Tectonics Study: Polished Rock

Michael Ackerson, Smithsonian.
May 14, 2021
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Polished rock under a microscope
Michael Ackerson, Smithsonian.

A thin, polished slice of a rock collected from the Jack Hills of Western Australia. Using a special microscope equipped with a polarizing lenses, the research team was able to examine the intricate internal structure of quartz that makes up the rock, including unique features that allowed them to identify ancient zircons (magenta mineral in the center of the red-outlined inset image).

Scientists led by Michael Ackerson, a research geologist at the Smithsonian’s National Museum of Natural History, provide new evidence that modern plate tectonics, a defining feature of Earth and its unique ability to support life, emerged roughly 3.6 billion years ago. The study, published May 14 in the journal Geochemical Perspective Letters, uses zircons, the oldest minerals ever found on Earth, to peer back into the planet’s ancient past.

To look billions of years into Earth’s past, Ackerson and the research team collected 15 grapefruit-sized rocks from the Jack Hills and reduced them into their smallest constituent parts—minerals—by grinding them into sand with a machine called a chipmunk. Fortunately, zircons are very dense, which makes them relatively easy to separate from the rest of the sand using a technique similar to gold panning.

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