Chemistry in Pictures: Earth’s oldest rock is pretty gneiss

The people at the Smithsonian Institution National Museum of Natural History were nice enough to give a couple members of the Chemistry in Pictures team a behind-the-scenes look at their mineral collection. For the next couple weeks, we’ll bring you photos of some of the minerals that wowed us, some of which are billions of years old.

This is a piece of the Acasta Gniess, one of Earth’s oldest known rocks. It formed about 4 billion years ago from cooling magma under the surface of the Earth, which had only been around for about 500 million years by that point. Billions of years of intense heat and pressure caused the igneous rock to metamorphose into a glassy gneiss with bands of silicate minerals including quartz, feldspar, and amphibole.

There are older rocks on Earth that didn’t originate on our planet (spoiler alert!) and older minerals enclosed within other rock formations. But the Acasta Gneiss is the oldest intact piece of Earth’s crust, according to Elizabeth Cottrell, chair of the department of mineral sciences at the National Museum of Natural History.

The formation was found in 1983 near the Acasta River in Canada’s Northwest Territories, around 300 km north of Yellowknife. There are pieces of it all over the world, including a large chunk outside of the National Museum of the American Indian. A team will return there next summer to collect more samples for the museum’s collection so that scientists can continue to study this slice of Earth’s past for generations to come.

Credit: Chip Clark/Smithsonian Institution National Museum of Natural History (tabletop specimen); Brianna Barbu/C&EN (gloved hands)

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