The chaos in the early Solar System was fiendish. Even after the planets had coalesced, there was more than enough rubble left behind to cause frequent and violent impacts that would have rocked the Earth’s youthful crust. After a phase of intense bombardment between about 4.1 and 3.8 billion years ago, things on the asteroid collision scene calmed down. Relatively speaking…

We don’t know exactly when life first developed on Earth, but we know it was present by 3.4 billion years ago. We don’t know if life was present to suffer from the earlier period of bombardment, but we know it was around for any impacts that followed. So what kinds of extraterrestrial punches did life take after 3.4 billion years ago?

A new study by Stanford’s Donald Lowe and Louisiana State University’s Gary Byerly examines a fascinating record of major impacts in South African rocks around 3.3 billion years old. Eight impact layers have been identified in these rocks, each containing sand-sized blobs of rock that solidified after the impact vaporized bedrock. The layers also show signs that they were hit by tsunamis shortly afterward.

It’s difficult to estimate the size of the bodies that hit the Earth during these events, but the researchers put their range from 20 kilometers across to more than 100 kilometers across. For comparison, the asteroid that helped do in the dinosaurs was probably about 5 to 10 kilometers across. We’re talking big, mean impacts—and eight of them within about 250 million years.

Impacts that large are getting into the range where the resultant heating of the atmosphere is so unfathomably extreme that a portion of the world’s oceans would boil away. In the case of these rocks near Baberton, South Africa, the researchers actually believe they see signs of that happening in two of the impact layers.

One of the layers of vaporized rock blobs settled onto the seafloor well offshore, where the water was probably tens of meters deep. Yet it looks like the sediments were exposed and eroded for a time. Afterward, smooth layers of silica (occasionally ripped up by wave action) precipitated to form a custom-fit cap.

The other layer was deposited closer to shore, atop some volcanic sediment. There, the researchers think that as the sediment was exposed, the water within it started to boil, hydraulically cracking it apart. The same sort of silica cap precipitated out over everything, and the deposition of seafloor sediment then resumed.

The researchers’ interpretation of all this— which they admit is a “speculative scenario”—is that up to a hundred meters of seawater evaporated away after the impact, exposing the shallow seafloor. That concentration of seawater led to the precipitation of silica like a bathtub ring. As sea returned to its previous level, the area turned to deeper seafloor once again.

This level of ocean evaporation is commensurate with atmospheric temperatures rising to over 500 degrees Celsius for a few weeks after the impact, and remaining above 100 degrees Celsius for over a year. That’s what models have predicted would occur for collisions with asteroids 50 to 100 kilometers across.

The silica cap looks a lot like what forms around super-hot springs, leading the researchers to describe the surface ocean as “essentially a global hot spring.” Like hot springs, there are also signs that microbes were living where this silica was precipitating. So while “catastrophe” would certainly be the right word to describe these events, we also see evidence of life’s resiliency written in the rocks.

Geology, 2015. DOI: 10.1130/g36665.1 (About DOIs).