Question

Pls give reasons of both parts

Answer 1

1) The volcanic material tends to remain within the rocks of the earth crust. However, when it stresses inside the crust develop due to earthquake global heating or natural or artificial causes the stresses may get extensively extended and the volcanic material reach the earth's surface evolving out with the great pressure. Month or year later when the reaction takes place again it comes out through the same hole. Due to deposition of volcanic material that shape of a heap and later a mountain like structure which we call a volcano.
2) Because the trenches have crustal friction near the surface and deep into the crust as subduction forces one plate under another. At oceanic ridges most of the crustal movement is near the surface as sea-floor spreading causes plates to move apart and new crust is formed by magma pushing up from the mantle.