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 physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or ...

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 atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

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 med ...

(~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 sup ...

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 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 legum ...

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, silica (silicon dioxide, ) is usually con ...

(usually

feldspar Feldspars are 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 ''plagioclase'' (sodium-calcium) felds ...

), consisting of very narrow planes of

glassy Glassy is an adjective meaning, of or resembling glass. Glassy may also mean: * Glassy, amorphous metal * ''Glassy'', a 2021 single album, or the title song by Jo Yu-ri * Glassy phase, amorphous solid * Glassy water, amorphous ice * Glassy carbon, ...

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 the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...

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

volcanic A volcano is a rupture 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 found where tectonic plat ...

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 ( polymorph) of silicon dioxide Si O2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the ...

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 ...

. Coesite occasionally occurs associated with

eclogite Eclogite () is a metamorphic rock containing garnet (almandine- pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, ...

s 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 Reidite is a rare polymorph of ZrSiO4 created when zircon experiences high pressure and temperature. Reidite is denser than zircon and has the same crystal structure as scheelite. All natural occurrences of reidite are associated with meteorite ...

, 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 K. Scheele (1742-1786). Well-formed crystals are sought by collectors ...

-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 t ...

, 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 with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolu ...

, 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 indices of refraction, and ranges from colorless to yellow, green, and da ...

-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, a ...

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 of matter, state, known as allotropes of the elements. Allotropes are different structural modifications o ...

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

, has been found associated with many impact structures, and both

fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...

s 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 solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

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