New research led by Curtin University geologists suggests that 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. The findings challenge the existing theory that Earth’s continental crust was solely formed by processes inside our planet.
Earth is unique among the known planets in having continents, whose formation has fundamentally influenced the composition of the mantle, hydrosphere, atmosphere, and biosphere.
Cycles in the production of continental crust have long been recognized and generally ascribed to the periodic aggregation and dispersal of Earth’s continental crust as part of the supercontinent cycle.
However, such cyclicity is also evident in some of Earth’s most ancient rocks that formed during the Hadean (over 4 billion years ago) and Archean (4-2.5 billion years ago) eons.
Frequencies of many geological processes become more challenging to decipher in the early Earth given that the rock record becomes increasingly fragmentary with age.
“As geologists, we normally think about processes internal to the Earth being really important for how our planet has evolved,” said lead author Professor Chris Kirkland, a researcher in the School of Earth and Planetary Sciences at Curtin University.
“But we can also think about the much larger scale and look at extraterrestrial processes and where we fit in the Galactic environment.”
Professor Kirkland and colleagues investigated the cyclicity in the addition of new crust and its subsequent reworking through the hafnium isotopic record of dated zircon grains from the North American craton in Greenland and the Pilbara craton in western Australia.
Using mathematical analysis, they uncovered the longer period pattern corresponding with the ‘galactic year.’
They observed a similar pattern when looking at oxygen isotopes, bolstering their results.
“Studying minerals in the Earth’s crust revealed a rhythm of crust production every 200 million years or so that matched our Solar System’s transit through areas of the Milky Way with a higher density of stars,” Professor Kirkland said.
According to the team, 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, there is a cloud of icy planetesimals — the Oort cloud — orbiting the Sun at a distance of between 0.03 to 3.2 light-years.
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.
“That’s important because more energy will result in more melting,” Professor Kirkland said.
“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.
The researchers 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.
“The findings challenged the existing theory that crust production was entirely related to processes internal to the Earth,” Professor Kirkland said.
“Our study reveals an exciting link between geological processes on Earth and the movement of the Solar System in our Galaxy.”
“Linking the formation of continents, the landmasses on which we all live and where we find the majority of our mineral resources, to the passage of the Solar System through the Milky Way casts a whole new light on the formative history of our planet and its place in the cosmos.”
The study is published in the journal Geology.Sci.News