The way volcanoes erupt usually takes a long time and this is how volcanoes erupt.  First a volcano makes something called magma from melted rock.  The magma goes through a circulation.  It has to form at the bottom of the volcano and then start its way up the main vent. The main vent is a hole that is in the volcano and when the volcano is ready to erupt the lava is at the top of the main vent.  The magma goes up the main vent slowly while it is still getting hotter.  When the magma is about half way up the main vent it turns into lava.  Lava is a very hot liquid which burns the remaining rocks from the magma.  The lava slowly continues up the main vent.  While going up the lava continues to get hotter and hotter. Ash and rocks are collected and the lava is getting hotter and hotter while the lava is continuing its way up the main vent.  When the lava is at the top of the main vent the volcano erupts.  The lava blasts out of the volcano along with ash, rocks, and a cloud of dust that is very thick.  The ash and rock crumble to the ground, but the lava is either moving down the volcano side very slowly or at a high speed.  The lava burns down almost everything in its way, and it sometimes leaves bits of things burning.  The lava from the volcano can cool fast, or sometimes  the lava will slowly cool down from its intense heat.  Lava that cools slowly forms igneous rocks.  There are many types of igneous rocks.  Volcanoes can damage themselves in the explosion.  A volcano literally blows its top off.   One of the volcanoes that has blown its top from an explosion is Mt. St. Helens.  Mt. St. Helens has erupted more than once.
Volcanoes can be under water or on land.  Volcanoes that are under water take a longer time than if they are on land because they are under water the water slows down the magma and lava but if the volcano is on land the lava and magma can move quicker up the main vent.  It just depends on the environment how fast the volcano can make the magma the magma makes lava and the volcano makes an explosion.  If the volcano is under water the cooled lava will probably make an island.  The Hawaiian Islands is an example of island made by a chain of volcanoes.  Now go back to the front page of our site and go to a different page on our site and of course be prepared to learn more about volcanoes.