METEOR IMPACT

Over the course of history many asteroids and comets have struck the Earth. These have ranged from tiny dust sized particles to so-called "extinction meteorites" such as the one that is theorised to have caused the extinction of the dinosaurs 65 million years ago. The majority of smaller meteors burn up in the Earth's atmosphere but the larger ones can leave huge craters in the surface of the planet.

The animation to the right shows a large asteroid striking the Earth. As it enters the atmosphere it heats up to temperatures of approximately 2000 Kelvin. The impact of the asteroid throws up a huge cloud of dust. Dust expelled into the troposphere will drop out within a matter of months, but if the impact is sufficient to send material through the tropopause (11km high at the equator, 5km high at the poles) into the stratosphere can take many years before the dust returns to the surface. Dust that enters the stratosphere, (be it from impacts or from other sources such as volcanoes or large scale fires) can circle the globe within days. High levels of dust in the atmosphere can cause a global cooling, as the dust particles reflect more of the Sun's energy back into space.
Unlike most objects in the solar system, the Earth's surface has a high level of erosion. Wind, water and tectonic processes all contribute to erasing old impact craters meaning that only the largest or the most recent are still visible on the Earth's surface.