New research paints a picture of what happened billions of years ago when a devastating meteorite crashed into the Earth
A devastating meteorite collision caused part of the Earth’s crust to flip
inside out billions of years ago and left a dusting of a rare metal
scattered on the top of the crater, says new U of T research.
The study, published in the June 3 issue of Nature, examines the devastating
effects of meteorite impacts on the Earth’s evolution. Researchers from the
University of Toronto and the Geological Survey of Canada studied the
remains of a 250-kilometre wide crater in Sudbury, Ontario, known as the
Sudbury Igneous Complex, caused by a collision with a Mount Everest-sized
meteorite 1.8 billion years ago. They discovered that the meteorite burrowed
deep into the Earth’s upper crust – which measures an average of 35
kilometres thick – and caused the upper crust to be buried under several
kilometres of melted rock derived from the lower crust.
The dynamics of meteorite impacts remain a source of debate among
researchers and, until now, there has been little hard evidence to prove a
meteorite could pierce through the Earth’s upper crust and alter its
compositional makeup. "It had not really been appreciated that large impacts
would selectively move material from the bottom of the crust up to the top,"
says lead researcher James Mungall, a U of T geology professor. "This has
been suggested for the Moon at times in the past but ours is the first
observational evidence that this process has operated on Earth."
In the study, Mungall, his graduate student Jacob Hanley and Geological
Survey researcher Doreen Ames concluded Sudbury Igneous Complex is
predominantly derived from shock-melted lower crust rather than the average
of the whole crust as has been previously supposed. The researchers
discovered a subtle but significant enrichment of iridium, an extremely rare
metal found mainly in the Earth’s mantle and in meteorites. Due to the low
magnesium and nickel content found in the samples they concluded that the
iridium came from the meteorite itself rather than the Earth’s mantle.
The discovery of the iridium allowed the researchers to paint a picture of
what happened billions of years ago, when a meteorite collided with the
earth at a velocity exceeding 40 kilometres per second and caused a shock
melting of 27,000 cubic kilometres of the crust. "The impact punched a hole
to the very base of the crust and the meteorite itself was probably
vaporized," says Mungall. This collision, he says, caused a plume of
iridium-enriched vaporized rock to surge up and recondense on top of the
impact site. Simultaneously, the cavity collapsed within minutes or hours to
form a multi-ring basin 200 to 300 kilometres in diameter and one to six
kilometres deep.
"Picture a drop falling into a cup of milk, thus producing a bowl-shaped
depression for a moment before the milk outside rushes back in to fill the
hole," says Mungall. "Now imagine that the falling drop of milk is a rock 10
kilometres in diameter, and the resulting depression is 30 to 40
kilometres deep."
The Sudbury Basin is the second oldest very large impact crater site in the
world but is one of the most accessible and well preserved. The oldest one,
South Africa’s two-billion year-old Vredefort Crater, has eroded over time
and only the basement remains. Another impact site, the Chicxulub Crater in
Yucatan Peninsula, believed to be responsible for the extinction of the
dinosaurs, lies buried under beds of limestone.
The study was funded in part by the Geoscience Laboratories of the Ontario
Geological Survey and the Geological Survey of Canada.
CONTACT:
James Mungall, Dept of Geology, ph: (416)-978-2975; e-mail:
mungall@geology.utoronto.ca
