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Even stable ice sheets are continually in motion as the ice gradually flows outward from the central plateau, which is the tallest point of the ice sheet, and towards the margins. The ice sheet slope is low around the plateau but increases steeply at the margins.<ref name="IPCC_AR6_AnnexVII" /> This difference in slope occurs due to an imbalance between high ice accumulation in the central plateau and lower accumulation, as well as higher [[ablation]], at the margins. This imbalance increases the [[shear stress]] on a glacier until it begins to flow. The flow velocity and deformation will increase as the equilibrium line between these two processes is approached.<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">{{cite book|author1=Greve, R. |author2=Blatter, H. |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}}{{pn|date=October 2021}}</ref> This motion is driven by [[gravity]] but is controlled by temperature and the strength of individual glacier bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on time scales ranging from hourly (i.e. tidal flows) to the [[wikt:centennial|centennial]] (Milankovich cycles).<ref name="GreveBlatter2009" /> |
Even stable ice sheets are continually in motion as the ice gradually flows outward from the central plateau, which is the tallest point of the ice sheet, and towards the margins. The ice sheet slope is low around the plateau but increases steeply at the margins.<ref name="IPCC_AR6_AnnexVII" /> This difference in slope occurs due to an imbalance between high ice accumulation in the central plateau and lower accumulation, as well as higher [[ablation]], at the margins. This imbalance increases the [[shear stress]] on a glacier until it begins to flow. The flow velocity and deformation will increase as the equilibrium line between these two processes is approached.<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">{{cite book|author1=Greve, R. |author2=Blatter, H. |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}}{{pn|date=October 2021}}</ref> This motion is driven by [[gravity]] but is controlled by temperature and the strength of individual glacier bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on time scales ranging from hourly (i.e. tidal flows) to the [[wikt:centennial|centennial]] (Milankovich cycles).<ref name="GreveBlatter2009" /> |
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On an hour-to-hour basis, surges of ice motion can be modulated by tidal activity. The influence of a 1 m tidal oscillation can be felt as much as 100 km from the sea.<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> During larger [[spring tide]]s, an ice stream will remain almost stationary for hours at a time, before a surge of around a foot in under an hour, just after the peak high tide; a stationary period then takes hold until another surge towards the middle or end of the falling tide.<ref name=Bindschalder2003>{{cite journal |last1=Bindschadler |first1=Robert A. |last2=King |first2=Matt A. |last3=Alley |first3=Richard B. |last4=Anandakrishnan |first4=Sridhar |last5=Padman |first5=Laurence |title=Tidally Controlled Stick-Slip Discharge of a West Antarctic Ice |journal=Science |date=22 August 2003 |volume=301 |issue=5636 |pages=1087–1089 |doi=10.1126/science.1087231 |pmid=12934005 |s2cid=37375591 |url=https://zenodo.org/record/1230832 }}</ref><ref name=Anandakrishnan2003>{{cite journal |last1=Anandakrishnan |first1=S. |last2=Voigt |first2=D. E. |last3=Alley |first3=R. B. |last4=King |first4=M. A. |title=Ice stream D flow speed is strongly modulated by the tide beneath the Ross Ice Shelf |journal=Geophysical Research Letters |date=April 2003 |volume=30 |issue=7 |page=1361 |doi=10.1029/2002GL016329 |bibcode=2003GeoRL..30.1361A |s2cid=53347069 |doi-access=free }}</ref> At neap tides, this interaction is less pronounced, and surges instead occur approximately every 12 hours.<ref name=Bindschalder2003/> |
