When a Giant Asteroid Impact Created Its Own Magma

http://www.wired.com/2014/09/when-a-giant-asteroid-impact-created-its-own-volcanoes-magma/



Landsat 8 image of the Sudbury Basin in Ontario, taken in September 2013.

When a Giant Asteroid Impact Created Its Own Magma
By Erik Klemetti
09.26.14

There are many ways we can form magma on Earth. Most of these processes involve the motion of the mantle and crust of the planet, creating places where hot mantle rock can decompress, like at a mid-ocean ridge, or where water coming off a piece of oceanic crust that is diving into the mantle lowers the melting point of rock, like in a subduction zone. These two processes — decompression and flux melting — are the main modes of generating the magma that drives volcanoes. However, occasionally very rare events like a massive asteroid impact can cause the rocks of the Earth’s crust to melt and even cause volcanoes and magma bodies to form that may have lasted over 50,000 years.

One of the best examples on Earth for this kind of cataclysmic volcanism in found in Canada. The Sudbury Basin in Ontario (see above) contains the remnants of a massive asteroid impact that occurred ~1.8 billion years ago. Today, the remains of the crater are perched up against some of the oldest rocks on Earth, namely the Canadian Shield, where the rocks are 2-2.6 billion years old. It is into these ancient rocks that the Sudbury asteroid slammed, creating what is thought to be a crater at least 200 kilometers across. All that remains today is an elliptical sequence of rocks ~50 kilometers across as most of the impact features have been long since eroded. However, this scar on the Earth’s surface might also be one of its most valuable, with over $500 billion worth of nickel, copper, platinum-group and other rare metals in its deposits.

Last week, our Denison University Geosciences field trip visited the Sudbury Basin and I got to see for the first time these results of this massive impact. You can find evidence for this impact in the rocks that surrounded the old impact structure. Namely, you can find “shatter cones” (see below) — areas in the rock that surrounded the impact crater that were suddenly and catastrophically fractured by the pressure wave released during the impact. The shatter cones around Sudbury vary from only a few centimeters to half a meter long (at least where we saw them). One cool feature is the sharp part of the cone points back towards the location of the impact (once you rotate the rocks to their original orientation). These shatter cones are some of the most important structures to support the hypothesis that the Sudbury rocks are formed from a massive impact.




Shatter cones found in the rocks surrounding the Sudbury impact structure. I’ve added some lines to help see the cones in this image.


The shatter cones are cool, but what I was most interested in seeing was one of the most astonishing products of the impact: magma and volcanoes. Now, most impacts aren’t going to produce much in the way of magma (melt). We do find small blebs of melt in the deposits from other impacts, but by volume, they are an insignificant part of the impact. However, at Sudbury, it seems that the force of the eruption was enough to create hundreds of cubic kilometers (if not more) of magma. Most of this magma stayed in the ground and formed what we call a large igneous intrusion. These features, like the Stillwater Complex in Montana, are one of the few times we see direct evidence of that “magma chamber” that is (incorrectly) envisioned beneath a volcano. The magma body caused by the Sudbury impact was large enough to stay hot for sufficient time to allow minerals to crystallize and separate (fractionate) in the chamber. That’s a little bit more into the gory details that I want to get, but one of the ramifications of all this melted crust is that you suddenly had volcanoes where there weren’t any mere moments before the impact!




Impact melt in the Sudbury Basin. You can see lighter grey chunks of the shattered crust into which the dark black impact melt/magma intruded