[meteorite-list] Did Dinosaur-Killing Asteroid Trigger Largest Lava Flows on Earth?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 19 May 2015 15:41:54 -0700 (PDT)
Message-ID: <201505192241.t4JMfsSm019810_at_zagami.jpl.nasa.gov>

http://newscenter.berkeley.edu/2015/04/30/did-dinosaur-killing-asteroid-trigger-largest-lava-flows-on-earth/

Did dinosaur-killing asteroid trigger largest lava flows on Earth?
By Robert Sanders, Media Relations
UC Berkeley
April 30, 2015

BERKELEY - The asteroid that slammed into the ocean off Mexico 66 million
years ago and killed off the dinosaurs probably rang the Earth like a
bell, triggering volcanic eruptions around the globe that may have contributed
to the devastation, according to a team of UC Berkeley geophysicists.

Specifically, the researchers argue that the impact likely triggered most
of the immense eruptions of lava in India known as the Deccan Traps, explaining
the "uncomfortably close" coincidence between the Deccan Traps eruptions
and the impact, which has always cast doubt on the theory that the asteroid
was the sole cause of the end-Cretaceous mass extinction.

"If you try to explain why the largest impact we know of in the last billion
years happened within 100,000 years of these massive lava flows at Deccan
- the chances of that occurring at random are minuscule," said team leader
Mark Richards, UC Berkeley professor of earth and planetary science. "It's
not a very credible coincidence."

Richards and his colleagues marshal evidence for their theory that the
impact reignited the Deccan flood lavas in a paper to be published in
The Geological Society of America Bulletin, available online today (April
30) in advance of publication.

While the Deccan lava flows, which started before the impact but erupted
for several hundred thousand years after re-ignition, probably spewed
immense amounts of carbon dioxide and other noxious, climate-modifying
gases into the atmosphere, it's still unclear if this contributed to the
demise of most of life on Earth at the end of the Age of Dinosaurs, Richards
said.

"This connection between the impact and the Deccan lava flows is a great
story and might even be true, but it doesn't yet take us closer to understanding
what actually killed the dinosaurs and the 'forams,'" he said, referring
to tiny sea creatures called foraminifera, many of which disappeared from
the fossil record virtually overnight at the boundary between the Cretaceous
and Tertiary periods, called the KT boundary. The disappearance of the
landscape-dominating dinosaurs is widely credited with ushering in the
age of mammals, eventually including humans.

He stresses that his proposal differs from an earlier hypothesis that
the energy of the impact was focused around Earth to a spot directly opposite,
or antipodal, to the impact, triggering the eruption of the Deccan Traps.
The "antipodal focusing' theory died when the impact crater, called Chicxulub,
was found off the Yucatan coast of Mexico, which is about 5,000 kilometers
from the antipode of the Deccan traps.

Flood basalts

Richards proposed in 1989 that plumes of hot rock, called "plume heads,"
rise through Earth's mantle every 20-30 million years and generate huge
lava flows, called flood basalts, like the Deccan Traps. It struck him
as more than coincidence that the last four of the six known mass extinctions
of life occurred at the same time as one of these massive eruptions.

Illustration of a hot mantle plume "head" pancaked beneath the Indian
Plate. The theory by Richards and his colleagues suggests that existing
magma within this plume head was mobilized by strong seismic shaking from
the Chicxulub asteroid impact, resulting in the largest of the Deccan
Traps flood basalt eruptions.

"Paul Renne's group at Berkeley showed years ago that the Central Atlantic
Magmatic Province is associated with the mass extinction at the Triassic/Jurassic
boundary 200 million years ago, and the Siberian Traps are associated
with the end Permian extinction 250 million years ago, and now we also
know that a big volcanic eruption in China called the Emeishan Traps is
associated with the end-Guadalupian extinction 260 million years ago,"
Richards said. "Then you have the Deccan eruptions - including the largest
mapped lava flows on Earth - occurring 66 million years ago coincident
with the KT mass extinction. So what really happened at the KT boundary?"

Richards teamed up with experts in many areas to try to discover faults
with his radical idea that the impact triggered the Deccan eruptions,
but instead came up with supporting evidence. Paul Renne, a professor
in residence in the UC Berkeley Department of Earth and Planetary Science
and director of the Berkeley Geochronology Center, re-dated the asteroid
impact and mass extinction two years ago and found them essentially simultaneous,
but also within approximately 100,000 years of the largest Deccan eruptions,
referred to as the Wai subgroup flows, which produced about 70 percent
of the lavas that now stretch across the Indian subcontinent from Mumbai
to Kolkata.

Michael Manga, a professor in the same department, has shown over the
past decade that large earthquakes - equivalent to Japan's 9.0 Tohoku
quake in 2011 - can trigger nearby volcanic eruptions. Richards calculates
that the asteroid that created the Chicxulub crater might have generated
the equivalent of a magnitude 9 or larger earthquake everywhere on Earth,
sufficient to ignite the Deccan flood basalts and perhaps eruptions many
places around the globe, including at mid-ocean ridges.

"It's inconceivable that the impact could have melted a whole lot of rock
away from the impact site itself, but if you had a system that already
had magma and you gave it a little extra kick, it could produce a big
eruption," Manga said.

Similarly, Deccan lava from before the impact is chemically different
from that after the impact, indicating a faster rise to the surface after
the impact, while the pattern of dikes from which the supercharged lava
flowed - "like cracks in a souffle," Renne said - are more randomly oriented
post-impact.

"There is a profound break in the style of eruptions and the volume and
composition of the eruptions,' said Renne. "The whole question is, 'Is
that discontinuity synchronous with the impact?'"

Reawakened volcanism

Richards, Renne and graduate student Courtney Sprain, along with Deccan
volcanology experts Steven Self and Lo?c Vanderkluysen, visited India
in April 2014 to obtain lava samples for dating, and noticed that there
are pronounced weathering surfaces, or terraces, marking the onset of
the huge Wai subgroup flows. Geological evidence suggests that these terraces
may signal a period of quiescence in Deccan volcanism prior to the Chicxulub
impact. Apparently never before noticed, these terraces are part of the
western Ghats, a mountain chain named after the Hindu word for steps.

"This was an existing massive volcanic system that had been there probably
several million years, and the impact gave this thing a shake and it mobilized
a huge amount of magma over a short amount of time," Richards said. "The
beauty of this theory is that it is very testable, because it predicts
that you should have the impact and the beginning of the extinction, and
within 100,000 years or so you should have these massive eruptions coming
out, which is about how long it might take for the magma to reach the
surface."

Since the team's paper was accepted for publication, a group from Princeton
University published new radioisotopic dates for the Deccan Traps lavas
that are consistent with these predictions. Renne and Sprain at UC Berkeley
also have preliminary, unpublished dates for the Deccan lavas that could
help solidify Richards' theory, Renne said.

Co-authors of the paper, in addition to Richards, Renne, Manga and Sprain,
are Walter Alvarez, a UC Berkeley professor emeritus of earth and planetary
science and the co-originator of the dinosaur-killing asteroid theory;
Stephen Self, an adjunct professor in the same department at UC Berkeley;
Leif Karlstrom of the University of Oregon; Jan Smit of Vrije Universeit
in Amsterdam, the Netherlands; Lo?c Vanderkluysen of Drexel University
in Philadelphia, Pennsylvania; and Sally A. Gibson of the University of
Cambridge in the United Kingdom.
Received on Tue 19 May 2015 06:41:54 PM PDT