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Evidence seen for early Earth layer differentiation

In a break from past thought on the evolution of the Earth, geologists Samuel Bowring of MIT and Todd Housh of the University of Texas at Austin have found evidence that early Earth quickly developed a crust and since then has been evolving toward an equilibrium state in composition and temperature. Predominant theory has the Earth slowly differentiating into crust and a complementary mantle over its whole history.

Professors Bowring and Housh's research on four-billion-year-old rocks (gneisses) from the Northwest Territories in Canada, described in the September 15 issue of Science, supports the theory that there is a limited amount of the very old continental crust because older crust has been continuously recycled, not because there was little crust early in Earth history.

Their work in studying the Earth's oldest known rocks suggests that plate tectonics, the theory that describes the surface of the Earth as a collage of moving plates, has acted to recycle existing crust as the edges of plates plunge beneath other plates and return to the underlying mantle. As the plates, which are the Earth's crust, return to the mantle, they efficiently recycle the old crust back to its ultimate place of origin.

"Crust from the Archean period may have been formed in large volume, but lacking a deep `root' beneath it which protected it from destruction, it may have been mostly recycled back into the mantle," Professor Bowring said.

It is only through the study of these old crustal rocks that geologists can get direct information about the Earth's processes four billion years ago. By studying the crust, scientists also can learn about the composition of the mantle of four billion years ago. These rocks from the Archean period (the time of formation of the planet until 2.5 billion years ago) were found by Professor Bowring in the 1980s when he was doing regional mapping to test geological models. It was later, in studying the rocks' ratios of two elements, samarium and neodymium, that the implications for the Earth's early history became evident, showing a decrease over the Earth's history in how differentiated its layers were.

The study of samarium and neodymium showed that early Earth was characterized by regions of very different chemical signatures. The best explanation for this, Professor Bowring said, is that the Earth formed a crust very early (generating the diversity) and subsequently continuously recycled that crust (progressively eliminating the diversity.)

The scientists also contrast early Earth history to early evolution of the Moon. "The isotopic and geochemical signature of the Earth's mantle is fundamentally different than that of our nearest neighbor, the Moon," Professor Bowring said, "because the process of plate tectonics has operated on Earth over much of its history whereas on the much smaller and colder Moon, it's never operated. Recycling of both continental and oceanic crust back into the Earth's mantle gives the Earth a distinctive geochemical signature that is absent in the Moon."

Dr. Housh was a postdoctoral associate under Professor Bowring from 1991 to 1992. This research was funded the National Science Foundation.

A version of this article appeared in MIT Tech Talk on September 20, 1995.

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