weathering

• Weathering is the breakdown of in-situ rock materials near the earths surface by exposure to the atmosphere, water and organic matter.
• Erosion is the removal of weathered products by gravity, water, wind and ice.
• Products of weathering include rock fragments, un-reactive quartz grains, clay minerals, and ions in solution.
• Mechanical/physical weathering leads to the disintegration of bedrock to smaller angular but chemically identical fragments, resulting in increased surface area of rock exposed to chemical weathering.
• Chemical weathering leads to the decomposition of the bedrock, only quartz is unreactive, resulting in the formation of clay minerals from the broken silicate minerals and ions are released into solution.
• Rocks and minerals chemically weather because they are out of equilibrium with the conditions that they formed from.
• Mechanical processes contributing to weathering include freeze-thaw, exfoliation, and pressure release.
• Freeze thaw involves water entering joints, bedding planes, cleavages, faults and pore spaces, ice freezes and expands, stressing the internal rock structure, and as the process repeats it breaks the rock, forming scree slopes.
• Exfoliation is common in areas with large diurnal temperature ranges, where outer layers of rock heat up and expand faster than deeper layers, cool at night, and contract faster, forming concentric fractures and causing the rock to peel off in layers like an onion.
• Pressure release occurs when rocks at depth under a large confining pressure are eroded, causing rocks to expand, fracture, and form horizontal joints, often occurring at quarries following blasting.
• Chemical processes contributing to weathering include carbonation, hydrolysis, and oxidation.
• Carbonation involves rain water being slightly acidic, water that enters the soil picking up more carbon dioxide, the weak acid rainwater dissolving carbonate minerals, and limestones made of calcite being most susceptible, often producing large cave systems.
• Hydrolysis involves silicate ions reacting with hydrogen ions from water, producing clay minerals and ions in solution, which are carried away into rivers by water runoff, with quartz being unreactive and feldspar being reactive.
• Oxidation involves the ability for minerals to incorporate oxygen atoms into their atomic structure, affecting iron rich minerals, resulting in red, brown, orange and yellow colouration of the soil.
• Spheroidal weathering involves rectangular blocks outlined by joints undergoing chemical weathering, with the corners and edges weathering more rapidly, eventually resulting in a spherical surface.
• Biological activity, such as tree roots widening joints and bedding planes, can exert immense stress within rocks.