The idea that our planet's early crust was shaped by cosmic bombardment is a fascinating one, and it's an idea that geologist Tim Johnson and his team are making a strong case for. Johnson argues that asteroid impacts were far more frequent in the early solar system, and that these impacts played a crucial role in the formation of Earth's continents. This theory challenges the conventional understanding of how our planet's crust formed, and it's a theory that could change the way we think about Earth's history.
The problem with studying the formation of continents is that the geological evidence is scarce. The oldest known continental-type rocks date back to around 4.03 billion years ago, and beyond that, there's not much to go on. So, scientists have had to rely on educated guesses, and there's been a lot of debate about what was going on in the early Earth. Johnson and his team are proposing a new idea: that asteroid impacts were a significant source of heat, and that this heat helped to create the conditions necessary for the formation of continents.
The team's research focused on the Moon, which doesn't have plate tectonics, and therefore has a continuous, solid crust. By studying the impact craters on the Moon, they were able to estimate the frequency and size of asteroid impacts in the early solar system. When they scaled this up to Earth, they found that the planet must have been hit by thousands of impactors greater than 10 kilometers in diameter. This bombardment delivered a lot of heat to Earth, and it helped to keep the early crust hot and thin.
The team's modeling showed that impact heating exceeded radiogenic and core heat for most of the Hadean period, and that this heat helped to create a thin, largely molten crust. This crust was less than 5 kilometers thick, and it was widespread partial melting that started just 2 to 3 kilometers below the surface. The key takeaway was that plate tectonics couldn't have worked in such conditions, as subduction and plate tectonics require a rigid lithosphere that can jostle around and subduct. With the localized effects of individual large impacts, the crust was recycled back into the mantle, and this recycling explains why so little Hadean crust survived to the present.
The impact flux didn't stay high forever; it declined exponentially, and between 3.9 and 3.5 billion years ago, internal heat sources took over as the dominant influence on the crust. As impact heating faded, the upper mantle cooled, and the once-thin basaltic crust thickened. The team's modeling suggests that crustal thickness reached around 30 kilometers by the early Archean, and this thicker, cooler, more rigid crust was able to support plate tectonics. It's around this time that the first continental rocks show up in the geological record.
Johnson admits that much of the argument rests on physics-based modeling rather than rock samples, but he thinks that reliance on modeling is justified in the absence of geological evidence. He's hopeful that ancient rocks will be found in the future, and that the Earth's history will continue to be uncovered. The team's research is an exciting development in our understanding of Earth's formation, and it's a reminder that there's still a lot to learn about our planet's past.
