Splosh! How the dinosaur-killing asteroid made its crater
It is hard to imagine
billions of tonnes of rock suddenly start to splosh about like a liquid - but
that is what happened when an asteroid struck the Earth 66 million years ago.
Scientists have now put
together a detailed picture of the minutes following the giant impact.
This, remember, is the
colossal event that wiped out the dinosaurs.
The analysis of rocks
drilled in 2016 from the leftover crater show they underwent a process of
fluidisation.
The pulverised material
literally began to behave as if it were a substance like water.
Models had predicted
what should happen when a 12km-wide stony object from space punched the ground.
Initially, a
near-instantaneous bowl would have been created some 30km deep and up to 100km
wide.
Then, instabilities
would have seen the sides collapse inwards and the base of the hole rebound
skyward, briefly reaching higher than the Himalayas.
When everything had
settled down, a crater roughly 200km wide and 1km deep would remain.
This is the feature
that is now buried under sediments in the Gulf of Mexico, close to the port of
Chicxulub.
The impact description
- scientists call it the dynamic collapse model of crater formation - is only
possible if the hammered rocks can, for a short period, lose their strength and
flow in a frictionless way.
And it is the evidence
for this fluidisation process that researchers now report after studying the
rocks they drilled from something called the "peak ring" -
essentially, a circle of hills in the centre of the remnant Chicxulub
depression.
"What we found in
the drill core is that the rock got fragmented. It was smashed to tiny little
pieces that initially are millimetre sized; and that basically causes this
fluid-like behaviour that produces in the end the flat crater floor, which
characterises Chicxulub and all such large impact structures, including those
we also see on the Moon," explains Prof Ulrich Riller, from the University
of Hamburg, Germany.
This is not rock being
melted; rather, it is rock being broken apart by immense vibrational forces,
says Prof Sean Gulick, the drill team co-lead from the University of Texas at
Austin, US.
"It is a pressure
effect; it's mechanical damage. The amount of energy moving through these rocks
is equivalent to Magnitude 10 or 11 earthquakes; the estimate for the whole
impact is something like 10 billion Hiroshima bombs."
Ultimately, the rocks
will regain their strength. They have to if they are to build the ring of
hills.
This return of
rigidity, again, is witnessed in the drill core samples.
A 12km-wide object dug
a hole in Earth's crust 100km across and 30km deep
This bowl then
collapsed, leaving a crater 200km across and a few km deep
The crater's centre
rebounded and collapsed again, producing an inner ring
Today, much of the
crater is buried offshore, under 600m of sediments
On land, it is covered
by limestone, but its rim is traced by an arc of sinkholes
"It is manifested
in what we call shear fractures - planar discontinuities where rocks can slide
past each other," Prof Riller added. "We see these fractures
over-printing the smashed rocks that formed beforehand. These planar structures
are evidence that the rock must have regained strength towards the end of
crater formation."
As well as bringing new
insights into one of the most catastrophic days in Earth history, and the mass
extinction that followed - the Chicxulub investigation will also help
scientists as they study big impact craters on other planetary bodies.
"We are explaining
a fundamental process that will occur on any rocky body," says Prof
Gulick.
"For the first
time ever, we now have rocks that tell us the kind of deformation they've
undergone to temporarily behave like a liquid and then become like a rock again
at the end - without melting. It's all done by overlapping deformation
mechanisms. This will be a fundamental process that resurfaces planets, not
just in our Solar System but presumably in all Solar Systems."
Profs Riller and Gulick
were part of the Expedition 364 drilling project, which was conducted in
April/May 2016 under the auspices of the European Consortium for Ocean Research
Drilling (ECORD) and the International Ocean Discovery Program (IODP).
Their latest findings
on "acoustic fluidisation" in the Chicxulub crater are published in
the journal Nature.
source:
https://www.bbc.com/news/science-environment-45986449
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