Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 235

transformation of the host quartz. The crystallisation and textural set-
ting of these phases are different in crystalline rocks and in porous
sedimentary rocks. Coesite is formed between 30 and 60 Gpa, while
stishovite is formed between 12 and 45 GPa.

(5) Melting and quenching to form lechatelierite. This is a product of the

highest degree of shock and is commonly found at impact craters formed
in sedimentary rocks or unconsolidated sediments as highly vesicu-
lated glass with flow structures. In non-porous, crystalline target rocks,
lechatelierite is only found in impact melt glasses and rocks in the
form of inclusions and schlieren that results from secondary melting
of highly shocked quartz mixed into the impact melt during the crater-
forming process.

Simplified shock wave profiles in quartz are characterised by three phases:
theloading phase, the compression phase and the unloading or decompression
phase. Complexities in the shock wave structure, however, can result in phase
transitions. Three temperatures are associated with shock metamorphism, these
being the pre-shock, the shock and the post-shock temperature. The latter two
are a function mainly of the shock pressure. Shock temperatures in porous rock
tend to be significantly higher than in crystalline rocks. Distal ejecta are ejecta
that are well removed from the impact crater, highly dispersed and quickly
cooled. Quartz and PDFs have been discovered in distal ejecta deposits at the
K--T boundary (Grieves et al. , 1996).

Impact ejecta and spherules

Extraterrestrial impacts can generate a layer of debris mantling the area
surrounding the crater (the ejecta blanket) to more distal ejecta fallout lay-
ers. These deposits may be preserved in the geological record and range from
alocal to global extent. The energy released by extraterrestrial impacts can
also directly or indirectly trigger a variety of high energy, surficial processes
that can rework sediments surrounding the impact site. These include wave sys-
tems, subaerial or subaqueous sediment gravity flows and possibly hyper storms.
The deposits generated by these catastrophic events are likely to be preserved
in the sedimentary record, particularly in otherwise low-energy environments
(Hassler and Simonson, 2001). The extent of the distribution of the impact
ejecta is related to the impact energy, and therefore also the final diameter
of the impact structure. To distribute impact ejecta globally, impact-shattered
and molten target rocks have to be ejected past Earth's atmosphere. This is
known as an atmospheric blow-out. The ejecta is further distributed by atmo-
spheric winds as it returns to the atmosphere by gravitation. Impact diameters as