NCERT Geography class 7 chapter- 3

The continental drift theory(Alfred Wegener, 1912) gave the first insight into the drifting apart of the continents from the supercontinent Pangaea. Due to the certain drawbacks of this theory like not able to explain the reason for the drifting of the continents; a new theory came into existence Plate Tectonics. The new theory gave importance to Seafloor spreading, it explained that all tectonic activities occur along the plate margins. Two forces act on the earth’s surface to evolve landforms- Endogenic forces(originate from within the earth) and Exogenic forces(originate on the surface of the earth). Earthquakes and volcanoes are the resultant of endogenic forces and are sudden in nature; they occur due to the collision along the plate margins. Volcano term is derived from the chimney of the forge of Roman God, Vulcan. It is a mountain of fire from where the hot and molten magma from the mantle erupts out into the surface which on solidification forms igneous rocks. Volcanoes are of three types- Active[erupt frequently; eg: stromboli and etna in italy], Dormant[do not show indication of erupting in near future; eg: Vesuvius in italy]; Extinct[ not erupted in hundreds of years; dead volcanoes; eg: Kilimanjaro in Africa and rainier in the USA]. Most of the active volcanoes form the Pacific ring of fire; another zone is Mid-world mountain belt along mediterranean sea. An earthquake is the sudden shaking of the ground surface due to tectonic movements. Focus is the point of origin; the vertical point to focus on the surface is Epicentre (highest frequency). There are three waves- Primary waves[P-wave; cause small displacement]; Secondary waves[S-wave; do not pass through liquids; produce strong shaking action]; Surface waves[L-wave; very powerful; cause most of the damage]. earthquakes are measured on the Richter scale; an increase of one point means 10 times greater intensity. The major distribution is along with three belts- Circum-Pacific belt[N. America, S.America and E.Asia]; Mid-Continental belt[Alpine mountains, mediterranean sea, Himalayas]; Mid-Atlantic ridge belt[mid-atlantic ridge and adjoining islands]. A severe earthquake hit the Kachchh region of Gujarat at 8:45 a.m. on 26 January 2001 of 8.1intensity. about 50,000 people died and more than 5,00,000 were adversely affected. Reasons for this earthquake — seafloor spreading of Indian Ocean; the gradual northward movement of Indian plate; reactivated faults below the surface in Kuchchh region. Please refer to my youtube link for further details-https://youtu.be/0xGNWJOEdUg