Plate Tectonics: Types of Plate Movements and Four Types of Plate Boundaries

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  • Plate Boundary: Surface trace of the zone of motion between two plates.

  • Plate Margin: Marginal part of a particular plate.

Types of Plate Movements

  • Constructive movement - New crust is created here by the upwelling of materials from the mantle

  • Destructive movement - represents the zone of convergent plate boundaries, along which two lithospheric plates are coming together and one plate is forced to plunge down into the mantle

  • Conservative margins - lithospheric plates slide past one another and that the plates neither gain nor lose surface areas

Four Types of Plate Boundaries

  • Divergent Boundaries - New crust is generated as the plates pull away from each other

  • Convergent Boundaries - Crust is destroyed here as one plate dives under another

  • Transform Boundaries - Crust is neither produced nor destroyed as the plates slide horizontally past each other

  • Plate Boundary zones - Involves at least two large plates and one or more microplates (smaller fragments of plates) caught up between them

Facts to be Remembered

  • Sea Floor Spreading - New Magma from deep within the earth rises easily through these weak zones and eventually erupts along the crust of the ridges to create new oceanic crust

  • Island Arcs - When an oceanic plate collides and subduct beneath another oceanic plate, an island arc is formed

  • Back-Arc Spreading - Behind the island arc, a basin is formed. Often a small spreading centre occurs, caused by pull of the subducting slab

  • Trenches - These are the deepest part of ocean floor created by subduction

  • Mid-Oceanic Ridge - Continuous submarine mountain chain that encircles the globe. Driven by convection currents in the earth’s mantle, molten basaltic rock is pushed up to the seafloor at the centre of the ridge and then is forced sideways

  • Rift Valley - It is 1-2km deep and several km wide splits many segments of the ridge crest

  • Dip - Defined as amount of inclination of a bed with respect to a horizontal plane

  • Strike - Direction of the line along which an inclined bed intersects a horizontal plane

  • Folds - Bending of rock strata due to compressional forces acting tangentially or horizontally towards a common point or plane from opposite directions is known as folding

  • Faults - These are well defined cracks along which the rock masses on either side have relative displacement

Multiple Choice Questions

1. Which among the following movements form new crust by the upwelling of materials from the mantle?

  1. Destructive movement

  2. Constructive movement

  3. Transform movement

  4. Earthquake

Answer: b

2. Oceanic trenches are formed by

  1. Plate Boundary zones

  2. Transform Boundaries

  3. Convergent Boundaries

  4. Divergent Boundaries

Answer: c

3. Rift Valley is formed by

  1. Destructive movement

  2. Constructive movement

  3. Transform movement

  4. None of the above

Answer: b

4. Bending of rock strata due to compressional forces is known as

  1. Dip

  2. Strike

  3. Fault

  4. Fold

Answer: d

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