Weathering is the process by which exposed rocks and other solid materials are decomposed ‘in situ’ (in that place) by the elements of weather at or near the Earth’s surface.
Chemical Weathering:
- Carbonation - The limestone solution process is known as carbonation. Limestone is made up of calcium carbonate. When carbon dioxide is dissolved in rainwater, it forms a weak carbonic acid CO₂ + H₂O → H₂CO₃. When carbonic acid comes into contact with limestone and passes through joints and bedding planes, it reacts with the rock to form calcium bicarbonate H₂CO₃ + CaCO₃ → Ca(HCO₃)₂. The calcium bicarbonate is soluble and is carried away in solution, gradually weathering the limestone.
- Hydrolysis - Hydrolysis is the chemical transition of one mineral to another. Hydrolysis is not only responsible for altering the chemical composition but also the size and resistance of rock materials to weathering. The process of hydrolysis is vitally important to feldspars. Clay began as feldspars and were dissolved through the reaction process of hydrolysis. So, hydrolysis converts feldspars to clay. This chemical breakdown of a compound is due to the presence of water.
Click here to look at this Prezi presentation that focuses on the location, processes and results of chemical weathering
Physical Weathering:
Click to view these short You Tube videos that explain the two physical weathering processes described above.
- Frost Action:This form of weathering is also known as freeze- thaw action. It occurs in areas where the temperature fluctuates around zero degrees Celsius for at least part of the year. Water may find its way into cracks and joints in the rock when the temperature rises above freezing. When the temperature dips below zero degrees Celsius, the water freezes and expands, exerting pressure on the rock and widening the cracks. This process is repeated over and over until the pressure exerted by the ice eventually shatters the rock.
- Exfoliation or 'onion skin peeling' is a process in which large flat or curved sheets of rock fracture and are detached from the outcrop due to pressure release. As erosion removes the overburden from a rock that formed at high pressure deep in the Earth´s crust, it allows the rock to expand, thus resulting in cracks and fractures along sheet joints parallel to the erosion surface.
Click to view these short You Tube videos that explain the two physical weathering processes described above.
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Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes.
Growing plant roots can exert stress or pressure on rock. Although the process is physical, the pressure is exerted by a biological process (i.e., growing roots). Biological processes can also produce chemical weathering, for example where plant roots or microorganisms produce organic acids which help to dissolve minerals.
Microbial activity breaks down rock minerals by altering the rock’s chemical composition, thus making it more susceptible to weathering. One example of microbial activity is lichen; lichen is fungi and algae, living together in a symbiotic relationship. Fungi release chemicals that break down rock minerals; the minerals thus released from rock are consumed by the algae. As this process continues, holes and gaps continue to develop on the rock, exposing the rock further to physical and chemical weathering.
Burrowing animals can move rock fragments to the surface, exposing the rock to more intense chemical, physical, and biological processes and so indirectly enhancing the process of rock weathering.
Humans also can rocks to break down by their activities for example, strip mining.
Although physical, chemical, and biological weathering are separate processes, some or all of the processes can act together in nature.
Try the activity below as a form of revision.