4-Billion-Year-Old Crystals Offer Clues to When Plate Tectonics Began
upstart writes:
Scientists have long known that plate tectonics, the movement of distinct, rigid plates that make up the Earth's crust, formed continents and mountains and was crucial to the evolution of the planet's surface from one of molten lava and rock to an environment hospitable to life.
What's been less apparent is when it began.
A team of Harvard University-led scientists has analyzed some very rare, ancient, and nearly indestructible crystals the size of small grains of sand called zircons for chemical clues about the onset of plate tectonics. The research study, published in recently in the journal AGU Advances, suggests that 3.8 billion years ago there was a major transition in the geochemistry of these zircons that make them look much more like the zircons that are formed today in the red-hot environments where plate tectonics happen.
"Prior to 3.8 billion years ago, the planet doesn't seem to be as dynamic," said Nadja Drabon, a Harvard assistant professor of Earth and Planetary Sciences and the paper's first author. "Today, there's lots of crust that gets constantly destroyed in what are called subduction zones, and new crust is created. Many [previous] zircons showed that back then once the early crust formed, it lived for a really long time -about 600 million years in this case. While there was some internal reworking, we never created new granitic crust.... Then 3.8 billion years ago, everything changes."
Think of Zircons as tiny time capsules that retain chemical clues of the Earth's first 500 million years. Some were formed in the magma of the planet more than 4 billion years ago when the Earth, geologically speaking, was still in its infancy. It makes them the oldest known materials on Earth. Their secrets can be understood by zapping them with lasers, which is what the researchers did for their analysis.
[...] Today, the Earth's outer shell consists of about 15 shifting blocks of crust, which hold the planet's continents and oceans. The process was key to the evolution of life and the development of the planet because the process exposed new rocks to the atmosphere, which led to chemical reactions that stabilized Earth's surface temperature over billions of years.
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