An impact of immense proportions
Roughly 1.85 billion years ago, a comet or asteroid between 10 and 15 kilometers wide slammed into the Earth's crust. The impact occurred in the Paleoproterozoic Era, when all life on Earth was single-celled. The resulting Sudbury Basin was originally a staggering 200 to 250 kilometers in diameter, making it the third-largest confirmed impact structure on Earth. Over billions of years, erosion and geological pressures have deformed the crater into its current, smaller elliptical shape of about 60 by 30 kilometers.
The energy released by the impact was immense, creating about 31,000 cubic kilometers of molten rock—enough to fill Lakes Huron and Ontario six times over. This impact melt sheet, now known as the Sudbury Igneous Complex, is up to 2.5 kilometers thick. The collision was so violent it punctured the crust, allowing magma from the mantle to well up and mix with the melted surface rock. As this molten mixture slowly cooled, heavy metals like nickel, copper, and platinum settled out, creating the incredibly large ore deposits that Sudbury is known for today.
Evidence of the impact's shockwave is permanently etched into the surrounding area. Geologists have identified shatter cones—unique, cone-shaped fractures in rocks—up to 15 kilometers away from the crater's edge. These features only form under the extreme pressures of a bolide impact or a nuclear explosion. Another sign is Sudbury Breccia, rock that was shattered and then fused together by the intense heat and pressure. The presence of these features was critical in confirming the basin's cosmic origin in the 1960s.
A record of a violent past
While the Sudbury impact happened 1.85 billion years ago, it is a valuable analogue for an even more violent period in our solar system's history: the Late Heavy Bombardment (LHB). The LHB is a hypothesized event that occurred between 4.1 and 3.8 billion years ago, when a disproportionately large number of asteroids and comets pummeled the inner planets. Evidence for this spike in impacts comes largely from Apollo mission moon rocks. Many samples, collected from different lunar basins, showed impact melt ages clustered around 3.9 billion years.
The leading explanation for this cataclysm is a theory known as the Nice model. It proposes that the giant planets—Jupiter, Saturn, Uranus, and Neptune—formed in a more compact configuration than they are in today. Gravitational interactions with a vast disk of planetesimals caused their orbits to shift. A key moment occurred when Jupiter and Saturn entered a 2:1 orbital resonance, meaning Jupiter orbited the sun exactly twice for every one of Saturn's orbits. This resonance destabilized the entire system, flinging Uranus and Neptune outward into the planetesimal disk and scattering asteroids and comets across the solar system.
Earth would have experienced this bombardment as well, but plate tectonics and erosion have erased almost all craters from that ancient time. The Sudbury Basin, though much younger, shows a well-preserved look at the geological consequences of a large-scale impact. Its structure, from the deep melt sheet to the widespread shock-metamorphism, helps scientists model the processes that shaped the surfaces of the Moon, Mercury, and Mars during the chaotic LHB era.