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March 22, 2007
SCRIPPS/UCSD
GEOPHYSICIST AMONG INTERNATIONAL TEAM FINDS EVIDENCE OF FIRST PLATE
TECTONICS
3.8 BILLION YEARS AGO
Identification
of the oldest preserved pieces of Earth's crust in southern Greenland
has provided evidence of active plate tectonics as early as 3.8 billion
years
ago, according to a report by an international team of geoscientists in
the
March 23 edition of Science magazine.
The
finding
pushes back the date of continent-forming processes previously
determined as
2.5 billion years ago to a much earlier era considerably closer to
Earth's formation
some 4.5 billion years ago. Geochemical analysis of rocks has
previously
suggested an earlier date for plate tectonics, but this is the first
study to
find physical evidence of tectonics among Earth's oldest known rock
structures,
according to Hubert Staudigel of Scripps Institution of Oceanography at
UC San
Diego.
"The fact that this rock structure is so well preserved is particularly
lucky," Staudigel said. "The materials were formed as seafloor along
a spreading center and accreted to a continental plate and just stuck
there,
surviving almost unscathed for as long as 3.8 billion years."
Coauthors of the report are Harald Furnes of University of Bergen,
Norway;
Maarten de Wit of University of Cape Town, South Africa; Minik Rosing
of the
University of Copenhagen, Denmark; and Karlis Muehlenbachs of the
University of
Alberta, Canada.
The study focuses on an area near the southwestern coast of Greenland
where there is a rare outcrop of ancient rock, called the Isua
Supracrustal
Belt, which have been dated at 3.8 billion years old. The Isua rocks
are
ophiolites, which have a green hue from the chlorite minerals within
them and
are found in all major mountain belts, usually located in areas
associated with
volcanism and plate tectonics. The Isua deposits were first described
in the
1960s. They also have been found to contain fossilized evidence of the
earliest
bacterial life on Earth, also about 3.8 billion years old, in studies
conducted
in 1999 by Minik Rosing.
The
new study
reveals the geological structure at Isua contains both seafloor pillow
lavas
and dikes, or sheets, of basalt that intruded into the pillow lavas
after they
formed. These features and the chemistry of the ophiolites indicate
that the
area was formed as the result of seafloor spreading, according to lead
author
Furnes. Even though the rocks have physically changed over time, it is
still
possible to see their original characteristics because of the
preservation of
fine-grained crystals that show they were cooled by contact with
surrounding
colder rocks, Furnes said.
"To what extent one is able to see an original structure in a highly
deformed rock depends basically on the experience of the observer,"
Furnes
said. "In our case we knew what we were looking for, and all of us who
did
the field work have reasonably good experience with identifying pillow
lavas
and associated dikes."
The finding of ophiolites in the oldest known rock structures leads the
scientists to believe that such rocks have formed throughout Earth's
nearly 4.5
billion year history, according to de Wit.
"Our work shows that some form of seafloor spreading and oceanic crust
formation occurs as far back in history as geological records go," de
Wit
said.
Rosing said, "Our paper describes large-scale structural relationships
that show the ancient oceanic crust was comparable to the modern crust
in its
structure and composition and that a section of ancient oceanic crust
could be
preserved by uplifting onto stable crust, similar to how more modern
ophiolite
complexes have formed."
The paper also sheds light on the ongoing debate about the oxygen
isotope
composition of seawater through geological time periods. The reactions
of
seafloor and seawater largely control the ocean's oxygen isotope
makeup, but
scientists have been polarized between those that maintain the oxygen
isotope
content has remained relatively constant and those that argue for
variation.
According to Muehlenbachs, this work shows that the early ocean had the
same or
slightly heavier oxygen isotope composition as that of the modern ocean.
"We can conclude from the oxygen isotope analyses of the pillows and
dikes
that the earliest ocean had already chemically reacted with the
seafloor,"
Muehlenbachs said. "This has great implications to the historical
chemical
composition of the oceans and may have played a role in the evolution
of
life."
The
geological
processes of the early Earth were largely responsible for the
distribution of
elements throughout the land, air and oceans, having fundamental
consequences
for the development of life, according to Staudigel. He said the
science team
was sampling the Isua Supracrustal Belt looking for chemical or
isotopic traces
of life in the pillow lavas when they realized the area supplied
geological
structures proving plate tectonics from the earliest history of Earth.
The Norwegian Research Council, the Nordic Center for
Earth
Evolution, GeoForschungsZentrum Potsdam, Agouron Institute and the
National
Sciences and Engineering Research Council of Canada provided funding
for the
research.
##
Contact:
Chuck
Colgan or Robert Monroe
Scripps
Institution of Oceanography
858-534-3624
scrippsnews@ucsd.edu
This
text
derived from:
http://scrippsnews.ucsd.edu/Releases/?releaseID=780
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