By Mark Shwartz
Volcanology is a dangerous profession.
Hot lava, ash and noxious fumes are a few of the hazards researchers face when they approach a volcano.
But rather than risk their lives on the ground, a growing number of geophysicists are using satellite radar images from outer space to detect volcanic activity on Earth. This innovative technology allows scientists to monitor remote volcanoes anywhere on the planet from the comfort and safety of their own office.
In a new study published in the journal Nature, a team of Stanford geophysicists demonstrates the power of radar imagery. Using satellites provided by the European Space Agency, the Stanford group has discovered that a cluster of volcanoes in the Gal·pagos Islands are rising
simultaneously — an indication that they could erupt some time in the future.
Located off the Pacific coast of Ecuador, the roughly 20 volcanic islands that make up the Gal·pagos archipelago have been an inspiration to scientists ever since British naturalist Charles Darwin made his famous voyage there in 1835.
"A number of volcanoes in the Gal·pagos have erupted since Darwin’s time," says Sigurjon "Sjonni" Jonsson, a graduate student in geophysics and co-author of the Nature study.
"But until we did our satellite research, people didn’t know that so much volcanic activity was occurring there all at once," he adds.
A volcano is considered active if it grows or shrinks over time. The slightest change in size or shape is an indication of underground activity, such as the build-up of magma, which eventually may result in an eruption.
Satellite imagery
Jonsson and his colleagues collected data on the Gal·pagos from specially designed satellites that bounce radar waves off the surface of the islands from 500 miles (800 kilometers) in space.
The technique, known as synthetic aperture radar interferometry (InSAR), creates remarkably detailed radar images of volcanoes and other land formations in cloudy or clear weather conditions.
By comparing individual satellite "snapshots" taken on different days, researchers are able to detect tiny changes in ground movement, which are often precursors of volcanic activity.
Some scientists advocate the creation of an InSAR satellite network to serve as an early warning system for predicting eruptions.
The Stanford team focused on the two most geologically active islands in the Gal·pagos chain: Fernandina and Isabela (see map,
Fernandina consists of a single volcano that erupted in 1995. Neighboring Isabela, the largest island in the Gal·pagos, is made up of six volcanoes, one of which — Cerro Azul — erupted in 1998.
Isabela’s other five volcanoes, including one named Darwin, were thought to be at rest until Jonsson and former Stanford postdoctoral candidate Falk Amelung demonstrated otherwise. Amelung, co-author of the Nature study, is now at the Hawaii Institute of Geophysics and Planetology.
"We compared satellite images of Fernandina and Isabela taken on five separate days between 1992 and 1999," Jonsson explains.
Their analysis revealed that four of Isabela’s volcanoes — Wolf, Darwin, Alcedo and Sierra Negra — had risen during the seven-year period.
The rate of change in each volcano varied considerably from the northern end of Isabela Island to the south. Wolf volcano in the north had grown just over 3 inches (9 centimeters) taller, but Sierra Negra in the south had risen nearly 9 feet (2.7 meters) — a substantial increase made all the more ominous because Sierra Negra experienced a major eruption in 1979.
"Earthquakes at Sierra Negra indicated some activity there," Jonsson observes, "but it was a total surprise that the three northern volcanoes [Wolf, Darwin and Alcedo] were also uplifting."
What caused the volcanoes to rise simultaneously?
The likely answer, according to the Oct. 26 Nature study, is the "accumulation of magma in reservoirs beneath the summit calderas of each volcano."
In addition to magma uplift, the authors suggest that an earthquake fault in Sierra Negra’s inner caldera slipped as much as 4 feet (1.2 meters) in recent years, causing the crust above the magma body "to hinge upward like a trap door."
Overall, the Sierra Negra volcano has experienced "particularly dramatic and variable ground deformation," write the authors.
Cerro Azul also demonstrated a great deal of activity, having risen prior to its 1998 eruption, then subsiding at least 12 inches (30 centimeters) afterward.
Of the seven volcanoes in the study, only Ecuador showed no increase in size and is therefore believed to be dormant.
Fernandina and Isabela Islands together are only about 100 miles (160 kilometers) long. That makes the Gal·pagos the only known place on Earth with six actively rising volcanoes concentrated in such a small area.
Howard Zebker, co-author of the Nature study and a pioneer of InSAR technology, is at a loss to explain why so much volcanic uplift is occurring simultaneously.
"We don’t really understand this at all, which is what makes it exciting," says Zebker, who holds joint professorships in geophysics and electrical engineering at Stanford.
Tortoise rescue
The 1998 Cerro Azul eruption made international headlines when the Ecuadoran army came to the rescue of giant Gal·pagos tortoises
threatened by rivers of molten lava. Because a single tortoise can weigh more than 500 pounds, army helicopters were required to airlift the endangered reptiles to safety.
In fact, neither Isabela nor Fernandina Islands has many human
inhabitants. But Paul Segall, co-author of the Nature study, believes that satellite radar imagery could be used to detect volcanic activity in other areas of the world densely populated by people.
A professor of geophysics at Stanford, Segall says that thousands of lives could be saved if satellite imagery determines that supposedly "dormant" volcanoes are indeed active. He also points out that several volcanologists have been killed at Mount St. Helens in the United States and at other sites in recent years, but InSAR gives scientists the ability to track volcanoes from a safe distance.
"Our dream is to monitor all 500 to 1,000 active volcanoes on Earth using satellite radar," notes Segall.
Jonsson, whose graduate advisers are Segall and Zebker, was born and raised in Iceland, where volcanoes are an ever-present threat.
"When I was growing up, there were nine volcanic eruptions less than 20 miles from my home," he recalls. "I remember going to bed as a small child and seeing the bright red sky out my window."
A typical family excursion for young Sjonni often meant driving to an erupting volcano to get a closer look. Now that he has turned his childhood fascination with volcanoes into a scientific passion, Jonsson is able to download data at a computer terminal instead of driving to the edge of a bubbling caldera half a world away.
"It’s amazing that a grad student at Stanford can learn about the Gal·pagos without ever going there," observes Segall.