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Earthquake and Volcanoes Notes for Geography Section of UPSC Exams

SSBInsights Team
11 Nov 2017

NOTES of Earthquake , Earth Movements and Volcanoes


The Mountains that come into existence due to folding are known as Fold Mountains. These mountains are by far the widest spread mountains. They are formed by sediments getting deposited into geosynclines. They are usually found along the edges continents because the thickest deposits of sedimentary rock generally accumulate along the edges of continents.


Horst or Block Mountains Due to tension or compression, two parallel faults come into existence and displacement of rock strata takes along both the fault planes and a block of earth’s crust stands like a mountain and is known as Horst or a Block Mountain. Sometimes, the land on the outer sides of the faults slides down along a fault plane leaving the area in between at a higher level. This portion is also known as Horst or Block Mountain. The slopes of horsts are very steep and they usually don’t have peaks. Examples include the Sierra Nevada Mountains in North America and the Harz Mountains in Germany.


Rift Valley When a block of land between two parallel faults subsides along a fault plane leaving the adjoining areas at a higher level, the subsided part is known as Rift Valley. These valleys have steep, wall-like slopes and the valley floors are comparatively wider. Example is East African Rift Valley system, which stretches from East Africa through Red Sea to Syria.


A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, such as an ocean or a large lake. They have a small amplitude offshore, and a very long wavelength, which is why they generally pass unnoticed at sea. They grow in height when they reach shallower water.

Homoseismal line is the line on the Earth's surface connecting points where the seismic wave arrives, generated by an earthquake, at the same time.


Isoseismal Line is a line on a map joining points of equal seismic intensity produced by an earthquake.

A seismic gap is a segment of an active fault  known to produce significant earthquakes that has not slipped in an unusually long time when compared with other segments along the same structure.

PLATE TECTONICS Tectonic plates move in relation to one another. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.65 to 8.50 centimeters per year. Tremendous energies are unleashed resulting in tremors that transform Earth’s surface. While all the plates appear to be moving at different relative speeds and independently of each other, the whole jigsaw puzzle of plates is interconnected. No single plate can move without affecting others, and the activity of one can influence another


 Whatever drives the movement, plate tectonic activity takes place at the boundaries of these plates


 Divergent boundaries: In such a boundary two plates are moving away from each other. An example of this is the Mid-Atlantic Ridge, where magma rises from deep within the Earth. This leads to an increase in volcanoes and earthquakes in the region


Convergent boundaries: They are also known as Destructive Plate Boundaries. There are two main ways that two plates can converge. In a collision boundary, the two plates push almost equally against each other, buckling the material up in the middle. This leads to the formation of mountain ranges, such as the Himalayas and the Alps. In geology, a nappe is a large sheetlike body of rock that has been moved more than 2 km to 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a large-scale recumbent fold.


 In a subduction boundary, one plate slips beneath the other, leading to the formation of a deep trench. The Mariana Trench in the Pacific Ocean, which at 11 km is the deepest point in the world's oceans, is an example where this is occurring.


Transform boundaries: These occur when two plates slip past each other. This slipping however generally doesn't happen smoothly - because of friction the plates remain in place for a while, building up a kind of stored potential energy from the forces trying to move them, and then this energy is released suddenly when the forces become too large. This sudden release of energy results in an earthquake. A notable example of a transform boundary is the San Andreas fault in California.


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