This document aims to summarise the various characteristics of impact craters, and show how these features can be used as evidence to identify an impact structure. It is also hoped that readers will gain an insight into what they could expect if a large extra-terrestrial body were to crash into Earth today. A disaster would begin to occur as soon as the comet , or asteroid entered Earth's atmosphere. These bodies, which have been calculated to travel at velocities of between 20km per second and 60km per second have so much energy that they create a huge fireball even before impact. As these hot bodies land they create a depression in the Earth, which is often far greater than the size of the objects themselves.
Small bodies landing on Earth generally only cause localised damage.
A body would need to have a diameter of at least 1km to produce global effects. Wherever the comet or asteroid lands, a vast amount of energy is produced, causing the meteorite to vaporise , and shock waves to move rapidly away from the impact site. Geologists believe that an impact such as that which formed the Chicxulub crater would be seismically equivalent to a magnitude 10 or 11 earthquake, and that the energy released would be at least 10 billion times greater than that produced by a nuclear bomb. The Chemistry of Impacts Because the energy of the impact usually causes the impacting body to vaporise, fragments of the meteorite or comet are not generally preserved in the geological record . However,because these impacting bodies have a composition which is exotic compared to the rocks found on Earth, the meteorite can leave a unique chemical signature which can be detected in contemporaneous sediment deposits.