Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 230

identification of impact events. Shock metamorphic effects on various minerals
such as quartz and zircon are some of the most reliable features used to identify
extraterrestrial impact events. These features are regarded as reliable indicators
of past impact events because shock features in minerals related to non-impact
processes have never been confirmed in nature. Impact-derived rocks (impactites)
form from terrestrial target rocks where the impactor is completely vaporised as
aresult of the enormous temperatures that are released during a hyper-velocity
impact. Minor amounts of recondensed meteoritic vapour may be mixed with
thevaporised, molten or shocked and brecciated target rocks and such debris
can be spread over vast distances across the Earth's surface. These processes are
only applicable for craters greater than 1.5 km diameter. For smaller events, only
small parts of the impactor may be preserved and here, isotopic signatures in
suspected impacted rocks and ejecta material are used to fingerprint an extrater-
restrial source. Most research on extraterrestrial impacts, therefore, is based on
stratigraphic and geochemical analysis, especially where the fossil record shows
major extinction events have occurred.

Impact craters: processes and effects

Craters are the most obvious features marking the impact of prehis-
toric bolides with the Earth and other planets. However, many features can be
mistaken for impact craters because, upon initial inspection, they can exhibit
asimilar morphological expression. Features such as solution depressions in
both limestone and other types of rock can resemble impact craters because
of their general circular shape and dish-like depression. Impact craters do have
distinctive features though that separate them from these other landforms. On
Earth, weathering and erosion can remove the crater-like morphology relatively
quickly compared to other planetary bodies such as the Moon where tectonic
activity is absent. Where preserved, these features are termed impact structures
or astroblemes.
Impact cratering can undergo three stages of formation. These are the initial
contact and compression, excavation and formation of a transient crater and
allochthonous breccia (rock fragments detached and displaced from their origi-
nal position), and modification with slumping and readjustment (central uplift
and annular collapse, Sturkell, 1998)(Fig.9.2). Rondot (1994)describes several
processes and features directly linked to the impact and formation of impact
craters. These are detailed below.

1. Effects of pressure on the rocks

At impact, the kinetic energy of the bolide (any natural object colliding
with Earth) is transferred to the target area resulting in compression. The shock