LBL Scientists study Possible Method to Predict Severity of Volcanic Eruptions

October 22, 1993

By Lynn Yarris,

A life-saving method for predicting the severity of a volcanic eruption could be in the offing with the discovery of a correlation between the chemistry and volume of the lava produced.

Don DePaolo and Chang-Hwa Chen, at LBL's Center for Isotope Geochemistry, led a study of Japan's Mount Unzen volcano that revealed significant differences in the neodymium isotope compositions of large and small lava eruptions during the past 300,000 years.

Says DePaolo, "Our results suggest that neodymium isotope compositions of early or precursory eruptive lava could be a qualitative indicator of the maximum size of a continuing or impending eruption and might be used in volcanic hazard evaluation."

Joining DePaolo and Chen on this study were Setsuya Nakada, of Kyushu University in Japan, and Yuch-Ning Shieh, of Purdue University.

Mount Unzen is located about 25 miles east of Nagasaki on the southern island of Kyushu. Since it began erupting explosively in 1991, the volcano has claimed nearly 50 lives and left some 2,300 people homeless.

The history of eruptions for Mount Unzen extends over a period of about 275,000 years and is divided into three main stages. DePaolo and Chen and their colleagues collected 18 samples of lava from all three stages and from pre-Unzen volcanic activity in the vicinity. The samples were analyzed for major elements and for the isotopic compositions of neodymium, strontium, and oxygen. Only the neodymium isotope concentrations were consistently different for different samples of lava.

Says DePaolo, "We found that the neodymium isotope composition is more mantle-like in large-volume eruptions (greater than 0.10 cubic kilometers) than in small-volume eruptions (0.02 cubic kilometers or less)."

The proposed explanation is that the volume-composition of lava is controlled by the rate at which magma (the name for lava when it is underground) rises up from the Earth's mantle and enters the crustal reservoir, the chamber underneath a volcano in which magma is stored, prior to an eruption.

A rapid rate allows little time for stored magma to melt and mix with the cold rock of the chamber walls. This results in large volumes of lava whose isotopic composition varies little from that of mantle rock.

A slower rate allows more storage time in the chamber which means more extensive mixing of magma and rock in the Earth's crust -- a process that cools and solidifies the magma. The result is smaller volumes of lava with an isotopic composition more like that of crustal rock.

The isotopic composition findings of DePaolo and Chen and their colleagues reflect volume-composition patterns that have been found in volcanoes across the western United States, particularly in Yellowstone National Park and around California's Mammoth Lakes. This has led at least one volcanologist to note that had the isotopic signature of lava eruptions been known back in May, 1991, scientists could have safely predicted that the eruption of Mount Unzen was going to last for at least another few months and would erupt twice as much more lava.

DePaolo agrees that the sensitivity of neodymium isotopes for assessing potential eruption size is consistent for volcanoes such as Mount Unzen, in which the time-scale for the formation of the magma chamber can be measured in decades, the isotopic contrast between mantle and crustal rock is moderate, and the amount of assimilation between the two is relatively great.

However, he cautions, "In island arcs where there is no underlying continental-type crust, there may be no neodymium isotopic contrast between crust and mantle and therefore no possibility of using such isotopic measurements for predicting eruption size."