|Clues of a violent eruption include ash, bombs,
and gas holes. Gases trapped in the molten rock infer that there is a
more violent eruption. Gas trapped in the rock gives a vesicular
texture or a rock with holes in it. Just like when you shake a bottle
of carbonated soda and you open the bottle cap. The gas was out!
Causing a mess. A volcano is not much different.
A quiet eruption would include lava rocks that
have no holes and are homogeneous throughout.
Another clue is the rock type itself. Rhyolite
is associated with violent eruptions, whereas basaltic rocks are
formed during quiet eruptions.
This has to do with the chemistry of the rocks.