Question

the volcanic material in some cases reaches the surface while in some other instances it gets deposited within the rocks of the earth's crust.why?

Answer 1

The volcanic material tend to remain within the rocks of the earth's crust. However, when the stresses inside the crust develop due to earthquakes, global heating or other natural or artificial causes, the stresses may get excessively exerted and the volcanic material reach the earth's surface, evolving out with great pressure. Months or years later, when the eruption takes place again, it comes out through the same hole. Due to deposition of volcanic materials that take shape of a heap and later a mountain like structure, which we call a volcano.