Prehistoric Seiche

Sixty-six million years ago, a meteorite impact in modern-day Mexico wiped out the dinosaurs and most other living species of the time. To call the event catastrophic feels like an understatement. At the site of impact, rocks and animals were vaporized. Further away, molten rock condensed into glass beads that form a geological layer found around the world.

Still further away, in what is now North Dakota and was then the bank of a freshwater river, scientists have discovered a deposit full of saltwater fish, sharks, and rays that would have lived in the vast inland sea (A) that stretched northward from Texas. The meteorite’s impact pushed these creatures kilometers upstream against the river’s natural flow.

One possible explanation for the inundation is a tsunami. But geological evidence indicates the deposit took place within 15 minutes to two hours of the impact, when glass beads were still raining down. To travel the 3,000 km from the point of impact would take a tsunami on the order of 18 hours – far too long.

Instead, the deposit is likely the result of a seiche (pronounced “saysh”) – a type of standing wave that occurs in an enclosed or partially enclosed body of water. If you imagine water sloshing in a cup or a tub, that’s essentially what a seiche is, but this was on a much larger scale. (For an example, check out this insane footage of an earthquake-induced seiche in a swimming pool.)

What set the seiche to sloshing are the seismic waves triggered by the meteorite impact. They would have reached this site 6-13 minutes after the impact and triggered waves on the order of 10m. As the waves drove up the riverways, they carried dead and dying sea creatures with them, leaving them stranded on the riverbank until scientists uncovered them tens of millions of years later. (Image and research credit: R. DePalma et al.; via The Conversation; submitted by Kam-Yung Soh)

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