Let’s start with the definition, “The shaking of the earth’s ground, typically causing great destruction due to the movements within the earth’s crust”.
Let us understand the definition. Shaking of the earth’s ground: as you all are aware of the earthquake, the ground shakes and causes the movement of the materials/objects on the surface, typically causing great destruction: many earthquakes cause destruction. However, both types of shaking of the earth’s ground are considered as earthquakes whether they create destruction or not, due to the movements within the earth’s crust: this is the thing we will discuss here.
The earth is divided into many horizontal layers from surface to its core viz. crust, mantle, outer core, inner core. The crust is the outermost layer of the earth. The thickness of the crust is 5-70 km and it is around 100 km in the Himalayas. And still, it is just 1% of the earth’s volume.
Do not try to understand the full picture. As we have now understood crust. Do not get confused between the crust and lithosphere. The lithosphere is sum total of crust and most upper part of the solid mantle. Now, according to the definition, we will now understand the movement of the crust.
The earth’s crust moves on the asthenosphere. The asthenosphere is the layer that is uppermost part of the lower mantle. This layer is made up of molten magma (lava). Whenever there is a volcanic eruption, the lava comes from this layer.
The crust is divide into many major and minor plates. There are 07 major plates. For example North America Plate, South America Plate, European Plate, Indo-Australia-New Zealand Plate, etc.
It is just for reference and quick understanding. Do not think deeply about this.We will deal with this tectonic plates in another post.
These different tectonic plates move in three ways.
When two tectonic plates move in away from each other, it is called the ‘Divergent Movement of plates’. Why does this happen? The reason will be given after ‘Transform’ is explained.
The earthquake does not happen because of this. It causes the formation of volcanoes. When two plates move away from each other, rupture and fracture happen and finally, the lava from the asthenosphere comes out forming volcanoes. Ex. Mid Atlantic ridges.
When two tectonic plates collide with each other, it is called the ‘Convergent Movement’. It causes both earthquakes and volcanic formation.
When two plates collide with each other, it causes ‘Earthquake’. After so much struggle, the denser plate subduct (go under) the less dense plate (the oceanic plate is denser than the continental plate). The subducting plate melts in the asthenosphere (layer of molten magma) and comes out on the surface as volcanoes.
Example: When East Pacific plate collided with the South American plate. The Pacific plate is the denser one went inside and giving rise to Andes Mountains (Volcanic) on the South American coast.
The other example you may think of collision of Indo-Australian Plate with Eurasian plate and resultant forming of Himalayas. But the Himalayas are not volcanic mountains. There is a reason behind it. As we all know that the Himalayas are formed due to the collision between Indo-Australian plate(continental plate) and Eurasian plate(continental plate) the subduction of the Indian plate is not so deep so that the subducted plate did not melt to form magma. So there is no volcanic eruption in the Himalayas
When to plate move in slide past each other in the opposite direction or same direction but with different speeds, it is called the ‘Transform Movement’ of plates. This movement does not cause volcanic formation but a destructive earthquake. For example, The San Andreas fault in western North America is known for its destructive earthquake.
Why does these tectonic plates move?
The tectonic plates move in three ways viz. opposite to each other (divergent), collide with each other (convergent), and slide past to each other (transform) as discussed above.
All these happen because of the conviction process inside the asthenosphere. The convection process takes place when there is hot liquid below and relatively less hot liquid above it. The hot liquid takes the place of relatively less hot liquid and moves in clockwise and anticlockwise.