Editing Glacier (section)

== 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>