The popular image of the early Earth as a magma-drenched ball of despair has taken yet another blow. New research affirms that our planet's first 500 million years were at times surprisingly similar to the present day — including the presence of oceans, continents, and crustal plates.
Up until about 30 years ago, most geologists agreed that conditions during this period were hellishly hostile to life — which is why it has been dubbed the Hadean Period, after Hades, the Greek word for hell and its ruler. The dearth of rock formations from the era led them to conclude that the early Earth was hellishly hot, if not entirely molten, and subject to such intense asteroid bombardment that any rocks that formed were rapidly melted.
But owing to the discovery of zircon crystals with ages exceeding 4 billion years old, a new picture began to emerge — that of a young Earth experiencing enough cool periods so that an established crust could form, and in turn (possibly) surface water. Today, there are two prevailing thoughts: Hadean Earth was (1) surprisingly similar to the present day, or (2) an alien and formidable place, similar to the hottest, most extreme, geological environments of today.
In regards to the latter vision, some geologists refer to Iceland as a prime example, a region where substantial amounts of crust are forming from basaltic magma. But no one has actually investigated Icelandic zircon and compared it to samples that are more than 4 billion years old.
That's now changed thanks to the efforts of geologist Tamara Carley. Since 2009, she's been collecting samples from volcanoes and sands derived from the erosion of Icelandic volcanoes. The samples represent Iceland's 18 million year history. With the help of her colleagues, she analyzed these 1,000 zircon crystals for their age and isotopic compositions.
"We discovered that Icelandic zircons are quite distinctive from crystals formed in other locations on modern Earth," said Carley in a press release. "We also found that they formed in magmas that are remarkably different from those in which the Hadean zircons grew."
Their analysis found that Icelandic crystals grew from much hotter magmas than Hadean zircons. Surface water did play an important role in the production of both, but in the Icelandic case the water was extremely hot when it interacted with the source rocks. The Hadean water-rocks, on the other hand, were at significantly lower temperatures.
"Our conclusion is counterintuitive," added study co-author Calvin Miller of Vanderbilt University. "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."
In other words, Iceland is not a magmatic analog for the Hadean — and the evidence from the zircon record proves it. And that's actually the title of the new study, which now appears in Earth and Planetary Science Letters.