Dante's Peak
(1997)

Dante's Peak makes its final explosion, and the nue ardente spreads at a high speed across the town, vaporizing it, just as Harry (Pierce Brosnan), Rachel (Linda Hamilton) and her two children are driving through it in an attempt to escape.  Fleeing the deadly burning cloud, they all end up stuck in a nearby cave.  The rest of Harry's team watches the blast far away from the town area in tears, believing Harry was lost in the catastrophe.

Harry, meanwhile, manages to turn on the tracking device provided by NASA, while they are all in the cave.  Three days later, Terry (Kirk Trutner) notices the device has been blinking, and joyfully realizes Harry is alive.  A search party is dispatched, and they are eventually found and rescued.
 

Thanks Dr Mat!