High viscosity and high gas content are the main characteristics
of an explosive magma. Viscosity refers to the resistance to flow; it is the
opposite of fluidity. When magma has high resistance in flowing it causes an
explosion. When the volcano finally erupts, bursting magma will fragment into
clots of liquid that will cool as they fall through the air.
High gas content also gives the magma its explosive character; this is because
the volume of gas expands as the pressure is reduced. When magma is at the
depth of the earth pressure inside the volcano is stuck or is built up. Once it
finds an opening this sudden reduction in pressure causes the gases to expand
which makes the magma explosive.
