Plate tectonics theory - Class 12 Geography (NCERT)

PLATE TECTONICS THEORY

WHAT IS PLATE TECTONICS THEORY?

The theory of Plate Tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.

WHAT IS A TECTONIC PLATE?

A Tectonic Plate or Lithospheric Plate is a massive , irregularly –shaped slab of solid rock ,generally composed of both continental and oceanic lithosphere.

Plates move over the asthenosphere . The lithosphere includes the crust and top mantle with thickness range varying between 5-100 km in oceanic parts and 200km in the continental areas.

There are two types of plates . They are:1)Continental &2) Oceanic plate

The plate may be referred to as continental or oceanic plate depending on which of the two occupy a larger portion of the plate.Pacific plate is an oceanic plate whereas Eurasian plate is a continental plate.

The seven major plates are: 1)Antarctica and the surrounding oceanic plate2)North American plate3)South American plate4)Pacific plate5)India-Australia-New Zealand plate6)Africa with the eastern Atlantic floor plate7)Eurasia with the adjacent oceanic plate.

Six important minor plates are:1)Cocos plate:between Central America & Pacific plate2)Nazca plate:between South America &Pacific plate3)Arabian plate:mostely Saudi Arabian landmass4)Philippine plate:between Asiatic &Pacific plate5)Caroline plate:between Philippine &Indian plate6)Fuji plate:north-east of Australia

There are three types of plate boundaries.

WHAT IS A DIVERGENT BOUNDARY?

A tectonic boundary where two plates are moving away from each other and new crust is forming from magma that rises to the earth’s surface between the two plates. They are also called passive margin or spreading zone. Eg: Middle of the Red Sea & the mid –ocean ridge(running the length of Atlantic ocean)

EFFECTS OF DIVERGENT PLATE MOVEMENT

-New Crust-Shallow earthquakes -Volcanoes-Mid-oceanic ridge system

WHAT IS A CONVERGENT BOUNDARY?

A tectonic boundary where two plates are moving toward each other. If the two plates are of equal density they push up against each other. If the plates are of unequal density one plate sinks beneath the other in the subduction zone. They are also called active margin or collision zone. Eg: Himalayan mountain & West coast of South America

The location where the sinking of the plate occurs is called a subduction zone. The three ways in which convergence can occur are:1)between an oceanic &continental plate 2)between two oceanic plates 3) between two continental plates.

EFFECTS OFCONVERGENT PLATE MOVEMENTS

-Earthquakes-Volcanic arcs-Deep-sea trenches-Great mountain ranges of the world like Alps , Andes , Rockies , Himalayas

WHAT IS ATRANSFORM BOUNDARY?

A transform boundary is a type of fault whose relative motion is predominantly horizontal in either sinistral or dextral direction. They are also called conservative plate boundary since they neither create nor destroy the earth’s lithosphere. Eg: San Andreas fault in California

EFFECTS OFTRANSFORM PLATE MOVEMENTS

Mostly Earthquakes

RATE OF PLATE MOVEMENT

Plates move at an average rate of between approximately 0.60cm/yr to 10cm/yr. In the North Atlantic the rate of movement is 1cm/yr while in the Pacific it is 4cm/yr . Plates in general travel 5-10cm/yr. the Eurasian plate is moving from the North American plate at the rate of 3cm/yr.

FORCE FOR THE PLATE MOVEMENT

The heated material rises to the surface , spreads , and begins to cool and then sinks back. The cycle is repeated to generate a convection cell or convective flow. Heat from the earth comes from radioactive decay and residual heat.

The slow movement of hot ,softened mantle that lies below the rigid plates is the driving force behind the plate movement.

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