[meteorite-list] Shiva: Another K-T Impact?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Nov 3 14:24:03 2004
Message-ID: <200411031924.LAA24254_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1281

Shiva: Another K-T impact?
Astrobiology Magazine
November 3, 2004

Summary: Most scientists believe a large meteorite impact in the
Yucatan Peninsula led to the extinction of the dinosaurs 65 million
years ago. But could a second, larger impact off the coast of India
share the blame?
  
Shiva: Another K-T impact?
by Leslie Mullen

According to the Earth Impact Database, there are two craters - the
180 kilometer-wide Chicxulub crater in Yucatan, Mexico and the much
smaller Boltysh crater in eastern Ukraine -- that date back to the
Cretaceous-Tertiary (K-T) extinction 65 million years ago.

Yet Sankar Chatterjee, a paleontologist at Texas Tech University in
Lubbock, says the catalog overlooks several craters, including Shiva, a
large, underwater crater off the coast of India. He says this crater
measures 600 by 400 kilometers, and was made by an enormous meteorite
measuring 40 kilometers across. The Shiva crater is shaped like a
teardrop, and Chatterjee thinks this is because the meteorite hit the
Earth at a low angle.

"The K-T extinction was definitely a multiple-impact scenario," he says,
pointing to two other craters not listed in the impact database: the
Small Point structure off the coast of Maine, and the Silverpit crater
in the North Sea.

These craters are not listed in the catalog because, despite the claims
of their discoverers, they have not been independently confirmed to be
the result of meteorite impacts. Doubt was cast on the impact origin of
the Silverpit crater earlier this year, when it was reported in the
journal Nature that Silverpit instead may be a sinkhole depression
caused by salt withdrawal.

Christian Koeberl, a geochemist at the University of Vienna in Austria,
says too many people are crying impact every time they see a round hole
in the Earth's crust. Craters can result from many other natural
processes, including volcanic eruptions.

Koeberl says that, ever since Chicxulub was confirmed as the likely
cause of the K-T extinction, "now everybody gets on the impact bandwagon."

"A lot of people who have not the foggiest idea about how to really
recognize an impact crater, and wouldn't be able to tell a shocked
quartz grain from a tectonically deformed one if their life depended on
it, call everything that is vaguely circular an impact crater," he says.

[Deccan]
The Deccan Traps are one of the largest volcanic provinces in the world.
It consists of more than 6,500 feet (>2,000 m) of flat-lying basalt lava
flows and covers an area of nearly 200,000 square miles (500,000 square
km) (roughly the size of the states of Washington and Oregon combined)
in west-central India. Estimates of the original area covered by the
lava flows are as high as 600,000 square miles (1.5 million square km).
The volume of basalt is estimated to be 12,275 cubic miles (512,000
cubic km)(the 1980 eruption of Mount St. Helens produced 1 cubic km of
volcanic material). The Deccan Traps are flood basalts similar to the
Columbia River basalts of the northwestern United States. This photo
shows a thick stack of basalt lava flows north of Mahabaleshwar.
Credit: Lazlo Keszthelyi


Other than being a round hole in the ground, an impact crater will have
evidence of the sudden violent force that punched a hole in the Earth's
crust. For instance, there will be impact breccia, which is lighter,
smashed-up rock that fills the crater after impact. Microscopic shards
of "shocked quartz"-crystals that shattered in the shock waves of an
impact - often will be present. Minerals other than quartz, such as
zircon, also may show signs of shock and exposure to high pressure. The
heat of impact can often produce glass as well.

Geologists also look for an ejecta blanket radiating out from the
crater. Above average amounts of iridium and other siderophile
("iron-loving") elements provide some of the strongest evidence for a
meteorite impact, since those elements are rare on the surface of the
Earth but can often be found in meteorites.

But not every impact crater will have all these attributes. Some
meteorites don't contain iridium, for instance, so not every impact
crater can be expected to be rich in that element.

Koeberl admits that identifying impact craters is neither easy nor
straightforward, but he is adamant that Shiva is not an impact crater.
Koeberl says not only is there no evidence of impact in the case of
Shiva, there is no crater structure. He calls Shiva, "a figment of
imagination."

"There's not even ambiguous evidence, or inconclusive evidence," says
Koeberl. "There are a couple of people that keep pushing for some crater
in the Indian Ocean, but this is inconsistent not only with the regional
geology and geophysics, but also with anything we know about impact
cratering."

