of the glacier.  This increases the rate of accumulation, since newly fallen snow is not transported away.  Consequently, the glacier thickens, with three consequences: firstly, the bed is better insulated, allowing greater retention of geothermal heat.<ref name=Clarke2005/>

Secondly, the increased pressure can facilitate melting.  Most importantly, τ<sub>D</sub> is increased.  These factors will combine to accelerate the glacier. As friction increases with the square of velocity, faster motion will greatly increase frictional heating, with ensuing melting – which causes a positive feedback, increasing ice speed to a faster flow rate still: west Antarctic glaciers are known to reach velocities of up to a kilometer per year.<ref name=Clarke2005/>
Eventually, the ice will be surging fast enough that it begins to thin, as accumulation cannot keep up with the transport.  This thinning will increase the conductive heat loss, slowing the glacier and causing freezing.  This freezing will slow the glacier further, often until it is stationary, whence the cycle can begin again.<ref name=Boulton2006/>
[[File:Lake_Vostok_drill_2011.jpg|thumb|Location and diagram of [[Lake Vostok]], a prominent subglacial lake beneath the East Antarctic Ice Sheet.]] 
The flow of water under the glacial surface can have a large effect on the motion of the glacier itself.  Subglacial lakes contain significant amounts of water, which can move fast: cubic kilometers can be transported between lakes over the course of a couple of years.<ref name=Fricker2007>{{cite journal| first1 = A.| last3 = Bindschadler| first2 = T.| last2 = Scambos| first3 = R.| first4 = L. | title = An Active Subglacial Water System in West Antarctica Mapped from Space| last1 = Fricker | journal = Science| last4 = Padman | volume = 315 | issue = 5818 | pages = 1544–1548 | date=Mar 2007 | issn = 0036-8075| pmid = 17303716 | doi = 10.1126/science.1136897| bibcode = 2007Sci...315.1544F| s2cid = 35995169}}</ref> This motion is thought to occur in two main modes: ''pipe flow'' involves liquid water moving through pipe-like conduits, like a sub-glacial river; ''sheet flow'' involves motion of water in a thin layer. A switch between the two flow conditions may be associated with surging behavior. Indeed, the loss of sub-glacial water supply has been linked with the shut-down of ice movement in the Kamb ice stream.<ref name=Fricker2007/> The subglacial motion of water is expressed in the surface topography of ice sheets, which slump down into vacated subglacial lakes.<ref name=Fricker2007/>

