Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 227

iridium are found is in metallic meteorites/asteroids. It was thought that a large
metallic asteroid impacted with Earth emitting large quantities of iridium laden
dust particles into the upper atmosphere. Eventually this dust settled uniformly
over the planet. Such an event could depress temperatures and the amount of
surface-reaching sunlight around the globe, leading to loss of food crops and
related problems over a number of years, creating inhospitable conditions for
the survival of dinosaurs (Zebrowski, 1997).
The impact signal of the K--T event is recognisable globally (Smit, 1999).
Large impact events have the capacity to blow a hole in the atmosphere above
theimpact site, permitting some impact materials to be dispersed globally by
the impact fireball, which rises above the lower atmosphere (Taylor, 2001). These
materials do not require atmospheric winds for dispersal and have the capacity
to encircle the globe in a relatively short time, before eventually returning to
thesurface. Relatively small impact events, resulting in impact structures in the
20 km size range, can also produce atmospheric blow-out (Jones, 1999). However,
at present the K--T is the only biostratigraphic boundary with a clear signal of
theinvolvement of a large-scale impact event. The involvement of impacts in
other geological boundary events in the terrestrial stratigraphic record has been
suggested but little evidence has been offered.
The riskofanasteroidimpact
Asteroids and comets have collided with the Earth throughout its
4.5 billion year history. The impact of near-Earth objects with our planet can
be catastrophic and still represents a natural hazard today (Wetherill and Shoe-
maker, 1982). The most dangerous asteroids are those capable of causing major
regional or global disasters; fortunately these are extremely rare. These bodies
impact the Earth only once every 100 000 years on average (NASA. 2004). The
greatest risk is associated with objects larger than 1--2 km diameter, which are
large enough to perturb the Earth's climate on a global scale. From estimates
of the terrestrial cratering rate, the frequency of the Cretaceous--Tertiary-sized
events occurring is about one every 50--100 million years (Earth Impact Database,
2003). Meteorites of about 0.5 km diameter impact the Earth once every 10 000
years and produce craters about 10 km in diameter (Wetherill and Shoemaker,
1982).
The formation of impact craters as large as 20 km could produce light reduc-
tions and temperature disruptions similar to a nuclear winter. Such impacts
occur on Earth around once every 2--3 million years (Earth Impact Database,
2003). The most recent known structure in this size range is 'Zhamanshin' in
Kazakhstan, with a diameter of 15 km and an age of 1 million years BP. Impacts