Somewhere on earth, a volcanic eruption is either happening or about to happen. About 1,500 of them have erupted at different times within the last 10,000 years and, because we never know when one becomes extinct, it is possible that any one of the 1,500 could spring into life again. Over five hundred have erupted within the past four hundred years. A volcanic eruption can occur at times because of a nearby earthquake triggering it but, in general, volcanic eruptions occur for a variety of reasons, all of which distinguish them from the world of earthquakes and tsunamis. From the times of Rome when Mount Vesuvius erupted and the entire town of Pompeii was smothered with lethal ash, volcanic eruptions have created intense interest. Pompeii’s ruins are still being studied at the present time, especially the findings of the outlines of the bodies of those who died because they were preserved by a special technique. Even the name of Pliny the Elder, a Roman leader who died in Pompeii’s destruction, has been taken to define the eruptions that are like the Vesuvius one. They are known as Plinian eruptions, continuous flows of pumice, ash, and volcanic gases forming a deadly cloud.
One of the easiest volcanic eruptions to study can be found in Hawaii. Mount Kilauea on the Island of Hawaii, often referred to as the Big Island, is an intra-plate volcano, that is to say it is erupting from inside a tectonic plate rather than at the junction of two plates. It is erupting almost daily at the present time and at such a low level of violence that its activity can be easily observed. Molten magma just oozes out. The entire island group that constitutes the state of Hawaii is formed from volcanic action and the Big Island, the island of Hawaii, is the principal actor at the present time. Other islands of the state were active in the past and this condition is a reminder of another feature of Hawaii—it is a hot spot. There are more than thirty of these hot spots around the world. What is a hot spot? They are places deep in the earth below the level of mountains and ocean crust from which magma is escaping to the surface. In sharp contrast to everything we see on the surface of the earth these hot spots do not move with respect to the surface of the earth. Instead the tectonic plates pass over them as they move.
The huge volume of molten rock that reaches the surface in the island of Hawaii over time from the hot spot is evident in the thousands of feet to which a volcano such as Kilauea has risen above sea level and its height is achieved after it has already risen many thousands of feet from the ocean floor up to sea level. Over a period of a few million years, Kilauea will move way from the hot spot as the Pacific Tectonic Plate on which it sits continues its westward movement. A new mountain will take shape over the hot spot and Mount Kilauea will gradually cool down to become inactive like the rest of the state of Hawaii. The long history of this process, over a period of more than seventy million years, can be observed today in the islands above and below sea level that stretch from Kilauea to the other islands of the state of Hawaii, and then across the Pacific all the way to the Kamchatka Peninsula of Russia. These islands form the Hawaiian-Emperor Chain and identification of their age, that is to say, the lapse of time since they were magma rising from the hot spot, tells the story of the movement of the Pacific Tectonic Plate over time.
The two tallest volcanic mountains on the chain lie on the Big Island of Hawaii. They are Mauna Kea and Mauna Loa. Each is more than 12,000 feet high but if their total individual heights, counting from the ocean floor, is calculated, they each stand more than 30,000 feet high, higher than that of Mount Everest in the Himalayas. The gentle rate at which magma flows upward into the mountains of Hawaii is common in intraplate volcanic eruptions. Yellowstone National Park stands on the remnants of an ancient volcano and it, like Kilauea, stands today on a hot spot on an intra-plate site. Furthermore, its activity is quite benign and so thousands of tourists visit it every year. But Yellowstone was not always so quiet. Three extremely large explosive eruptions have occurred there in the past 2.1 million years and scientists have estimated Yellowstone’s recurrence interval as about 600,000 to 800,000 years. It is difficult to be more precise with such long intervals. The most recent of these explosive eruptions was about 640,000 years ago. Given the estimates for recurrence rates it is possible, despite assurances from USGS experts that there are no indications now of the likelihood of such an event, that another explosive eruption could come within the next one or two hundred years.
