Earth hasn’t always been an oasis of blue and green life in an otherwise inhospitable solar system. During our planet’s first 50 million years, roughly 4.5 billion years ago, its surface was a hellish landscape of magmatic oceans, bubbling and belching with the heat of the Earth’s interior.
The subsequent cooling of the planet from this molten state and the crystallization of these magmatic oceans into solid rocks was a critical step in assembling our planet’s structure, the chemistry of its surface, and the formation of its early atmosphere. .
It was speculated that these primitive rocks, containing clues that could explain the habitability of the Earth, had been lost due to the ravages of plate tectonics. But now my team have discovered the chemical remnants of Earth’s magmatic oceans in 3.7 billion-year-old rocks in southern Greenland, revealing a tantalizing snapshot of a time when Earth was almost completely melted.
Hell on earth
Earth is the product of a chaotic early solar system, which is said to have exhibited a number of catastrophic impacts between Earth and other planetary bodies. The formation of the Earth culminated in its collision with a Mars-sized impactor planet, which also resulted in the formation of the Earth’s moon about 4.5 billion years ago.
These cosmic clashes are believed to have generated enough energy to melt the earth’s crust and almost all of our planet’s interior (mantle), creating planetary-scale volumes of molten rock that formed “magmatic oceans.” Hundreds of kilometers deep. Today, on the other hand, the earth’s crust is entirely solid, and the mantle is considered a “plastic solid”: allowing slow and viscous geological movement away from the liquid magma of the earth’s first mantle.
As Earth recovered and cooled from its chaotic collisions, its deep magmatic oceans crystallized and solidified, beginning Earth’s journey to the planet we know today. Volcanic gases escaping from Earth’s cooling magmatic oceans may have played a decisive role in the formation and makeup of our planet’s primitive atmosphere – which would eventually support life.
Finding geological evidence of the ancient molten state of the Earth is extremely difficult. Indeed, oceanic magmatic events likely occurred over 4 billion years ago, and many rocks from this period of Earth’s history have since been recycled by plate tectonics.
But if rocks from this period no longer exist, their chemical traces can still be stored deep in the Earth. The solidified crystals from the Earth’s cooling period would have been so dense that they would have sunk at the base of the Earth’s mantle. Scientists even believe that these mineral residues can be stored in isolated areas deep inside the Earth’s mantle-core border.
If they exist, these ancient crystal cemeteries are inaccessible to us – hiding far too deep for us to directly sample. And if they were ever to rise to the Earth’s surface, oceanic magma crystals would naturally undergo a process of melting and solidification, leaving only traces of their origins in the volcanic rocks that reach the earth’s crust.
We knew Greenland would be a good place to search for these traces of Earth’s molten past. Our samples come from the supracrustal belt of Isua in southwest Greenland, which is a well-known region for geologists. At first glance, Isua rocks look like any modern basalt you will find on the seabed. But these rocks are among the oldest in the world, ranging from 3.7 to 3.8 billion years old.
By analyzing the rocks of Isua, we discovered unique iron isotope signatures. These signatures showed that the mantle region from which the rocks had formed had been subjected to very high pressure, more than 700 kilometers below the Earth’s surface. This is exactly where the minerals formed during the crystallization of ocean magma would have been located.
But if these rocks did indeed carry traces of ocean of crystallized magma, how did they end up on the surface of the Earth? The answer lies in how the Earth’s interior is melting, producing volcanic rocks on the planet’s surface.
As regions of the Earth’s semi-solid mantle warm and melt, they float upward toward the Earth’s crust, ultimately producing volcanic rocks as the magma reaches the surface and cools. By studying the chemistry of these rocks on the surface, we can probe the composition of the material that melted to form them.
The isotopic composition of the Isua rocks revealed that their journey to the Earth’s surface involved several stages of crystallization and reflow inside the planet – a sort of distillation process on their way to the surface. But the rocks that emerged, located in present-day Greenland, still retain chemical signatures that link them to Earth’s magma-covered past.
The results of our work provide some of the first direct geological evidence for the signature of oceanic magmatic crystals in volcanic rocks found on the Earth’s surface. Now, we’d like to know if other ancient volcanic rocks around the world can tell us more about Earth’s ancient magmatic oceans, or if we’ve come across a geological quirk instead: a more point-in-time clue.
While other volcanoes may have spat out similar geological artifacts, we could also look to modern eruption hotspots such as Hawaii and Iceland for other isotopic novelties that speak to Earth’s ancient past. It is possible that more primordial rocks could be found in the future, which could help us better understand the violent and magma-covered past of Earth.
This article by Helen M Williams, Reader in Geochemistry, University of Cambridge is republished from The Conversation under a Creative Commons license. Read the original article.
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