=== Speed ===
[[File:Wendleder 2024 Baltoro supraglacial acceleration.jpg|thumb|The formation of supraglacial lakes at Baltoro Glacier in April 2018 (top) had substantially accelerated its melting and motion in the following summer months (bottom)<ref name="Wendleder2024">{{Cite journal |last1=Wendleder |first1=Anna |last2=Bramboeck |first2=Jasmin |last3=Izzard |first3=Jamie |last4=Erbertseder |first4=Thilo |last5=d'Angelo |first5=Pablo |last6=Schmitt |first6=Andreas |last7=Quincey |first7=Duncan J. |last8=Mayer |first8=Christoph |last9=Braun |first9=Matthias H. |date=5 March 2024 |title=Velocity variations and hydrological drainage at Baltoro Glacier, Pakistan |journal=The Cryosphere |volume=18 |issue=3 |pages=1085–1103 |doi=10.5194/tc-18-1085-2024 |doi-access=free |bibcode=2024TCry...18.1085W }}</ref>]]
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 {{convert|1|m|ft|0|abbr=on}} per day.<ref>{{cite web |url=http://www.geo.hunter.cuny.edu/tbw/ncc/Notes/chap3.landforms/erosion.deposition/glaciers.htm |title=Glaciers |website=www.geo.hunter.cuny.edu |access-date=2014-02-06 |archive-url=https://web.archive.org/web/20140222172708/http://www.geo.hunter.cuny.edu/tbw/NCC/Notes/chap3.landforms/erosion.deposition/glaciers.htm |archive-date=2014-02-22 |url-status=dead}}</ref> 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 {{convert|20|–|30|m|ft|-1|abbr=on}} per day, such as in Greenland's [[Jacobshavn Glacier|Jacobshavn 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.<ref>[http://earth.esa.int/pub/ESA_DOC/gothenburg/154stroz.pdf T. Strozzi et al.: ''The Evolution of a Glacier Surge Observed with the ERS Satellites''] {{Webarchive|url=https://web.archive.org/web/20141111175824/http://earth.esa.int/pub/ESA_DOC/gothenburg/154stroz.pdf |date=2014-11-11 }} (pdf, 1.3 Mb)</ref> These surges may be caused by the failure of the underlying bedrock, the pooling of meltwater at the base of the glacier<ref>{{cite web|url=http://www.hi.is/~oi/bruarjokull_project.htm |title=The Brúarjökull Project: Sedimentary environments of a surging glacier. The Brúarjökull Project research idea|publisher=Hi.is |access-date=2013-01-04}}</ref>&nbsp;— perhaps delivered from a [[supraglacial lake]]&nbsp;— or the simple accumulation of mass beyond a critical "tipping point".<ref>Meier & Post (1969)</ref> Temporary rates up to {{convert|300|ft|m|-1|order=flip|abbr=on}} 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]]s occur. These are large scale earthquakes that have seismic magnitudes as high as 6.1.<ref name="people.deas.harvard.edu">[http://people.deas.harvard.edu/~vtsai/files/EkstromNettlesTsai_Science2006.pdf "Seasonality and Increasing Frequency of Greenland Glacial Earthquakes"] {{webarchive|url=https://web.archive.org/web/20081007062935/http://people.deas.harvard.edu/~vtsai/files/EkstromNettlesTsai_Science2006.pdf |date=2008-10-07 }}, Ekström, G., M. Nettles, and V.C. Tsai (2006) ''Science'', 311, 5768, 1756–1758, {{doi|10.1126/science.1122112}}</ref><ref name="TsaiEkstrom_JGR2007 2007">[http://people.deas.harvard.edu/~vtsai/files/TsaiEkstrom_JGR2007.pdf "Analysis of Glacial Earthquakes"] {{webarchive|url=https://web.archive.org/web/20081007050046/http://people.deas.harvard.edu/~vtsai/files/TsaiEkstrom_JGR2007.pdf |date=2008-10-07 }} Tsai, V. C. and G. Ekström (2007). J. Geophys. Res., 112, F03S22, {{doi|10.1029/2006JF000596}}</ref> The number of [[glacial earthquake]]s 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.<ref name="TsaiEkstrom_JGR2007 2007"/>

=== Ogives ===
[[File:Forbes Bands on Mer de Glace in France.jpg|thumb|Forbes bands on the [[Mer de Glace]] glacier in France]]
Ogives or Forbes bands<ref>{{cite book|last=Summerfield |first=Michael A. |title=Global Geomorphology |year=1991 |page=269}}</ref> 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.<ref>{{cite book |last=Easterbrook |first=D.J. |title=Surface Processes and Landforms |publisher=[[Prentice-Hall]], Inc. |year=1999 |edition=2 |location=New Jersey |page=546 |isbn=978-0-13-860958-0}}</ref> Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives.<ref>{{cite web|url=http://pubs.usgs.gov/of/2004/1216/no/no.html |title=Glossary of Glacier Terminology |publisher=Pubs.usgs.gov |date=2012-06-20 |access-date=2013-01-04}}</ref>

