“To see a world in a grain of sand,” the opening phrase of William Blake’s poem, is a widely used phrase that also captures some of what geologists do.
We observe the composition of the mineral grains, smaller than the width of a human hair.
We then extrapolate the chemical processes they suggest to reflect on the very construction of our planet.
Now, we’ve taken this meticulous attention to new heights, connecting tiny grains with Earth’s place in the galactic environment.
Looking out into the universe
On an even larger scale, astrophysicists seek 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.
Although we can think of the planet’s surface as something shaped by processes entirely within the Earth, our planet has certainly felt the effects of its cosmic environment. This includes periodic changes in the Earth’s orbit, variations in sunrise, gamma ray bursts and, of course, meteorite impacts.
Just looking at the Moon and its scarred surface should remind us of this, given that Earth is more than 80 times more massive than its gray satellite.
Indeed, recent work has pointed to the importance of meteorite impacts in the production of continental crust on Earth, helping to form floating “seeds” that floated in 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 rhythm points to a really large driving mechanism. This work has just been published in the journal Geology.
The rate of crust production on Earth
Many rocks on Earth are formed from molten or semi-molten magma. This magma is derived either directly from the mantle, the predominantly solid but slowly flowing layer beneath the planet’s crust, or from the recovery of even older pieces of pre-existing crust. As liquid magma cools, it eventually freezes into solid rock.
Through this cooling process of magma crystallization, large minerals grow and can trap elements such as uranium that decay over time and produce a kind of stopwatch, recording their age.
Not only that, but the crystals can also trap other elements that track the composition of the parent magma, like how a surname might trace a person’s family.
With these two pieces of information, age and composition, we can reconstruct a timeline of crust production. We can then decode its main frequencies, using the mathematical magic of the Fourier transform.
This tool basically decodes the frequency of events, much like deciphering ingredients that have gone into the mixer for a cake.
Our results from this approach suggest an approximate 200 million year pace for crust production on the early Earth.
Our place in the cosmos
But there is another process with a similar rhythm. Our Solar System and the four spiral arms of the Milky Way are revolving around the supermassive black hole at the center of the galaxy, but they are moving at different speeds.
The spiral arms orbit at 210 kilometers per second, while the Sun advances at 240 km per second, meaning our Solar System is sailing in and out of the galactic arms.
You can think of spiral arms as dense regions that slow the passage of stars much like a traffic jam, only to clear further down the road (or through the arm).
Geological events, including major crustal formation events prominent in the transit of the Solar System through the galactic spiral arms. NASA/JPL-Caltech/ESO/R. injured)
This model results in approximately 200 million years between each entry that our Solar System makes into a spiral arm of the galaxy.
So there seems to be a possible connection between the production time of the crust on Earth and the time it takes to orbit the galactic spiral arms, but why?
Shots from the cloud
At the far reaches of our Solar System, a cloud of icy rocky debris called the Oort cloud is thought to orbit our Sun.
As the Solar System periodically moves in a spiral arm, the 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 hit Earth.
Earth experiences relatively frequent impacts from rocky bodies in the asteroid belt, which on average reach speeds of 15 km per second. But comets ejected from the Oort cloud arrive much faster, averaging 52 km per second.
We argue that it is these periodic high-energy impacts that are tracked by the crustal production record preserved in small mineral grains.
Comet impacts excavate large volumes of the Earth’s surface, resulting in decompression melting of the mantle, not too dissimilar to popping a cork in a bottle of gas.
This molten rock, enriched in light elements such as silicon, aluminum, sodium and potassium, effectively floats on top of the denser mantle.
Although there are many other ways to generate continental crust, it is likely that the impact on our first planet formed floating seeds of crust. Magma produced from later geological processes would adhere to these early seeds.
Harbingers of doom or gardeners of earthly life?
The continental crust is vital in most of the Earth’s natural cycles: it interacts with water and oxygen, forming new weathered products, which host most metals and biological carbon.
Large meteor impacts are cataclysmic events that can obliterate life. However, the impacts may 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 that they carried life across the cosmos.
However we came here, it’s impressive 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 large minerals, rock and continental crust. below, all tied together through a very large beat.
Chris Kirkland, Professor of Geology, Curtin University and Phil Sutton, Senior Lecturer in Astrophysics, University of Lincoln
This article is republished from The Conversation under a Creative Commons license. Read the original article.