he very origins of the word volcano are founded in myth: Ancient civilizations believed that the lava and ash that erupted from the Earth came from the forge of Vulcan, the blacksmith of the Roman gods, who created thunderbolts for Jupiter, king of the gods, and weapons for Mars, god of war. Even as science has replaced the myth, volcanoes remain among the more mysterious phenomena in nature.
"It's very difficult to see what's going on under the earth," says Don Swanson, geologist with the U.S. Geological Survey, working out of the Hawaiian Volcano Observatory in Hawaii Volcanoes National Park. "We can talk about volcanoes in general terms, but we certainly don't understand everything there is to know about them."
That talk has increased in recent months, as eruptions at Mount St. Helens, a U.S. Forest Service site, garnered headlines, and geologists singled out volcanoes within the National Park System that are primed and ready to erupt, including Mauna Loa in Hawaii Volcanoes National Park. What's brought them to this point, and what prompts scientists to make such predictions?
Like earthquakes, volcanic eruptions are caused by incredible friction in the Earth's plates, miles below the surface, which explains why it's tough to record conditions or predict the timing of the eventual outcome. Most of the information that scientists can gather comes from seismometers, which measure tremors miles underground, and global positioning satellites, which measure swelling in the volcano and shifts in the Earth's surface as small as a millimeter.
Whereas most earthquakes are the result of two plates shifting and rubbing against each other as they pass in opposite directions, volcanoes are the result of a direct collision that forces one plate deeper into the Earth, making for a more violent impact. When one plate is pushed down about 100 miles into the Earth's hot mantle, complex chemical changes result in new magma, or liquid rock, which rises to the Earth's surface.
Of course, every rule has its exceptions. "Hawaiian volcanoes are very different from Mount St. Helens and all other volcanoes in the Cascade Mountain Range and most other national parks," says Swanson. "The Hawaiian islands are situated on top of a hot spot, kind of a Bunsen burner that causes magma to rise from the moving Pacific plate above."
Many of us have seen the brilliant images of Hawaiian volcanoes erupting, as bright orange geysers of lava bubble over the landscape; but most of us also watched as Mount St. Helens cleared its throat recently, spurting ash and rock into the air, even turning the skies gray for a few months in 1980. What makes these eruptions so different? Two major factors: the amount of gas generated by the underground activity, combined with the viscosity (or thickness) of the liquid rock.
"If you have a relatively low gas content and low viscosity [or thin liquid], you'll get the Hawaiian type of eruption, with slow lava coming out and gas escaping," says Swanson. "But with the opposite-lots of gas and viscous, sticky liquid-gas bubbles form as the magma rises, but they can't easily escape, which can lead to a great buildup of pressure and a violent eruption." Geologists like to compare the mechanics to a shaken bottle of soda-the shaking activates the gases in the liquid, and as they emerge, so too does the soda. If the liquid is more like cold molasses, more pressure is needed to expel the contents, and the process can be much more sudden and violent.
In spite of the progress of science, myths and legends still have a place in the way many people interpret and understand volcanoes. According to early Hawaiian traditions, a violent struggle between the goddess Pele and her sister created the Hawaiian islands, the trail of destruction and renewal left in their wake. Scientists have even adopted the myth, in a way: To this day, long thin strands of volcanic glass created by eruptions on Hawaiian islands are called Pele's hair, a fitting tribute to the Hawaiian goddess of volcanoes.
