[meteorite-list] Extraterrestrial Impacts Tranform Earth's Surface In An Instant

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:33 2004
Message-ID: <200206152304.QAA08572_at_zagami.jpl.nasa.gov>

http://www.sciencenews.org/20020615/bob9.asp

Presto, Change-O!

Extraterrestrial impacts transform Earth's surface in an instant
Sid Perkins
>From Science News, Vol. 161, No. 24, June 15, 2002, p. 378.

Most geological processes unfold at less than a snail's pace. The tectonic
plates that cover Earth's surface slog along, crashing into and sliding over
one another at rates of only a few millimeters per year. Over millions of
years, however, these unhurried liaisons raise mountain ranges. Wind, rain,
and natural chemical erosion gradually rework the mountains into silt, clay,
and dissolved minerals. Slowly, this inorganic detritus wends its way to the
sea, where it joins a languid rain of dead marine organisms to form thick
layers of ocean-floor ooze.

Every now and again, however, things happen in a flash. Asteroids, comets,
and smaller objects smack into the planet at clips of thousands of
kilometers per hour. When this happens, the impacts can gouge sizable holes
in Earth's outer crust. Within milliseconds, rocks at the impact site
vaporize. The rapid expansion of this superheated gas blows melted and
pulverized material into the atmosphere or back into space.

The immense seismic vibrations from an impact can create temperatures high
enough to melt or demagnetize some rocks in and near the crater. Farther
away, the sudden changes in pressure triggered by shock waves shatter and
otherwise transform mineral crystals as no other geological process does.

Although these planetary bruises and black eyes have significantly shaped
the planet's surface, many have remained hidden. Scientists are taking
advantage of the magnetic and gravitational scars of these impacts to
identify the sites of the most dramatic bombardments this planet has ever
experienced.

When worlds collide

Many of the smallest objects on a collision course with Earth burn up in the
atmosphere before they reach the surface. A meteoroid-an interplanetary
object ranging in size from a dust grain up to a mountain-needs to be at
least the size of a child's marble to blaze all the way to Earth's surface.
Anything that survives the fall is, by definition, a meteorite. The kinetic
energy of the meteorite when it strikes the ground-a function of the mass of
the space rock and its velocity-strongly influences the size of the hole or
the splash it creates.

Tiny meteorites are slowed by the atmosphere so much that they simply drop
to the ground, sometimes making no more than a dent. When these dark objects
fall on frozen, snow-covered terrain, they're particularly easy to find.
Residents of Canada's Yukon Territory recovered pieces of a rare carbon-rich
meteorite soon after it fell in January 2000 (SN: 4/8/00, p. 235), and
scientists visiting Antarctica routinely use snowmobiles to hunt for the
extraterrestrial rocks.

More-massive meteoroids are slowed less by air resistance and therefore pack
a bigger punch when they land. They typically gouge out classic, bowl-shaped
craters. Arizona's Meteor Crater-also known as Barringer Crater, after the
Philadelphia mining engineer who began studying the site in 1902-is the
best-preserved terrestrial example of such a so-called simple crater.

The impact scar, located about 20 kilometers west of Winslow, Ariz., was
formed nearly 50,000 years ago when an iron-nickel meteorite about 45 meters
in diameter punched through the region's rocky plain. The impact energy of
20 million tons of TNT was roughly equivalent to the power of a hydrogen
bomb. The sudden collision vaporized the meteorite, pulverized rocks at
ground zero, and heaved large blocks of limestone, some the size of small
homes, out of a 200-m deep, 1.2-km-diameter hole. That debris formed an
elevated rim that still rises above the Arizona plain.

On Earth, craters that range up to about 5 km across have this simple
structure, says Harrison H. Schmitt, a geologist and retired astronaut who
trained at Meteor Crater before walking on the moon during the Apollo 17
mission.

Meteoroids larger than 200 m or so across create a different type of impact
scar when they slam into Earth, says Thomas Kenkmann, a geologist at
Humboldt University in Berlin. These complex craters have a flat floor
marked with a central uplift, which typically is either a single or ring
peak. This uplift forms as the rocks beneath the deepest portion of the
crater floor rebound from the compressive shock of the meteorite's impact.

