Editing Glacier (section)

=== 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.