Living With a Volcano

Living near an active volcano can be beneficial as well as dangerous. The soil is fertile, and a lot of volcanic products can be used in everyday life. Sulphur, for example, can be used as an ingredient in matches, or in medicine, while the finer volcanic deposits, such as the gravels and sands found in rivers, can be used in building materials. In addition, the thermal energy from some volcanoes can be used to generate electric power.

But if you live too close to a volcano—and it erupts—it can be lethal.

green hill with houses and narrow road in foreground, smoking volcano in background
Courtesy Barry Voight

If you live with a volcano like Merapi, it';s wise to watch it closely. Volcanoes provide fertile soil--as well as deadly hot hurricanes of lava and ash.

Mount Merapi in Central Java, Indonesia, is one of the most active volcanoes in the world. It is the most feared volcano in a country that has 129 volcanoes known to be active: About one million people live within 20 miles of Merapi, and the population of the surrounding towns is growing. Because of what he calls "the distinct potential for catastrophe," Barry Voight, a professor of geosciences, has been studying Merapi since 1988. I entered graduate school at Penn State in 1996 and joined him on the Merapi project a year later.

I grew up 300 miles west of Merapi, far enough away that only a major eruption would affect my hometown. But we were within 50 miles of another active volcano—within the risk limits of a major eruption. Since I entered college, earthquake study had been my primary interest. Earthquake activity around volcanoes has been linked directly to volcanic activity, and seismology is one of the main tools used to study volcanoes around the world. Currently, Voight and I, in collaboration with the Volcanological Survey of Indonesia, are using seismology to try to answer the following questions: What do we know about the interior workings of volcanoes like Merapi? What can we learn about the processes of magma flow? Besides attempting to forecast the timing of an eruption, can we anticipate the type of eruption?

Not all volcanoes are the same. Some are almost flat and others are cone-shaped. The shape of the volcano depends largely on the kinds of lava that have erupted. Cone-shaped volcanoes like Merapi commonly build up from repeated eruptions of viscous lava. Inside are layers of thick lava and broken clasts and ash from previous eruptions. Because viscous lava tends to plug up the volcanic vents and make it difficult for water vapor to boil off, the gas pressure inside the conduit can cause violent eruptions. Pieces of rock and a great deal of ash are hurled high into the air. Blocks of lava and clouds of ash flow like hot hurricanes down the sides of the volcano. Is there a way to predict a volcanic eruption like this?

From January 16 to February 23, 1998, we collected earthquake data from four seismic instruments deployed on the crater's rim. The instruments were 150 to 300 meters from the estimated center of the dome. Putting instruments on top of Merapi was not impossible: People have gone up to its top for years when Merapi is in its "quiet" time. Hiking up the volcano takes about three to four hours. Normally we'd start walking at about 1 a.m. to get to the top at dawn. We started in cloud forest and ended up in dwarfed vegetation and then on barren rock. It's warm at the bottom but could be cold on the top, though it's normally rather pleasant.

Going up without carrying heavy equipment was easy—as long as we did not mind climbing rock slopes of 45 degrees built of unconsolidated volcanic materials. Taking the instruments up was another story. We had about 20 boxes of equipment for the installation, each weighing about 50 pounds. Fortunately, I was accompanied by staff from the Volcanological Survey of Indonesia who helped me find locations to install the instruments. We hired local people who were used to climbing and carrying stuff to the top of the volcano to transport our equipment and supplies. The deployment involved roughly 25 people, and we had to stay over for three days on the top of the volcano. After that, I hiked up almost once every two weeks for one and a half months.

During the time our instruments were deployed, earthquakes occurred about three or four times a day. (As Voight says, "Being dangerous very often, Merapi offers the opportunity to test hypotheses: If you install instruments, an eruption will come.") The experiment yielded a fascinating data set of very unusual waveforms associated with most of the earthquakes. We refer to these low-frequency, long-period waveforms as very-long-period, or VLP, pulses.

These waveforms had never been recognized at Merapi before and only at a very few other volcanoes, since special broadband seismographs deployed near the crater are needed to see them. The epicenter of the quakes clustered in a central region of the volcano's lava dome complex, about 20 meters south of the north crater wall. The source was a few hundred meters under the top of the dome.

These seismic signals, we hypothesize, could have been caused by gas pressure associated with the escape of gases from rising magma. The gas might accumulate slowly within a shallow pocket in the conduit, building up pressure against the rock wall. When the pressure is greater than the rock's strength, the gas pocket can push the rock surrounding it, causing what's known as radially outward tilt, or inflation. As the gas is released, the pocket shrinks and the rock returns to rest, causing radially inward tilt, or deflation.

Another potential cause of these seismic signals is called the "stick-slip rebound" model. This occurs when a brief, episodic, and unsteady upward movement of magma occurs in the volcano's conduit. Earthquakes could be produced when the mostly crystalline, highly viscous magma coming up the conduit rubs against the rocks in the conduit wall. What we have detected as multiphase earthquakes could be the vibrations of the volcano wall caused by this magma movement.

Both mechanisms indicate that we have detected shallow magma movement near the summit of the volcano.

What does our research mean for the million people living around Merapi? First, as Voight says, "we have to separate our research objective from the practical objective." Right now, we are trying to understand a particular seismic process. If we are successful, it may convert to practical consequences.

Our study of earthquakes at Merapi is part of an attempt to learn what happens beneath the volcano before it erupts. So far, we have shown that our temporary seismic network at the rim crater can detect better earthquake signals from within the volcano. Particularly, we found that earthquakes having this very-long-period signature may be caused by shallow magma activity. If a lot of these earthquakes occur, we think it means that the volcano activity has become shallower and an eruption could occur in the near future.

Seismic studies alone are not sufficient to provide early warning. But in combination with other methods, such as deformation and volcanic gas monitoring and geological study, they will yield a better understanding of Merapi's behavior and will help the volcanology team to better alert people living around the volcano.

Dannie Hidayat is a graduate student in geosciences in the College of Earth and Mineral Sciences. His adviser is Barry Voight, Ph.D., professor of geosciences, 334A Deike Bldg., University Park, PA 16802; 814-865-9993; Their work is funded by the National Science Foundation.

Last Updated January 01, 2000