Sudbury Basin: Ancient Impact Crater and Nickel Mining Region

The Sudbury Basin in Ontario, Canada, is the remnant of one of the largest asteroid impacts known on Earth, which took place about 1.849 billion years ago. The collision melted a vast volume of rock, and that melt later gave rise to some of the world’s richest deposits of nickel, copper and platinum-group metals. Today the basin lies within the city of Greater Sudbury, a mining centre for more than a century.

The impact

During the Paleoproterozoic Era, an asteroid or comet struck the southern edge of the ancient Superior craton, at the boundary with younger rocks to the south. Estimates of the impactor’s size range from about 10 to 15 kilometres. Uranium-lead dating of zircon and other minerals in the melt rocks places the event at about 1.85 billion years ago, refined to about 1.849 billion years.

The impact excavated a crater originally estimated at roughly 130 to 260 kilometres across. Later mountain-building squeezed the structure from the south-east, turning it into the elongated oval seen today, about 60 kilometres long and 30 kilometres wide. Erosion has since removed much of the original surface.

The centre of the basin is now a low, fertile plain known locally as the Valley, surrounded by a rim of rocky hills formed by the more resistant rocks of the melt sheet. Lake Wanapitei, just to the north-east, occupies a much smaller and younger impact crater, which is unrelated to the Sudbury event.

Debris from the explosion was flung far across the region. In 2005 researchers reported an ejecta layer near Thunder Bay and in Minnesota, lying directly on top of the Gunflint Chert, hundreds of kilometres away.

The Sudbury Igneous Complex

The most important legacy of the impact is the Sudbury Igneous Complex, a sheet of once-molten rock up to about 2.5 to 3 kilometres thick that lines the basin. Most geologists now interpret it as an enormous pool of impact melt formed from crustal rocks.

As the melt cooled, dense sulphide liquids rich in nickel, copper, cobalt and platinum-group metals separated, like oil from water, and settled near the base of the sheet and into fractures beneath it. These became the ore bodies of the Sudbury district. Above the melt sheet lies the Onaping Formation, a thick layer of broken and partly melted rock that fell back into the crater.

How the impact origin was established

For many years the Sudbury structure was explained as a volcanic feature. In 1964 the American geologist Robert Dietz described shatter cones around Sudbury, cone-shaped fractures produced by shock waves, and argued that the basin was an “astrobleme”, or ancient impact scar.

The idea was debated for decades, partly because the ores seemed to call for a deep volcanic source. Further discoveries of shocked quartz with planar deformation features, impact breccias such as the Sudbury Breccia, and the distant ejecta layer gradually built a strong case. The impact origin is now widely accepted, though scientists still discuss how much of the metal came from the impactor itself; most think the bulk came from melted crust.

Mining and the city of Sudbury

Copper-nickel ore was discovered in 1883 when workers building the Canadian Pacific Railway cut through the area. Mining expanded rapidly, and by the early twentieth century Sudbury was one of the world’s most important nickel producers, supplying metal for steel and armour during both world wars.

Smelting caused severe environmental damage: sulphur dioxide emissions and logging stripped much of the landscape of vegetation. Since the 1970s, emission controls and a long community regreening programme, including the planting of millions of trees, have helped the land recover.

The area also attracted space scientists. Because it shares features with lunar craters, Apollo astronauts visited Sudbury for geology field training in the early 1970s.

Comparison with other impacts

Sudbury is among the three largest confirmed impact structures on Earth, alongside the Vredefort crater in South Africa, estimated at 250 to 300 kilometres across, and the younger Chicxulub crater in Mexico, about 180 kilometres across. Its ores make it the most economically important of them.

Timeline

• c. 1.849 billion years ago: An asteroid or comet strikes the edge of the Superior craton, forming the Sudbury structure.
• c. 1.849 billion years ago: Impact melt cools to form the Sudbury Igneous Complex and its sulphide ores.
• 1883: Copper-nickel ore is found during construction of the Canadian Pacific Railway.
• 1964: Robert Dietz describes shatter cones and proposes an impact origin.
• Early 1970s: Apollo astronauts carry out geology training in the Sudbury area.
• 1984: A major Ontario Geological Survey volume on the Sudbury structure reports precise dating of about 1.85 billion years.
• 2005: Distant ejecta from the impact are reported near Thunder Bay and in Minnesota.

Why it matters

The Sudbury Basin shows how a catastrophic event nearly two billion years ago can shape human history today. The impact created a metal-rich treasure house that built a city and supplied industries worldwide, and its well-exposed rocks remain a natural laboratory for understanding large impacts on Earth and other planets.