Science Still Puzzled: How Do Volcanoes Really Work?
A Soviet scientist, climbing over lava beds still warm from the continuing eruption of Kilauea Volcano, paused as he tried to explain why volcanologists are unable to figure out just what makes some volcanoes explode with spectacular fury.
S. A. Fedotov of the Soviet Union’s Institute of Volcanology kicked at a chunk of lava and summed up the frustration shared by volcano experts around the world:
“We are trying to learn how the engine operates by studying the exhaust.”
Fedotov was one of more than 400 scientists who gathered here recently for a major international symposium held to commemorate the 75th anniversary of the Hawaii Volcano Observatory. The symposium, possibly the largest gathering of top volcanologists ever held, was titled “How Volcanoes Work,” but the title might have been more appropriate if it had ended with a question mark.
Move Into Laboratories
To be sure, scientists in recent years have moved more and more of their research from the mountain slopes into the laboratories where the tools of modern chemistry and geophysics have helped volcanology evolve from the purely observational to the analytical and experimental. This has shed some light on volcanoes, but much remains unknown and is left to guesswork.
By examining the evidence, such as rocks formed by volcanic activity--the “dead bodies in the crater,” as one geologist put it--scientists are beginning to understand much more about how molten rock moves up through the Earth’s crust and spills onto the surface, sometimes violently, sometimes benevolently, but always spectacularly.
And that research has forced them to re-examine ideas once thought absurd. For example, many years ago volcanologists rejected the idea that some volcanoes could be affected by long-term weather changes because, they thought, there was no link between weather patterns and the deep Earth forces that create volcanoes. But new research presented during the Hilo meeting indicates that climatic changes such as the end of an ice age indeed may have caused some volcanoes to erupt and others to die.
New technologies, such as better methods for dating volcanic activity, are making it possible for scientists to chart past volcanic eruptions with considerable accuracy, and chemical analysis is telling them more and more about the nature of those events.
May Have Common Source
They have learned through chemistry, for example, that the lava deposited on the sea floor of the mid-Pacific over a period of 70 million years is so similar that the entire chain of volcanoes that created the Hawaiian Islands must have sprung from a common source, although there is still considerable debate over the nature of that source.
The hope for the future is that other volcanoes around the world can be instrumented as extensively as those in Hawaii to further illuminate the dynamics of volcanoes.
But one major limitation appears to be an insurmountable barrier. The forces that power volcanoes are hidden so deeply that scientists can only study the results of those forces; it simply is not possible to bore holes 40 or 50 miles into the ground and place instruments several times deeper than the deepest wells in the world.
And that has left the scientists with only a “pretty good understanding” of how magma moves up through the Earth’s crust through what geologists call volcanic “plumbing.” But it does not tell them much about the deep forces that drive it, according to Robert W. Decker of the U.S. Geological Survey, who organized the Hilo symposium.
“As you go from surface to depth, the resolution gets less clear,” he said.
Grinding of Plates
In the last couple of decades, volcanologists have generally come to accept the theory that most volcanoes are created by the crunching and grinding along the borders of the Earth’s tectonic plates--the massive slabs of rock that make up the planet’s outer skin. The theory seems reasonable chiefly because most volcanoes are found along those borders, and the kind of violent geological activity that would spring from the collision of such massive structures is consistent with the explosive nature of most volcanoes.
Mountains, seas and the continents rest on the surface of the tectonic plates, which drift slowly around the globe on the molten mass that lies about 40 to 50 miles beneath the surface.
The movement of the huge Pacific Plate, which underlies most of the Pacific Ocean, is believed responsible for the “ring of fire” of volcanoes and earthquake faults that rim the Pacific.
On the West Coast of the United States, the Pacific Plate grinds horizontally against the North American Plate, creating the enormous fault system of California. To the north, the Pacific Plate is “subducted” beneath the North American Plate in the Aleutian Trench, creating the chain of volcanoes known as the Aleutian Islands.
According to the most widely accepted theory, the subduction zones are heated when the edge of one plate crunches beneath the other, causing a partial melting of rock on the bottom surface of the crust. Although the pressure caused by the grinding of the plates is widely accepted as the most probable mechanism for melting the rock, at least 20 other mechanisms have been proposed, Decker said.
Hawaii Far Removed
There is no single explanation that would fit all volcanoes. The Hawaiian volcanoes, for example, are more than 2,000 miles from the nearest plate boundary.
The heat, whatever its cause, melts the rock on the lower surface of the Earth’s crust, which rises to form an underground lake of magma, called a reservoir, several miles below the surface. As the magma intrudes into the reservoir, pressure builds up, straining the rocks around it. That, in turn, causes hundreds of small earthquakes as the surrounding rock fractures under the pressure.