== Geography ==
{{Further|topic=this topic|List of glaciers}}
[[File:Fox-Gletscher1.jpg|alt=|thumb|[[Fox Glacier]] in New Zealand finishes near a rainforest]]
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, Pakistan,<ref>{{Cite web |date=2023-07-11 |title=10 Countries With The Most Glaciers |url=https://www.dailyo.in/visualstories/webstories/10-countries-with-the-most-glaciers-48276-11-07-2023 |access-date=2024-07-03 |website=www.dailyo.in |language=en}}</ref> Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in the [[Andes]], the [[Himalayas]], the [[Rocky Mountains]], the [[Caucasus Mountains|Caucasus]], [[Scandinavian Mountains]], and the [[Alps]]. [[Snezhnika]] glacier in [[Pirin]] Mountain, [[Bulgaria]] with a [[latitude]] of 41°46′09″ N is the southernmost glacial mass in Europe.<ref name="grunewald-129">Grunewald, p. 129.</ref> Mainland Australia currently contains no glaciers, although a small glacier on [[Mount Kosciuszko]] was present in the [[last glacial period]].<ref>{{cite web|url=http://www.ga.gov.au/education/facts/landforms/auslform.htm |title=C.D. Ollier: ''Australian Landforms and their History'', National Mapping Fab, Geoscience Australia |publisher=Ga.gov.au |date=2010-11-18 |access-date=2013-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20080808081441/https://www.ga.gov.au/education/facts/landforms/auslform.htm |archive-date=2008-08-08 }}</ref> In New Guinea, small, rapidly diminishing, glaciers are located on [[Puncak Jaya]].<ref>{{cite conference |first=Joni L. |last=Kincaid |author2=Klein, Andrew G. |url=http://www.easternsnow.org/proceedings/2004/kincaid_and_klein.pdf |title=Retreat of the Irian Jaya Glaciers from 2000 to 2002 as Measured from IKONOS Satellite Images |location=Portland, Maine, USA |pages=147–157 |year=2004 |access-date=2009-01-05 |conference= |archive-date=2017-05-17 |archive-url=https://web.archive.org/web/20170517095529/http://www.easternsnow.org/proceedings/2004/kincaid_and_klein.pdf |url-status=dead}}</ref> 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]]<ref>{{cite web|url=http://geology.com/press-release/hawiian-glaciers/ |title=Hawaiian Glaciers Reveal Clues to Global Climate Change |publisher=Geology.com |date=2007-01-26 |access-date=2013-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20130127143044/http://geology.com/press-release/hawiian-glaciers/ |archive-date=2013-01-27}}</ref> and [[Tenerife]] also had large alpine glaciers, while the [[Faroe Islands|Faroe]] and [[Crozet Islands]]<ref>{{cite web|url=http://www.discoverfrance.net/Colonies/Crozet.shtml |title=French Colonies – Crozet Archipelago |publisher=Discoverfrance.net |date=2010-12-09 |access-date=2013-01-04}}</ref> 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]]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 line]]s reach above {{convert|6500|m|ft|-1|abbr=on}}. Between 19˚N and 19˚S, however, precipitation is higher, and the mountains above {{convert|5000|m|ft|-1|abbr=on}} usually have permanent snow.
[[File:Black-Glacier.jpg|thumb|Black ice glacier near [[Aconcagua]], Argentina]]
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 desert]]s 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]],<ref>{{cite book|last=Collins |first=Henry Hill |title=Europe and the USSR |page=263 |oclc=1573476}}</ref> and [[Alaska Interior|central]] and [[northern Alaska]],<ref>{{cite web |url=http://www.beringia.com/centre_info/exhibit.html |title=Yukon Beringia Interpretive Center |publisher=Beringia.com |date=1999-04-12 |access-date=2013-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20121031054552/http://www.beringia.com/centre_info/exhibit.html |archive-date=2012-10-31}}</ref> though extraordinarily cold, had such light snowfall that glaciers could not form.<ref>{{Cite web|url=http://www.eas.slu.edu/People/KChauff/earth_history/4EH-posted.pdf|title=Earth History 2001 |date=July 28, 2017 |page=15 |archive-url=https://web.archive.org/web/20160303183327/http://www.eas.slu.edu/People/KChauff/earth_history/4EH-posted.pdf |archive-date=March 3, 2016 |access-date=July 28, 2017}}</ref><ref>{{cite web |url=http://www.wku.edu/~smithch/biogeog/SCHM1946.htm |title=On the Zoogeography of the Holarctic Region |publisher=Wku.edu |access-date=2013-01-04 |archive-date=15 October 2009 |archive-url=https://web.archive.org/web/20091015114943/http://www.wku.edu/~smithch/biogeog/SCHM1946.htm |url-status=dead }}</ref>

In addition to the dry, unglaciated polar regions, some mountains and volcanoes in Bolivia, Chile and Argentina are high ({{convert|4500|to|6900|m|ft|-2|abbr=on|disp=or}}) 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 ==

=== Erosion ===
[[File:Arranque glaciar-en.svg|thumb|Diagram of glacial plucking and [[Abrasion (geology)|abrasion]]]]
Glaciers erode terrain through two principal processes: [[Plucking (glaciation)|plucking]] and [[abrasion (geology)|abrasion]].{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|pp=263–264}}

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.{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|p=263}} 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{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|p=263}} 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&nbsp;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 striation]]s. 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 mark]]s, 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.[[File:PluckedGraniteAlandIslands.JPG|thumb|right|Glacially plucked granitic bedrock near [[Mariehamn]], [[Åland]]]]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.<ref>{{Cite journal |last1=Dühnforth |first1=Miriam |last2=Anderson |first2=Robert S. |last3=Ward |first3=Dylan |last4=Stock |first4=Greg M. |date=2010-05-01 |title=Bedrock fracture control of glacial erosion processes and rates |journal=[[Geology (journal)|Geology]] |language=en |volume=38 |issue=5 |pages=423–426 |doi=10.1130/G30576.1 |issn=0091-7613 |bibcode=2010Geo....38..423D}}</ref> 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.<ref>{{Cite journal |last1=Koppes |first1=Michéle |last2=Hallet |first2=Bernard |last3=Rignot |first3=Eric |last4=Mouginot |first4=Jérémie |last5=Wellner |first5=Julia Smith |last6=Boldt |first6=Katherine |title=Observed latitudinal variations in erosion as a function of glacier dynamics |journal=[[Nature (journal)|Nature]] |volume=526 |issue=7571 |pages=100–103 |doi=10.1038/nature15385 |pmid=26432248 |bibcode=2015Natur.526..100K |year=2015 |s2cid=4461215}}</ref>

