What about Pumice, or PMCE? Created when lava from volcanoes solidifies with gas bubbles in it, making in so light it can float on water! And the ASH would move upwards from the volcano as long as there is pressure. If the updraft disappears or weakens, the ash solidifies and falls down. If the plume of ash is to heavy to be carried upwards at all, it will fall down and race down the side of the mountain, incinerating everything in its path. That is called a pyroclastic flow.

Sorry for lame english.