SCIENCE / GEOLOGY : Much of Asia Relatively New, Expert Says

TIMES SCIENCE WRITER

Evidence from parts of Russia that recently opened to Western research indicates that volcanoes may have burped up more than 2 million square miles of Asia within the last 650 million years, challenging theories that the continental crust was created much earlier in Earth's history.

The findings, reported in today's edition of the journal Nature, indicate that the great mountain ranges of Central Asia are not unrelated anomalies, as some geologists assert, but part of a coherent chain of volcanoes similar to those that created Japan and Micronesia.

Celal Sengor of Istanbul Technical University and his colleagues said that these volcanoes, fueled by the violent melting that results when one large piece of the Earth's crust is forced under an adjoining slab, apparently produced huge volumes of molten rock that became a part of the modern Asian continent.

They concluded that the volcano chain, which was most likely undersea, produced the equivalent of two football fields of new crust every year for about 350 million years. About 55% of this material was recycled from old continental crust, but the remainder was rock fresh from the mantle, the partially melted rock layer between Earth's molten iron core and its solid crust.

If Sengor and his colleagues are correct, the creation of Asia--and maybe other continents--was far different from mainstream theories that say continents were formed by the collision of much older material broken off the prehistoric supercontinent of Pangea.

"Continental drift alone merely rearranges the crustal cards on the global table," said University of Arizona geology professor William R. Dickinson, "but (this) example shows how new cards can be drawn from the mantle deck in the process of making a composite continent."

Sengor's new tectonic model differs from other theories in several ways:

By suggesting the creation of a significant amount of "juvenile crust newly extracted from the mantle," it contradicts notions that all continental crust was created in the Proterozoic eon from 2.5 billion to 600 million years ago and has simply been rearranging itself ever since. (This movement of continental crust is driven by the creation of new sea floor at undersea ridges.)

By contending that these volcanoes were part of one chain--and, thus, essentially a single phenomenon--that created new crust, it challenges theories that the bulk of present-day Asia was built up by a series of separate volcanic events that recycled existing crustal material.

Some geologists expressed skepticism of Sengor's conclusion, although they acknowledged his experience and expertise in the field. Others welcomed the findings, which they said may help resolve a decades-long debate over whether continents have grown significantly over the eons or merely rearranged themselves by skating around the globe atop tectonic plates.

"It suggests some very significant tectonic activity . . . in a period of geologic history that we don't have a good understanding of," said John Baumgardner, a geophysicist at the Los Alamos National Laboratory in New Mexico. "It could give us important clues to the way plates were moving back then."

Baumgardner added that the report also is exciting because it indicates "the vast amount of geologic information available in the (former) Soviet Union," which covered one-eighth of the planet's land surface.

Copyright © 2019, Los Angeles Times
EDITION: California | U.S. & World
59°