For the first 500 million years of its existence, our planet was believed to literally be a hell on Earth. But new research shows that this early Earth may have been surprisingly similar to the present day, complete with oceans, continents and active crustal plates.
This alternate view of Earth’s first geologic eon, called the Hadean, is based on a comparison of zircon crystals formed four billion years ago with those formed during the same time period in Iceland. This icy country is supposedly what early Earth geological conditions were like, and so serves as a sort of blueprint for scientists studying the beginnings of our planet.
"We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals — and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments," lead researcher Calvin Miller of Vanderbilt University said in a statement.
Until 30 years ago, scientists thought the Hadean period was hellishly hot, and Earth was covered by a giant “magma ocean.” This view was based on the fact that they could never find rock formations from that time period, jumping to the conclusion that the intense heat melted the rocks, leaving behind no trace.
But then geologists discovered zircon crystals - a mineral typically associated with granite - preserved in younger sandstones. Radiometric dating and other analytical techniques allowed the researchers to study early Earth’s crust via these four-billion-year-old crystals, as well as extract information about the environment in which the crystals formed, including the temperature and whether water was present.
And after comparing these crystals with about 1,000 ancient zircons sifted from volcano and sand samples off Iceland, the researchers found that Icelandic zircons grew from much hotter magmas than Hadean zircons.
Despite the assumption that Earth was insanely hot, their analysis revealed that at some points during the Hadean period Earth’s crust cooled enough so that surface water could form - possibly on the scale of oceans.
"Our conclusion is counterintuitive," said Miller. "Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even ‘cooler’ and ‘wetter’ than those being produced today."
The findings were published in the journal Earth and Planetary Science Letters.