Upwardly Mobile

Natural History, April, 2001 by Henry S. F. Jr. Cooper

A Museum scientist analyzes what's inside volcanoes to predict how high they'll blow.

In the Gottesman Hall of Planet Earth is a ten-foot, grayish black cube: a cast of part of a debris flow from Mount Vesuvius that helped bury Pompeii in A.D. 79. Pieces of real rocks that stuck to the latex mold provide the cast with an authentic veneer, and missing bits have been painted in. At the bottom are the remains of a brick column crushed by the weight of the volcanic material.

"It's the stuff that buried the bodies found at Pompeii," says James Webster, a geochemist interested in volcanoes and a curator of earth and planetary sciences in the Museum's Division of Physical Sciences. "The level where the pillar is, is the same layer where the bodies were found. The interpretation is that the eruption, which had been going on for several hours, seemed to calm down, and the people who had left returned to their homes to collect their belongings. Then flow upon flow of pyroclastic [violently ejected] material came pouring down the side of Vesuvius."

Webster collected many of the volcanic rocks displayed in the Hall of Planet Earth. His research may lead to ways of predicting which of the world's 500 to 600 active volcanoes will produce the most explosive eruptions. Clearly the citizens of Pompeii would have appreciated some advance warning of these catastrophic events. So would the neighbors of Indonesia's Mount Tambora in 1815 and Krakatoa in 1883 and those of the Philippines' Mount Pinatubo in 1991, as well as the more than 20,000 Colombians killed by a lahar (a mudflow of hot volcanic material) when Colombia's Mount Ruiz erupted in 1985.

In his laboratory on the Museum's fourth floor, Webster shows me a large blue vessel capable of reproducing the temperatures and pressures corresponding to depths inside Earth where mag-mas form in the crust and upper mantle. The two-week experiment going on behind the tightly closed hatch has just started. It involves heating and squeezing powdered rock from a lava flow; gauges on the wall indicate that the current pressure is 30,000 pounds per square inch (equivalent to the pressure four and a half miles down) and that the temperature is about 1,000 [degrees] C. Webster is trying to remelt the rock particles to measure the concentration of one of the flow's volatiles, sulfur. Volatiles--the water and dissolved gases in magma--are what drive eruptions, propelling the molten material upward and eventually out.

Despite the miniature volcano seething inside the pressure vessel, the laboratory is a quiet place for a talk. Some geologists, Webster tells me, are attempting to predict eruptions by measuring seismic waves (a method that has achieved some success, particularly at Kilauea in Hawaii), while others measure the volatiles released from volcanoes. Webster tries to measure volatiles in the magma before an eruption. The most common ones--and those that provide the most explosive force--are water and carbon dioxide. Depending on the composition of the magmas, however, smaller quantities of sulfur, chlorine, and fluorine, as well as traces of nitrogen, hydrogen, and carbon monoxide, may also be involved.

The deeper the magma, the greater the pressure and the greater the quantity of volatiles that will be dissolved in it. As magma rises, the pressure decreases and the volatiles increasingly turn to vapor, pushing the magma onward and upward with ever increasing force. The richer the magma is in dissolved volatiles, the faster they are released as gases and the more explosive the eruption. The problem with looking for volatiles in the original melt is that by the time magmas reach the surface, cool into solid lava flows, and find their way into Webster's machine, the rocks have lost their volatiles. It's a little like looking for carbon dioxide in flat ginger ale. As the lava cools and crystallizes, however, crystals that measure between half a millimeter and tens of millimeters in diameter can retain droplets of "melt," or still-molten volcanic glass. The melt hardens as glass inclusions in the lava, trapping the volatiles inside what amount to tiny enclosed bottles and providing the scientists who open them with a measurable source.

One result of discovering this source has been to allow geochemists to take a closer look at the role of some of the less abundant volatiles. "We used to look at the rocks, and when we saw they didn't have much chlorine, we assumed that chlorine was not important," says Webster. "But over the last decade, looking at the melt inclusions, we find higher abundances of chlorine in many cases. One of the things we've been doing at the Museum in the last ten years is looking at the behavior of chlorine-rich magmas, and we've been focusing on volcanoes around the world that have higher chlorine contents."

In particular, Webster and Museum colleague Federica Raia, together with Benedetto DeVivo, of the University of Naples, have been looking at lavas from Somma-Vesuvius (Mount Somma is an older cone that forms a summit and ridge partly encircling Vesuvius). Over the past 25,000 years, Somma-Vesuvius has erupted dozens of times, but only seven or eight of these eruptions have been as catastrophic as the one that buried Pompeii. "We have been looking at melt inclusions in these magmas that date from 3,500 to 14,000 years ago, which is when we believe the break between Somma and Vesuvius occurred," Webster says. "What we have found is that the most highly explosive eruptions--the ones that bury communities and cause the greatest loss of life--show very different ratios of sulfur to chlorine in their melt inclusions and therefore in the magma." He is quick to add that other factors, such as the strength of the rock surrounding the magma, are important too.