May 11th, 2001

Ancient sea floor slab said big geologic find


WASHINGTON -- A massive slab of rock near the Great Wall of China is actually a section of ocean floor dating back 2.5 billion years, scientists said on Thursday in announcing an important geological discovery that also could yield clues about early life on Earth.

The finding has huge implications for the study of plate tectonics -- the ongoing movement of continent-sized plates of rock on the planet's surface -- said Timothy Kusky, professor of geology at St. Louis University.

The discovery of the so-called Dongwanzi ophiolite pushes back by 500 million years the known beginning of plate tectonics, Kusky said, and suggests that the process has been going on since Earth's infancy.

An ophiolite (pronounced OH-fee-oh-lite) is a distinctive complex of rock layers created when the Earth's tectonic plates are pulling apart and magma from deep inside the planet pushes through to the surface. It is made of dark-colored rocks including basalt, diabase and gabbro.

"The surface of the Earth is broken into a number of fairly large, rigid plates that are moving around on the planet relative to each other," Kusky said. "The continents are parts of these plates, but they don't form the plate boundaries in all cases."

While other types of rocks that are older have been found, the Dongwanzi ophiolite represents the oldest complete section of oceanic sea floor ever located.

Scientists previously had believed the movement of the plates on the Earth's surface -- which can be witnessed in the jigsaw puzzle-like match of the east coast of South America and the west coast of Africa -- was a comparatively recent development.

But Kusky said this dynamism probably has been occurring for 4 billion of Earth's 4.5-billion-year history.

Kusky and Jiang-Hai Li of Peking University in Beijing made the discovery in May 2000 in a mountainous region in the Eastern Hebei Province near the border with inner Mongolia about 155 miles (250 km) northeast of Beijing. It was close to a section of the Great Wall, which dates back a mere 2,200 years. Their findings appear in the journal Science.

Relic from earliest geological period

The ophiolite is about three miles (five km) wide, up to 30 miles (50 km) long and nine miles (14 km) thick. It dates back 2.505 billion years to the Earth's earliest geological time period -- the Archean. Its age was determined by Robert Tucker of Washington University in St. Louis.

"I brought pieces back in a suitcase -- carry-on luggage. They were surprised at the airport," Kusky said. "They asked, 'What do you have in there, rocks?' I said, 'Uh-huh."'

The oldest previous such rock formation, thus the oldest evidence for plate tectonics, dated to 1.95 billion years ago.

Kusky said the findings also could have a big impact on theories related to the development of life on Earth. There is evidence that life first arose on Earth 3.8 billion years ago -- simple, single-celled organisms in the oceans.

But when those organisms evolved into more complex ones has been debated for years.

Kusky said researchers are now checking the ophiolite's rocks for evidence of life. He noted that hot volcanic vents on parts of the sea floor much like the section he has discovered may have provided the nutrients and temperatures needed for life to flourish and develop.

File Date: 5.11.01