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Increasing global air temperatures due to climate change take around 10,000 years to directly propagate through the ice before they influence bed temperatures, but may have an effect through increased surface melting, producing more [[supraglacial lake]]s. These lakes may feed warm water to glacial bases and facilitate glacial motion.<ref name=IPCCc4>Sections 4.5 and 4.6 of {{IPCC4/wg1/4}}</ref> Lakes of a diameter greater than ~300 m are capable of creating a fluid-filled crevasse to the glacier/bed interface. When these crevasses form, the entirety of the lake's (relatively warm) contents can reach the base of the glacier in as little as 2–18 hours – lubricating the bed and causing the glacier to [[surge (glacier)|surge]].<ref name=Krawczynski2007>{{cite conference |last1=Krawczynski |first1=M. J. |last2=Behn |first2=M. D. |last3=Das |first3=S. B. |last4=Joughin |first4=I. |title=Constraints on melt-water flux through the West Greenland ice-sheet: modeling of hydro- fracture drainage of supraglacial lakes |date=1 December 2007 |pages=C41B–0474 |bibcode=2007AGUFM.C41B0474K |url=http://www.agu.org/cgi-bin/wais?jj=C41B-0474 |archive-url = https://archive.today/20121228013531/http://www.agu.org/cgi-bin/wais?jj=C41B-0474 |url-status=dead |archive-date=2012-12-28 |access-date=2008-03-04 |book-title=Eos Trans. AGU |volume=88 |issue=52 }}</ref> Water that reaches the bed of a glacier may freeze there, increasing the thickness of the glacier by pushing it up from below.<ref name="Bell2011">{{Cite journal | last1 = Bell | first1 = R. E. | last2 = Ferraccioli | first2 = F. | last3 = Creyts | first3 = T. T. | last4 = Braaten | first4 = D. | last5 = Corr | first5 = H. | last6 = Das | first6 = I. | last7 = Damaske | first7 = D. | last8 = Frearson | first8 = N. | last9 = Jordan | first9 = T. | last10 = Rose | doi = 10.1126/science.1200109 | first10 = K. | last11 = Studinger | first11 = M. | last12 = Wolovick | first12 = M. | title = Widespread Persistent Thickening of the East Antarctic Ice Sheet by Freezing from the Base | journal = Science | volume = 331 | issue = 6024 | pages = 1592–1595 | year = 2011 | pmid = 21385719| bibcode = 2011Sci...331.1592B | s2cid = 45110037 }}</ref> |
Increasing global air temperatures due to climate change take around 10,000 years to directly propagate through the ice before they influence bed temperatures, but may have an effect through increased surface melting, producing more [[supraglacial lake]]s. These lakes may feed warm water to glacial bases and facilitate glacial motion.<ref name=IPCCc4>Sections 4.5 and 4.6 of {{IPCC4/wg1/4}}</ref> Lakes of a diameter greater than ~300 m are capable of creating a fluid-filled crevasse to the glacier/bed interface. When these crevasses form, the entirety of the lake's (relatively warm) contents can reach the base of the glacier in as little as 2–18 hours – lubricating the bed and causing the glacier to [[surge (glacier)|surge]].<ref name=Krawczynski2007>{{cite conference |last1=Krawczynski |first1=M. J. |last2=Behn |first2=M. D. |last3=Das |first3=S. B. |last4=Joughin |first4=I. |title=Constraints on melt-water flux through the West Greenland ice-sheet: modeling of hydro- fracture drainage of supraglacial lakes |date=1 December 2007 |pages=C41B–0474 |bibcode=2007AGUFM.C41B0474K |url=http://www.agu.org/cgi-bin/wais?jj=C41B-0474 |archive-url = https://archive.today/20121228013531/http://www.agu.org/cgi-bin/wais?jj=C41B-0474 |url-status=dead |archive-date=2012-12-28 |access-date=2008-03-04 |book-title=Eos Trans. AGU |volume=88 |issue=52 }}</ref> Water that reaches the bed of a glacier may freeze there, increasing the thickness of the glacier by pushing it up from below.<ref name="Bell2011">{{Cite journal | last1 = Bell | first1 = R. E. | last2 = Ferraccioli | first2 = F. | last3 = Creyts | first3 = T. T. | last4 = Braaten | first4 = D. | last5 = Corr | first5 = H. | last6 = Das | first6 = I. | last7 = Damaske | first7 = D. | last8 = Frearson | first8 = N. | last9 = Jordan | first9 = T. | last10 = Rose | doi = 10.1126/science.1200109 | first10 = K. | last11 = Studinger | first11 = M. | last12 = Wolovick | first12 = M. | title = Widespread Persistent Thickening of the East Antarctic Ice Sheet by Freezing from the Base | journal = Science | volume = 331 | issue = 6024 | pages = 1592–1595 | year = 2011 | pmid = 21385719| bibcode = 2011Sci...331.1592B | s2cid = 45110037 }}</ref> |
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