The evaporating of harmful gases, earthquakes, and some evidences from the manhole are the manifestations showed that an underground volcano was forming under the city. This means that there was already lava under the city and the ground cannot take the pressure anymore so it formed a volcano.
The physical changes in the movie are the breaking of glasses from buildings, breaking of buildings, cracking of the ground, boiling of water from the pond, injured people, the melting of metal, and the falling down of some properties. The chemical changes are the burning of trees and people, formation of the volcano and its eruption, the ashfall,