Mechanical weathering is common in areas with access to water. Liquid form water can easily seep into cracks in rocks and freeze there, creating wedge-like effects which break them apart over time.
Gravity and glaciers (see Chapter 10, Mass Wasting) can transport rock and sediment to new places; geologists use the term rounding to refer to how transport wears away at sharp corners of rock fragments, gradually rounding them off.
Physical
Water, whether liquid or frozen, plays an essential role in mechanical weathering. If it seeps into cracks in rock, it can expand and act like a wedge to push out. This process has given sidewalks their characteristic craters while road crews must regularly patch potholes in asphalt roads.
Plant roots may enlarge cracks in rocks through root wedging. Some of these roots become fossilized over time and are then known as rhizoliths; burrowing animals such as earthworms and ants also help with mechanical weathering processes.
Time exposure plays an integral part in how quickly rocks erode, with those exposed for extended periods eroding more quickly than those covered quickly by earth or other materials. Rocks buried quickly tend to experience significantly less erosion.
Chemical
As bedrock disintegrates through chemical and mechanical breakdown, its particles become more easily fragmented into smaller pieces that can then be transported elsewhere as sediment.
Minerals with higher crystallization points, such as quartz and feldspar, tend to weather more slowly than minerals lower on the Bowen reaction series. Furthermore, rocks that have been subjected to weathering for an extended period are more prone to erosion than recently formed unweathered rocks.
Chemical weathering relies heavily on water; when combined with oxygen in the air, water forms acids which break down minerals present in rocks and soil over time. Other agents, like carbon dioxide can combine with water to form weak acids which dissolve large quantities of limestone and form caves such as Carlsbad Caverns in New Mexico.
Hydration
Chemical weathering involves the interaction of minerals with external agents like air or water to alter their shape. Oxygen can oxidize some minerals to alteration products while water can dissolve silicates entirely or transform silicates into clays.
Mineralogy and structure of rocks determine their susceptibility to mechanical and chemical weathering, with massive rocks such as granite having fewer planes of weakness than sedimentary layers with bedding planes.
Fluid systems like wind or flowing water tend to move and deposit coarser sand than fine silt sediment, leading to its deposition along with other materials, creating a sandstone formation with groove casts, tool marks, load casts or load marks as results of further weathering. Plant roots also play an integral part in mechanical weathering by burrowing through cracks in bedrock to pry it apart.
Hydrolysis
Hydration and chemical weathering are two processes used to break down rocks, with the former often taking place where there is plenty of water – an universal solvent. Water in its various forms promotes chemical weathering by softening rock surface layers and unleashing forces that break apart the material.
Chemical weathering also alters the surface of rocks. For instance, granite undergoes mechanical weathering forces before being altered chemically by water to form feldspars, quartz and mica that eventually break down over time and turn into gravel, sand and silt.
Weathering may also turn a rock that contains iron into rust, an oxidation process which weakens and breaks apart rocks it touches. Rusty boulder piles may be found on landscapes where hydration and chemical weathering have broken down more durable materials or unfractured parts of subterranean blocks selectively over time.
Haloclasty
Weathering occurs through two major forms: physical and chemical. Both types are equally important and must be differentiated in your exam answers for maximum marks! Failing to distinguish these two may result in missed marks!
Physical weathering occurs when water or ice seeps into cracks in rocks and expands, swelling them up until they split apart the rock formations. It’s a leading cause of road damage, as well as being the catalyst behind some cave formations like Timpanogos Cave National Monument in northern Utah.
Mechanical weathering processes also include exfoliation and salt weathering (haloclasty). Salt weathering occurs when saline water seeps into cracks in rocks, then evaporates leaving behind crystallized salt particles that gradually break apart the rock through pressure from expanding salt crystals, gradually dissolving it away and eventually creating honeycomb-shaped formations of hundreds or even thousands of pits on it. Honeycomb weathering is particularly prevalent near coastal areas.