Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 225

Chicxulub
c rater
Figure 9.1. Distribution of craters globally. The Chicxulub (Yucatan Peninsula) crater
is also shown (after Professor D. A. Kring, University of Arizona).
entered the Earth's atmosphere. The effects can range from damage to property
as a result of a meteorite landing on a house or car, to the mass extinction
of many species. The impact of asteroids can cause shock waves around the
impact site, fill the atmosphere with dust and generate tsunamis capable of
travelling whole oceans. To date no recorded human deaths from meteorite
impacts have occurred. In more recent geologic time, there is evidence that at
least one mass extinction event, notably that of the dinosaurs and many other
species 65 million years ago, is linked to the global effects caused by a major
asteroid impact. Impacts also have some economic significance; for example, the
vast copper--nickel deposits at Sudbury, Canada are possibly related to a large-
scale impact 1850 million years ago and several impact structures in sedimentary
rocks have formed suitable reservoirs for economic oil and gas deposits (Earth
Impact Database, 2003).
Stony meteorites with a diameter less than about 100 m generally do not
reach the Earth's surface. These objects usually explode several kilometres above
the surface and are known as an 'airburst' (NASA, 2004). The kinetic energy
involved is substantial; a typical impact by a 50 m diameter object releases
energy equivalent to about 10 megatons (Mt) of TNT and 100 m diameter object
releases the equivalent of about 75 Mt. These are equivalent in energy to large
thermonuclear explosions and could cause devastation over thousands of square
kilometres. Larger asteroids that survive the journey through the Earth's atmo-
sphere can have devastating effects and leave a definite signature on the Earth's
surface and atmosphere.
Over 160 impact craters have been identified on Earth (Fig. 9.1). Almost all
known craters have been recognised since AD 1950 and several new structures