[meteorite-list] Holes in the Earth: 170 and Counting

From: Mike Groetz <mpg444_at_meteoritecentral.com>
Date: Wed, 16 Apr 2008 14:58:22 -0700 (PDT)
Message-ID: <29762.55940.qm_at_web33003.mail.mud.yahoo.com>

http://www.space.com/scienceastronomy/080414-mm-earth-holes.html

Holes in the Earth: 170 and Counting
By Clara Moskowitz
Staff Writer
posted: 14 April 2008
07:00 am ET
 

When a meteorite struck Earth before humans were
around to watch, did it still make a "splat?" Although
it's too late to witness the many pummelings our
planet has already seen, scientists are still finding
the humongous holes left here by long ago impacting
space rocks.

At last count, there were more than 170 known impact
craters on our planet, according to the Earth Impact
Database maintained by the University of New Brunswick
in Canada. These puncture wounds are littered over
every continent, as well as the seafloor.

There would be countless more if it weren't for
Earth's constant remodeling. Plates shift, mountains
form, volcanoes erupt and erosion washes over the
planet's surface, continually hiding the evidence of
most craters.

"If there was no erosion or tectonic activity, we
would look like the moon," said Lucy Thompson, a
geologist at the University of New Brunswick. "The
moon is just pockmarked with impact craters."

Puzzling differences

Scientists think the Earth was bombarded more heavily
earlier in the solar system's history, when planets
were still forming and bushels of debris were flying
madly around. Luckily for us, things have quieted down
lately and meteorite impacts are few and far between.

One of Earth's most recently-formed holes is Arizona's
Barringer Meteor Crater, created around 50,000 years
ago. Though this crater, one of the most famous, awes
tourists with its roughly three-quarters of a mile
(1.2 km) diameter, it is considered quite dinky on the
geological scale.

"That's a nice, simple bowl-shaped crater," Thompson
said.

Geologists get really excited about complex craters,
such as Manicouagan in Quebec, Canada. Scientists
estimate this crater is more than a hundred times
wider than Barringer, and was made more than 200
million years ago.

"With large impacts, you have complex craters forming,
and instead of having a nice bowl shape, you get a
central uplift," Thompson told SPACE.com. "It's like
if you drop something in water, you get rings forming,
but the middle comes back up."

Scientists want to understand how the rock achieves
this without actually becoming liquid or shattering
into pieces.

Big and bad

A major heavyweight is South Africa's Vredefort
crater, which at 186 miles (300 km) wide, is said to
be Earth's largest verified impact crater. At more
than 2 billion years old, it is also one of the most
ancient.

Other contenders are the 155 mile-wide (250 km-wide)
Sudbury Basin in Ontario, Canada, and the roughly 110
mile-wide (180 km-wide) Chicxulub crater, half
submerged off the coast of the Yucat?n Peninsula in
Mexico.

The latter can claim fame as the landing spot of the
asteroid that purportedly killed the dinosaurs, along
with most life on Earth.

If it weren't for erosion and other geological
processes that erase evidence of craters, there would
likely be hundreds of thousands of impact craters on
the Earth, Thompson said. Scientists are still
discovering new craters, especially in remote areas
and on the seafloor where evidence of them is easily
missed.




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Received on Wed 16 Apr 2008 05:58:22 PM PDT