When it comes to material hiding in plain sight, so to speak, Earth has its own fair share. For example, scientists hypothesized that the dinosaurs went extinct, in part, due to the collision of a huge asteroid with Earth. There was a lot of evidence for this fact, mostly in the form of the K-T boundary layer, which can be found all over our planet. This boundary is a thin layer full of iridium, and that just does not occur naturally here in the quantities present in that layer. It’s space dust that is found in asteroids. But where was the evidence of that impact?
Earth isn’t like the Moon or Mars. It doesn’t keep its record of impacts there for all the world to see and find for eternity. We have weather and plate tectonics. Earth’s surface is constantly changing. Trees grow. Rocks erode. And soon, a crater is lost to time, at least as far as our surface-bound eyes can tell.
But then we started taking sonar measurements of our oceans and seas. Some of it was to protect us during wartime, and those measurements led to the evidence scientists needed to accept plate tectonics as fact. Some of it was to find oil, and while the oil companies were a little stingy on sharing, when they did, that’s how we found the evidence to pinpoint just where our dinosaur-killing asteroid smacked into Earth — Chicxulub crater off the coast of the Yucatán peninsula in Mexico. Combined with gravity measurements and satellite imagery, the crater was finally revealed.
And now, scientists have sampled granitic rocks in and around the crater and found something else hiding in plain sight — the remnants of a volcanic arc. Using zircon crystals found in the rocks, scientists measured the lead content as a way to date when uranium present in the crystals began to radioactively decay. This process begins when the crystals solidify, so we can then date the age of the crystals.
Graduate student Catherine Ross did what graduate students do. She measured the age of 835 individual zircon crystals, boring into each of them with a laser to get the data needed, and then she analyzed the data and broke the ages into three groups. One group was 1.3 to 1 billion years old, making it a part of the supercontinent Rodinia. The next group was 550 million years old, and that meant the supercontinent Gondwana. The final group was between 500 and 400 million years old, and these crystals are the evidence for that volcanic arc that formed when the Rheic ocean subducted under the edge of that Gondwana plate. As we have discussed before, subduction – where one plate slips under another plate – causes volcanic arcs.
And voila! Another piece of the puzzle literally fits into place in the history of Earth’s continental movements and formation. Science is amazing.
More Information
Vestiges of a Volcanic Arc Hidden Within Chicxulub Crater (Eos)
“Evidence of Carboniferous arc magmatism preserved in the Chicxulub impact structure,” Catherine H. Ross et al., 2021 April 30, GSA Bulletin
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Given the Tonga eruption event from January, 2022–and the problematically low levels of iridium detected and significantly thinner melt deposits than expected of a major impactor–it seems likelier that the Chicxulub Structure is a consequence of super-volcanism rather than an asteroid / meteorite strike.