4.4-billion-year-old crystals proving Earth had liquid water oceans within 150 million years of formation. Water appeared almost immediately after our planet was born.
Gretarsson, CC BY 4.0, via Wikimedia Commons
A message in a crystal
In the Jack Hills of Western Australia, an 80-kilometer-long belt of ancient metamorphosed rock holds the oldest known pieces of our planet. These are not rocks, but microscopic crystals of the mineral zircon (Zirconium silicate, ZrSiO4). Geologists have found thousands of these durable crystals, with the very oldest dated to 4.404 billion years old, a mere 150 million years after Earth's formation. The zircons are found within a much younger metaconglomerate, a type of sedimentary rock, meaning the parent rocks that first formed the zircons have long since eroded away. The resilient crystals survived this erosion, becoming embedded in new sediment to form a new rock, as records from a lost world.
The age of these minerals is determined through uranium-lead (U-Pb) dating. Zircon's crystal structure readily incorporates uranium atoms but rejects lead. Over billions of years, the radioactive uranium within a crystal decays into lead at a fixed, predictable rate. By measuring the ratio of uranium to lead, scientists can calculate the precise age of the crystal's formation.
The isotopic evidence for water
The Hadean Eon, from Earth's formation to 4 billion years ago, was long pictured as a hellscape of molten magma oceans. The Jack Hills zircons present a different picture. Scientists analyze the ratios of oxygen isotopes within the zircon crystals to reconstruct the environment in which they formed. Oxygen exists in heavier (Oxygen-18) and lighter (Oxygen-16) forms. Zircons that crystallize from magma in a purely volcanic, water-free environment have a specific, predictable O-18/O-16 ratio.
Many of the Hadean zircons from Jack Hills show a significantly higher concentration of Oxygen-18. This isotopic signature shows that the magma from which the zircons grew had interacted with large volumes of liquid water. Rocks altered by surface water become enriched in Oxygen-18; when these rocks are melted and recycled into new magma, the resulting zircons lock in that isotopic evidence. These δ18O values, some reaching over 7.5‰, are much higher than typical mantle values of 5.3‰ ± 0.6‰. This finding suggests the presence of a hydrosphere—and possibly oceans interacting with continental-type crust as early as 4.3 billion years ago. This "Cool Early Earth" theory, which posits cooler temperatures and liquid water far earlier than previously assumed, is built substantially on the evidence contained within these tiny Australian crystals.
💡Fun Facts
The Hadean Eon is named after Hades, the Greek god of the underworld, reflecting the old belief that the young Earth was a fiery, uninhabitable world.
Zircon is so chemically and physically tough that it can survive the complete destruction of its parent rock through erosion and even survive being melted and incorporated into new magma.
The specific discovery site for the oldest zircons is Erawondoo Hill, which is now on Australia's National Heritage List and recognized as a world geological heritage site.
Some of these ancient zircons also contain the oldest diamond crystals ever found, which were encased inside the zircon as it formed.
24/7. The site is a remote, unfenced area of wilderness.
Admission
Free. However, access is restricted for scientific preservation purposes, and casual tourism is discouraged.
Accessibility
The Jack Hills are remote, located about 800 km north of Perth. Reaching the area requires a 4WD vehicle and experience with remote travel. The terrain is rugged and undeveloped.
