Definition Poem!

A huge rock. Wider on the bottom and gradually gets smaller near the top.
Shooting lava up into the air.
As everyone runs for cover,
Black smoke emerges, giving you a warning before the explosions start.
There are thousands still on this planet, some are underwater, just waiting to burst.
Nothing can stop it from exploding.
The scary thing is, some don't explode for thousands of years and people do not see it as a threat.
What is this ?