Oct 07 2022

Chicxulub Impact Triggered Mega-Earthquake

At this point there is little question that a giant asteroid, 10 kilometers across, impacted the Earth about 66 million years ago. Evidence for this impact began with an iridium layer discovered at the Cretaceous-Paleogene, or K-Pg, boundary. Something deposited an unusually high level of iridium in a brief event all around the world. Later the likely crater resulting from this impact was found in Chicxulub, Mexico. Multiple other discoveries have supported this conclusion, including the fact that this impact was the likely cause of the dinosaur extinction. There was also massive volcanic activity at that time, and dinosaur populations may have been in decline, but that was likely a side show. The main event was the impact.

Such an impact would have released a tremendous amount of energy (10^23 joules), equivalent to a 100 million megaton bomb. There were multiple effects of that impact. One is that a lot of Earth crust material would have been melted and thrown up into the atmosphere, but at less than escape velocity so ultimately raining back down to Earth. Some of these molten droplets cooled into glass spherules as they fell, raining tiny glass beads onto the Earth – creating another geological marker for the impact.

The asteroid impact was essentially in the Gulf of Mexico, causing a massive tsunami that swept over North America. My favorite geological find resulting from this is at the Tanis site in Hell’s Creek. The massive tsunami washed lots of fish and other sea life across the continent, and deposited them in a valley, creating a large jumble of fossils all deposited at once. Scientists know they are from the day of the impact because the fish have glass spherules stuck in their gills – they breathed them in while still alive.

The impact would have also caused a shockwave in the atmosphere, resulting in a superheated plasma 10,000 degrees C with winds up to 1,000 kph, killing everything in its path for 900-1800 km.

Finally, the Earth itself would have rung like a bell from the impact, but for how long? That is the subject of a new study, presenting evidence that the impact triggered a mega-earthquake that lasted weeks to months. First let’s talk about the evidence for an earthquake, and then we look at the evidence for the timing. A powerful shaking from seismic activity would have the effect of liquefying water-infused sedimentary layers. This happens with modern earthquakes, which can cause the ground beneath building to flow leading to their collapse. In the geological record we can see the flowing of sediments resulting from liquefaction. There is also soft-sediment deformation – lines that appear in the sedimentary layer from being strongly shaken. Finally there are other rock deformations, like cracks, that are evidence of an earthquake.

All of these lines of evidence are present at the K-Pg boundary layer, indicating that the impact created a mega-earthquake. But how long did it last? To answer this the new study looks again at the glass spherules. These are an extremely useful geological feature, because they can only be created by a massive impact that threw up molten material into the atmosphere. The glass spherules would then rain back down onto the Earth fairly quickly, lasting about 30 minutes. However, in the deep ocean these tiny glass beads would then slowly filter down to the ocean floor. That process would take weeks to months.

Therefore, if we look at geological deposits from the deep ocean floor at the time of the impact we should see a much thicker layer of glass spherules, deposited over weeks to months (not just in one day). Study author, Hermann Bermúdez, and his co-authors did just that. They also looked for the signs of earthquake activity in that layer. They found evidence of sediment disruption through the layer containing the glass spherules, suggesting that the Earth was shaking strongly for weeks to months after the impact, while the glass spherules were still slowly falling to the ocean floor.

It’s hard to imagine the destruction of that day. The impact site was vaporized, surrounding by a ring of superheated air, and a larger ring scoured by a massive tsunami. A rain of glass and fire would have set the world’s forests afire. Acid rain fell for 5-10 years after the impact. The dust cloud would have blotted out the sun, reducing photosynthesis and causing an 18 month winter. It’s amazing anything survived. We often hear that 75% of species went extinct from the impact, but >99% of all individual animals were killed. A tiny sliver of life eked through this event, and it’s not hard to imagine a slightly worse impact where nothing survived.

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