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 erratic]]s". 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 ===
[[File:MorainesLakeLouise.JPG|thumb|Glacial moraines above [[Lake Louise (Alberta)|Lake Louise]], Alberta, Canada|left]]
Glacial [[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 a glacier meets its tributary glacier and merge, and the lateral moraines of each coalesce to form a moraine in the middle of the combined glacier. Less apparent are [[drift (geology)|ground moraines]], also called ''glacial drift'', which often blankets the surface underneath the glacier downslope from the equilibrium line.<ref name="NeuendorfOthers2005a">Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. {{ISBN|0-922152-76-4}}</ref><ref name=dkp/> 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]]. 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 ===
[[File:Drumlins_around_Horicon_Marsh_in_Wisconsin.jpg|thumb|Drumlins around [[Horicon Marsh]], Wisconsin, in an area with one of the highest concentration of drumlins in the world. The curved path of the [[Laurentide Ice Sheet]] is evident in the orientation of the various mounds.]]
[[Drumlin]]s are asymmetrical, canoe-shaped hills made mainly of glacial sediments.<ref name=dkp/> Their heights vary from 15 to 50&nbsp;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 field]]s'' 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.

=== {{anchor|Glacial valleys}}Glacial valleys, cirques, arêtes, and pyramidal peaks ===
[[File:Glacial landscape.svg|thumb|Features of a glacial landscape|left]]

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.<ref>{{cite web |url=https://nsidc.org/cryosphere/glaciers/gallery/troughs.html |title=Glacial Landforms: Trough |publisher=[[National Snow and Ice Data Center]] |website=nsidc.org}}</ref> 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 lake]]s. If a glacial valley runs into a large body of water, it forms a [[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 valley]]s.<ref name=dkp/>

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 called the "lip". 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]] is left. This structure may result in a [[mountain pass]]. If multiple cirques encircle a single mountain, they create pointed [[pyramidal peak]]s; particularly steep examples are called [[Glacial horn|horns]].<ref name=dkp/>

=== 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]],''{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|pp=271}} 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.<ref>{{Cite book|title=Glaciers & Glaciation |publisher=Arnold |location=London |date=1998 |first1=Douglas |last1=Benn |first2=David |last2=Evans |pages=324–326}}</ref> 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&nbsp;m to 13&nbsp;km, with depths of up to 45&nbsp;meters. Most are circular in shape because the blocks of ice that formed them were rounded as they melted.<ref name="britannica">{{cite web |title=Kettle geology |publisher=Britannica Online |url=https://www.britannica.com/EBchecked/topic/315739/kettle |access-date=2009-03-12}}</ref>

=== Glacial deposits ===
[[File:Receding glacier-en.svg|thumb|Landscape produced by a receding glacier|300x300px]]
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 ''[[kame]]s''. 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 ''[[esker]]s''. 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,<ref name=dkp/> with heights exceeding 100&nbsp;meters and lengths of as long as 100&nbsp;km.

=== Loess deposits ===
Very fine glacial sediments or rock flour{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|p=264}} 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 wind]]s can be important in this process.

