Movement of the solar system through the Milky Way’s galactic spiral arms
helped form Earth’s first continents
Boulder, Colo., USA: A new study of zircon crystals from two of Earth’s
oldest continents indicates that the formation of Earth’s continental crust
goes through cycles, with periods of increased crust production roughly
every 200 million years, corresponding to the solar system’s transit
through the four primary spiral arms of the Milky Way galaxy. According to
the study published in the journal Geology yesterday, regions of
space with dense interstellar clouds may send more high-energy comets
crashing to the surface of the Earth, seeding enhanced production of
continental crust.
“As geologists, we normally think about processes internal to the earth
being really important for how our planet has evolved. But we can also
think about the much larger scale and look at extraterrestrial processes
and where we fit in the galactic environment,” explains Chris Kirkland,
lead author of the study.
Among its many unique features, Earth remains the only planet we know of
that hosts 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 came from two places where Earth’s earliest
continental history is preserved: the North American craton in Greenland
and the Pilbara craton in western Australia. In both locations, decay of
uranium in zircon crystals has been used to establish a timeline of
formation, spanning the period from roughly 2.8–3.8 billion years ago,
during the Archean eon. Hafnium isotopes measured within the zircon were
used to identify periods of time when there were influxes of juvenile
magmas associated with crust production. Using mathematical analysis, the
researchers uncovered the longer period pattern corresponding with the
“galactic year.” They observed a similar pattern when looking at oxygen
isotopes, bolstering their results.
The researchers point to galactic traffic as the likely source of this
pattern. Our Solar System and the spiral arms of the Milky Way are both
spinning around the galaxy’s center, but they are moving at different
speeds. While the spiral arms orbit at 210 km/second, the sun is cruising
along at 240 km/second, meaning it is surfing into and out of spiral arms
over time. At the outer reaches of our solar system, astronomers believe
there is a cloud of icy planetesimals—named the Oort cloud—orbiting our sun
at a distance of between 0.03 to 3.2 light years (for comparison, the Earth
is 8.3 light minutes from the Sun). As the solar system moves into a spiral
arm, interaction between the Oort cloud and the denser material of the
spiral arms could send more icy material from the Oort cloud hurling toward
Earth. While Earth experiences more regular impacts from the rocky bodies
of the asteroid belt, comets ejected from the Oort cloud arrive with much
more energy. Kirkland explains, “That’s important because more energy will
result in more melting. When it hits, it causes larger amounts of
decompression melting, creating a larger 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 rained out from rock-vapor plumes
after impact. The authors of the study observed that the ages of spherule
beds are well-correlated with the solar system’s movement into spiral arms
around 3.25 and 3.45 billion years ago. Determining the ages for more
deposits of spherule beds could add more evidence to the story.
Phil Sutton, an astrophysicist and coauthor of the study, explains that
these findings should motivate more investigation of how forces outside the
solar system have shaped our planet. “It’s very hard to prove these things;
we want to make that link and start the conversation to look at geological
processes beyond the Earth, beyond the Solar System, and what might drive
those. We didn’t just form in isolation.”
FEATURED ARTICLE
Did transit through the galactic spiral arms seed crust production on
the early Earth?
C.L. Kirkland; P.J. Sutton; T. Erickson; T.E. Johnson; M.I.H. Hartnady; H.
Smithies; M. Prause
Author contact: Chris Kirkland,
C.Kirkland@curtin.edu.au
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G50513.1/616377/Did-transit-through-the-galactic-spiral-arms-seed