Earth’s crust is the ultimate survivor. Bits of crust that sink into the planet’s interior can last for billions of years, geologists report in the April 25 Nature.
Rita Cabral of Boston University and colleagues discovered traces of ancient crust in one of the South Pacific’s Cook Islands while analyzing lava that erupted 20 million years ago. The rock contains different forms, or isotopes, of sulfur in ratios that could have only originated in an atmosphere with little oxygen, sometime before 2.45 billion years ago.
The researchers suspect that the sulfur was originally part of a slab of oceanic crust that slid beneath another tectonic plate and plunged into the mantle more than 2.45 billion years ago. The crust sank so low in the mantle that it was effectively in a crustal graveyard, the team suggests, where convection was weak and the crust could stay intact. A couple billion years later, parts of that crust rose back to the surface in a plume of buoyant mantle material and fueled the volcanic eruptions that gave rise to the Cook Islands.