Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during

impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...

s. The formation of similar features during explosive volcanism is generally discounted due to the lack of

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

effects unequivocally associated with explosions and the difficulty in reaching sufficient pressures during such an event.

Effects

Mineral microstructures

Planar fractures

Planar fractures are parallel sets of multiple planar cracks or cleavages in

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

grains; they develop at the lowest pressures characteristic of

shock waves In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

(~5–8 GPa) and a common feature of quartz grains found associated with impact structures. Although the occurrence of planar fractures is relatively common in other deformed rocks, the development of intense, widespread, and closely spaced planar fractures is considered diagnostic of shock metamorphism.Chapter 4, 'Shock-Metamorphic Effects in Rocks and Minerals'
of the online book, French, B.M. 1998. '' Traces of Catastrophe, A handbook of shock-metamorphic effects in terrestrial meteorite impact structures'',

Lunar and Planetary Institute The Lunar and Planetary Institute (LPI) is a scientific research institute dedicated to study of the Solar System, its formation, evolution, and current state. The Institute is part of the Universities Space Research Association (USRA) and is ...

120pp.

Planar deformation features

Planar deformation features, or PDFs, are optically recognizable microscopic features in

grain A grain is a small, hard, dry fruit (caryopsis) – with or without an attached husk, hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and ...

s of

silicate minerals Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...

(usually

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

. PDFs are only produced by extreme shock compressions on the scale of meteor impacts. They are not found in

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

environments.

Brazil twinning in quartz

This form of twinning in quartz is relatively common but the occurrence of close-spaced Brazil twins parallel to the basal plane, (0001), has only been reported from impact structures. Experimental formation of basal-orientated Brazil twins in quartz requires high stresses (about 8 GPa) and high strain rates, and it seems probable that such features in natural quartz can also be regarded as unique impact indicators.

High-pressure polymorphs

The very high pressures associated with impacts can lead to the formation of high-pressure polymorphs of various minerals. Quartz may occur as either of its two high-pressure forms,

coesite Coesite () is a form (polymorphism (materials science), polymorph) of silicon dioxide (silicon, Sioxide, O2) that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first ...

and

stishovite Stishovite is an extremely hard, dense tetragonal form ( polymorph) of silicon dioxide. It is very rare on the Earth's surface; however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle. Stishovite w ...

. Coesite occasionally occurs associated with eclogites formed during very high pressure regional metamorphism but was first discovered in a meteorite crater in 1960. Stishovite, however, is only known from impact structures. Reidite, the high-pressure

scheelite Scheelite is a calcium tungstate mineral with the chemical formula Ca W O4. It is an important ore of tungsten (wolfram). Scheelite is originally named after Swedish chemist Carl Wilhelm Scheele (1742–1786). Well-formed crystals are sought ...

-structure polymorph of

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

, is known only from impact structures. Wells Creek shatter cones 1

Two of the high-pressure polymorphs of

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...

, one with a

baddeleyite Baddeleyite is a rare zirconium oxide mineral (ZrO2 or zirconia), occurring in a variety of monoclinic prismatic crystal forms. It is transparent to translucent, has high Index of refraction, indices of refraction, and ranges from colorless to yel ...

-like form and the other with a α-PbO₂ structure, have been found associated with the

Nördlinger Ries The Nördlinger Ries is an impact crater and large circular depression in western Bavaria and eastern Baden-Württemberg. It is located north of the Danube in the district of Donau-Ries. The city of Nördlingen is located within the depression ...

impact structure. Diamond, the high-pressure

allotrope Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: the ...

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

, has been found associated with many impact structures, and both fullerenes and carbynes have been reported.

Shatter cones

Shatter cones have a distinctively conical shape that radiates from the top of the cones repeating cone-on-cone, at various scales in the same sample. They are only known to form in rocks beneath

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

s or underground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the range of 2-30 GPa.

Occurrence

The effects described above have been found singly, or more often in combination, associated with every impact structure that has been identified on Earth. The search for such effects therefore forms the basis for identifying possible candidate impact structures, particularly to distinguish them from volcanic features.

References

External links

Chapter 4, 'Shock-Metamorphic Effects in Rocks and Minerals' of the online book, French, B.M. 1998. Traces of Catastrophe, A handbook of shock-metamorphic effects in terrestrial meteorite impact structures, Lunar and Planetary Institute 120pp.

{{Glass production techniques Impact geology Metamorphic petrology Glass in nature