Written by Michael Poelchau.
Granite. I’ve started to develop a relationship with the granite we’re currently drilling through. Our group was excited about the first few meters of this rock; it was beautifully cored and would have made a nice countertop for your kitchen, but after about the tenth core of granite, you could feel the enthusiasm decline a tiny bit. As an impact geologist, I had to appreciate the fact that the repetitive occurrence of the granite is a perfect blank slate that records the brutal force that the impact threw at it.
In our lab, we can make crude preparations of the rock for the microscope. You shave off some tiny bits of a granite sample from the last core, put some glue and a cover glass on it and bake it under a UV-lamp for a few minutes. Under the microscope, the minerals light up in a full rainbow of colors. But their beauty is deceptive. On closer inspection, you notice that they’ve seen things they can’t unsee, they’ve been traumatized by the shock wave and are now bearing the scars of this event. Quartz is no longer its bland self, but has been squeezed into submission and now shows the tell-tale stripes of what we call shock metamorphism. The other minerals, feldspars and micas, have similarly been badly mistreated by forces so swift and strong that they can only stem from impacts.
Often the core curator will tear me away from my musings at the microscope to look at the next core that was pulled on deck. We scan the liner and sometimes see half meter-thick black dikes that have shot in to the granite. The granite is not very happy about this blazing injection and has expressed this by turning green (much the same way I did when the flight to Merida got turbulent). We believe that these injections must have come from what became a 3km deep lake of liquid rock in the crater, which took possibly over one hundred thousand years to cool off. Their emplacement must have occurred within the first few seconds to minutes after the asteroid touched the Earth’s surface. While these molten dikes were being pressed into the ground over 10 kilometers below, the crater rim was expanding at a rate of roughly five kilometers each second in all directions. That’s about 50 football fields in the time it takes to say “Chicxulub crater”. After all these years of studying craters, I still find the dimensions and the corresponding violence, destruction and heat of their formation hard to comprehend. This truly must have been the most horrifying event of the last 100 million years.
Feature Image: ELeBer@ECORD_IODP
Middle Left: MMowat@ECORD_IODP
Middle Right: ELeBer@ECORD_IODP
Bottom Image: MPoelchau@ECORD_IODP