Scientists can define the reservoir by plotting the epicenters of those earthquakes, since earthquakes do not occur in a molten mass. The plot shows the rocks around the reservoir, thus revealing its size.
As Decker put it, the intrusion of magma tells scientists that the gun is loaded, but it does not tell them when it is going to fire. “All that lets you know is that there’s an increased probability it will go,” he said.
As magma builds up in the reservoir, the temblors increase in frequency, a strong indicator that an eruption is near.
‘Don’t Know the Answer’
No one knows for sure what sets it off. “What triggers the eruption?” Stanford University geologist Gail Mahood said, repeating a question. “We don’t know the answer to that.”
The best understood volcanoes in the world are the ones that created the Hawaiian Islands because the volcanoes have been intensely studied for three-quarters of a century, and they erupt frequently enough for scientists to study them while they are actually erupting, rather than after the fact.
Much of the pioneering research on volcanoes has been carried out at the Hawaii Volcano Observatory. In the early days, the science consisted mostly of observing and logging seismic activity, volcanic eruptions and lava flows, aided by crude instruments.
One way to detect approaching volcanic activity is to measure any change in the tilt of the ground caused by the movement of magma toward the surface. The early instruments consisted of a small level platform with a pendulum suspended above, somewhat akin to a grandfather’s clock. As the ground tilted, a needle on the platform moved relative to the tip of the pendulum, thereby recording the tilt. But scientists were plagued by distortions caused by wind and earthquakes.
Today, electronic instruments can measure any change in tilt on almost a microscopic level, and that information is radioed instantly to scientists in the observatory.
Accurate Predictions
Tiltmeters have proven extremely valuable to volcanologists, but they can sometimes be misleading. Erroneous readings led to a warning a few years ago that the Long Valley area near Mammoth Lakes, Calif., was bulging, suggesting that a major eruption was near. That warning has since been lifted. However, tiltmeters are among the instruments used by scientists studying Mt. St. Helens in Washington, and they have been able to predict eruptions there with great accuracy since the initial blast in 1980.
Early researchers in Hawaii also used seismometers to measure earthquake activity to try to determine if an eruption was near, but scientists later learned that many of the “earthquakes” were actually trucks passing on a nearby road.
Today, the Hawaii Volcano Observatory uses orbiting satellites, computers and highly sophisticated instruments to study some of the Earth’s most benign active volcanoes, and scientists usually are able to determine whether magma is once again intruding into a volcanic reservoir, and whether an eruption is near.
The volcanoes here ooze rather than explode because the magma breaks through the thin layer on the surface during the early stages of magmatic intrusion, and it flows down the gentle slopes almost as quickly as the reservoir is resupplied from below, Decker said.
Recent evidence supports a generally accepted theory that the Hawaiian Islands were formed by a mysterious “hot spot” deep within the Earth that remains fixed in one spot as the plate moves over it, although no one knows why it stays in one place or what causes its intense heat. As the Pacific Plate moves over the fixed “hot spot,” the lower layer of rock is melted and pushed up through the crust, forming reservoirs of magma, which eventually become volcanoes.
Fit Hot Spot Theory
Scientists have found that the Hawaiian Islands are pretty much as they should be if they were in fact created by such a hot spot.
David A. Clague and G. Brent Dalrymple of the U.S. Geological Survey have documented some of the evidence that they say is consistent with the hot spot theory. If the theory were correct, “the volcanoes should become progressively older to the west and north as a function of distance from the hot spot,” the scientists reported during the Hilo symposium. In other words, the youngest volcanoes should be those at the southeastern tip of the chain, now over the hot spot, while those farther west should be the oldest. Dating of rocks from the volcanoes shows that to be the case, the scientists said.
The theory is further supported by the fact that volcanic rocks are similar throughout the Hawaiian chain, suggesting they were all formed by the same process.
The scientists concluded that “there is now little doubt” that the chain owes its origin to a fixed hot spot, but they said they are still mystified by the feature.
Hot spots are believed to have created volcanoes elsewhere, including scores of seamounts that dot the floor of the Gulf of Alaska, but those volcanoes do not conform to the tidy, linear pattern of the Hawaiian Islands, leaving scientists to wonder if they are dealing with one fickle hot spot beneath the gulf that wanders around, popping up volcanoes on an erratic basis, or with many hot spots.
