Introduction to The Biggest Eruptions on Earth
When Mount Pinatubo in the Philippines began erupting in June 1991, the catastrophe made headlines around the world. Almost 300 people died during the initial blasts, and eventually hundreds of thousands lost their livelihoods. It was the fourth-largest volcanic eruption of the 1900's. Geologically speaking, however, the Pinatubo eruption was comparatively small in scale. Thousands of times during Earth's history, volcanic eruptions have erupted at least 100 times more lava, ash, and dust.
Fortunately, eruptions on this scale occur at widely spaced intervals. According to most estimates, only one has happened since recorded history began. That disaster struck in April 1815, when Indonesia's Mount Tambora exploded. The volcano blasted into the sky a column of rock and ash so huge that it completely blocked the sun for 600 kilometers (370 miles) around the volcano. Even larger eruptions occurred during prehistoric times at Yellowstone Lake in Wyoming, Long Valley in California, and Valles volcano in New Mexico's Jemez Mountains. At these sites—and in virtually every other country where there are volcanoes—eruptions from time to time have buried the landscape deep in ash and debris. Some of those eruptions disrupted weather patterns around the globe, and some may even have killed off entire species of plants and animals.
At present, no one can tell where the next massive eruption will occur, or whether any person living today will be around to see it. But geologic history indicates that, sooner or later, another gigantic blast or huge flood of lava will break out somewhere in the world.
Evidence of Giant Eruptions
Experts say that if they understood the processes leading up to an eruption better, they could identify in advance the danger signs, and people could take steps to avoid the worst of the catastrophe. Many researchers worldwide devote their careers to this study. Some concentrate on finding evidence of prehistoric eruptions, a branch of geology known as tephrochronology. Other scientists learn about the smaller eruptions that occur at various sites every year. Their studies concentrate on the geologic activity that produces all eruptions, no matter what size.
In the late 1800's, geologists first began looking for evidence of huge eruptions. Since then, they have identified about 80 volcanoes that at one time have erupted at least 100 cubic kilometers (24 cubic miles) of rock and gas, which is about 20 times the volume erupted by Mount St. Helens in Washington state in 1980.
Many more sites of massive eruptions go undetected, because the largest and most violent eruptions produce the least visible volcanoes. Only relatively weak eruptions allow rock, ash, and other debris to pile up around a crater, building the steep, cone-shaped formation that typically marks a volcano. An extremely violent eruption, on the other hand, blasts its lava far away. Such an eruption will most likely create a depression in the ground where the surface collapsed to partially fill the void where molten rock had been. Volcanic depressions, called calderas, can be so large—some 100 kilometers (60 miles) across—that the complete circular outline can be detected only by looking at a satellite image of the Earth.
Once geologists discover the site of an ancient eruption, they begin studying the volcanic rock on the ground. Mapping the distance of the deposits from the volcano enables geologists to determine the force of the eruption. By identifying the layers of ash, lava, and other material in the rock, geologists can determine the order in which the debris erupted. Measuring the thickness of the layers indicates how much material was thrown out.
Finally, dating the deposits provides an estimate of when an eruption occurred and how much time elapsed between eruptions if there was more than one. The best method of dating volcanic rock is a technique called argon-argon dating, in which scientists measure the ratio of two radioactive forms of argon in the rock. One form of argon changes into another over time at a known rate, so measuring relative amounts of each is a way of calculating the rock's age.
The Forces Underground
These examinations, as well as studies of present-day eruptions, have helped geologists develop theories about the forces that cause massive eruptions. The eruptions probably begin in the same way that smaller ones do—with currents of heat rising from especially hot points far below Earth's surface. These hot points are thought to exist along the boundary between Earth's inner core and the mantle (the middle layer, sandwiched between the core and the outer crust).
As a current of heat rises slowly through the mantle over periods lasting millions of years, some of the mantle rock melts. The molten rock, called magma, contains dissolved gases, which make it lighter than the solid rock surrounding it. Being lighter, the magma worksits way upward. It continues until it reaches the base of Earth's crust, which varies from about 8 to 80 kilometers (5 to 50 miles) below the surface. Crustal rock is less dense than the magma, so the crust often acts like a ceiling, blocking the magma's path and causing it to poolat the top of the mantle.
