Plucking (glaciation)

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Zone of plucking in the formation of tarns and cirques.
Glacially-plucked granitic bedrock near Mariehamn, Åland Islands.

Plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the erosion and transportation of bedrock especially large joint blocks. As a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints in the bedrock. The freezing and thawing action of the ice causes cracks in the bedrock through hydraulic wedging. Eventually these joint blocks come loose and become trapped in the glacier.

In this way, plucking has been linked to regelation.[1] Rocks of all sizes can become trapped in the bottom of the glacier. Joint blocks up to three meters have been "plucked" and transported.[2] These entrained rock fragments can also cause abrasion along the subsequent bedrock and walls. Plucking also leads to chatter marks, wedge shaped indentations left on the bedrock or other rock surfaces.[3] Glacial plucking both exploits pre-existing fractures in the bedrock and requires continued fracturing to maintain the cycle of erosion.[4] Glacial Plucking is most significant where the rock surface is well jointed, fractured, or contains exposed bed planes as this allows meltwater and clasts to penetrate more easily.[2]

Plucking mechanisms[edit]

Glacial plucking is largely dependent on the amount of stress exerted on a clast overlain by glacial ice. This relationship is a balance between the sheer stress exerted on the clast and the normal pressure on the clast by a body of ice. Plucking is increased where there are preexisting fractures in a rock bed. As the glacier slides down a mountain, energy from friction, pressure or geothermal heat causes glacial meltwater to infiltrate the spaces between rocks.[4] This process, known as frost wedging, puts stress on the rock structure as water expands when it freezes. Impacts from large clasts carried in the bedload can cause additional stress to the bedrock.[5] Additionally, plucking can be seen as a positive feedback system in which the increased action of rock removed from the landscape entrained in the glacier causes larger scale fracturing further down the glacier because of a heavier load of force pushing down on the rock bed.[4]

Mechanical erosion[edit]

Glacial plucking is the main mechanism of other small scale mechanical glacial erosion such as striation, abrasion and glacial polishing. The heavier the sediment load, the more extreme the erosion of the downhill landscape. Erosion is largely dependent on the amount of water flow and its velocity, the clast size and hardness with relation to the stability of the slope.[4]

Glacial striation[edit]

A rock that has been subject to glacial erosion will often show a striation pattern in which the rock appears scratched. Long parallel lines will cover the rock and show the appearance of something having been dragged along the top of it. Although striations can form on any sort of rock, they are usually present on more stable bedrock such as quartzite or granite where erosion processes are more readily preserved.[6] Striations, because of their nature of erosion, can also tell geologists the path and movement of the glacier.

Polishing[edit]

Glacial polishing is the result of clasts embedded in glacial ice passing over bedrock and grinding down the top of the rock into a smoother surface. The small rocks entrained by plucking act like sandpaper to the downhill slope.[7] This creates an almost mirror like surface in the rock. Polish indicates a more recent process as it is often lost to weathering of the rock surface.

Glacial till[edit]

The joint blocks and rock fragments that are entrained and carried down the slope can be deposited as till. This leads to a whole set of depositional glacial landforms such as moraines, roche moutonnée, glacial erratics and drumlin fields.

References[edit]

  1. ^ Rothlisberger, Hans, and Almut Iken. "Plucking As an Effect of Water-Pressure Variations at the Glacier Bed." Annals of Glaciology 2.1 (1981): 57-62. Web. 5 Oct. 2013.
  2. ^ a b Whipple, Kelin; Hankock G; Anderson R.S. (2000). "River incision into bedrock: Mechanics and relative efficacy of plucking, abrasion, and cavitation". Geological Society of America Bulletin 112 (3): 490–503. doi:10.1130/0016-7606(2000)112<490:riibma>2.0.co;2. 
  3. ^ Essentials of Geology, 3rd ed., Stephen Marshak
  4. ^ a b c d Harbor, Jonathan (2011). Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 332–340. ISBN 978-90-481-2641-5. 
  5. ^ Bierman, Paul R (2013). Geomorphology. W H Freeman & Co. pp. 189–191. ISBN 978-1429238601. 
  6. ^ McCalla, Carolle. "Glacial Striations and Slickensides". Utah Geological Survey. Retrieved 9 October 2013. 
  7. ^ Nelson, Stephen A. "Glaciers and Glaciation". Tulane University. Retrieved 9 October 2013.