Earth's path in galaxy found in crust

"To see a world in a grain of sand", the opening sentence of the poem by William Blake, is an oft-used phrase that also captures some of what geologists do. We observe the composition of mineral grains, smaller than the width of a human hair. Then, we extrapolate the chemical processes they suggest to ponder the construction of our planet itself. Now, we've taken that minute attention to new heights, connecting tiny grains to Earth's place in the galactic environment.

Looking out to the universe

At an even larger scale, astrophysicists seek to understand the universe and our place in it. They use laws of physics to develop models that describe the orbits of astronomical objects. Although we may think of the planet's surface as something shaped by processes entirely within Earth itself, our planet has undoubtedly felt the effects of its cosmic environment. This includes periodic changes in Earth's orbit, variations in the Sun's output, gamma ray bursts, and of course meteorite impacts.

Just looking at the Moon and its pockmarked surface should remind us of that, given Earth is more than 80 times more massive than its grey satellite. In fact, recent work has pointed to the importance of meteorite impacts in the production of continental crust on Earth, helping to form buoyant "seeds" that floated on the outermost layer of our planet in its youth.

We and our international team of colleagues have now identified a rhythm in the production of this early continental crust, and the tempo points to a truly grand driving mechanism. This work has just been published in the journal Geology. Many rocks on Earth form from molten or semi-molten magma. This magma is derived either directly from the mantle – the predominantly solid but slowly flowing layer below the planet's crust – or from recooking even older bits of pre-existing crust. As liquid magma cools, it eventually freezes into solid rock. Through this cooling process of magma crystallisation, mineral grains grow and can trap elements such as uranium that decay over time and produce a sort of stopwatch, recording their age.

Not only that, but crystals can also trap other elements that track the composition of their parental magma, like how a surname might track a person's family. With these two pieces of information – age and composition – we can then reconstruct a timeline of crust production. Then, we can decode its main frequencies, using the mathematical wizardry of the Fourier transform. This tool basically decodes the frequency of events, much like unscrambling ingredients that have gone into the blender for a cake. Our results from this approach suggest an approximate 200-million-year rhythm to crust production on the early Earth.

Strikes from the cloud

In the distant reaches of our Solar System, a cloud of icy rocky debris named the Oort cloud is thought to orbit our Sun. As the Solar System periodically moves into a spiral arm, interaction between it and the Oort cloud is proposed to dislodge material from the cloud, sending it closer to the inner Solar System. Some of this material may even strike Earth. Comet impacts excavate huge volumes of Earth's surface, leading to decompression melting of the mantle, not too dissimilar from popping a cork on a bottle of fizz. This molten rock, enriched in light elements such as silicon, aluminium, sodium and potassium, effectively floats on the denser mantle. While there are many other ways to generate continental crust, it's likely that impacting on our early planet formed buoyant seeds of crust. Magma produced from later geological processes would adhere to those early seeds.

Harbingers of doom?

Continental crust is vital in most of Earth's natural cycles – it interacts with water and oxygen, forming new weathered products, hosting most metals and biological carbon. Large meteorite impacts are cataclysmic events that can obliterate life. Yet, impacts may very well have been key to the development of the continental crust we live on.

With the recent passage of interstellar asteroids through the Solar System, some have even gone so far as to suggest they ferried life across the cosmos. However we came to be here, it is awe-inspiring on a clear night to look up at the sky and see the stars and the structure they trace, and then look down at your feet and feel the mineral grains, rock and continental crust below – all linked through a very grand rhythm indeed.

(Kirkland is Professor of Geology, Curtin University and Sutton is Senior Lecturer in Astrophysics, University of Lincoln)