Deep weathering is the weathering of underground rocks by chemical means as water and humic acids percolate downwards.
Factors Affecting Deep Weathering:
Deep weathering is most common in the humid tropics because:
- Abundant vegetation provides humic acids as they decay.
- Abundant rainfall percolates through the soil and therefore increase soil moisture accelerating deep weathering.
- High temperatures speed up the rates of chemical reactions.
- Dense vegetation holds the weathered mantle in situ
- Occurrence of faulted rocks provides already made fractures through which acids can penetrate.
The deep weathered profile is more visible on igneous rock types such as granite that are weathered in boulders.
On soluble rocks such as limestone the deep weathered profile may not be sharply visible as these rock types are completely washed away in solution.
- Bedrock/ basal surface
- The bedrock is the fresh unweatherd rock lying deep down where water and acids fail to reach.
- Weathering front
- Is the boundary separating the unweathered bed rock from the weathered regolith.
- This layer represents the last stage of weathering and is composed mainly of angular boulder.
- The saprolite layer which is composed of big and small rounded corestones.
- Mobile layer
- The top layer is called the mobile layer and consists of sand and finely weathered clay/ silt.
- All the unconsolidated weathered material (saprolite, saprock, sand, silt) covering the bedrock or basal surface. Sometimes called the weathered mantle.
Regional Differences in Deep Weathered Profiles
Water in the humid tropics infiltrate and percolate deep down as prolonged rainfalls are more pronounced.
Lush vegetation provides the necessary humic acids for deep weathering. High temperatures speed up the rate of chemical weathering.
Sub-tropical or savannah regions are similar to the humid tropics except that they have a wet and a dry season.
During the wet season water may percolate to considerable depth promoting deep weathering, whereas in the dry season the ground is completely dry and lack of vegetation means less humic acids will be provided limiting deep weathering.
During dry seasons the deep regolith can be stripped away revealing weathered boulders or basal surfaces as tors or bornhardts respectively.
Temperate regions are rainy, but mild and cooler temperatures are more dominant limiting water and chemical reactions to take place.
In arid regions water is scarce limiting deep weathering.
There’s also lack of vegetation which limits humic acids necessary for deep weathering.
Rocky and baked surfaces impede water to infiltrate down.
Polar Regions, like deserts, also have very low rainfalls and scarce vegetation. Also the ice in polar regions create an impermeable layer through which water cannot infiltrate.
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