The magma may remain trapped under the crust for a very long time. As fresh magma wells up from below, the pool gradually becomes larger. The reservoir eventually becomes so huge and so hot that the lower crust begins to melt. Because molten crustal rock is buoyant, it begins moving upward, through the crust. The magma eventually rises toward the surface, where it is stopped by upper crustal rocks. Barred from further upward movement, the magma collects in a pool called a magma chamber.
Massive eruptions owe their size to the enormity of their magma chambers, a fact scientists learned by “measuring” the chambers below various volcanoes. To do that, geophysicists monitored seismic waves (vibrations from earthquakes). The vibrations are affected differently by passing through molten rock compared with solid rock. By monitoring the seismic waves that pass through the region below a volcano, scientists can map out the contours of the volcano's magma chamber. In the 1960's and 1970's, for example, seismic studies of the huge volcano in Yellowstone National Park revealed that its magma chamber was about 100 kilometers (60 miles) across. A chamber that size could hold about 80,000 cubic kilometers (20,000 cubic miles) of magma.
Clues to the Type of Eruption
Just as seismic studies tell scientists how much magma a volcano may hold, examining the composition of volcanic rocks gives clues to exactly how the magma reached the surface. Certain types of rock indicate the volcano erupted in an explosive blast, and other kinds show that the lava poured out more smoothly, in a fiery flood.
Pumice rock is the telltale sign of an explosive blast. This light-weight, frothy looking rock is solidified lava. The rock contains tiny holes where gas escaped violently from the lava as it was erupted.
Pumice forms from types of magma called rhyolite and dacite. Both contain high levels of silica, which makes magma “sticky,” in that it holds dissolved gases. Before an explosive eruption, this sticky magma carries trapped gases with it as it moves up through the Earth. Eventually, huge quantities of hot magma pool inside the magma chamber. The reduced pressure in the top few kilometers of the Earth's crust frees gases from the magma. As the gases begin to escape at a furious rate, the pressure of the gas trapped inside the chamber becomes greater and greater. Finally, the pressure causes an explosion, ripping a hole in the surface of the Earth.
If geologists discover that all or most of the rock near a volcano is a type called basalt, on the other hand, they assume that the eruptions were much less explosive. Basalt, which forms chiefly in the mantle, contains much less silica than rhyolite and dacite and therefore is not very sticky. Another difference is that basaltic magma is hotter than the other types. A magma chamber filled with basaltic magma does not develop the high gas pressure of a chamber containing rhyolite and dacite, and thus it is much less likely to explode. Instead, the growing pool of magma continues melting the rock above it until it finally breaks through to the surface, often at several deep cracks many kilometers long.
An Explosion That Shakes the Earth
What would it be like to live through a massive eruption? Historical records of the Tambora eruption, as well as evidence gathered from present-day smaller volcanic blasts, have helped geologists understand how the catastrophe would unfold. For an explosive eruption, the first signs of disaster would be an intense swarm of earthquakes. Then there would be a tremendous rush of gas out of the vent. The gas, dense with rock particles and propelled by the force of the explosion, travels so fast that it creates a sonic boom. The highly destructive blast levels almost everything in its path, flattening trees and wiping bare most of the landscape for perhaps 1,500 to 2,000 square kilometers (600 to 800 square miles) around the vent.
Fragments of lava, ash, and other debris explode from the vent at the same instant. Rocks fly out to distances of more than 5 kilometers (3 miles). Fast-moving clouds of steam, gas, and rock particles burst horizontally from the vent. Most of the debris, however, is propelled straight up into the sky in a gigantic column. This spectacular formation of hot, buoyant debris is called a Plinian column after the Roman scholar Pliny the Elder, who died when Mount Vesuvius erupted in A.D. 79.
