Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 226

are found each year (Pesonen, 1996; Earth Impact Database, 2003). The basic types
of impact structures on Earth are simple structures up to 4 km in diameter, with
uplifted and overturned rim rocks surrounding a bowl-shaped depression, par-
tially filled by breccia. Complex impact structures and basins are generally 4 km
or more in diameter, with a distinct central uplift in the form of a peak and/or
ring, an annular trough, and a slumped rim. The interiors of these structures
are partially filled with breccia and rock melted by the impact (Earth Impact
Database, 2003).
Impacts may also induce chemical changes in the atmosphere. These are
related to the vaporisation of the impacting body and a portion of the target.
Relatively small impacting bodies < 0.5 km in diameter produce impact craters
on the scale of 10 km in diameter and inject five times more sulphur into the
stratosphere than occurs there at present. Larger impact events occurring on
thetimescale of a million years will inject enough sulphur to produce a drop in
temperature of several degrees and a major climatic shift. There are additional
effects on atmospheric chemistry, including the potential for the destruction
of the ozone layer from shock heating atmospheric nitrogen and the injection
of fluorides from the vaporised impacting body. The threshold for these effects
appears to be on the time scale of 100 000s of years (Earth Impact Database,
2003).
In some circumstances an ocean impact might even be less hazardous to
humankind than a land impact because less debris will be thrown into the
atmosphere and indirect effects might be reduced. This was true for the 'Eltanin'
impact, a 1--4 km diameter asteroid that landed in the ocean near Chile approx-
imately 2 million years ago. It did not create a crater on the seabed and appar-
ently did not result in mass extinctions (NASA, 2004). There is the risk though of
destruction from asteroid-induced tsunamis when they strike oceans. For exam-
ple, an impact anywhere in the Atlantic Ocean by a body 400 m in diameter
would devastate the coasts on both sides of the ocean with wave run-ups poten-
tially greater than 60 m. An impact-generated tsunami would be 10 times more
powerful than the largest earthquake-generated tsunami and occur with a recur-
rence interval of a few thousand years.
The most devastating impact of an asteroid collision is a mass extinction.
In 1980, Louis Alvarez and co-workers theorised that the Cretaceous--Tertiary
(K--T) boundary sediments and elemental anomalies worldwide were due to a
major asteroid impact 65 million years ago. There is an unexpectedly high con-
centration of the rare element iridium in geological samples taken from the
narrow boundary between the Cretaceous and Tertiary periods and this time
period also marks the disappearance of the dinosaurs from the fossil record
(Zebrowski, 1997). One of the only other places where high concentrations of