== Retreat of glaciers due to climate change ==
{{further|Retreat of glaciers since 1850}}
{{multiple image | total_width=650
| image1 = Glacer_Retreat_Photo.png| caption1=[[South Cascade Glacier]] in Washington photographs from 1928 to 2003 showing the recent rapid glacier retreating
| image2 = 2000- Glacier mass change - global.svg |caption2= Globally, the mass of glaciers has persistently declined (blue line), with the rate of decline increasing in recent years (red bars).<ref name=Nature_20250219>{{cite journal |author1=The GlaMBIE Team |title=Community estimate of global glacier mass changes from 2000 to 2023 |journal=Nature |date=19 February 2025 |doi=10.1038/s41586-024-08545-z}} Fig. 2.</ref>
| image3 = 2015-2100 Impacts of global warming on glaciers and sea level rise.svg| caption3= Based on current national pledges, global warming is projected to cause loss of ~half of Earth's glaciers by 2100 and raise sea level by ~115 mm (~4.5 in.)<ref name=Science_20230105>{{cite journal |last1=Rounce |first1=David R. |last2=Hock |first2=Regine |last3=Maussion |first3=Fabien |last4=Hugonnet |first4=Romain |last5=Kochtitzky |first5=William |last6=Huss |first6=Matthias |last7=Berthier |first7=Etienne |last8=Brinkerhoff |first8=Douglas |last9=Compagno |first9=Loris |last10=Copland |first10=Luke |last11=Farinotti |first11=Daniel |last12=Menounos |first12=Brian |last13=McNabb |first13=Robert W. |display-authors=4 |title=Global glacier change in the 21st century: Every increase in temperature matters |journal=Science |date=5 January 2023 |volume=379 |issue=6627 |pages=78–83 |doi=10.1126/science.abo1324 |pmid=36603094 |bibcode=2023Sci...379...78R |s2cid=255441012 |url=https://www.science.org/doi/10.1126/science.abo1324|hdl=10852/108771 |hdl-access=free }}</ref> (not counting rise from melting [[ice sheet]]s).
}}
Glaciers, which can be hundreds of thousands of years old, are used to track climate change over long periods of time.<ref name=NOAAiceCores_20230128/> Researchers melt or crush samples from glacier [[ice core]]s whose progressively deep layers represent respectively earlier times in Earth's climate history.<ref name=NOAAiceCores_20230128/> The researchers apply various instruments to the content of bubbles trapped in the cores' layers in order to track changes in the atmosphere's composition.<ref name=NOAAiceCores_20230128/> Temperatures are deduced from differing relative concentrations of respective gases, confirming that for at least the last million years, global temperatures have been linked to [[carbon dioxide]] concentrations.<ref name=NOAAiceCores_20230128>{{cite web |last1=Dusto |first1=Amy |title=Climate at the core: how scientists study ice cores to reveal Earth's climate history |url=https://www.climate.gov/news-features/climate-tech/climate-core-how-scientists-study-ice-cores-reveal-earths-climate |website=climate.gov |publisher=National Oceanographic and Atmospheric Administration (NOAA) |archive-url=https://web.archive.org/web/20230128231031/https://www.climate.gov/news-features/climate-tech/climate-core-how-scientists-study-ice-cores-reveal-earths-climate |archive-date=28 January 2023 |date=28 January 2023 |url-status=live }} Reviewed by Erich Osterberg and David Anderson. Applied instruments include [[Mass spectrometry|mass spectrometers]], [[scanning electron microscope]]s, and [[Gas chromatography|gas chromatographs]].</ref>

Human activities in the industrial era have increased the concentration of carbon dioxide and other heat-trapping [[greenhouse gas]]es in the air, causing current [[Climate change|global warming]].<ref name=NASAcauses_2019/> Human influence is the principal driver of [[Effects of climate change|changes]] to the [[cryosphere]] of which glaciers are a part.<ref name=NASAcauses_2019>{{cite web | title=The Causes of Climate Change  | url=https://climate.nasa.gov/causes/ | website=climate.nasa.gov | publisher=NASA | archive-url=https://web.archive.org/web/20191221010331/https://climate.nasa.gov/causes/ | archive-date=2019-12-21 | url-status=live | date=2019}}</ref>

