Volcanoes can erupt at any time.  First we need to know where do volcanoes get their lava.  Volcanoes  get their lava from the core its when two plates collide and one plate slides under the other one.  Then it causes the mantle to melt and it melts more rock and when it has too much pressure it cracks open and explodes.  When the lava cools down and it makes a volcano.  Next when the volcano erupts it blat out lava or magma,  bombs,  glass,  and pumice in the air.  Sometimes volcanoes can explode with only smoke.  The volcanoes have a magma chamber where it sits before it shoots out.