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Transcript

Process Involved

Explanation

1.

  • There are rocks beneath the Earth’s surface (e.g. intrusive igneous rocks)
  • And other rocks lying over them on the Earth’s surface.
  • The underlying rocks help support the weight of the overlying column of rock.
  • Therefore, the underlying rocks are subjected to high pressure.

3.Eventually, after continuous erosion, the underlying rocks are exposed to the surface, causing slabs of rocks to break off along fractures.

2.

  • When the overlying rocks are eroded over time, there is a decrease in pressure, known as ‘unloading’, on the underlying rocks.
  • These underlying rocks respond to this change in pressure by expanding upwards.
  • This results in the formation of pressure release fractures (cracks) that are parallel to the surface. (Also called SHEET JOINTS).

SHEET JOINTS

About the Diagram

  • Confining pressure = Weight of all overlying rocks
  • Similar to the pressure on your eardrums under a deep pool (=weight of water above you)
  • 'Confining' as it is forceful in ALL directions

Pressure Release

VS Exfoliation

1. Pressure release:

  • When underlying rocks expand after the overlying rock materials have been eroded.

2. Exfoliation:

  • Follows pressure, where the outer layer of rock peels off.

Definition

Factors affecting Pressure Release

  • Form of MECHANICAL/PHYSICAL weathering of rocks
  • Rocks undergoing mechanical/physical weathering are broken down into SMALLER FRAGMENTS
  • No change in MINERAL and CHEMICAL composition in rocks

1. Rock Structure :

  • Distribution,
  • Size
  • Quantity of pressure release fractures – The BIGGER and the GREATER the number of fractures, the FASTER the rate of weathering and formation of exfoliation domes and sheets

2. Amount of Water:

  • EROSION takes place at a FASTER rate in the presence of a GREATER amount of water

3. Climate:

  • Weathering MORE prevalent in COLD, DRY regions

4. Gradient:

  • Weathering takes place FASTER on STEEPER slopes – Rock slides off EASILY

5. Vegetation:

  • PROMOTES weathering due to INCREASED WATER RETENTION

Weathering: PRESSURE RELEASE

Animations and Videos

Link to Animations: http://www.mhhe.com/earthsci/geology/plummer/student/olc/chap05animation.mhtml

Independence Rock: Wyoming

Other Examples of Exfoliation Domes

Stone Mountain, GA

Enchanted Rock, TX

Half-Dome, Yosemite

Example of Exfoliation Dome

Features of Pressure Release

Note the rounded surfaces and the bare rocks

  • Bare rock surfaces that may be more resistant than surrounding rocks known as exfoliation domes
  • Exfoliation sheets: Slabs of rocks that break off.
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