Complex craters also have terraced rims, which form when the initially steep
walls of the crater collapse downward and inward. An analysis of twisted
rocks taken from the central uplift of the 7-km-wide Crooked Creek crater in
Missouri suggests that this collapse is very quick, says Kenkmann.

The roughly 320-million-year-old impact occurred in sediments composed of
mineral grains 10 to 100 micrometers in diameter bound into rock. As many as
40 percent of the boundaries between individual grains were fractured, and
rock deformation typically took place in bands between 10 and 500
micrometers wide. None of the grains seem to have been stretched before they
broke. All these clues point to the crater collapsing in less than 30
seconds, says Kenkmann. His analyses of several complex craters between 5
and 15 km in diameter suggest that their rims collapsed within a minute of
the impact. He reports his findings in the March Geology.

The pressure's off

Thick sheets of melted rocks line the bottom of many large meteor craters.
Some of these impact melts derive from the kinetic energy of the impact, a
large part of which is converted to heat when the meteorite smacks Earth and
grinds to an abrupt stop. However, the sudden excavation of a large crater
probably plays a bigger role in forming impact melts, says Schmitt.

Rocks lying kilometers deep within Earth are often on the verge of melting
but are prevented from doing so by the immense pressure of all the material
above them. When meteorites blast that weight away, the pressure in the
rocks beneath the crater floor drops precipitously and the underlying
minerals melt. The impact melts may not fully cool for hundreds of thousands
of years. In the meantime, water from the environment and the heat from the
newly exposed rocks can combine to form hydrothermal systems in the heavily
fractured rocks in and around the crater. Scientists believe such warm,
mineral-rich venues could have played a role in the early development of
life on Earth (SN: 3/9/02, p. 147: Available to subscribers at
http://www.sciencenews.org/20020309/fob2.asp).

The 200-m-thick impact melts found within an ancient crater surrounding the
town of Sudbury in central Ontario are more than a sign of extraterrestrial
impact: They're a treasure trove of minerals. More than $1 billion of metal
ores including those bearing nickel, platinum, and copper are mined from the
melts each year, says Richard Grieve, a geologist at Natural Resources
Canada in Ottawa. Isotopic analyses show that the metals come from Earth's
crust, not from the meteor that fell from space. Before the impact melts
solidified, the deep, thick blend of light silicates and dense metal
ores-which didn't mix well with each other-separated into two layers,
according to density, just like oil and vinegar do. This ancient segregation
makes mining today much easier.

The hydrothermal system created by the Sudbury impact also dissolved
minerals containing copper and other metals from a broad area and then
concentrated them in rich veins. One large outcrop of ore alone holds
minerals valued around $100 billion, says Grieve. The economic interest in
the area has proven a boon to scientists, who have attained access to deep
rock cores originally extracted to determine the best locations to sink
mining shafts.

Radioactive dating of the melts and the hydrothermal deposits indicates the
Sudbury impact occurred about 1.85 billion years ago. The original crater
probably was between 250 and 300 km across, says Grieve. It's tough to tell
because erosion, including the ravages of several ice ages, has scraped away
up to 4 km of Earth's surface from the crater site. That has erased many of
the impact's effects.

A somewhat older impact crater provides a deeper view. The Vredefort impact
structure, named after the city in South Africa that was built in the center
of the ancient bull's-eye, was created by a collision about 2.02 billion
years ago. The rocks now at Earth's surface there were once between 7 and 10
km belowground, says Roger Gibson of the University of the Witwatersrand in
Johannesburg. That much overlying material, including all of the crater's
impact melts, has eroded away since the crater formed. However, that loss is
science's gain: The erosion has made it easy for geologists to get samples
of rock that formed deep within the crater's central peak, now a dome of
exposed material.

