Question

how we can say that earth is active

Answer 1

Everyday there are certain changes happening in the continents like the increasing of coasts or decreasing and even there are many volcano eruption and the earthquake that occurs.
So, we can say that the earth is still active..

Answer 2

Yes the earth is yet active. We can say this because constantly the matter from the deep layers of the earth shoots up through volcanoes and fissures on ocean floors and cools down to form the earth's crust. The constant process of formation and destruction of the earth's crust explains the fact that our earth is still active. The movement of the tectonic plates even tell us that the earth is yet active.