A pyroclastic flow from Mount Oyama on the Japanese island of Miyakejima crashes into the sea.

AFP/Getty Images

It starts out as just another afternoon in the shadow of a volcano, full of everyday commerce, humor and toil. Then suddenly everything takes a turn for the worse. The Earth shakes, and the air trembles with a monstrous explosion. Before you know it, the sky darkens and thick torrents of volcanic ash begin to fall from the sky, covering everything in a heavy, gray blanket of dust.

All that ash may sound like more of a mess than a real danger, but just consider the sheer destructive power of a pyroclastic flow. This mass of ash, gas and rock fragments can travel at speeds approaching 125 mph (200 kph). And with internal temperatures of 752 to 1,472 degrees F (400 to 800 degrees C), it can pretty much bake anything in its path.

Archaeologists studying the 79 A.D. eruption of Mount Vesuvius frequently discovered fractured skulls among Pompeii's dead -- signs of an exceedingly swift and brutal end. The heat from pyroclastic flow had simply boiled their brains until the pressure shattered their heads like eggshells. Pyroclastic flows can cause tremendous destruction, but they only occur in explosive volcanic eruptions (as opposed to the nonexplosive eruptions you might see in Hawaii).

Fortunately, most people haven't had to contend with the civilization-destroying, head-popping effects of a volcanic eruption. Armed with a better understanding of how volcanoes work, we've managed to read the warning signs and largely avoid the massive loss of life that volcanic activity inflicted on past centuries.

Nevertheless, ash fall still threatens widespread areas. Winds can carry the falling particles across thousands of miles, posing a risk to plants and animals wherever it falls. Following the 1980 eruption of Washington's Mount St. Helens, one­ of North America's biggest, ash fell as far away as Montana.

­B­ut how does volcanic ash form and what happens to it after the eruption? In this article, we'll blow the roof off these questions and learn just what this material is made of.