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

[Ron Baalke]

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 Loÿc 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; Loÿc Vanderkluysen of Drexel University 
in Philadelphia, Pennsylvania; and Sally A. Gibson of the University of 
Cambridge in the United Kingdom.