Yet Chatterjee feels sure that Shiva is an impact crater. One indication
of an impact origin, he says, is that the floor of the Shiva crater is
missing most of the lithosphere - the brittle outer shell of the Earth
that includes the crust (the continents and the ocean floor) and the
uppermost part of the mantle.

Chatterjee says the large meteorite that created the Shiva crater could
have easily shattered the lithosphere, and by doing so may have
triggered plate tectonics. He says the rate of India's northward
movement increased around 65 million years ago, and he suspects this was
due to the Shiva impact.

Geologists who study plate tectonics agree that the Indian plate's
northward movement did speed up, but say this acceleration probably
occurred before the K-T extinction. For instance, Jerome Dyment, a
geologist with the Institut de Physique du Globe de Paris, says the
plate sped up about 69 million years ago - moving from 8 to 18
centimeters per year. This faster rate was sustained for about 20
million years, and then slowed as India began to plow into the Eurasian
continent.

At the time of the K-T extinction, India was an island located over the
Reunion hotspot. Hotspots are fixed points where hot material from the
mantle rises to the Earth's surface. This underground welling flooded
portions of India with a vast amount of lava. Today, these cooled lava
fields are called the Deccan Traps.

The slow outpouring of Deccan lava probably began a few million years
before the K-T extinction. Then about 65 million years ago, the trickle
became a torrent.

Around the same time, says Steve Cande from the Scripps Institution of
Oceanography in La Jolla, California, the India-Africa ridge jumped
northwards to the edge of western India. This geologic "jump" caused a
sliver of continent to split off, forming the Seychelles.

"These events are probably associated with the Deccan Traps," says
Cande. "Now, I suppose you might say that all of these events were
triggered by a meteorite impact, but I think most people believe that
the Deccan Traps was the culmination of a mantle plume that was long in
the making -- millions if not tens of millions of years."

While geologists haven't pinned down the exact connection between the
ridge jump and the volcanic event that triggered the Deccan Traps,
Dyment says that most believe both events may be the result of the head
of the Reunion hotspot finally reaching the Earth's surface.

"Such large volcanic events and associated ridge jumps also have been
observed in the Atlantic, " notes Dyment, pointing to similar activity
in the north Atlantic near Iceland 54 million years ago and in the
central Atlantic between North America and Africa 180 million years ago.

But Chatterjee believes the geologic activity in India is best explained
by a massive meteorite impact. For further proof, he points to alkaline
igneous rock spires that are encased in the Deccan Traps. These spires
are rich in iridium, but the Deccan lava did not contain iridium. How
else, he asks, could the spires have formed if not by a nearby meteorite
impact?

In addition, Chatterjee says there is an underwater mountain as high as
Mount Everest within the Shiva crater. He says this structure has been
dated to be 65 million years old, and he thinks it could be the central
peak that is often seen within large impact craters.

Finally, Chatterjee says the crater contains shocked quartz, a key sign
of impact. And because the K-T clay boundary layer in India is one meter
thick - the thickest in the world - Chatterjee thinks a meteorite impact
must have been close by.

While all this evidence seems compelling, Chatterjee has so far failed
to convince a majority of scientists that it adds up to proof of impact.
One problem, says Simon Kelley, a geologist at the Open University in
England, is that there is not very much information about Shiva
available in the peer-reviewed journals. Chatterjee published a paper
discussing Shiva eight years ago in the journal "Memoirs of the
Queensland Museum," and Shiva is mentioned in a book about global
tectonics that he edited. Most other information about the crater has
appeared in conference abstracts or proceedings.

"The advance of science normally goes ahead by postulating hypotheses
and then testing them with colleagues by publishing the work," says
Kelley. "The lack of published work on Shiva means I can't really
evaluate it against the normal criteria -- so it has to be classed as a
hypothesis to be tested."

Until Shiva can be studied more intensively, the crater will remain a
tantalizing possibility rather than hard evidence of another K-T
meteorite impact. Chatterjee says that oil companies and the Indian
government control the site where Shiva is located, and access is
extremely limited.

"It's very frustrating," says Chatterjee. "We are so close to solving
the riddle, and yet so far because of the lack of critical drilling and
geophysical data. If Shiva were indeed an impact crater, it would be the
largest crater so far preserved on Earth."
Received on Wed 03 Nov 2004 02:24:01 PM PST