Shock Metamorphism

Shock metamorphism

Except for certain laboratory experiments and outdoor detonations of high explosives (including nuclear weapons), evidence of shock metamorphic conditions of extreme pressure and heat on Earth exist only within and around impact craters. Only hypervelocity impact between objects of substantial size moving at cosmic velocity (at least several kilometers per second) can produce these conditions. Meteorites larger than approximately 246 ft (75 m) in diameter, asteroids , and comets can pass through Earth's atmosphere and yet retain a very high percentage of their original velocity. In such events, this energy of motion is converted into seismic and heat energy almost instantaneously. On planetary bodies with no atmosphere, even smaller impacting bodies (even micrometeorites) can produce shock metamorphic effects. Meteorites recovered on Earth, which are fragments of larger bodies shattered by impact elsewhere in the solar system , also show shock metamorphic features and effects.

Shock metamorphism involves changes wrought by instantaneously applied extreme pressure and heat. This contrasts sharply with metamorphic changes accompanying development of most of Earth's metamorphic crustal rocks via long-term contact, cataclastic, and regional metamorphic conditions. Typical shock metamorphic pressures range from 72,519 to 14,503,774 psi (0.5 to 100 Gpa). Usually shock temperatures range from a few hundred to a few thousand degrees Fahrenheit or Celsius.

Shock metamorphism manifests itself through unique physical changes in mineral characteristics. There are five recognized shock stages, which are numbered 0, Ia, Ib, II, and III. Quartz (SiO₂), a very common mineral on Earth, displays a range of shock effects that make it one of the most studied minerals in shock metamorphism. At shock stage 0, Ia, and Ib, quartz displays progressively greater numbers of planar features (PFs), numbers of planar deformation features (PDFs), and extent of mosaicism. PFs form at threshold shock temperatures and pressures. PFs are microscopically thin fissures, spaced at about 20 microns or more, which are parallel to selected atomic planes within the quartz crystal. At higher temperatures and pressures, PFs give way to PDFs and mosaicism. PDFs are microscopic, parallel zones within the quartz cystal, spaced at 2–10 microns. PDFs are strongly planar and are arranged in specific crystallographic orientations. The number of PDFs and the specific orientation of PFs and PDFs are diagnostic of approximate levels of shock pressure. PFs and PDFs are found mainly in quartz, but also occur in some other silicate minerals as well. Mosaicism is a microscopic type of shock metamorphism in quartz and some other silicate minerals. It is an irregular, mottled pattern, as revealed under polarized light. Mosaicism is shock-induced expansion of crystal volume that results in multiple crystal development within the original (pre-shock) crystal.

At shock stages II and III, high-pressure forms (polymorphs) of pre-existing minerals and shock-produced melts can form. For quartz, the high-pressure polymorphs are stishovite and coesite, both SiO₂ like quartz, but with different internal structures of atoms (and higher densities). Other common minerals and their high-pressure polymorphs (in parentheses) are: olivine (ringwoodite); plagioclase (jadite); pyroxene (majorite); and graphite (lonsdaleite, a polycrystalline, impure diamond ). For pressures over 60 gigapaschals, rocks can undergo complete melting and thus form impact melts. These melts may have very high temperatures (due to shock-wave passage); temperatures tend to be much higher than normal crustal processes or volcanic activity would produce. These extreme temperatures generate high-temperature polymorphs of common minerals such a lechatelierite (SiO₂, like quartz), which forms at temperatures over 3,092°F (1,700°C), and baddeleyite (ZrSiO₄, like zircon), which forms at temperatures over 3,452°F (1,900°C). Lechatelierite is not found in any other natural material, except fulgarites (fused soil or sand from lightning strikes). Impact melt temperature may exceed 18,000°F (10,000°C) and thus, pre-existing rock like limestone , which has a very high melting temperature, may become liquid. Rapid cooling of such melts produces various kinds of impact glassses.

See also Impact crater; Metamorphic rock