Question: Are the particles from the ash heavier than air?

This is almost the same question that I was asked in my PhD exam and it was really hard to answer! 🙂

it’s not really a straight forward yes or no answer, but i’ll try…

have you seen pictures of volcanoes with huge ash clouds coming out of them? do you see how the ash cloud goes right up into the sky and seems to stay there for ages? well the ash and pumice is sent up into the air because more and more material is being pushed up from underneath during the eruption. Gravity should bring the ash and pumice straight back down to the earth but it doesn’t. Why not? Well, when the ash and pumice are erupting, it is full of gas and it’s really hot. This hot gas makes the ash and pumice less dense than the cold air around it and so it almost floats in the air. But as the ash and the pumice start to cool down, the become more dense than the air around them (because they are indeed heavier than air) and so the can no longer float and fall to the ground.

phew! i hope that made some sense? 🙂