Geologists have discovered a long-term cycle that may link the formation of Earth's continents to increased comet impacts and the planet's journey through the Milky Way. The team linked fluctuations in tiny grains in stable parts of the crust to the time our solar system passed through the galaxy's spiral arms.
Earth's crust is made up mostly of magma that cools and crystallizes into solid rock as the mantle cools. Geologists can trace the timeline of crust formation by studying the decay of trapped radioactive isotopes, as well as the ages of different grains based on their composition.
In doing so, the scientists observed patterns that suggested crust production spikes every 170 to 200 million years. Interestingly, this was consistent in both cratons, even though they are on opposite ends of the planet, indicating that the cycle is not due to local factors. Far from it – the researchers found that it could be the result of a galactic-scale cycle going on.
Everything in the galaxy is moving around the supermassive black hole at the center, but it's not all moving at the same speed. Our Sun, and the Solar System as a whole, is traveling at about 240 km (150 mi) per second, while the larger spiral arms orbit at a slower, about 210 km (130 mi) per second. . This means that from time to time, the Solar System will pass through these more dense spiral arms. How from time to time? About every 200 million years.
The team says that when our neighborhood moves into one of the spiral arms, the gravitational effects of the high density of material can shake off material released from the Oort cloud, which encircles the Solar System. These objects can then migrate to the inner Solar System as comets, increasing their chances of colliding with Earth.
Comets travel at much higher speeds than asteroids, and the team says that when they hit Earth, they excavate and melt large parts of the mantle, causing the molten rock to rise to the surface and become denser. The mantle floats on top of the material. As these sections cool, they form the "seeds" of continental crust, which can grow over time as more magma sticks to them.
"Our study reveals an exciting connection between geological processes on Earth and the motion of the Solar System in our galaxy," said lead researcher Professor Chris Kirkland. "Linking the formation of the continents on which we all live and where we find most of our mineral resources, to the passage of the Solar System through the Milky Way sheds a whole new light on our planet and its early history in the universe. in place."
Further evidence may exist in the era of globular beds, layers that contain concentrations of material produced when the pressure and heat of impacts vaporize the rock which then rains. The team says a correlation was found between some of the circular beds and the time when the solar system may have moved into a spiral arm between 3.25 and 3.45 billion years ago. The last time we encountered a spiral arm was famously—about 66 million years ago, according to research—when a giant asteroid or comet slammed Earth, wiping out the dinosaurs.
While it's certainly an interesting story, much more evidence will be needed before it becomes an accepted theory for how continental crust occurred. Currently, it is believed that much of it is the result of internal processes.
