Impact spherule
An impact spherule is a tiny droplet of melted rock thrown out by a large impact, which can travel around the world.
Impact spherules are small glassy or crystalline beads, usually less than a millimetre across, formed when rock melted or vaporised by a large impact condenses into droplets. They can be thrown far from the crater, sometimes across the whole planet.
Spherule layers in rocks are signs of large ancient impacts. The layer at the Cretaceous–Paleogene boundary, 66 million years ago, contains spherules from the Chicxulub impact, found in sediments around the world. Even older spherule layers in South Africa and Western Australia, more than 3 billion years old, record impacts far larger than any in recent history, probably by asteroids tens of kilometres across.
Spherules can be told from volcanic beads or cosmic spherules by their shape, chemistry and associated features, such as shocked minerals or high levels of iridium. They are valuable because they can preserve evidence of impacts whose craters have long been destroyed by erosion and plate tectonics.
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