Plate Tectonics - Physical Geography, UPSC, IAS. | Geography (Prelims) by Valor Academy PDF Download

Plate Tectonics

The earth movements which bring vast changes are called tectonic movements. The concentration of great internal forces within the earth raises local areas upwards or cause than sinking downwards.

They are divided into- Sudden and Slow or secular movements.

1.  SUDDEN MOVEMENTS

They are commonly noticed during an earthquake.

2.  SLOW OR SECULAR MOVEMENTS

These movements continue much longer as compared to our life span e.g. the raised sea beaches along east coast of India to a height of 15-30 metre above sea level are observed.

VERTICAL MOVEMENTS

These movements are responsible for a rise or a fall of a portion of the earth’s surface. When a part of the earth’s crust rises in relation to surrounding position, it is known as uplift. Conversely, when the sinking of a part earth’s crust relative to surrounding position takes place, it is called subsidence.

These earth movements on a large scale build up continents and plateaus. They do not disturb the horizontality of the strata.

HORIZONTAL MOVEMENTS

These movements responsible for greatly disturbing the horizontal arrangement of layers of rock. They involve both the forces of compression as well as tension.

Folding

Folding occurs through compression, which is when two forces act towards a common point or a plane from opposite directions. After the folding the strata occupy a smaller area of the earth’s surface than what the original rocks or rock- forming material occupied.

Folds have two limbs along the slope. It is called an anticline when the limbs incline in different directions from the top. It is syncline when the limbs incline to the same direction into the bottom of the trough.

Faulting

When forces act horizontally in opposite direction away from a given plane or a point. The simple breaks in rocks involving no movements are turned as fractures. On the other hand, when a movement occurs along a rock and breaks, it is called a fault. Sometimes, one side of a fault is vertically thrown upwards and the other side moves downwards.

The faulted strata are sometimes pushed forward far away from the parent strata. Faulting gives rise to relief features totally contrasted from the formation associated with folding. The escarpment viz., fault scarps of Western Ghats overlooking the Arabian Sea and of the Vindhyas facing Narmada Valley are primarily due to faulting

VOLCANOES
A volcano is a vent or opening through which heated materials consisting of gases, water, lava and fragments of rocks are comes out from the highly heated interior to the surface of the earth.
Volcanoes are classified under different scheme:

A. On the basis of period of eruption. 

1. Active Volcanoes.
2. Dormant Volcanoes.
3. Extinct Volcanoes.

B.  on the basis of mode of eruption-

1. Central eruption.
2. Fissure eruption.

Active volcanoes
The volcanoes which continue to erupt periodically are called active volcanoes. e.g. Mona Loa in Hawaii Island, Mt Etna in Sicily

Dormant volcanoes
It is the volcanoes which have been quiescent for a long time but in which there is a possibility of eruption are called dormant volcanoes.
e.g. Mt Fuji of Japan, Karakota of Indonesia

Extinct volcanoes
It is the volcanoes in which the eruption has completely stopped and is not likely to occur are called Extinct Volcanoes.
E.g. Popa Mountain in Myanmar, Mt. Kilimanjaro of Africa

Products of volcanic Activity

The materials derived from a volcanic eruption are liquid, solid and gaseous in nature. The liquid matter is the lava. Very often, the lava in the vertical passage gets solidified. The next volcanic eruption through the pipe is accompanied by explosion of highly compressed gases throwing out blocks of solidified lava from inside the choked pipe.

The solid material consists of fine ash and dust particles and angular fragments mostly of lava rock blown up from within the vent.

The gaseous substances are mainly composed of steam, products being hydrochloric acid, ammonium chloride, sulpher dioxide, hydrogen sulphide, hydrogen and carbon dioxide.

Topography produced by Volcanoes

• Cinder Cone: - they are of low height and are formed of volcanic dust, ashes and pyroclastic matter. Its formation takes place due to the accumulation of finer particles around the volcanoes vent.
• Composite Cones:- they are formed due to the accumulation of different layers of various volcanic materials.
• Lava Domes:- these are formed due to accumulation of solidified lavas around the volcanic vents.
• Craters:- the depression formed at the mouth of a volcanic vent is called a crater. When it is filled with water it becomes a ‘crater lake’.
• Calderas: - generally enlarged form of craters is called caldera.
• Geysers: - they are intermittent hot springs that from time to time spout steam and hot water from their craters.

EARTHQUAKES

Any sudden distance below the earth’s surface produces vibration or shaking in the crust is called earthquake. When the rocks break, the particles next to the break are set in motion. It is the movement of one rock mass against another that causes vibration. Some of these vibrations reach the surface and are known as earthquakes.

Focus / Hypocentre: Place of origin of earthquake inside the earth.

Epicenter: Point on the earth’s surface vertically above the focus, most affected area.

Seismograph: The instrument to record the earthquake and the intensity of earthquake is measured on Richter scale.

Isoseismic Line: The line joining places which experience the earthquakes at the same time.

Types of Seismic Waves:-

There are three types of waves:

1. P or Longitudinal or Primary waves.
2. S or Transverse or Secondary waves
3. L or Surface or Long waves.

The L waves do not pass and do not deeper in the 

earth. The S waves can not pass through a liquid and are transmitted only through a rigid or a solid body. The P waves can pass through solid, liquid and gaseous bodies. Their velocity increases as they pass through solid body.

Causes:

1. Collapse of roof of cavities, mines, tunnels etc.
2. Volcanic eruption.
3. development of faults in the crust which results from the plate collisio

Effects:

1. Landslides and damming of rivers in highlands regions.
2. Causes depression forming lakes.
3. Formation of cracks or fissures in the epicenter region and sometimes water, mud, gas are ejected from it.
4. Causes the raising and lowering of parts or the sea floor.
5. Devastation of cities, fires, disease etc.