Dante's Peak (1997)

Volcanologist Harry Dalton (Pierce Brosnan) and Mayor Rachel Wando (Linda Hamilton), finally convince the unbelieving populace that the big one is about to hit and that they need to evacuate immediately, only to discover her two children have gone up the mountain to get their grandmother. With Earth's clock racing against them, they must rescue the kids and grandma before the volcano explodes in a fury of flame and ash a million times more powerful than an atomic bomb.