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pressure reaches hundreds of GigaPascals (GPa) and the temperature rises thou-
sands of degrees. The increased pressure results in rocks diminishing (volume,
thickness) as they exceed their Hugoniot elastic limit (HEL). The HEL is generally
below 1 GPa for sedimentary rocks and between 3 and 4.5 GPa for crystalline
rocks. At higher pressures, when the volume of rock diminishes more slowly
than it does immediately after the HEL is exceeded, the two denser minerals
stishovite and coesite may form at 15 and 25 GPa, respectively. At pressures
above 40--50 GPa, these minerals disappear to form silica glass. At even higher
pressures, the whole rock melts and vaporises.
2.
Effects of the shock wave on minerals in crystalline rocks
The peak pressure during an impact occurs during a fraction of a sec-
ond. The effects of the shock wave in minerals are complex as rock is typi-
cally heterogenous and usually contains several minerals that possess different
shock reactions. For example, quartz develops planar deformation features dur-
ing shock impact. Different types of planar features (four in total) have been
identified; their occurrence depends upon the pressure during impact.
3.
Distribution of shock metamorphosed rocks
At impact, the shock wave continues to propagate in the impact area
taking the shape of a hemisphere centred roughly one projectile diameter below
theground surface. As the shock wave expands, it degrades into a plastic stress
wave that is preceded by an elastic precursor wave before it changes to an entirely
elastic wave below 5 GPa. The reaction of the rock to these waves depends upon
the strength of that rock.
4.
Fine, regularly spaced fractures, shatter cones
Impact craters are usually characterised by an abundance of fractures.
This is not a direct result of the shock wave, but of tensile interactions with
subsequent refraction waves.
5.
A-type pseudotachylite and B-type pseudotachylite
Thin veins of pseudotachylite form almost without displacement in the
centre of astroblemes. A-type pseudotachylite forms as a result of local fusion
of the rock material. B-type pseudotachylites are thick autochthonous breccia
dykes. They are highly varied in nature and can be white, grey, black, green or
red when altered by weathering near the surface.
6.
Impactite dyke or impact melt dykes
These also characterise large astroblemes and consist of dykes of fine
crystalline rock, locally, with fragments of shocked rock and quench textures.
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