Scripps/UCSD geophysicist among international team finding evidence of first plate tectonics

DF? Response?

http://www.eurekalert.org/pub_releases/2007-03/uoc--sga032207.php

Observations indicate that plate tectonics began before any currently known
structural geological record on Earth
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."

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.

(oh, wait. According to EE, there is no oceanic crust older than 250
million years. Oops. O gues they are proven wrong, yet again!)

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.

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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.

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.


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