[meteorite-list] Ancient Crater Is A Window On Past

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:01 2004
Message-ID: <200101191710.JAA02344_at_zagami.jpl.nasa.gov>

http://www.deseretnews.com/dn/view/0,1249,245014897,00.html?

Ancient crater is a window on past
By Joe Bauman
Desert News
January 18, 2001

      An enormous asteroid or comet that ripped into South Africa 2 billion
years ago gives a window into the formation of the continents, according to
scientists from the University of Utah and South Africa.
      Their report, "Birth of the Kaapvaal Tectosphere 3.08 Billion Years
Ago," is published in Friday's edition of the journal Science.
      Lead scientist is Desmond Moser, research assistant professor at the
U. His co-authors are Rebecca M. Flowers, who earned her master's at the U.
and is now working on a doctorate at the Massachusetts Institute of
Technology; and R.J. Hart of the University of Witwatersrand, Johannesburg,
South Africa.
      The trio studied a huge crater structure in South Africa called the
Vredefort impact structure, located in the Witwatersrand Basin. The crater
is the largest known remnant of "the asteroid bombardment that affected this
part of the solar system" in an early stage of Earth's history, Moser said.
      An intense bombardment of space material pounded Earth in those days,
he said. The theory is that "the planets themselves are kind of like big
snowballs that have accumulated from materials in space." This swept up much
of the primordial material around Earth.
      Immediately after the impact, the crater was about 9 miles deep and
185 miles across. "The Earth welled up to fill the hole," he said.
      Deeper layers filled the crater. Moser proposed to the National
Science Foundation that they should go to South Africa and examine the
remnants of the crater, mapping the geologic strata that pushed in from
deeper in the Earth.
      They also used sophisticated dating techniques to determine the age of
the rocks, discovering that the underlying rocks had solidified 3.5 billion
years ago and that the material hit 2 billion years ago.
      Heat released by the impact melted some of the rocks, which changed
the mineral, in effect resetting its clock. "I dated one of these impact
melts," which helped to date the asteroid impact, Moser said.
      Throughout the world, huge continental plates are continuously
shifting, pushing up mountains and carving out rift valleys. When the
asteroid or comet hit, it provided a snapshot of what rocks were at the
time.
      "The meteorite impact basically exposed rocks from very deep levels,"
and these tell about the early types of plate tectonics, he said.
      The team learned that this part of the African continent, which is one
of the oldest known, "formed from the top down." Earth's geology formed the
upper parts of the plate first, and they thickened over time, the scientists
learned.
      Although the impact was nearly twice as large as the one that wiped
out the dinosaurs 65 million years ago, it didn't kill any sophisticated
species. At the time, as far as scientists know, Earth had no life beyond
the single-cell level, he noted.
      An interesting sidelight to the research is that these prototype
tectonic plates were important to the formation of diamonds. "Something in
the processes that formed these earlier plates also generated diamonds,"
Moser said.
      "When we look at the younger plate material, they don't generate
diamonds." This is why diamonds are found mostly in southern Africa, as it
is where the early type of tectonic plate remains.
Received on Fri 19 Jan 2001 12:10:53 PM PST