[[File:Vatnajökull glacier.jpg|350px|thumb|left|[[Jökulsárlón|Ice lagoon Jökulsárlón]] at the foot of the [[Vatnajökull|Vatnajökull Glacier]], [[Iceland]], 2023]]
Global warming creates [[positive feedback|positive feedback loops]] with glaciers.<ref name=NatureComms_20201027/> For example, in [[ice–albedo feedback]], rising temperatures increase glacier melt, exposing more of earth's land and sea surface (which is darker than glacier ice), allowing sunlight to warm the surface rather than being reflected back into space.<ref name=NatureComms_20201027>{{Cite journal |last1=Wunderling |first1=Nico |last2=Willeit |first2=Matteo |last3=Donges |first3=Jonathan F. |last4=Winkelmann |first4=Ricarda |date=27 October 2020 |title=Global warming due to loss of large ice masses and Arctic summer sea ice |journal=Nature Communications |language=en |volume=11 |issue=1 |page=5177 |doi=10.1038/s41467-020-18934-3 |pmid=33110092 |pmc=7591863 |bibcode=2020NatCo..11.5177W }} Source mentions ice-albedo and melt-elevation feedbacks.</ref> Reference glaciers tracked by the [[World Glacier Monitoring Service]] have lost ice every year since 1988.<ref name=WGMS_thru2022>{{cite web |title=Global Glacier State |url=https://wgms.ch/global-glacier-state/ |publisher=World Glacier Monitoring Service ("under the auspices of: ISC (WDS), IUGG (IACS), UN environment, UNESCO, WMO") |archive-url=https://web.archive.org/web/20230129051518/https://wgms.ch/global-glacier-state/ |archive-date=29 January 2023 |date=January 2023 |url-status=live }} See [[:File:1950- Glacier annual mass change - World Glacier Monitoring Service.svg|chart on Wikimedia]].</ref> A study that investigated the period 1995 to 2022 showed that the flow velocity of glaciers in the Alps accelerates and slows down to a similar extent at the same time, despite large distances. This clearly shows that their speed is controlled by the climate change.<ref>{{Cite journal |last1=Kellerer-Pirklbauer |first1=Andreas |last2=Bodin |first2=Xavier |last3=Delaloye |first3=Reynald |last4=Lambiel |first4=Christophe |last5=Gärtner-Roer |first5=Isabelle |last6=Bonnefoy-Demongeot |first6=Mylène |last7=Carturan |first7=Luca |last8=Damm |first8=Bodo |last9=Eulenstein |first9=Julia |last10=Fischer |first10=Andrea |last11=Hartl |first11=Lea |last12=Ikeda |first12=Atsushi |last13=Kaufmann |first13=Viktor |last14=Krainer |first14=Karl |last15=Matsuoka |first15=Norikazu |date=2024-03-01 |title=Acceleration and interannual variability of creep rates in mountain permafrost landforms (rock glacier velocities) in the European Alps in 1995–2022 |url=https://iopscience.iop.org/article/10.1088/1748-9326/ad25a4 |journal=Environmental Research Letters |volume=19 |issue=3 |pages=034022 |doi=10.1088/1748-9326/ad25a4 |bibcode=2024ERL....19c4022K |issn=1748-9326}}</ref>

Water runoff from melting glaciers causes global sea level to [[sea level rise|rise]], a phenomenon the [[IPCC]] terms a "slow onset" event.<ref name="IPCC AR6 WGII Summary for Policymakers 20"/> Impacts at least partially attributable to sea level rise include for example encroachment on coastal settlements and infrastructure, existential threats to small islands and low-lying coasts, losses of coastal ecosystems and ecosystem services, groundwater salinization, and compounding damage from tropical cyclones, flooding, storm surges, and land subsidence.<ref name="IPCC AR6 WGII Summary for Policymakers 20">{{cite web |title=IPCC AR6 WGII Summary for Policymakers |url=https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_SummaryForPolicymakers.pdf |website=ipcc.ch |publisher=Intergovernmental Panel on Climate Change (IPCC) |archive-url=https://web.archive.org/web/20230122220619/https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_SummaryForPolicymakers.pdf |archive-date=22 January 2023 |date=2022 |url-status=live}}</ref>

== Isostatic rebound ==
[[File:Glacier weight effects LMB.png|Isostatic pressure by a glacier on the Earth's crust|left|thumb|300x300px]]
Large masses, such as ice sheets or glaciers, can depress the crust of the Earth into the mantle.<ref>{{Cite book|url=https://books.google.com/books?id=2UYrRG2ocHoC&q=Large+masses,+such+as+ice+sheets+and+glaciers,+can+depress+the+crust+of+the+Earth+into+the+mantle.&pg=PT95 |title=Global Warming Cycles: Ice Ages and Glacial Retreat |last=Casper |first=Julie Kerr |date=2010 |publisher=Infobase Publishing |isbn=978-0-8160-7262-0 |language=en |via=[[Google Books]]}}</ref> 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 other planets ==
{{main|Glaciers on Mars|Sputnik Planitia}}
[[File:Wide_view_of_glacier_showing_image_field.JPG|thumb|[[Protonilus Mensae]], [[Ismenius Lacus quadrangle]], Mars]]
The polar ice caps of [[Mars]] show geologic evidence of glacial deposits. The south polar cap is especially comparable to glaciers on Earth.<ref>{{cite web|url=http://www.lpi.usra.edu/meetings/polar2003/pdf/8112.pdf |archive-url=https://web.archive.org/web/20080227091117/http://www.lpi.usra.edu/meetings/polar2003/pdf/8112.pdf |archive-date=2008-02-27 |url-status=live |title=Kargel, J.S. et al.:''Martian Polar Ice Sheets and Mid-Latitude Debris-Rich Glaciers, and Terrestrial Analogs'', Third International Conference on Mars Polar Science and Exploration, Alberta, Canada, October 13–17, 2003 (pdf 970 Kb) |access-date=2013-01-04}}</ref> Topographical features and computer models indicate the existence of more glaciers in Mars' past.<ref>{{cite web|url=http://www.esa.int/SPECIALS/Mars_Express/SEMS3PMZCIE_0.html |title=Martian glaciers: did they originate from the atmosphere? ESA Mars Express, 20 January 2006 |publisher=Esa.int |date=2006-01-20 |access-date=2013-01-04}}</ref> 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 apron]]s (LDAs).<ref>Head, J. et al. 2005. Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars. Nature: 434. 346–350</ref><ref>Plaut, J. et al. 2008. Radar Evidence for Ice in Lobate Debris Aprons in the Mid-Northern Latitudes of Mars. Lunar and Planetary Science XXXIX. 2290.pdf</ref><ref>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</ref>

