MIT geologists have made the first reliable estimates of the amount of water dissolved in the underground molten rock associated with the world's most violent volcanoes-those that make up the so-called "Ring of Fire" in the Pacific.
They have done so by recreating in the lab the conditions under which this molten rock, or magma, forms.
Scientists have long known that water dissolved in this magma contributes to the volcanoes' explosiveness, but until now they had no way to estimate how much was there. "That's because the water is completely released as a gas during the explosion. As the liquid magma approaches the surface, the solubility of water in the magma decreases and it boils off," said Timothy L. Grove, a professor in the Department of Earth, Atmospheric and Planetary Sciences (EAPS).
As a result, Professor Grove said, "people could only guess at the amount of water present, although conventional wisdom held that it was low."
The MIT scientists found, however, that Ring of Fire magmas actually contain from two to four weight percent water. "That's four to eight times more than previously thought," Professor Grove said.
Graduate Student Glenn A. Gaetani of EAPS, Wilfred B. Bryan of the Woods Hole Oceanographic Institute, and Professor Grove reported their results in the September 23 Nature. In addition, Mr. Gaetani will present a paper on the subject next month at a meeting of the American Geophysical Union.
To arrive at their conclusion, the geologists developed new techniques that allowed them to recreate in the lab the high temperatures and high pressures that exist in magma chambers under the surface of the Earth. Using a special furnace and pressure vessel, they melted natural lava-the end product of an eruption-under a variety of experimental conditions. The procedure "directly reproduces the conditions under which [Ring of Fire] magmas form," said Professor Grove.
Overall, Professor Grove said, the work and the estimates it has produced "will allow people to develop realistic models of the processes that lead to the generation of these magmas."
TO UNDERSTAND A VOLCANO
The Earth's crust is divided into several plates that slowly move against and under each other. The volcanoes along the Ring of Fire form when the cold, dense, water-laden oceanic crust of one plate sinks down into the Earth's mantle at the border of another, lighter plate. "But when the dense crust gets to a certain depth," Professor Grove said, "the minerals that store the water become unstable, and the water is released and rises up into the overlying mantle."
That water interacts with and induces melting in the mantle, and the resulting water-bearing magma then rises through the mantle and into the overlying crust. The water finally boils off as a gas as the magma ascends through the last 10 kilometers of the shallow crust and the pressure drops.
While such subduction volcanoes-named for the "subducted slab" that takes the water down-account for only 20 percent of the volcanic activity on Earth, they are the most violent. "It's this decompression and boiling of water that makes them so dangerous," Professor Grove said. Hence the interest in defining how much water is dissolved in these volcanoes' magma.
To answer this question, the MIT geologists recreated the magma in the lab by melting samples of solidified lava under a variety of experimental conditions-with and without water, and at high and low temperatures and pressures. Because such initial conditions determine the mineral compositions in the resulting rock, the scientists could match the compositions produced in their experiments with those of natural rock from subduction volcanoes.
And when they did this, they found that the composition of minerals preserved in the natural volcanic rock closely resembled the experimental mineral compositions produced by melting the lava with relatively high amounts of water and at a relatively low pressure and temperature.
After reviewing the MIT scientists' work, Trevor Falloon of the University of Bristol in the United Kingdom wrote in an accompanying News and Views article in Nature, "their conclusion seems inescapable: [these] magmas must have high and significant water contents."
HELP FROM MATERIALS SCIENCE
Professor Grove notes that one of the most difficult parts of the work was developing the technology that allowed them to carry out the experiments. For example, he said, it was difficult to find a material that could be used to make the vessel that contains the sample. It turns out that the water and hydrogen gases critical to the experiments can escape through most materials at such high temperatures and pressures.
In tackling such problems the geologists turned to MIT researchers in materials science. "That's one of the great things about this place-you can talk to people doing different things, and often get an idea for doing an experiment in a better way," Professor Grove said.
The materials science researchers "turned us on to a new fabrication process for the pressure vessels that we were using that significantly increased their upper pressure and temperature limits," Professor Grove said.
Professor Grove is excited about applying the new technologies to the study of very old volcanic rock (by geologic standards, the volcanoes that were the subject of the current study are quite young).
Questions he is interested in include how volcanic activity took place during the first 500 million years of Earth's history, and how water might have been involved. Research in such areas, he concluded, will contribute to our understanding of the early Earth.
A version of this
article appeared in the
November 3, 1993
issue of MIT Tech Talk (Volume