Question

how can volcanoes be classified on the basis of their activity?

Answer 1

by time of their apperance

Answer 2

Volcanoes form in areas where magma has made its way to the Earth's surface.  Magma in the asthenosphere has a lower density than the rocks of the Earth's crust.  This difference in density, combined with its extreme heat makes magma rise upwards pushing into the Earth's crust.   

Sometimes, the magma is able to push and melt its way up all the way through the rocks of the Earth's crust and makes it to the surface.  Here the magma cools forming a mound we call a volcano.