Editing Glacier (section)

=== 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>