Into the Upper Stratosphere
By measuring the distance of debris from the vents of ancient volcanoes, geologists learned that Plinian columns may rise up to 50 kilometers (30 miles) into the air—higher than jet airplanes fly, higher even than the ozone layer in the upper stratosphere. At the top of the Plinian column, the debris is spread by the wind into an immense canopy, which acts like a huge blanket, obliterating the sun and creating total darkness for hundreds of kilometers around the volcano.
A day or two after the start of the eruption, the gas pressure in the magma chamber drops, and the Plinian column finally begins to collapse. About half the ash and rock in the column falls to the ground and then sweeps out in all directions in what geologists call pyroclastic flows. Each pyroclastic flow pours over the landscape on a cushion of turbulent hot gases. It races at speeds of up to 300 kilometers (190 miles) per hour, and its tremendous momentum can easily carry it over hills and may propel it for distances of 50 to 150 kilometers (30 to 90 miles) or more.
After the passage of a large pyroclastic flow, the countryside appears totally changed, with all structures and all living things destroyed. The flow leaves behind hot ash that levels off the landscape by filling valleys and depressions to great depths while thinly covering hills. If this ash is hot enough, and if it accumulates rapidly enough, it forms a dense rock called welded tuff, which looks like concrete when it has cooled.
If a pyroclastic flow slams into a lake or river, a huge mudflow will surge downstream and over low-lying ground, destroying everything in its path. If the pyroclastic flow sweeps into a very large lake or an ocean, it may create giant waves called tsunami that swamp low-lying coastal areas and islands.
Scientists call airborne volcanic debris tephra, a term that describes fine ash as well as boulders tens of meters across. The boulders and other large particles fall quickly back to the ground close to the volcano. Finer tephra falls to Earth after being dispersed by the wind, and the ash may travel an amazing distance from the vent. About 50 kilometers (30 miles) away, the ashfall from a massive eruption of 50 cubic kilometers (12 cubic miles) will cover the ground to a thickness of 10 meters (33 feet). At 175 kilometers (110 miles), the ash may be 1-meter (3-feet) thick. Even as far as 2,250 kilometers (1,400 miles) away—the distance between New York City and Houston—the ashfall may be 1-centimeter (0.4-inch) thick.
The Aftermath
A single volcano may erupt in this fashion several times, though one eruption in a single period of activity is usually much bigger than the others. Eventually, unless the supply of magma is renewed from deeper levels, most of the gas-rich magma is ejected. The remainder then cools and solidifies underground over a period that may be as long as several thousand years. Shrinkage cracks form as the rock cools, opening the way for the underlying basaltic magma to break through. This cycle of events has happened at least three times at Yellowstone, with lesser volumes of basalt following eruptions of thousands of cubic kilometers of rhyolite. Where the supply of basaltic magma is especially large, basaltic flood eruptions may take place.
Lava Floods and Fire Fountains
Although flood eruptions do not produce Plinian columns or pyroclastic flows, a massive flood eruption would be a spectacular sight. The basaltic magma pours through surface cracks in huge glowing red sheets. “Fire fountains” of lava shoot up high into the air. The heat can cause adjacent forestland to erupt into flame. If the lava flows into rivers and lakes, it creates mudflows, tsunami, and gigantic eruptions of steam. Airfall tephra and poisonous gases are a hazard to plant and animal life even at great distances.
The largest known flood eruptions in North America began 17 million years ago and continued, on and off, for the next 3 million years. These eruptions produced a total of about 200,000 cubic kilometers (50,000 cubic miles) of lava. That rock is the Columbia Plateau, which covers large portions of Idaho, Oregon, and Washington.
By dating some of the layers of lava produced during these eruptions, scientists estimated that up to 100 cubic kilometers (24 cubic miles) erupted in a single week, which means that lava was flowing from the volcano on average at about 165,000 cubic meters (6 million cubic feet) per second. At such a rate of flow, one week would be enough time to cover an area the size of the state of Washington knee-deep in lava.