In 2015, as ''[[New Horizons]]'' flew by the [[Pluto]]-[[Charon (moon)|Charon]] system, the spacecraft discovered a massive basin covered in a layer of nitrogen ice on Pluto. A large portion of the basin's surface is divided into irregular polygonal features separated by narrow troughs, interpreted as convection cells fueled by internal heat from Pluto's interior.<ref name = "Pluto updates"/><ref name="McKinnon2016"/> Glacial flows were also observed near Sputnik Planitia's margins, appearing to flow both into and out of the basin.<ref name="Umurhan2016"/>

== See also ==
* {{annotated link|Glacial landform}}
* {{annotated link|Glacial motion}}
* {{annotated link|Glacier growing}}
* {{annotated link|Glacier morphology}}
* {{annotated link|Ice jam}}
{{Clear}}

== References ==
{{reflist|refs=
<ref name = "Pluto updates">{{cite web | url = http://www.planetary.org/blogs/emily-lakdawalla/2015/12211538-pluto-updates-from-agu.html
 | title = Pluto updates from AGU and DPS: Pretty pictures from a confusing world | last = Lakdawalla | first = Emily | author-link=Emily Lakdawalla | date = 21 December 2015 | publisher = [[The Planetary Society]] | access-date = 24 January 2016}}</ref>
<ref name="Umurhan2016">{{cite web
| url = https://blogs.nasa.gov/pluto/2016/01/08/probing-the-mysterious-glacial-flow-on-plutos-frozen-heart/
| title = Probing the Mysterious Glacial Flow on Pluto's Frozen 'Heart'
| last = Umurhan| first = O.| date = January 8, 2016
| website =blogs.nasa.gov| publisher =NASA| access-date = January 24, 2016}}</ref>
<ref name="McKinnon2016">{{cite journal|last1=McKinnon|first1=W. B.|display-authors=etal|title=Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigour|journal= Nature|volume=534|issue= 7605|date=1 June 2016|pages= 82–85|doi= 10.1038/nature18289|pmid=27251279|arxiv=1903.05571|bibcode=2016Natur.534...82M|s2cid=30903520}}</ref>
}}

=== Bibliography ===
{{refbegin|colwidth=30em}}
* {{Cite book |last=Huggett |first=Richard John |title=Fundamentals Of Geomorphology |edition=3rd |series=Routledge Fundamentals of Physical Geography Series |publisher=[[Routledge]] |date=2011 |isbn=978-0-203-86008-3}}
{{refend}}

