Yes. Crater lakes atop volcanoes are typically the most acid, with pH values as low as 0.1 (very strong acid). Normal lake waters, in contrast, have relatively neutral pH values near 7.0.
Yes. Encounters between aircraft and clouds of volcanic ash are a serious concern.
Not usually. Earthquakes associated with eruptions rarely exceed magnitude 5, and these moderate earthquakes are not big enough to destroy the kinds of buildings, houses, and roads that were demolished in the movie.
Lahars are commonly initiated by: 1) large landslides of water-saturated debris, 2) heavy rainfall eroding volcanic deposits, 3) sudden melting of snow and ice near a volcanic vent
Lahars can be of any size. They may be as small as several centimeters wide and deep, flowing less than one meter per second.
Pyroclastic flows can be extremely destructive and deadly because of their high temperature and mobility.