HOUSTON, Texas, Feb. 19, 1998 -- Research led by Rice University geologists estimates that within 400,000 years a new volcano could erupt in northern California, relatively soon in geologic terms.
The findings suggest that magma located in chambers about 20 kilometers, or 12 miles, into the earth's crust could rise to the surface in the Lake Pillsbury area, about 120 miles north of San Francisco.
The research is published in the February issue of Geology in an article titled, "Fluids in the lower crust following Mendocino triple junction migration: Active basaltic intrusion?" Authors include Alan Levander and Timothy Henstock of Rice University, Anne Meltzer and Bruce Beaudoin of Lehigh University, Anne Trehu of Oregon State University (OSU) and Simon Klemperer of Stanford University. The research is part of the Mendocino Triple Junction Seismic Experiment (MTJSE), a National Science Foundation Continental Dynamics project involving Rice, Lehigh, OSU, Stanford and the United States Geological Society, and focusing on the Mendocino Triple Junction, a region where the Pacific, Gorda and North American plates meet and slide past each other in northwestern California.
"These are the brightest features I have ever seen in the Earth's crust," said Levander, professor of geology and geophysics at Rice and leader of this study, looking at the data from the study. "After the '93 experiment when we had preliminary reports, the first thing we thought was that we found a magma chamber. We knew we had something interesting, but it took the second experiment to actually verify it."
Levander and the team suggest that the seismic reflections result from a complex system of basalt melt created from upwelling layers of rock after the triple junction moved northward.
As the triple junction system moves north, it makes a hole in the mantle, which fills with a partially molten mantle rock including basaltic elements. One of the consequences of the plate geometry in the area is that south of this triple junction region, the base of North America is exposed to the deeper levels of the mantle.
There is no intervening shield of partially solidified mantle so this hot rock comes straight up, reaches the base of the crust, and in the process undergoes a phenomenon called decompression melting, producing some volume of basalt, an intermediate rock. This process goes on as this region moves upward and it has left behind a string of volcanoes, all but one of which are extinct. The Clear Lake area, just 50 miles south of Lake Pillsbury, is the last active site, with modern geothermal activity--now used as hot springs and a spa. Moving south, the sites get older and older.
The study covered a 90 square mile area, and the researchers have seen evidence of magma everywhere under that patch. Interlaced basalt is intruded into about 10 percent of the lower crustal layer--a layer 3-4 miles thick.
The researchers collected data in 1993 and 1994, using a technique similar to what oil companies use to assess an oil prospect, but they studied much deeper levels. Using 150 portable seismographs from the IRIS/PASSCAL Program, they created a seismic array at 400 recording sites. With the seismographs placed about 200 feet apart over 18 miles in an east-west direction, they fired one thousand pounds of chemical explosives into a 100-foot well. They repeated the experiment on a northwest-southeast line, parallel to the geologic structures.
Studying the speed and direction of the seismic waves, the researchers found that the bodies causing the reflections extend no more than 1.5 miles across, and are less than 440 yards thick. However, although most of the bodies are grouped near the top of a basalt later, they do occur throughout the layer, down to the Moho, the boundary between the crust and the mantle.
"The prognosis," Levander said, "depends upon global plate movement. If the plates choose one direction to move in, they could shut the system off. If they choose another they could accelerate it and if they continue with the current one, my guess would be that it might produce a Clear Lake-type volcanic field."
Says Henstock, a senior research fellow at Rice, "There are some reasons why Lake Pillsbury could be a good place for it to happen, but it could equally skip Lake Pillsbury and not show up at the surface until another 31 miles north, where there's another region that's a candidate." Still, "an eruption could happen anytime," Levander said, "although it won't be in the next few years."
When basalt erupts it can be sudden. Although geologists do not know exactly how long it takes magma to rise to the surface, and each case varies, it can take 1,000 years or less for magma to move from the mantle to the crust.
This part of the MTJSE, which was a $1.5 million program, was done for $15,000, funded by the NSF's Continental Dynamics Program.