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.