The Earth was once entirely covered by an ocean of magma, scientific accounts tell. We've never had direct proof of this other than deducing it from the conditions of the era. Now, however, rocks have been found containing crystals that appear to have cooled out of this molten sea, taking us as far back in the chronology of the planet as it is possible to go. If correct, these observations would change our thinking about our present as well as our past, particularly hotspot volcanoes, of which Hawaii is the most famous example.
The magma ocean theory derives from the heat produced in planetary formation. Before this could wane, the Earth was pummelled by immense rocks from outer space, including the one (or more) responsible for the Moon. The heat of their impact would have long delayed the cooling process.
Eventually, however, things settled down, impacts became rarer and temperatures abated. Plate tectonics took over, and relics of these extreme conditions were drawn down into the planet's interior to be recycled beyond the point of recognition. Or so we thought. Now it appears at least one such legacy survived.
Greenland's Isua supracrustal belt competes with Australia's Jack Hills for the title of the Earth’s oldest rocks. "There are few opportunities to get geological constraints on the events in the first billion years of Earth's history,” Dr Helen Williams of Cambridge University said in a statement.” “It's astonishing that we can even hold these rocks in our hands - let alone get so much detail about the early history of our planet,"
Williams and co-authors have announced in Science Advances that crystals within the Isua rocks have distinctive chemistry indicating they were formed at the bottom of the magma ocean, where the mantle met the core. The conclusions were only possible because of recent advances in understanding the way iron isotope ratios vary depending on the temperatures and pressures under which crystals containing them form.
“Those samples with the iron fingerprint also have a tungsten anomaly - a signature of Earth's formation - which makes us think that their origin can be traced back to these primeval crystals," Williams added.
These crystals then took a winding path to the surface, partially melting and recrystallizing several times in a process that did not completely erase their previous transformations before being incorporated into surviving rocks.
The authors noted the similarities between the Isua rocks and those laid down by hotspot volcanoes. “Something we are interested in is whether [hotspots] might also be tapping into the depths and accessing regions of the interior usually beyond our reach," said Cambridge’s Dr Oliver Shorttle.
Answering this may be easier than the authors’ other quest; to find further examples of holdovers from the magma ocean. The oceanic crust gets recycled every few hundred million years. Continental crust lasts longer, but the modern continents are still mostly much younger than the planet. There are only a few spots, coincidentally sited in places whose climates make them remote from major cities, which preserve evidence of the first lifeforms, let alone the origins of geology.