Most of the crystalline mineral grains in the dome's rocks measure between 1
and 5 millimeters across, which matches the grain size for similar rocks in
the area. However, rocks found within 5 km of the center of the Vredefort
dome typically have grains no more than 100 micrometers across. Because
grain size is related to the length of time that the crystal took to grow,
Gibson contends that the rocks in the center of the dome experienced a short
burst of terrific heat before they rebounded toward Earth's surface.

His analyses indicate that the rocks were between 15 and 20 km below ground,
at around 400C, before the impact occurred. Then, during the strike from
space, temperatures in the rocks directly beneath the impact briefly rose to
between 1,000C and 1,400C, primarily due to intense shock waves. At sites
about 25 km from the impact, shock waves had dissipated somewhat, and the
rocky material there got only a small boost in temperature, Gibson says. His
team's analyses appear in the May Geology.

New finds, old tools

Extraterrestrial impacts leave distinct calling cards. For instance, when a
rock's temperature rises above its so-called blocking temperature, any
magnetic fields in the minerals are disrupted and then realign to match the
strength and direction of the magnetic fields in the rock's environment.
This phenomenon takes place in molten rocks spewing from volcanoes and
undersea ridges, but it also takes place in the wake of meteor strikes. If
the magnetic field at the location of an extraterrestrial impact is
significantly different from the one in place when those rocks last cooled,
then the cosmic bruise will produce magnetic anomalies.

Those irregularities can be quite extensive, says Jasper Halekas, a
geophysicist at the University of California, Berkeley. He and his
colleagues have analyzed data collected from lunar craters during the Apollo
moon missions and the more recent Lunar Prospector probe. Those studies show
magnetic anomalies that often extend up to several crater radii from an
impact site. That finding implicates temperature boosts from seismic shock
rather than exposure to vaporized material from the meteorite. The team
presented its results last December at a meeting of the American Geophysical
Union in San Francisco.

Impacts also can produce gravitational anomalies. Even long after an impact
scar becomes heavily eroded, the pulverized rock that fills the crater
bottom is much less dense than the solid rock from which it's derived. The
precise force of gravity at any location depends, in part, on the density
and amount of material in the neighborhood. Impact melts and a central
uplift, if any, also can affect local gravitational patterns.

Other geological processes can produce magnetic and gravitational anomalies,
but when these two hallmarks occur together, or are backed up with other
geologic evidence, it's a strong hint that scientists may have found an
ancient impact site. At the meeting in San Francisco, Dallas Abbott and her
colleagues at Lamont-Doherty Earth Observatory in Palisades, N.Y., described
a possible impact crater southeast of Hawaii. They found two strong magnetic
anomalies, possibly related to impact melts, inside an unusually shallow,
150-km-diameter crater that lies in water about 3.8 km deep.

The team also found small spherules of glassy material in sediments all
around the proposed impact site. The tiny orbs ranged up to 200 micrometers
in diameter, a size characteristic of those produced by meteorites that
create craters 55 km or more across. The crater may be uncharacteristically
shallow for a couple of reasons, the researchers say. First, the deep water
probably cushioned the blow of the meteorite. Also, chemical analyses of the
spherules, which are high in potassium and low in silicon, suggest that the
impact landed on an undersea mountain rather than flat ocean floor.

A group of scientists from the University of South Carolina in Columbia says
that they've used geological anomalies, as well as clues from rock samples,
to identify an ancient crater buried beneath the piedmont sediments of their
state. A magnetic anomaly about 10 km across is nearly superimposed on a
12-km-diameter gravitational irregularity near the town of Johnsonville,
says geologist Christopher D. Parkinson. A 290-m borehole, drilled when
other scientists were studying the area's aquifers, shows that sediments at
the proposed impact site are about 275 m thick. The deepest sediments were
laid down about 90 million years ago, and they lie directly on top of
basement rock that is a little less than 300 million years old.

Shocked quartz and other
metamorphic changes in the basement rocks indicate the minerals were
subjected to the intense pressures and strong seismic waves generated by a
meteorite impact, says Parkinson. Some of the changes suggest that
temperatures in the rocks rose to at least 1,300C.

