The movement of the solar system through the galactic spiral arms of the Milky Way helped form Earth’s first continents

Spiral galaxy from Kirkland et al. CREDIT Chris Kirkland and colleagues.


A new study of zircon crystals from two of Earth’s oldest continents indicates that the formation of the Earth’s continental crust goes through cycles, with periods of increased crust production approximately every 200 million years, corresponding to the transit of the solar system through the four primary spiral arms of the Milky Way Galaxy. According to the study published yesterday in the journal Geology, regions of space with dense interstellar clouds could send more high-energy comets crashing into Earth’s surface, seeding increased production of continental crust.

“As geologists, we normally think that processes inside the earth are really important to the evolution of our planet. But we can also think on a much larger scale and look at extraterrestrial processes and our place in the environment. galactic,” says Chris Kirkland, lead author of the study.

Among its many unique features, Earth remains the only planet we know of that harbors continents and active plate tectonics. Plate tectonic processes have helped make our planet hospitable to life, shaping the composition and behavior of the hydrosphere, atmosphere and biosphere.

The data used in this new study comes from two places where Earth’s oldest continental history is preserved: the North American Craton in Greenland and the Pilbara Craton in Western Australia. In both places, the decay of uranium in zircon crystals has been used to establish a chronology of formation, covering the period from about 2.8 to 3.8 billion years ago, during the Archean eon. Hafnium isotopes measured in zircon were used to identify periods of time when there were influxes of juvenile magmas associated with crustal production. Using mathematical analysis, the researchers discovered the longer-period pattern corresponding to the “galactic year.” They observed a similar trend when examining oxygen isotopes, strengthening their findings.

The researchers point to galactic traffic as the likely source of this pattern. Our solar system and the spiral arms of the Milky Way both revolve around the center of the galaxy, but they move at different speeds. While the spiral arms orbit at 210 km/second, the sun moves at 240 km/second, which means it surfs in and out of the spiral arms over time. At the edge of our solar system, astronomers believe that there is a cloud of icy planetesimals – called the Oort cloud – orbiting our sun at a distance of between 0.03 and 3.2 light years (at for comparison, the Earth is 8.3 light-minutes from the Sun). As the solar system moves in a spiral arm, the interaction between the Oort Cloud and the denser material in the spiral arms could send more icy material from the Oort Cloud hurtling towards Earth. As Earth experiences more regular impacts from rocky bodies in the asteroid belt, comets ejected from the Oort cloud arrive with much more energy. Kirkland explains, “It’s important because more energy will mean more melting. When it strikes it causes greater amounts of decompression melting, creating greater uplift of material, creating a larger crustal seat.

Spherule beds – rock formations produced by meteorite impacts – are another key piece of evidence linking periods of increased crust generation to comet impacts. Spherule beds are deposits of small spheres formed either as ejected impact melt or condensed and fallout from post-impact rock vapor plumes. The study authors observed that the ages of the spherule beds correlate well with the movement of the solar system in spiral arms around 3.25 and 3.45 billion years ago. Determining the ages for more spherule bed deposits could add more evidence to the story.

Phil Sutton, astrophysicist and co-author of the study, says these findings should motivate further investigation into how forces outside the solar system have shaped our planet. “It is very difficult to prove these things; we want to make that connection and start the conversation to look at the geological processes beyond Earth, beyond the solar system, and what might be driving them. We didn’t just train in isolation.

Did it transit through the seed crust production of galactic spiral arms on early Earth?
CL Kirkland; PJ Sutton; T. Erickson; TE Johnson; MIH Hartnady; H. Smithies; M. Prause


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