Early Earth was Bombarded by Series of City-Sized Asteroids

upstart writes:

Early Earth was bombarded by series of city-sized asteroids:

Scientists know that the Earth was bombarded by huge impactors in distant time, but a new analysis suggests that the number of these impacts may have been 10 times higher than previously thought. This translates into a barrage of collisions-similar in scale to that of the asteroid strike that wiped out the dinosaurs-on average every 15 million years between 2.5 and 3.5 billion years ago.

[...] Earth's early years were unimaginably violent in comparison to today. Scientists believe that Earth was struck by a significant number of large asteroids (greater than 10 km in diameter), and this would have had significant effect on the Earth's near-surface chemistry and ability to support life. The effect of just one such collision was shown comparatively recently by the Chicxulub impact 66 million years ago, which led to the extinction of the dinosaurs. The early Earth, however, was very different to the Earth at the time of the Chicxulub impact, and so were the effects of collisions.

Impact craters from similar collisions can be seen on the Moon and other rocky planets, but atmospheric weathering and plate tectonics have tended to mask any direct evidence for ancient impact craters on Earth. However, echoes of these distant impacts can be seen in the presence of "spherules" found in ancient rocks; the huge impacts threw up molten particles and vapors which then cooled and fell to Earth to be embedded in rock as small spherical glassy particles. The greater the impact, the more these particles would have spread from the impact site, so global distribution of a thick spherule layer shows a huge impact.

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