Other boreholes drilled in the area during the aquifer study were spaced 20
to 50 km apart and, like the Johnsonville borehole, extended all the way to
the basement rocks. All these other sediment cores include a layer of
volcanic basalt, dozens of meters thick, that was laid down about 200
million years ago.

Parkinson suggests that the Johnsonville core doesn't contain this basalt
because it was blown away by an impact that occurred between 90 million and
200 million years ago. The team is now conducting detailed analyses of the
melt glasses in the sediments, which should provide a more specific date for
one of the Piedmont's worst days in the last few thousand millennia.

So far, scientists have identified fewer than 200 impact craters on our
planet. However, one look at the pockmarked moon-which shares Earth's orbit
around the sun-suggests that many of our planet's scars have faded or remain
hidden. Finding ancient craters and unveiling their geophysical histories
will help fill in the blanks of Earth's continuing story.


References:

Abbott, D.H., et al. 2001. Ewing structure: A possible abyssal impact crater
(Abstract P22D-04). American Geophysical Union Fall Meeting. December 10-14.
San Francisco.

Gibson, R.L., et al. 2002. Metamorphism on the moon: A terrestrial analogue
in the Vredefort dome, South Africa? Geology 30(May):475-478. Abstract

Grieve, R., and A. Therriault. 2000. Vredefort, Sudbury, Chicxulub: Three of
a kind? Annual Review of Earth and Planetary Science 28:305.

Halekas, J.S., et al. 2001. The role of shock in lunar paleomagnetism
(Abstract GP32A-0191). American Geophysical Union Fall Meeting. December
10-14. San Francisco.

Kenkmann, T. 2002. Folding within seconds. Geology 30(March):231-234.
Abstract

Parkinson, C.D., P. Talwani, and E. Wildermuth. 2002. The Johnsonville
Impact Crater, South Carolina: Petrologic evidence of shock metamorphism
from core samples (Abstract T21A-10). American Geophysical Union Spring
Meeting. May 28-31. Washington, D.C.

Wildermuth, E., P. Talwani, and C.D. Parkinson. 2002. Potential field
analysis of the Johnsonville Impact Crater, South Carolina. Presentation
T21A-09 at American Geophysical Union Spring Meeting. May 28-31. Washington,
D.C.

Further Readings:

Cowen, R. 2000. Rocks on the ice. Science News 157(April 8):235.

______. 2001. A meteorite's pristine origins. Science News 160(Sept.
29):203. Available to subscribers at
http://www.sciencenews.org/20010929/note12.asp.

Perkins, S. 2002. Space Rocks' Demo Job: Asteroids, not comets, pummelled
early Earth. Science News 161(March 9):147. Available to subscribers at
http://www.sciencenews.org/20020309/fob2.asp.

______. 2002. Mangled microfossils may mark impact sites. Science News
161(June 15):382. Available to subscribers at
http://www.sciencenews.org/20020615/note14.asp.

Sources:

Dallas H. Abbott
Lamont-Doherty Earth Observatory of Columbia University
103 Oceanography, 61 Route 9W
P.O. Box 1000
Palisades, NY 10964-8000

Roger L. Gibson
Impact Cratering Research Group
School of Geosciences
University of the Witwatersrand, WITS
Johannesburg 2050
South Africa

Richard Grieve
Earth Sciences Sector
Natural Resources Canada
Ottawa, Ontario K1A 0E8
Canada

Jasper S. Halekas
University of California, Berkeley
Space Physics Research Group
Space Science Laboratory #7450
Berkeley, CA 94720-7450

Thomas Kenkmann
Institute of Minerology
Museum of Natural History
Humboldt University-Berlin
D-10115 Berlin
Germany

Christopher D. Parkinson
Department of Geological Sciences
700 Sumter Street
University of South Carolina
Columbia, SC 29208

Harrison H. Schmitt
University of Wisconsin, Madison
Department of Engineering Physics
437 Engineering Research Building
1500 Engineering Drive
Madison, WI 53706
Received on Sat 15 Jun 2002 07:04:55 PM PDT


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