Pyroclastics are the solid and liquid materials that are ejected from volcanoes. Most commonly they are associated with composite cones like the Cascade volcanoes or Etna or Vesuvius, to mention a few common ones. The explosion takes place when gas pressure builds up below the vent of the volcano. When the pressure can no longer be contained, an explosion takes place. If the erupted material is mostly dust and gas, a pyroclastic flow can sweep down the side of the volcano.
