Magmatic Entry

Magmatic Entry deals damage to target creature
equal to the number of instant and sorcery spells
in your graveyard.
If no creatures are on the battlefield, reveal the
top card of your library. If it is an instant or
sorcery card, you may cast it without paying its
mana cost.
I've never heard a story about someone who
saw a volcano erupt. I wonder why that is.