Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 229

Impact crater
rim

Impact
melt sheet

Impact breccias

Central uplift

Peak ring

Figure 9.2. Schematic of the Chicxulub impact crater (after Professor D. A. Kring,
University of Arizona).

The biggest asteroid impact on Earth to have occurred in modern times
is known as the Tunguska event. On the 30th June 1908 a small asteroid or
meteroid, 100 m in diameter, exploded 8 km above the remote region of Tun-
guska in Siberia. The explosion produced a shock wave that knocked people to
theground in the town of Vahaurara 70 km away (Vasilyer, 1998). The object
wasreduced to dust by the explosion and stratospheric winds spread the dust
around the globe. Seismic records of the event recorded it as having an explosive
energy of 12.5 million tons of TNT. The explosion destroyed about 2150 km 2 of
Siberian forests felling some 60 million trees (Vasilyer, 1998). The event inspired
aplethora of scientific investigations and debate over the last century as to the
cause of the event but the evidence to date suggests that it was an asteroid, not
a comet as first suggested, that vaporised as it entered the Earth's atmosphere.
On the 9th October 1992 a meteorite (H6) of 12.4 kg landed on a parked car
in Peekskill, USA. As the meteorite entered the Earth's atmosphere it created a
fireball that was reported to be as bright as a full moon and was seen streaking
across the sky from Kentucky to New York. At least 14 people captured part of
thefireball on videotape. On the evening of the 26th March 2003 meteorites
punched through roofs in Park Forest, Illinois, USA. It was believed that the
meteorites came from a larger mass that weighed no less than 900 kg before it
hit the atmosphere (Earth Impact Database, 2003).

palaeoasteroid impacts with earth

Despite the fact that approximately 160 impact craters have been iden-
tified on Earth, such features are difficult to detect because terrestrial pro-
cesses such as volcanism, tectonics and erosion can erase the impact record.
The presence of meteoritic components in geological strata is important in the