The consequences of such an event are unimaginable. The eruption of about 640,000 years ago, among other things, covered most of what is now North America with six feet of hot ash. Thus the USGS, from time to time, issues reminders of the kinds of things that need to be considered so that the area is as prepared as possible for the future. There are potential future hazards that could affect as many as 70,000 people even though the area is sparsely populated and this fact has to be kept in mind by all who are responsible for the care of the park. The plateau on which the park sits was built by one of the earth’s youngest, but largest, volcanic systems. It has been the scene of eruptions for more than two million years. The three largest of these eruptions sent out ash that was so hot that it welded into sheets of rock. Each of the three produced a crater-like depression, a caldera, tens of miles wide, formed by the collapse of the ground surface into the partly emptied magma chamber beneath. Faults within the present caldera are small and they produce small earthquakes from time to time that reflect strains in the earth’s crust. The active hydrothermal system of Yellowstone is one of the largest on earth and, although accidents involving hot water occasionally injure visitors, these can be avoided if park regulations are followed.
At the places where tectonic plates interact, especially at subduction zones, most of the most violent eruptions of the twentieth century have occurred. The interaction of the Nazca and South American plates is one general location that experienced catastrophic eruptions, particularly because the coastal area is high in elevation and lahars rush down from eruptions to overwhelm the towns and cities below. The 1985 eruption of Colombia’s Nevado del Ruiz volcanic mountain carried plenty of warnings of the coming event, both in the minor eruptions that were observed for days before the main explosion and in the opinions given by local authorities. Despite these warnings, there seems to be little understanding of when one must escape to some protected place. In the case of Nevado del Ruiz, the lahars rushed down from the mountain, from 17,500 feet above sea level, devastating everything in their path. About 21,000 people lost their lives in one town, Armero, which only had a population of 28,000. The USGS scientists, particularly horrified by the things that happen to people at volcanic sites, decided to do something to alert people to the dangers and to inform them about how to avoid an imminent event. They devised a three-part action plan.
First, they produced a video depicting the typical phases of volcanic eruptions and what happens to people, buildings, and environment when they erupt. It was quite a scary video, deliberately so, not suitable for younger people. Second, they selected fifteen volcano sites from around the world to study intensely and to examine along with the various local authorities in order to have the details of a collection of representative case studies. The third element was instrumental usage for predicting when a volcano would blow given that the advance signals were evident. In 1991 the video was rushed to the Philippines when Mount Pinatubo was threatening to erupt. The day after it was shown on television, about 50,000 chose to evacuate voluntarily. A few days later the volcano erupted. Tens of thousands of lives had been saved. Convincing people to evacuate as they had done in the Philippines was the biggest challenge everywhere it was attempted but it was the key to survival. What also was secured by Mount Pinatubo’s eruption was a dramatic lowering of temperature worldwide for two years, more than countering the amounts that would have risen in that time as part of global warming.
Just as earthquakes pose enormous risks for big cities if their epicenters are nearby, so the proximity of cities to volcanic sites is an equally great risk and there are many cities in that kind of setting. Historically there are good reasons for such a condition. We tend to think of the destructive aspect of eruptions, and rightly so, but we need to remember that volcanic soils are among the most fertile anywhere. People were drawn to those great farming locations in earlier times and cities grew up there over the years. At least 500 million people live today under the shadow of a potential volcanic eruption and many big cities are included in that number—Tokyo, Manila, Jakarta, Mexico City, and Quito are examples of these. Mount St. Helens that erupted in 1980 was a good example of how to do the right thing. Advance indicators in the form of small vibrations alerted local officials to move access to the mountain farther back that had been customary. A day before the violent eruption a general clearance was ordered for everyone to move far away from the mountain and people did as they were told. The only casualties close to the mountain was a reporter and a man who had lived for much of his life near the mountain and refused to leave.
To compare the magnitude of volcanic eruptions geologists have developed a Volcanic Explosivity Index (VEI), similar in principle to the Richter Scale for measuring earthquake magnitudes. This index is based on the volume of explosive products and the height of the eruption cloud. Each category in the index represents a ten-fold increase in power over the previous one. Eruptions with magnitudes of 0 or 1, common patterns in Hawaii, ooze lava with little or no violent activity. Tristan da Cunha, 1961, was a 2; Iceland, 1973, was 3; Martinique, 1902, was 4; and Mount St. Helens, 1980, was 5. Mount Vesuvius, 79, and Krakatau, 1883, had VEIs of 6, and Tambora had 7. Tambora erupted in 1815 in Indonesia, presenting us with the greatest eruption known in history and giving us a glimpse into the incredible power and potential of such events. Their destructive power is frightening when we examine one like Tambora. Toba was a supervolcano of VEI 8 and it is the only one from ancient times for which we have a large volume of data, a valuable reference if our planet ever experiences a supervolcano.