Sudbury
Key facts
- Country
- Canada
- Diameter
- 130 km
- Age
- about 1.85 billion years
- Location
- 46.60° N, 81.18° W
An elliptical basin about 60 km long and 30 km wide, rimmed by hills of the Sudbury Igneous Complex, with the city of Greater Sudbury and its mines around the edge.
Sudbury in Ontario is one of the largest and oldest impact structures on Earth, and almost certainly the most valuable. About 1.85 billion years ago an impactor perhaps 10 to 15 km across struck here, forming a crater that was probably around 130 km wide, and possibly larger. The crater melted so much rock that it left a molten sheet several kilometres thick, and from that melt came one of the world's great deposits of nickel and copper.
Squeezed out of shape. The original crater was round, but it did not stay that way. Later mountain-building to the south-east squashed it, so the central Sudbury Basin is now an ellipse about 60 km by 30 km. Erosion has also removed the upper parts. What survives are the rocks that formed in and beneath the crater: shattered and melted target rock, the thick Sudbury Igneous Complex (the cooled melt sheet), and a layer of fallback debris called the Onaping Formation, made of material that fell back into the crater after the blast.
A railway discovery. In 1883 workers building the Canadian Pacific Railway noticed rusty, metal-rich rock in a cutting near Sudbury. That find led to one of the largest mining districts on Earth. For decades nobody suspected an impact. Then in the 1960s the American geologist Robert Dietz looked for shatter cones, the distinctive fracture patterns made by shock waves, and found them in abundance around the basin. Further work found shocked quartz and other shock features, and an impact origin became widely accepted. Debris from the impact has since been identified in rocks hundreds of kilometres away, in Ontario and Minnesota.
Why so much metal? Scientists still debate whether the nickel came from the impactor or from the Earth's crust. The most widely held view is that the vast pool of impact melt, cooling slowly, allowed heavy metal sulphides to separate and sink to its base, concentrating metals already present in the target rocks. Whatever the source, Sudbury has produced enormous quantities of nickel, copper, cobalt and platinum-group metals for more than a century.
Training for the Moon. In the early 1970s NASA sent Apollo astronauts to Sudbury to study impact rocks, so they would recognise similar breccias (rocks made of broken, cemented fragments) on the Moon. The crews of Apollo 16 and 17 both trained in the area.
Visiting today. Greater Sudbury is a busy city built in and around the structure. The surrounding landscape was once badly damaged by smelter pollution, and a long regreening programme has replanted millions of trees. A science centre, Dynamic Earth, explains the area's mining heritage, and a giant replica coin, the Big Nickel, is a local landmark. Shatter cones can be seen in road cuttings around the basin, though collecting on private or mining land requires permission.
Did you know?
Sudbury's ore deposits have made it one of the world's leading sources of nickel for more than a century.
Apollo 16 and 17 astronauts practised fieldwork in Sudbury before going to the Moon.
The impact basin was squeezed from a circle into an ellipse by later mountain-building.