The Big Boom : Scientists Are Gaining a Better Understanding of How Geological Developments Deep Inside the Earth Affect Volcanic Eruptions. : Science File / An exploration of issues and trends affecting science, medicine and the environment
Earthquakes, it is well known, frequently are the end products of the slow, mammoth motions of the planet’s subsurface tectonic plates. In recent years, scientists have also come to appreciate the vital role played by shallow subduction zones--where 50-mile-thick “slabs” of one great tectonic plate dive under another, in indirectly generating many volcanic eruptions. The volcanoes of the Cascade Range are a well-known example.
Now--as an ever-more sophisticated scientific view is emerging of the Earth’s interior--much more has become known, or at least theorized, about how geological developments of very deep origin may affect major, long-term volcanic activity.
Gradually, such information is accumulating about the way the Earth works. Ultimately, though much more research must be done, researchers hope some catastrophic events may become predictable, if not preventable.
A team of scientists from the United States, Russia, China and India have now postulated a process by which volcanic plumes rise over periods of 20 million years from the core-mantle boundary, 1,800 miles below the Earth’s surface, eventually to explode into the atmosphere with the most massive eruptions ever known.
The largest of at least 11 “flood basalt” extrusions--250 million years ago in Siberia--covered 966,000 square miles and contained so much magmatic material (molten rock) that if it were spread evenly, it would cover the entire surface of the Earth to a depth of 10 feet.
Basalts are a distinctive type of dark lava, low in silica content, rich in iron and magnesium. They are often spewed out in such volume that they form massive plateaus. These products of deep-origin volcanoes are distinct from the materials ejected by many individual, shallow-origin volcanoes in smaller volumes.
In a recent article in the journal Science, geologist Paul R. Renne, director of the Berkeley Geochronology Center, along with several other scientists, advances the theory that the Siberian eruptions caused the greatest mass extinction of life in Earth’s history. The blast, they theorize, saturated the atmosphere with sulfate aerosols, cooling the Earth and causing devastating acid rains, destroying life on a gigantic scale.
Such eruptions, they write, follow the slow spread of magma under the surface that has risen in the volcanic plumes.
The volcanic ejection of these vast basaltic reservoirs can then result in uplifts of more than 3,000 feet in land masses. Great quantities of ice may form at the higher elevations, freezing water that would otherwise run out to sea, and thus lowering sea levels. So the amount of the Earth’s land surface may periodically be affected by the processes of deeply derived volcanism, asserts Renne’s colleague, Mark A. Richards.
An earlier Science article reported that researchers had confirmed that the basalts contained a primordial helium that appeared to come from the depths of the Earth. Helium is so light that almost all of it on the surface floats off into space, and only when it is trapped im bubbles in minerals does it persist for scientists to observe.
These theories are not universally held. A Caltech geophysicist, Don L. Anderson, has challenged them, maintaining that the plumes come from no deeper than a few hundred miles.
But Renne, in an interview, said it has become clear to most scientists that the tectonic movements of the great continental plates themselves demonstrate that they originate at far deeper levels.
The plumes from deep in the Earth, he noted, are in a constant location. As the plates move, and land surfaces with them, the volcanic activity generated by these plumes, or so-called “hot spots,” makes “trails” on the surface. In Hawaii, gradual volcanic eruptions have formed islands in a pattern progressing from northwest to southeast.
Even now, as the plates move, the older islands to the northwest of the Hawaiian chain are eroding after the cessation of eruptions, volcanic activity continues on the island of Hawaii, the island most to the southeast. And a new island, still under water, is forming in the Pacific to the southeast.
Another example of a hot spot that appears on the surface to be similarly migrating over many thousands of years is at Yellowstone National Park. Proof of earlier eruptions can be seen in Idaho, and scientists believe that the massive Columbia River basalts even further west in Oregon and Washington show the beginning of the system currently active in Yellowstone.
Robert A. Duncan of the College of Oceanography at Oregon State University said the plume theory has come a long way toward acceptance. “The speculative writings of 1991 have been confirmed,” he said in an interview.
“Basaltic eruptions are infrequent but spectacular,” Duncan observed. “They indeed are quite unusual events, and we have become aware of their connection to periodic mass extinctions, because of their dramatic effects on oceans and atmospheres.”
Duncan said that such an eruption can, over a relatively short period, extrude thousands of cubic miles of basalt in an area hundreds of miles wide. By contrast, the Mt. St. Helens eruption of 1980, while widely thought by the public to be huge, was triggered by relatively shallow tectonic movements and expelled less than a cubic mile of nonbasaltic material.
The scientists believe the most recent huge eruption of flood basalts anywhere in the world was the Columbia River episode 17 million years ago.
Researchers are virtually certain that more such eruptions will occur in the future. Just recently, Daniel Dzurisin, director of the Cascades Volcano Observatory, said he thinks it is possible that a small example of a basaltic eruption could occur relatively soon.
An adjunct to the theories now being advanced is that beneath the surface, in the upper reaches of the mantle, the large storehouses of magma could be thrust up to the surface by some geologic event.
One theory of such a causative event held by some scientists is that asteroids colliding with the Earth can rip the surface open so that reservoirs of basaltic magma explode outward. But most of the scientists expounding the plume theories hold that this would not be necessary for most such eruptions, which they maintain are entirely terrestrial in origin. According to this view, an earthquake triggering a landslide, for example, could uncover the underlying magma and cause it to explode upward.
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Hot Spots
Scientists believe they have discovered a new kind of deeply based volcanism. According to theories that have been developed recently, the surface system of volcanic hot spots in the world is connected to volcanic plumes rising over millions of years from the core- mantle boundary- 1,800 miles below the Earth’s surface.
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The plumes are responsible over millions of years for the deposit of flood basalts above at scattered locations- vast quantities of magma that cover up to thousands of square miles.
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This very deep volcanism contrasts with shallow volcanism that derives from subducting tectonic plates, where a leading slab of one plate dives under another, causing the rise of magma usually from 100 miles down or less.
