Geologists have long debated how the Earth’s continents formed. A new study suggests that intense asteroid bombardment may be the key to understanding this process.
The Earth is the only planet known to have buoyant, silica-rich continents. However, despite decades of research, scientists still do not agree on how these continents were created. The oldest continental-type rocks date back around 4.03 billion years ago, but the geological evidence from this period is scarce. Scientists have had to rely on educated guesses to understand what happened during this time.
One dominant idea has been that plate tectonics was already running in the Hadean era, with continental crust forming above subduction zones. However, another theory suggests that early Earth was too hot for rigid plates and that crust instead formed above mantle plumes rising from deep within the planet.
The issue with both these ideas is that they do not fit with most models of the Earth’s heat budget. Scientists have tried to understand how much energy was coming into the Earth from internal sources such as radioactive decay, but this has proven challenging. The reason for this difficulty lies in the fact that plate tectonics effectively recycles the surface of the planet back into the mantle.
To get around this problem, Tim Johnson and his team turned to the Moon. Because the Moon does not have plate tectonics, its crust is a single, solid shell that has been peppered with impact craters. By calibrating crater counts on the Moon against dated lunar samples, the team was able to estimate how frequently large bodies were hitting our closest celestial neighbor shortly after the Earth had formed.
When they scaled this flux up to the Earth’s larger size and stronger gravity, it became clear that the planet must have been hit by thousands of impactors greater than 10 kilometers in diameter. The team then calculated how much energy these impacts delivered to the Earth and how much heat was produced.
Most prior modeling of early Earth’s heat budget focused on internal sources such as radioactive decay and core formation. However, Johnson’s space bombardment model showed that these were not dominant. Instead, impact heating exceeded radiogenic and core heat for most of the Hadean by roughly an order of magnitude.
The team then fed this reworked heat budget into geodynamic simulations to see what would happen. The results suggested that the Earth’s crust in the Hadean was thin and largely molten underneath. The models showed that melt fractions exceeded 30 percent by volume at around 5 kilometers depth, making it impossible for rock to hold together as a coherent slab.
This study provides new insights into how the Earth’s continents were formed. It suggests that asteroid impacts may have played a key role in creating the world we live on today.
Source: Original article