By Will Dunham
 WASHINGTON
(Reuters) - 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 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 TIME PERIOD
The ophiolite is about three miles (five km) wide,
up to 30 miles long and nine miles 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. |
