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At the surface gas bubbles expand explosively and send magma out as a fine spray, which cools to ash.
Near the surface the lower weight of magma allows gas bubbles to grow.
Deep in the vent the magma is under pressure, and gas bubbles are tiny.
(Below) Volcanic ash from the Mount Saint Helens eruption.
(Above) A volcanic bomb.
Explosive eruptions occur when gassy magma is very viscous. Under these conditions the gases cannot escape from the magma until it is close to the surface. When the gases finally burst from the magma, they fragment the liquid content of the magma, throwing it into the air, where it cools into ash or bombs. Magma also erodes the sides of
the vent, so some of the ash will be material from previous eruptions.