Scientists at the Smithsonian National Museum of Natural History have provided new evidence that modern plate tectonics, which defines the Earth’s characteristics and its unique ability to support life, originated approximately 3.6 billion years ago, rather than 3.2 billion years ago, as was thought by the latest research.
Earth is the only known planet to have complex organics, and this ability is due in part to another feature that makes the planet unique – plate tectonics. No other planetary body known to science has a dynamic crust like the Earth, which is divided into continental plates that move, break and collide with each other for eons. Plate tectonics provides a link between the chemical “reactor” inside the Earth and its surface that created a habitable planet: from oxygen in the atmosphere to the concentrations of climate-regulating carbon dioxide. But when and how plate tectonics began remains a mystery.
The new study uses zircons, the oldest minerals ever found on Earth, to provide a glimpse into the planet’s ancient past. The oldest zircon in the study, which was sourced from Jack Hills in Western Australia, was about 4.3 billion years old, which means that these nearly indestructible minerals were formed when the Earth itself was in its infancy for only about 200 million years. Along with other ancient zircons collected in Jack Hills in the earliest history of the Earth up to 3 billion years ago, these minerals are the closest researchers to continuous chemical records of the nascent world.
“We have reconstructed how the Earth changed from a molten ball of stone and metal to what we have today. None of the other planets have continents, liquid oceans, or life. In a sense, we are trying to answer the question of why the Earth is unique, and we can answer this question with the help of these zircons. ”
Michael Akerson, exploratory geologist at the Smithsonian National Museum of Natural History
To get a glimpse into the Earth’s past for billions of years, a team of researchers collected 15 grapefruit-sized rocks in Jack Hills and crushed them to the smallest constituent parts, grinding them into sand. Fortunately, zircons are very dense, so it is relatively easy to separate them from the rest of the sand using a technique similar to gold washing.
The team tested over 3,500 zircons, each just a couple of human hairs wide, by detonating them with a laser and then measuring their chemical composition with a mass spectrometer. These tests revealed the age and chemical composition of each zircon. Of the thousands tested, about 200 were studyable due to the ravages of the billions of years these minerals have endured since their inception.
The age of zircon can be determined with a high degree of accuracy, since every zircon contains uranium. The known radioactive nature of uranium and the well-defined decay rate allow scientists to reconstruct the lifetime of this mineral.
The research team also became interested in the aluminum content of each zircon. Tests of modern zircons show that zircons with a high aluminum content can only be obtained in a limited number of ways, allowing researchers to use the presence of aluminum to infer what might have happened, geologically speaking, during the formation of the zircon.
After analyzing the results of studies of hundreds of useful zircons out of thousands tested, the scientists found a noticeable increase in the concentration of aluminum approximately 3.6 billion years ago. Ultimately, they concluded that rocks are melting deeper below the Earth’s surface, which means that the planet’s crust is getting thicker and begins to cool, and that this thickening of the earth’s crust was a sign that a transition to modern plate tectonics is taking place.