=== General references ===
{{SPATRAcite|:es:Glaciar|24 July 2005}}
{{refbegin|colwidth=30em}}
* {{cite book
 | first1 = Michael
 | last1 = Hambrey
 | last2 = Alean
 | first2 = Jürg
 | title = Glaciers
 | url = https://archive.org/details/glaciers0002hamb
 | url-access = registration
 | edition = 2nd
 | publisher = Cambridge University Press
 | year = 2004
 | isbn = 978-0-521-82808-6
 | oclc = 54371738
}} A less-technical treatment of all aspects, with photographs and firsthand accounts of glaciologists' experiences. All images of this book can be found online (see Weblinks: Glaciers-online)
* {{cite book
 | first1 = Douglas I.
 | last1 = Benn
 | last2 = Evans
 | first2 = David J.A.
 | title = Glaciers and Glaciation
 | publisher = Arnold
 | year = 1999
 | isbn = 978-0-470-23651-2
 | oclc = 38329570
}}
* {{cite book
 | first1 = M.R.
 | last1 = Bennett
 | last2 = Glasser
 | first2 =  N.F.
 | title = Glacial Geology: Ice Sheets and Landforms
 | publisher = John Wiley & Sons
 | year = 1996
 | isbn = 978-0-471-96344-8
 | oclc = 33359888}}
* {{cite book
 | first = Michael
 | last = Hambrey
 | title = Glacial Environments
 | publisher = University of British Columbia Press, UCL Press
 | year = 1994
 | isbn = 978-0-7748-0510-0
 | oclc = 30512475
}} An undergraduate-level textbook.
* {{cite book |last=Knight |first=Peter G. |title=Glaciers |location=Cheltenham |publisher=Nelson Thornes |year=1999 |isbn=978-0-7487-4000-0 |oclc=42656957 |url-access=registration |url=https://archive.org/details/glaciers0000knig}} A textbook for undergraduates avoiding mathematical complexities
* {{cite book |last=Walley |first=Robert |title=Introduction to Physical Geography |publisher=Wm. C. Brown Publishers |year=1992}} A textbook devoted to explaining the geography of our planet.
* {{cite book |last=Paterson |first=W.S.B. |title=Physics of Glaciers |edition=3rd |publisher=[[Pergamon Press]] |year=1994 |isbn=978-0-08-013972-2 |oclc=26188 |author-link=Stan Paterson}} A comprehensive reference on the physical principles underlying formation and behavior.
{{refend}}

== Further reading ==
*  Gornitz, Vivien. ''Vanishing Ice: Glaciers, Ice Sheets, and Rising    Seas'' (Columbia University Press, 2019) [http://www.h-net.org/reviews/showrev.php?id=56939 online review]
* [[Twila Moon|Moon, Twila]]. [https://www.science.org/doi/full/10.1126/science.aam9625 Saying goodbye to glaciers], ''Science,'' 12 May 2017, Vol. 356, Issue 6338, pp.&nbsp;580–581, {{doi|10.1126/science.aam9625}}

== External links ==
{{Wikibooks |Historical Geology|Glaciers}}
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{{wiktionary}}
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* [https://physicalgeography.org/erosion-carried-out-by-glaciers-work-of-glaciers/ Erosion carried out by Glaciers (Work of Glaciers)]
* {{cite web|url=http://www.grid.unep.ch/glaciers/|title=Global Glacier Changes: Facts and Figures|publisher=[[United Nations Environment Programme]] (UNEP)|year=2008|access-date=2014-11-10|archive-url=https://web.archive.org/web/20181225130137/http://www.grid.unep.ch/glaciers/|archive-date=2018-12-25|url-status=dead}}, a report in the [[Global Environment Outlook]] (GEO) series.
* {{cite web|url=http://www.grid.unep.ch/glaciers/|title=Global Glacier Changes: Facts and Figures|publisher=[[United Nations Environment Programme]] (UNEP)|year=2008|access-date=2014-11-10|archive-url=https://web.archive.org/web/20181225130137/http://www.grid.unep.ch/glaciers/|archive-date=2018-12-25|url-status=dead}}, a report in the [[Global Environment Outlook]] (GEO) series.
* [http://danbbs.dk/~stst/glaciologi/ Glacial structures – photo atlas] {{Webarchive|url=https://web.archive.org/web/20121006041109/http://danbbs.dk/~stst/glaciologi/ |date=6 October 2012 }}
* [https://www.pbs.org/now/shows/516/index.html NOW on PBS "On Thin Ice"]
* [http://www.asiasociety.org/onthinnerice Photo project tracks changes in Himalayan glaciers since 1921]
* Short radio episode ''[https://web.archive.org/web/20040523194932/http://californialegacy.org/radio_anthology/scripts/muir.html California Glaciers]'' from ''The Mountains of California'' by John Muir, 1894. [[California Legacy Project]]
* [http://donlehmanjr.com/Mountain/08%20The%20Dynamics%20of%20Glaciers.htm Dynamics of Glaciers]
* [https://geo.badw.de/en/the-project.html Mountain glaciers and their role in the Earth system]
* [http://www.gletschervergleiche.ch/Pages/ImageCompare.aspx?Id=6 GletscherVergleiche.ch] – Before/After Images by Simon Oberli

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[[Category:Glaciology]]
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[[Category:Bodies of ice]]
[[Category:Montane ecology]]
[[Category:Water ice]]