While there is a popular assumption that volcanic activity is always the same, eruptions come in all sorts and sizes, ranging from hissing gas to gushing molten or “melted” lava and huge explosions of ash and dust; and the side effects of eruptions can produce great sea waves, mudflows, brilliant sunsets, and fires. These many kinds of eruptions occur when hot materials, similar to molten rock and containing gases and steam, rise up from the nearly molten layer that lies under great pressure below the cool and solid crust of Earth. These materials reach the surface via chimneylike vents or along fissures that crack the crust. Specialists have spent their whole lives arguing about exactly how this happens. Basically, as the mass approaches the surface, the pressures acting upon it are reduced and the gases and steam can separate out. When the materials are fluid and hot, the gases and steam can hiss out fairly easily to the surface. Most of the molten material runs across the land as lava flows, although mild gas explosions can shatter some of it into ash and cinders that pile up as little volcanic cones usually rising less than 250 meters above the chimney. This kind of activity, with small variations, is described in the accounts of such famous eruptions as Stromboli, Etna, Lanzarote, Laki, and Paricutin.
On the other hand, the rising materials can often be much more viscous-glue-like rather than fluid-and much cooler. In this case, the steam and gas cannot easily escape as the pressure is reduced. They form bubbles that honeycomb and weaken the molten rock until it becomes a stiff, frothy mass. Suddenly the multitude of bubbles explodes with enormous force. The molten froth shatters into an infinity of small pieces that are thrust into the air, where they cool to dust, ash, or pumice (a spongy form of volcanic glass). Sometimes, these fragments form huge, incandescent clouds that can race down a volcano at 500 kilometers an hour. Even more frequently, eruptions hurl colossal, billowing, gray columns of gas and fine fragments into the stratosphere.