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Crinoid Saccocoma A
Geological structures are structures in the Earth's crust that have geological causes. There are many types of geological structures and these can have several causes. For example, tectonics caused widespread deformation of the crust like fractures and folds. Volcanism can also cause various geological structures such as volcanoes or intrusions.
A fault is a zone in rocks where movement has occurred. Due to high pressures caused by tectonics, a fault in the rock can be formed. There are several types of faults. A normal fault is caused by pulling strength and downward slip. With a reverse fault one block is pushed over the other due to compression. Along a strike-slip fault horizontal movement occurs. Because of the high friction of faultplanes, movement occurs not gradually, but in short events. Hereby earthquakes occur.
Grabenstructure of two normal faults. Hellisheidi, Iceland
A fold in a rock is usually the result of major tectonic forces. By a mountain formation (orogeny) rocks are deformed and folded on a large scale. A fold (in cross section) downwards is called a syncline, a top an anticline.
If the tectonic movement continues after folding, a fold can break, and becomes a thrust, a kind of fault. Folding can also occur on a smaller scale by pressure from glaciers or ice sheets.
In sediments many structures occur as a result of the deposition and the environment. Sedimentary structures are formed during the formation of the sediment. Examples are cross stratification and wave ripples. See also our article on Sedimentary structures.
Wave ripples in rock. Between Igherm and Tafraoute, Morocco.
Erosion is the abrasion of the Earth's surface. Most erosion is caused by wind (aeolian), running water (rain, rivers, seacurrents, waves etc.), or due to the works of ice (freezing/melting, glaciers, ice sheets). Erosion through volcanism or earthquakes is much rarer. Because of these erosion processes many different kinds of geomorphological structures can be formed, wich can be preserved in sediments when the eroded layers are again covered by new sediments. Geological structures like glacier marks, gullies and unconformities can than later be found in the sediments.
School example of erosion by rainwater. The soft sediment is easily eroded. This kind of badland erosion occurs mainly in dry areas without much vegetation. Taroudant, Morocco. Photo: Herman Zevenberg
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