Scientists trace Earth's path through the Milky Way through tiny crystals found in Earth's crust

Scientists trace Earth’s path through the Milky Way through tiny crystals found in Earth’s crust

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“Seeing a world in a grain of sand,” the opening line of William Blake’s poem, is an often-used phrase that also captures some of what geologists do.

We observed a composition of mineral grains smaller than the width of a human hair. We then extrapolated the chemistry they suggested to think about the structure of our planet itself.

Now, we’ve turned our attention to new heights, linking tiny particles to Earth’s place in a galactic environment.

look into the universe

On a larger scale, astrophysicists try to understand the universe and our place in it. They use the laws of physics to develop models that describe the orbits of astronomical objects.

While we may think that the Earth’s surface is entirely shaped by processes within the planet, our planet has undoubtedly felt the effects of its cosmic environment. This includes periodic changes in Earth’s orbit, changes in the sun’s output, gamma-ray bursts, and, of course, meteorite impacts.

Given that Earth is more than 80 times the mass of its gray moons, just looking at the Moon and its pockmarked surface should remind us of this. In fact, recent work has pointed to the importance of meteorites hitting the continental crust on Earth, helping to form the young “seeds” that float in our planet’s outermost layers.

We and our international team of colleagues have now identified the rhythm of the creation of this early continental crust, and the rhythm points to a truly great driving mechanism.This work has just been published in the journal geology.

The rhythm of crustal formation on Earth

Many rocks on Earth are formed from molten or semi-molten magma. This magma either comes directly from the mantle — the mostly solid but slow-moving layer beneath the crust — or from re-cooked or even older pre-existing crust. As the liquid magma cools, it eventually freezes into hard rock.

Through the cooling process of magma crystallization, mineral grains grow and can trap elements such as uranium, which decay over time and produce a sort of stopwatch that records their age. Not only that, but the crystals can also capture other elements that track the composition of their parents’ magma, such as how a surname traces a person’s family.

With these two pieces of information — age and composition — we can reconstruct the timeline of crustal production. We can then decode its dominant frequency using the mathematical magic of the Fourier transform. This tool basically decodes the frequency of events, like unraveling the ingredients that have gone into the blender to make a cake.

Our results from this approach suggest that crustal production on the early Earth had a rhythm of about 200 million years.

Scientists trace Earth's path through the Milky Way through tiny crystals found in Earth's crust

Geological events, including major crust-forming events highlighted as the solar system crosses the galactic spiral arm. Image credit: NASA/JPL-Caltech/ESO/R.harm

Our place in the universe

But there is another process with a similar rhythm. Both our solar system and the Milky Way’s four spiral arms revolve around the supermassive black hole at the center of the Milky Way, but they move at different speeds.

The spiral arms travel at 210 kilometers per second, while the sun travels at 240 kilometers per second, which means our solar system is moving in and out of the Milky Way’s arms. You can think of a spiral arm as a dense area that slows the passage of stars, like a traffic jam, it will only clear further down the road (or through the spiral arm).

This model results in about 200 million years between each entry of our solar system into the spiral arms of the Milky Way.

So there seems to be a possible connection between the time Earth’s crust was created and the length of time it took to wrap around the Milky Way’s spiral arms — but why?

A blow from the cloud

In the far reaches of our solar system, a cloud of icy rocky debris known as the Oort Cloud is thought to orbit our Sun.

As the solar system periodically moves into a spiral arm, its interaction with the Oort cloud is proposed to remove material from the cloud, bringing it closer to the inner solar system. Some of this material might even hit Earth.

Earth is relatively frequently hit by rocky bodies in the asteroid belt, with an average velocity of 15 kilometers per second. But comets ejected from the Oort Cloud arrive much faster, averaging 52 kilometers per second.

We believe that it is these periodic high-energy impacts that are tracked by records of crustal production preserved in tiny mineral grains. The comet impact excavated a lot of Earth’s surface, causing the mantle to decompress and melt, not unlike popping a cork on a soda bottle.

This lava, rich in light elements such as silicon, aluminum, sodium and potassium, effectively floats on the denser mantle. While there are many other ways to create continental crust, the effects on our early planet likely formed the buoyant seeds of the crust. Magma from later geological processes would attach to those earlier seeds.

A harbinger of doom, or a gardener of life on Earth?

The continental crust is critical in most of Earth’s natural cycles – it interacts with water and oxygen to form new weathering products, and hosts most metals and biological carbon.

Large meteorite impacts are catastrophic events that can destroy life. However, the impact is likely to be the key to the development of the continental crust on which we depend.

With the recent passage of interstellar asteroids through the solar system, some have even suggested that they are transporting life in the universe.

Yet here we are, looking up at the stars on a clear night, seeing the stars and the structures they depict, and then looking down at the mineral grains, rocks and continental crust beneath our feet, is awe-inspiring – all through a very grandiose rhythm linked together.

What created the continent?New evidence points to giant asteroid

More information:
CL Kirkland et al. Transit via galactic spiral arm seed crust on early Earth? geology (2022). DOI: 10.1130/G50513.1

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