volcano lovers
nothing Energetic little buggers, those volcanoes...
Volcanoes are a reminder that Earth is not a static hunk of rock, but a living planet that retains much of the heat from its formation. Recall that our home planet coalesced from a hot cloud of gas and dust 4.6 billion years ago. The surface radiated its heat to space, and cooled pretty quickly, but the high-temperature rock on the inside could not. In fact, it's generated extra heat over the years from radioactive decay and gravitational energy.

The long and the short of it is this: We're sitting on a big ball of fiery rock wrapped by a dozen or two kilometers of relatively cool rock. That's unstable. In the long run, you can't keep a bezillion (defined) tons of fiery rock down.

"Earth is dynamic," notes Arizona State University volcanologist Stanley Williams. "It's not a pile of cold rock."

You'll also recall that heat rises -- because hot substances are less dense than colder ones. So all that magma is looking for a way to escape up through the crust. And since heat is a form of


energy, rising magma brings up oodles of energy -- enough to create steam and cataclysmic explosions. Some people even think volcanoes have changed climate over the millennia.

Details, please
The biggest clue to the origin of volcanoes is their peculiar distribution. Most are located around the edge of the Pacific Ocean, in a band called the "Ring of Fire."

Most above-water volcanoes are on the Ring of Fire, which marks the intersection between tectonic plates surrounding the Pacific Ocean.

ring-o-fire The Ring is a zone of geologic instability where various tectonic plates (defined) collide and the heavier ocean floor plates sink beneath the lighter continental crust plates. The Ring, and other areas where one plate similarly dives beneath another, are called subduction zones.

To find the origin of the magma flow, dig down -- say about 100 kilometers -- beneath a subduction zone. James Luhr, director of the Global Volcanism Project at the Smithsonian Institution in Washington stated, "In 1996, most geologists would agree that what is triggering the melting is seawater being released from the ocean crust" in this deep, hot part of the Earth. (Geologists still believe that this is the mechanism for subduction zone melting).

He explains that the sinking oceanic crust contains hydrous minerals (defined) and some seawater in pores in the rock under fantastic pressure and temperature (about 20-30,000 times atmospheric pressure and temperatures up to 1,000 degrees Celsius). When this hot high-pressure water is released and flows through or mixes with hot rock in the upper mantle (defined) and lower crust, the rock undergoes partial melting and yields the magma which then rises through the crust. It's a phenomenon that Luhr compares to melting ice on the roads in winter: just as salt lowers the melting point of ice, water lowers the melting point of rock.

Magmatic limbo: The lower you go, the hotter it gets.

The upper mantle is thought to be the source of magma columns that feed volcanoes.

Rising magma gathers in the magma chamber a few kilometers below the vent. As more magma enters, pressure inside the chamber increases.

Then, bingo -- an eruption drops the pressure and spreads a mess of hot rock onto the surface.

Volcano diagram

Since the hot, molten rock is less dense than the surrounding rock, it rises through buoyancy. And, after a few thousand years, the molten rock, called magma, rises toward the surface through cracks it finds or creates in the crust.

This upward flow of magma can take many forms, depending primarily on the chemical composition of the magma and the local geology. Both of these factors vary widely and somewhat unpredictably throughout the crust and upper mantle. Those complications make volcano forcasting an inexact science, but still a necessary one, as the 1985 death toll in Colombia (25,000) so visibly attests. (Volcanologists hate to call it "predicting," which implies a certainty they cannot attain. Volcano "forecasting," in contrast, carries the same sense of probability as weather forecasting.)

So, what's the forecast?


The Why Files
©1997, University of Wisconsin, Board of Regents.