Gigantic eruptions of basalt on the sea floor dwarf even those that created the Columbia Plateau. Sea-floor eruptions tend to be of monstrous size, probably because the crust on the ocean floor is thinner, and thus more easily melted through, than the crust that makes up the continents. For example, seismic studies of two underwater plateaus formed by flood eruptions about 115 million years ago—Ontong Java, north of the Solomon Islands in the southwestern Pacific, and Kerguelen in the southern Indian Ocean—revealed that those features were each formed by eruptions of about 250 times more lava than created the Columbia Plateau.
Eruptions That Warm the Earth
Submarine eruptions such as these may discharge so much volcanic gas that they significantly alter the chemistry of the oceans and the atmosphere. The eruptions at Ontong Java and Kerguelen, for example, released vast amounts of carbon dioxide, sulfur, phosphorus, and nitrogen.
According to a theory proposed by scientists Ken Caldeira and Michael Rampino of New York University in New York City, those eruptions may have indirectly led to the creation of much of Earth's petroleum deposits. Petroleum deposits are formed from layers of dead plant and animal matter subjected to extreme pressure as they are buried in sedimentary rock over millions of years.
Caldeira and Rampino set forth the following scenario. First, the eruptions released gases including carbon dioxide, one of the so-called greenhouse gases. In the atmosphere, greenhouse gases trap energy from the sun somewhat like the windows in a greenhouse. According to Caldeira and Rampino, the extra carbon dioxide in the atmosphere helped cause an increase in average global temperatures and spurred plant growth. As polar ice melted and as lava spilled out on the sea floor, sea levels rose. Tiny marine organisms called plankton flourished in the warming waters, which now contained large amounts of nitrogen and other chemical nutrients released by the volcanoes. When those marine organisms died and settled to the sea floor, they provided the raw material that eventually became the petroleum deposits.
Eruptions That Cause A Cooling Effect
Several massive eruptions of the explosive type, on the other hand, have been followed by unusually cool weather. The process begins as sulfur dioxide and other volcanic gases sent high into the atmosphere combine with hydrogen and oxygen to form droplets of sulfuric acid. These drops, which may remain aloft for years, scatter incoming radiation from the sun. At the same time, airborne dust particles absorb heat radiating from the Earth. Both lead to lower average temperatures at the surface.
Many scientists believe that even the relatively small eruption of Mount Pinatubo temporarily cooled the world's weather. The more severe Tambora eruption caused a much greater temperature decrease. Historical records show that the year after the blast was known as “the year without a summer.” In the Northern Hemisphere, summertime snows and frost ruined crops.
Volcanoes and Ice Ages—cause and Effect?
A few geologists have even proposed that series of gigantic explosive eruptions may have triggered the ice ages (periods during which enormous sheets of ice covered huge areas of Earth). Geological records do show a link between the timing of some of the largest known eruptions and the beginning of the most recent ice age about 2 million years ago.
Another small group of experts argue that the relationship is reversed: the ice ages triggered volcanic activity. Those researchers say that the build-up of ice creates great stress on Earth's crust, which leads to the eruptions. It may even be possible that both theories are correct. The ice ages may trigger eruptions, which further cool the climate, causing the ice ages to be more intense and prolonged than they would have been otherwise.
The debate will probably not be resolved until scientists discover a way to determine much more precisely when the ice ages began and when the known massive eruptions occurred. Until then, determining whether there is a relationship between the two—and, if so, what it is—is virtually impossible.
A Link to Mass Extinctions and Dinosaurs
Massive eruptions may be involved in yet another geologic mystery—why large numbers of plant and animal species suddenly died out during at least five periods in the past 245 million years. One of the most recent of these extinction periods occurred about 65 million years ago, when the last of the dinosaurs died off.
Many theories have been proposed to explain one or more of the extinctions. The most popular current theory is that the extinctions happened after a comet or asteroid struck the Earth, throwing up huge clouds of dust that blocked the sun and cooled Earth's climate or destroyed the ozone layer. In the cold and darkness that followed, many plants and animals perished. A second and less-popular theory is that massive volcanic eruptions causing dramatic temperature changes were to blame.
