Scientists have detected more than 160 asteroid impact craters on Earth's surface as of July 2014. Most of these were cataloged before 1950, and a few new ones are discovered each year. The largest impact crater is 236 miles across in South Africa due to an impact that happened more than 2 billion years ago. Asteroids more than 6 miles across create layers of pulverized rock on the surface.
Impact craters on Earth come in two different types. Smaller craters are less than 2 miles in diameter with a bowl-shaped depression and some rock fill in the middle. Larger craters are more than 2 miles across with a central peak, melted rocks and a slumped rim.
The Vredefort Crater in South Africa is a UNESCO World Heritage Site. The impact crater is the largest and one of the oldest on Earth. The Sudbury Basin in Ontario, Canada, is 1.8 billion years old and 81 miles in diameter. This part of Canada is the second-largest impact crater in the world.
Large asteroids more than 6 miles wide that strike the Earth create plumes of pulverized rock, called spherules, that are ejected into outer space due to the violence of the impact. Spherule layers settle back to the surface, and these layers are detected by scientists. Spherule layers are dated by surrounding layers of rock to give geologists an idea of when large asteroid impacts occurred.