Asteroids that hit Earth roughly 3.3 billion years ago may have caused the ocean to boil

Ars Technica : After a period of extremely frequent impacts from large asteroids that occurred between 4.1 billion and 3.8 billion years ago, there were still occasional major bombardments. Now Donald Lowe of Stanford University and Gary Byerly of Louisiana State University believe they’ve found evidence of eight large impacts in South Africa. The impact layers date to 3.3 billion years ago and span 250 million years. Each layer contains evidence that the bedrock was vaporized by the impact and was hit by tsunamis soon after. Lowe and Byerly estimate that the asteroids ranged from 20 km to 100 km across. They also propose that two of the impacts were large enough to boil away the ocean at the impact sites. In what they admit is speculative, they suggest that the presence of silica, which precipitated into the layers, is a potential sign that the sea floor was exposed and cracked open by the boiling of the water.