Scientists have determined, by using evidence gathered from the oldest mineral grains yet discovered on Earth, that in the planet’s earliest days, land was scarce, flat, and barren. In fact, Earth was a waterworld, but where there was any land at all, flat earth prevailed. And it remained relatively the same for over a billion years.
Phys.org reported this week that research conducted by scientists at Australia National University (ANU), after studying zircon mineral grains found preserved in sandstone rocks from the Jack Hills in Western Australia, believe that the Earth was likely covered with water — except for a few islands here and there — about 4.4 million years ago. Those islands, the team determined, were likely flat and barren as well. And from the time the Earth was 700 million years old until roughly 2.3 billion years ago, the topography of the planet changed very little. Effectively, there were no mountains and tectonic plate collisions on the young waterworld.
Dr. Antony Burnham, of the ANU Research School of Earth Sciences, said of the research, “The history of the Earth is like a book with its first chapter ripped out with no surviving rocks from the very early period, but we’ve used these trace elements of zircon to build a profile of the world at that time.”
“Our research indicates there were no mountains and continental collisions during Earth’s first 700 million years or more of existence — it was a much more quiet and dull place. Our findings also showed that there are strong similarities with zircon from the types of rocks that predominated for the following 1.5 billion years, suggesting that it took the Earth a long time to evolve into the planet that we know today.”
Burnham said that the zircon grains studied eroded out of the oldest rocks and likened them to skin cells found at a crime scene. Using the types of granite found in southeast Australia to figure out the link between zircon composition and the magma type, Burnham and the research team built a “picture of what those missing rocks were.”
The zircon formed from melting older igneous rocks, as opposed to sediments.
“Sediment melting,” Dr. Burhnam said, “is characteristic of major continental collisions, such as the Himalayas, so it appears that such events did not occur during these early stages of Earth’s history.”
The Earth is approximately 4.54 billion years old, give or take about 50 million years, according to scientists’ best estimates. As Space reported in 2014, improvements in radiometric dating have made estimating the age of the Earth an accurate process by measuring the decay rate of isotopes in the samples being studied. The radioactive elements themselves decay into other elements at rates that can be predicted. To gauge the age of a rock, scientists calculate the initial quantity of a particular element in said rock by determining the amount of the existing element and just how long it took for the element to decay.
Thus far, the oldest rocks discovered on Earth are the 4.03-billion-year-old Acasta Gneisses, which were discovered in northwestern Canada near the Great Slave Lake. The zircon mineral grains found in Australia and studied by Dr. Burnham are older, nearly as old as the Earth itself, but the samples are not rocks. In fact, the source rocks of the Australia mineral grains have yet to be found.
The research, according to Burnham, was made possible by building on zircon samples collected from Jack Hills over a number of decades. He also credited “chemical analyses carried out by an ANU research group 20 years” before that had proved invaluable to the study, which was published in Nature Geoscience.
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