Geologists have known for a decade that the UK experienced a major meteor strike more than a billion years ago. However, they have not been sure where the impact happened. Now, an answer has been provided, and the unusual nature of the proposed site means there will be more opportunity than usual to learn about the event.
Meteors bring with them elements that are rare on Earth but more common in asteroids, such as the iridium that alerted scientists to the asteroid that ended the Cretaceous era. Their impact also throws up distinctively shocked materials.
Dr Ken Amor of the University of Oxford concluded in 2008 that an impact had occurred based on the green glass and platinum group metals mixed in with red sandstone in a distinctive layer of debris (known as the Stac Fada Member) in British rocks 1.2 billion years old. The distribution of this debris suggested the impact must have been in, or near, the British Isles, and the impactor around 1-kilometer (0.6-mile) across – smaller than the “dinosaur killer” but large enough to create a major crater. The passage of time can wipe craters away or bury them beneath kilometers of sediment, however, so efforts to find the site were largely speculative.
Now, Amor believes he knows where it is. In the Journal of the Geological Society, he argues that the impact site lies 15-20 kilometers (9-12.5 miles) off Enard Bay on the west coast of Scotland, in what is known as the Minch Basin. The site was identified by comparing the depth and composition of the Stac Fada Member at numerous locations. The thicker the layer – up to 30 meters (100 feet) in some places – and the greater the concentration of droplets with shorter flight paths, the closer the point of origin. Some sites also show signs the debris came from a particular direction, and the alignment of magnetic particles in the layer support Amor's suspicions.
Based on the amount of material ejected, the paper estimates the crater as having originally been 13-14 kilometers (8-9 miles) across and 3 kilometers (1.8 miles) deep.
The site of impact was a rift valley at the time, back when Scotland lay near the equator. “The material excavated during a giant meteorite impact is rarely preserved on Earth, because it is rapidly eroded, so this is a really exciting discovery," Amor said in a statement. "It was purely by chance this one landed in an ancient rift valley where fresh sediment quickly covered the debris to preserve it.”
The sandstone represents the pre-existing material at the site the asteroid hit, which the impact distributed far and wide, along with some of the space rock and glass formed in the strike.