| Sudbury's
mining activities occur in about 15 nickel-copper mines in the unique and enigmatic
geological structure known as the Sudbury Basin. The 60 km by 27 km elliptical
formation shown is all that is left of a once much larger crater nearly 70 km
in diameter blasted out of the Canadian Shield about 1.8 billion years ago. Most
scientists who have studied Sudbury's geology believe the crater was formed by
a meteorite of diameter between 1 and 3 km, travelling at 15 km/s. The impact
shock energy created a huge explosion and vaporized much of the shattered country
rock and the meteorite. Other rock was either melted and sprayed on to the crater
walls or broken and flung far from the impact site. Between 1000 and 2000 km3
of fragments which fell back into the crater are still preserved. The Apollo 17
astronauts examined Sudbury surface rock, because of its similarity to moon crater
debris. Two or three minutes after the explosion, the floor of the crater, 20
km deep originally, rebounded elastically like the peak in the centre of a bouncing
raindrop, and then collapsed to form the first Sudbury Basin. The release of overburden
pressure on nickel and copper-rich rock many km below the impact site caused this
rock to melt and rise into the crater, then cool to form the valuable ore as well
as the rugged rim around the edges of today's basin. Mountain-building stresses
in the intervening years and several km of erosion as well as glacier action have
distorted the crater into its present elliptical shape, reduced its size and created
fertile farmland in its centre. |
