UNIT 7. GEOLOGICAL PROCESSESGeological processes are the transformation of rocks by external geological agents: water, ice,the atmosphere, wind and living beings. 1) WEATHERING:Is the process of alteration and fragmentation of rocks by the action of the atmosphere, waterand living things. Weathering can be physical, chemical or biological: a) Physical Weathering: is the disintegration of rocks into smaller fragments without changing its mineral composition: - Cryofracturing: when water accumulated in cracks in the rock freezes, and its volume expands creating pressure against the walls of the crack which fractures the rock. - Thermal stress weathering: as a result of the strong contrast between the temperatures during the day and at night, rocks continuously contract and expand, causing them to fracture. b) Chemical Weathering: it occurs when minerals in rocks are changed into other minerals which are more stable under the conditions of the surface. The most frequent transformations are: - Oxidation: when minerals contain iron. The most common result is ferrous oxide, which gives a red colour to rocks. - Carbonation: when atmospheric CO2 combines with water, producing a weak acid which dissolves limestone rocks. - Dissolution: it happens to soluble rocks. - Hydrolysis: when water reacts with silicate minerals, dissolving them.
c) Biological Weathering: it is caused by living beings, and can be physical or chemical. 2) TRANSPORTATION:Is the transporting of eroded material. Depending of the transporting agent, there can bedifferent processes: a) The particles transported by the water in rivers and oceans move in different ways: dissolved, floating on the surface, suspended or even jumping, rolling and dragged along the river. The type of transport depends on the size and the weight of the materials and on the speed of the water. b) The particles carried by the wind may be transported by traction (particles are carried without losing contact with the Earth’s surface), saltation, and by suspension (when small, light particles remain in the air without touching the ground for a long time). c) Rock material eroded by a glacier may be transported at the base of the glacier, or within the glacier.
3) SEDIMENTATION AND FORMATION OF SEDIMENTARY ROCKS:Sediments are deposited when the agent transporting loses energy, in areas calledSedimentary Basins. Sedimentary rocks are formed from these materials through a processcalled Diagenesis, which consist of two stages: 1. Compaction: sediments volume is reduced as the result of the pressure exercised by the weight of sedimentary layers. It closes any gaps in the sediment, and the water they contain is expelled. 2. Cementation: is the precipitation of natural cements, such as calcium carbonate, among the grains of the sediment. These salts were dissolved in the water that circulates through the holes in the sediment.