The seeds of continental crust

Featuring image: Lava lake in Hawaii Volcanoes National Park, May 1954. Photo by J.P. Eaton, Public Domain (C0).

Paper: Did transit through the galactic spiral arms seed crust production on the early Earth?

Authors: C.L. Kirkland, P.J. Sutton, T. Erickson, T.E. Johnson, M.I.H. Hartnady, H. Smithies, M. Prause

Plate tectonics reshape the face of Earth over long periods of time, but how the first continental crust evolved is still unclear. Now, a new investigation of very old rocks showed that Earth structure might have been influenced by the galactic dance of our solar system through the Milky Way.

The dating of old continental crust from the Precambrian (2.8 – 3.6 billion year old rocks) indicates that the formation of continental crust happened in cycles. Scientists discovered these cycles, which indicate that the crust didn’t form continuously, decades ago by dating minerals contained in continental crust all over the globe. Now, new research suggests that these cycles correspond to the periods where Earth passed through the spiral arms of our galaxy, the Milky Way.

The cyclicity of continental crust formation was long thought to be connected to the so-called supercontinent cycle. When looking at the global rock record, supercontinents seem to form and break apart in a surprisingly constant pattern of 300 to 500 million years. But modern tectonics and supercontinents didn’t exist before 2.5 billion years ago.

To better understand the earlier crust forming processes, Kirkland and co-workers looked into zircons from locations where very old continental crust is preserved: the Archean North Atlantic craton and the Pilbara craton. Both sites contain continental crust which is older than 3 billion years. And still these rocks show hints of a cyclic crust formation, preserved as variations of the isotopic composition in the trace element hafnium at both locations. Moreover, the cycles of crust formation also seem to be very similar at the Pilbara and Archean North Atlantic cratons. Both sites showed cycles with periods between 170 and 200 million years.

Many different astronomical and geological processes have cyclic behaviour, like changes in the Earth’s orbit or magnetic field. But they don’t match with the periodic patterns found in those old rocks. Only one parameter seems to fit: the orbit of our solar system around the centre of our galaxy. Approximately every 200 million years, our solar system passes through one of the four spiral arms of the Milky Way.

Our galaxy, the Milky Way, is a spiral galaxy. The sun orbits the centre of the Milky Way at a different speed than the spiral arms. Thus, our solar system passes a spiral arm approximately every 200 million years. NASA/JPL-Caltech/ESO/R. Hurt Public Domain (C0).

How can passage through a spiral arm influence the geology of our planet? The spiral arms of the Milky Way are not static structures, but instead are a kind of density wave in our galaxy. They mark areas where the concentration of matter (dust, stars, planets) is much higher than in the space between arms. This higher concentration can disturb the orbits of the comets orbiting our sun in the far outside regions of our solar system and might set one on a collision course with Earth.

These comets can be quite large, much larger than the meteorite that killed the dinosaurs. On impact, they could melt a large portion of the upper crust around the impact site and even the mantle under the crust. This new study suggests that these melting events might have helped to form more differentiated rocks and building-up the continental crust from the primordial basaltic crust on early Earth. Moreover, the fragments of the comets themselves could have functioned as crystallisation nuclei to form large scale, contiguous continental crust slabs.

When a big comet would strike Earth, parts of the upper crust would start to melt. On ancient Earth, this could have triggered a partial melting of the primordial basaltic crust. From this melt, silicate rich crustal rocks like granite could have formed. An impact like this can also induce melting in the underlying mantle. The prior formed silicate rich segments can than act as crystallisation nuclei for continental crust. Changed after Kirkland et al. 2022 figure 3d, Creative Commons (CC-BY).

Still, the mystery around the dynamics on our young planet remains unsolved, and we need more astronomical and geological investigations to confirm or reject Kirkland and co’s theory. However, this work is an exciting example of how interwoven the nature of our home planet is with our galactic heritage.

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‘The seeds of continental crust’ by Max Winkler is licensed under a Creative Commons Attribution-ShareAlik

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