Geologist Chris Kirkland from Curtin University and his colleagues have put forward an unusual hypothesis: the growth of Earth's first continents may have been partly linked to the Solar System's passage through the spiral arms of the Milky Way. The authors found evidence for this in the isotopic composition of the oldest minerals — zircons. The work was published in the journal Earth and Planetary Science Letters.

How this could have happened

The Solar System orbits the center of the Milky Way approximately every 250 million years and crosses the galaxy's spiral arms roughly every 150–200 million years. These are regions of increased gas and dust density, where new stars are most actively born.

When crossing a spiral arm, gravitational disturbances could destabilize the Oort Cloud — a vast spherical shell of icy bodies at the far periphery of the Solar System. As a result, streams of comets are directed into the inner Solar System. The authors suggest that it was these impacts that helped form the first stable fragments of continental crust.

What the zircons showed

The team studied zircons from eleven Archean cratons on different continents. Hafnium isotopes in these minerals record episodes of new crust formation, while oxygen isotopes track the recycling of previously existing material.

Synchronous shifts in isotopic ratios were identified in sections from different cratons. These were matched to the timeline of predicted spiral arm crossings and independently supported by crater data from Earth and the Moon.

Important caveats

The authors emphasize that this is currently a correlation of independent datasets, not a proven causal relationship.

Kirkland explained that if changes in the galactic environment can trigger star formation, it would be surprising if the Solar System's repeated passages through these environments had no effect on it at all, adding that the scientific question is whether these effects are strong enough to leave a detectable geological trace on Earth.

The hypothesis can be tested by looking for similar signals in minerals from Mars and the Moon. If the pattern is genuinely galactic in nature, it should appear there as well.

In brief

Scientists have proposed a hypothesis that the formation of Earth's first continents may have been linked to the Solar System's passage through the spiral arms of the Milky Way. Gravitational disturbances destabilized the Oort Cloud and caused comet streams that, according to the authors, contributed to continental crust growth. Evidence was found in synchronous isotopic shifts in ancient zircons. This is currently a correlation of data, not a proven causal link. The work was published in the journal Earth and Planetary Science Letters.