SCIENCE / MEDICINE : The Mellow Volcano : Geology: Hawaii’s Kilauea and Washington’s Mt. St. Helens are miles apart in more ways than one. Islanders have learned to go with the flow.
Fiery tongues of lava from Kilauea Volcano have ravaged this coastal community. But compared to some of its cousins around the world, the Hawaiian volcano is downright polite. “At least it gives you time. It lets you take what you want, and you can watch it,” said Leslie Doctor, who stood mesmerized as the searing lava bore down on his family’s home.
Since the eruption began seven years ago, Kilauea has torched more than 160 houses, buried a highway and smothered more than 30 square miles in a blanket of solid rock. But the disaster has been played out in slow motion and nobody has been caught in the lava’s lethal grasp.
Compare that with the explosive blast of Mt. St. Helens, which devastated more than 200 square miles of forest in a matter of hours on May 18, 1980, felling trees like toothpicks. The eruption spewed 540 million tons of ash over 22,000 square miles and left 57 people dead.
Clearly, the two volcanoes are breeds apart. A glance at their profiles is enough to distinguish them. Kilauea has a gently sloping silhouette; Mt. St. Helens had a sharp, conelike peak.
But the differences run deeper, as deep as the molten rock of the Earth’s mantle.
Mt. St. Helens has its roots in friction, appropriately enough for such a mean-tempered mountain. The theory of plate tectonics holds that the Earth’s crust is divided into huge slabs that float on its hot, fluid mantle. As the plates collide or grind past each other they cause a commotion on the Earth’s surface.
In St. Helens’ case, scientists believe the Pacific Plate is forcing its way under the North American plate, generating the heat for the mountain’s outbursts.
Hawaii is a different story. The island chain lies smack in the middle of the Pacific Plate, 2,000 miles from its nearest boundary. There are no collisions here. The oceanic plate is skating toward Asia at a leisurely pace of only four inches a year, hardly enough to rattle an island riding on it.
Instead, scientists believe that an extremely hot area deep in the Earth’s molten mantle, called a fixed “hot spot,” is fueling Hawaii’s creation.
Scientists are engaged in a continuing debate over the nature of Hawaii’s hot spot. Despite the fact that Hawaii’s volcanoes are the most extensively studied in the world, no one is sure why there should be such a thing as a hot spot, or why it should remain fixed in one location as the Pacific Plate slides over it. There are other hot spots around the globe, but the Hawaii spot, which created the islands as well as a chain of undersea hills called “seamounts” that extend across the western Pacific, is the most dramatic.
The hot spot melts the Pacific Plate that is slowly drifting above it, and piles up one volcanic island after another. As the volcanoes age, they float away from the furnace, gradually extinguishing their flames.
The process is literally older than the hills. The hot spot, estimated to be 25 to 40 miles below the Earth’s surface, has been gurgling up molten rock steadily since the time of the dinosaurs. Kilauea, on the island of Hawaii, simply is the most recent to breach the waves.
The hot spot also is feeding Mauna Loa, Kilauea’s active neighbor, as well as a new island forming off shore. Kilauea’s youngest sibling, Loihi, won’t emerge from the ocean for at least 10,000 years.
“People call this destruction, but it’s the way the Earth was developed,” said Harry Kim, a civil defense administrator grappling with the effects of today’s flow. “Mankind has simply put himself in the way.”
Such geological forces may hold clues to the nature of the volcanic dramas on the Earth’s surface. Because the ingredients of the continental and oceanic plates differ, the molten rock they produce also varies.
Scientists say the chief factor determining the force of eruptions is the content of the magma stored in volcanic chambers. The more viscous, or sticky, the magma, the more explosive the eruption.
“If the lava is fluid, gas can escape more easily, like bubbles from a soda,” said Ken Hon, staff geologist at Hawaii Volcano Observatory, which is perched on the lip of Kilauea’s summit crater. “Kilauea’s lava is fluid, like syrup. At Mt. St. Helens, the lava is stiffer, more like taffy. The gas bubbles expand but they can’t rise through it.”
Instead, he said, pressure builds until finally the magma explodes with great force into tiny bits. Such blasts are called pyroclastic, literally “broken fire.” It takes much longer for such “stratovolcanos” to build up the energy for an eruption. Mt. St. Helens usually rests for hundreds of years between blasts. Kilauea’s clock runs in tens of years.
Hawaiian lava, known as basalt, contains less silicon and aluminum and more iron, magnesium and calcium than what erupted at Mt. St. Helens. It flows more freely, both below and above ground, gradually shaping the volcano’s gentle profile.
Eruptions in Hawaii typically begin with “curtains of fire,” vertical sheets of lava that spurt along cracks in the Earth. The fountains often settle on one vent and spout like enormous hoses. In the 1950s, Kilauea was called “the drive-in volcano” because it was so obliging to people who wanted to watch its fireworks.
Although the fountains can be spectacular (one shot nearly 2,000 feet in the air), the force behind them is relatively weak. By comparison, the 1980 St. Helens eruption sent ash more than 12 miles into the atmosphere.
When the fountains in a Hawaiian eruption subside, either they die or the magma migrates down-slope to a site of less pressure, where it oozes continuously from the opening. Today’s flow is welling up out of the East Rift Zone, a weakened swath on Kilauea’s flank.
“Kilauea’s lava fountains and flows can be seen, usually in plenty of time to get out of the way,” said Steve Brantley, public information scientist for the Cascades Volcano Observatory. “With explosive volcanoes like Mt. St. Helens, the sequence of events can begin very, very quickly.”
Although there were warning signs, including a bulge on the face of the mountain, no one anticipated the speed or force of the 1980 St. Helens blast. The eruption, according to Cascades Volcano Observatory scientists, was preceded by a landslide that dug a gash in the face of the volcano, allowing the highly pressurized magma to burst through, like a pressure cooker whose lid was knocked off too soon.
“You’d never have that circumstance in Hawaii if you had a landslide that exposed part of the plumbing system,” said Tom Wright, scientist in charge of the Hawaii Volcano Observatory. “You’d get some release, but it wouldn’t be anywhere near the force of St. Helens.”
Explosive eruptions, however, are not impossible even at Hawaii’s seemingly gentle volcanoes. Occasionally, as magma drains out of a vent, ground water interacts with the hot rock and flashes into steam, causing explosions.
In 1790, such an eruption blanketed Kilauea’s summit with as much as 30 feet of pyroclastic debris. About 80 Hawaiian warriors trekking across the volcano’s flank suffocated in the hot gas. Their footsteps, still discernible in the ash deposits, serve as a warning that, even in easygoing Hawaii, no volcano is entirely predictable.