Sharp Debate
Hot spots are so mysterious, in fact, that scientists attending the Hilo symposium sometimes engaged in sharp debate over whether the hot spot beneath Hawaii results from a plume of heat originating deep within the Earth’s core, or whether it is a shallow feature limited to the area just below the crust.
“There is scant information concerning the exact mechanism involved,” Clague and Dalrymple said. Even the term hot spot may be misleading because pressure, rather than heat, may be the driving force that causes the underbelly of the Earth’s crust to melt and push its way up to the surface.
Advances in volcanology have forced scientists to take another look at one idea they discarded long ago.
Gudmundur E. Sigvaldason, director of Iceland’s volcano observatory, startled fellow volcanologists during the Hilo symposium when he offered evidence that some volcanic activity may be influenced by such seemingly unrelated events as changes in long-term weather patterns.
He said changes such as the end of an ice age may have caused volcanoes thousands of miles apart to either erupt or die. Sigvaldason reached this conclusion after determining that receding glaciers may have triggered one of the most active periods in the history of the volcanoes that created his island nation.
‘Overburden’ Reduced
The receding glaciers, brought on by warmer weather, reduced the weight of the “overburden” that caps the volcanoes of Iceland, he said.
When the weight of the glaciers is removed through recession or melting, the pressures from molten reservoirs beneath the surface overpower the crustal strength and break through the surface, just like popping the lid on a pressure cooker, he said.
Sigvaldason said the discovery “started out with routine dating of lava. We found out that a large majority was deposited shortly after the glaciers retreated” in a major climatic episode 10,000 years ago. Lava production was greatest in the earliest years after the retreat began, he added, but it lasted for more than 3,000 years as the ice continued to melt.
The link between changing weather patterns and volcanic activity was in itself an important development, Sigvaldason said, but it became especially intriguing when he was discussing it last year with Mahood, the Stanford geologist.
Mahood, who had been studying a volcano in the Mediterranean Sea called Pantelleria, was struck by the fact that her volcano appeared to be in opposite phase with Sigvaldason’s in that when one was active, the other was dormant.
Role of the Weather
Both scientists now believe that weather changes played a major role in volcanic activity thousands of miles apart, and any partially submerged volcano could have been affected.
The huge glaciers in Iceland shut off the volcanoes there during the ice age because the weight of the glaciers was greater than the thrust of the magma, Sigvaldason said.
When the warmer weather came, the glaciers retreated and melted, thus removing the weight from the volcanoes and ushering in a new era of intense volcanic activity.
Similarly, when the ice melted it raised the water level of the world’s oceans dramatically. In the Mediterranean, the water rose on the undersea slopes of Pantelleria, in effect choking off its volcanic activity, Mahood said.
In the past, volcanologists had scorned climatic effects because they thought that such explanations offered a simplistic view of a complex event. When Sigvaldason first presented his findings in the Hilo symposium, several of his colleagues said they thought he had been joking, but Mahood said she believes that more and more scientists are beginning to accept it.
Dramatic Differences
Volcanoes differ dramatically around the world, and no one expects to discover a comprehensive formula for what makes volcanoes erupt. “They all have their own personalities,” Mahood said.
Many scientists are worried about volcanoes that are poorly understood but could have a major--possibly even worldwide--effect if they erupt. For instance, the volcano that lies beneath what is now Yellowstone National Park has shown signs of restlessness in recent years, including land deformation in some areas. But it has been studied so briefly, on the geological time scale, that scientists are unsure what the activity means.
Three times in the not-too-distant geological past, Yellowstone has exploded with a fury seldom seen on Earth.
The last time it erupted, according to Mahood, it covered much of what is now the Middle West with ash several feet thick.
“That would wipe out most of our agriculture for a year, maybe more,” she said.
Could Go Anytime
Yellowstone erupted 2 million years ago, 1.6 million years ago, and 600,000 years ago. It may not blow its stack for another 200,000 years, or it could go anytime in the next few years. No one knows for sure.
“So if you believe these things follow any sort of pattern, you might want to watch this one,” Mahood said.
Similarly, some of the volcanoes that make up the Cascade Range in the Pacific Northwest could devastate much of that area if they were to erupt, as Mt. St. Helens did in 1980.
The same threat can be found throughout much of the world, including on the slopes of Italy’s Mt. Vesuvius, where 70,000 people now live.
Although the volcanoes differ, the technology developed in Hawaii that lets scientists closely monitor subsurface activity could be transferred to other areas, thus greatly increasing the understanding of how volcanoes work, Decker said.
At the very least, he said, scientists should now be able to tell when an eruption is imminent, even if they do not understand exactly what is causing it.
“That information could save a lot of lives,” Decker added.