In the late 1980's, some experts attempted to combine the two arguments by linking comet showers to flood eruptions. These scientists proposed that several mass extinctions were caused by comet showers that jolted Earth severely enough to trigger intense outpourings of lava. The combination of eruptions and impacts would have disrupted the environment so severely that many species and even entire families of organisms would have become extinct.
In 1990, geologists reported the discovery of what appeared to be a huge crater on the coast of Mexico's Yucatan Peninsula, a finding that supports the theory that an asteroid impact led to the mass extinction about 65 million years ago. But scientists continued to evaluate evidence that volcanoes may have played a direct role. One clue was reported in a 1991 study of one of the largest known eruptions, an outpouring of about 1.6 million cubic kilometers (400,000 cubic miles) of basalt lava in Siberia. Researchers Asish R. Basu of the University of Rochester in New York and Paul R. Renne of the Institute of Human Origins in Berkeley, Calif., found that this eruption occurred between 248.3 million and 247.5 million years ago, about the time of Earth's most devastating mass extinction, when up to 95 percent of all marine animal species died out.
Predicting the Next Big One
A huge eruption occurring in the future is likely to cause serious problems. An explosive blast at Yellowstone, for example, would produce enough ash to cover places as far away as New York City, Boston, and Montreal to a depth of more than 3 centimeters (1 inch). The falling ash would create intense electrical disturbances, blacking out television, radio, and telephone signals. Electric power systems would shut down as ash collapsed transmission lines and clogged generators. And because tephra causes jet engines to stall, travel by air would be impossible.
These problems would only trouble people living outside an approximately 40-kilometer (25-mile) ring around the volcano. Those inside that ring would almost certainly perish in the initial blast or the later pyroclastic flows. But even people living on the other side of the world might suffer from the eventual temperature drop and poor harvests.
Fortunately, scientists see no immediate threat from Yellowstone or any other volcano. But experts do believe there is a need to identify likely sites of future massive eruptions, in order to recognize early signs of activity and give emergency workers time to plan.
New Instruments For Detection
Since the 1960's, geophysicists have developed new instruments and techniques for monitoring the activity of volcanoes before and during eruptions. These advances have greatly improved scientists' ability to forecast eruptions both large and small. For example, using gas analyzers, geochemists can measure changes in the composition and amount of gases escaping from a vent, changes that may signal an impending eruption. Tiltmeters and strain gauges can detect the expansion of the surface of a volcano. Such expansion means that gas pressure is increasing, a sign that an eruption may soon occur. Extremely sensitive seismometers, instruments that record seismic waves, allow experts to track the movement of magma under a volcano and estimate when it may begin to break through to the surface. These techniques enabled geologists to forecast in advance all eruptions at Mount St. Helens after the initial blast in May 1980. Geologists had similar success in forecasting the major eruption of Mount Pinatubo and the ongoing eruptions at Kilauea in Hawaii.
A Quiet—but Haunting—menace
Where will the next massive eruption occur? One possible candidate is the little-known Curtis volcano in the Kermadec Islands near New Zealand, whose surface rose 7 meters (23 feet) between 1929 and 1964. Another is the volcano that formed the Japanese island of Iwo Jima, where a similar uplift of the surface has amounted to about 8 meters (26 feet) since 1945. Other possibilities include volcanoes at Long Valley in California, the Phlegraean Fields caldera in Italy, Rabaul in Papua New Guinea, and Yellowstone.
There are doubtless many other sites, as yet undetected, where geological unrest will one day lead to a massive eruption. The real question is when, because big eruptions happen at intervals so long that a species like ours may evolve, flourish, and die off without ever experiencing one. Some 600,000 to 800,000 years elapsed between each of the three most recent major eruptions at Yellowstone, for example. The last big eruption there took place about 600,000 years ago. Although the numbers indicate that Yellowstone might erupt on a massive scale at any time, no one should be surprised if it remains quiet for tens of thousands of years.
But the menace of dormant volcanoes continues to haunt us. After all, Earth is ringed with thousands of volcanoes, and geologists have identified only some of these sleeping giants. It is entirely possible that somewhere on Earth, sometime in our lifetime, a volcano will erupt with such fury that Mount Pinatubo will pale into insignificance.