Meteorite shock stage is a measure of the degree of fracturing of the matrix of a common

chondrite A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...

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 ...

. Impacts on the parent body of a meteoroid can produce very large pressures. These pressures heat, melt and deform the rocks. This is called

shock metamorphism Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorp ...

. Meteorites are often given a rating from 1 to 6 showing the level of shock metamorphism. However, the degree of shock can vary within a meteorite on the scale of centimeters. Smaller bodies colliding with one another would not have sufficiently great impact velocity to produce the pressures and temperatures required to produce shock effects, due to their lesser

gravitational attraction In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force be ...

for one another. High instantaneous pressures, in excess of 5

GPa Grading in education is the application of standardized measurements to evaluate different levels of student achievement in a course. Grades can be expressed as letters (usually A to F), as a range (for example, 1 to 6), percentages, or as num ...

(1 GPa = 10,000 atmospheres), are necessary to produce shock metamorphism.

Shock grade

The fracturing of crystals and other features must be observed under a

microscope A microscope () is a laboratory equipment, laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic ...

with shock effects observed under

polarized light , or , is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. One example of a polarize ...

. Larger structures, such as shock veins, are visible to the eye. Many of the shocked veins formed at the boundaries of polished surfaces of

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

ted specimens exhibit intricate spider-web-like structures. Following is a summary of the shock grades:Stoffler, D., Keil, K., and Scott, E. R. D., (1991); Shock metamorphism of ordinary chondrites; Geochimica et Cosmochimica Acta, 55, pp. 3845-3867. *S1: completely unshocked (up to 5

) *S2: very weakly shocked (5-10 GPa); uneven darkening of

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

as seen under

; planar and irregular fractures (breaks in other than a natural cleavage plane.) *S3: weakly shocked (15-20 GPa); weak fractures in olivine seen under polarized light; dark shock veins and some melt pockets *S4: moderately shocked; (30-35 GPa); weak planar fracturing of olivine under polarized light; some pockets of melted material, dark interconnected shock veins *S5: strongly shocked (45-55 GPa); very strong planar fracturing and deformation features in olivine; alteration of

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

into

maskelynite Maskelynite is a glassy material found in some meteorites and meteorite impact craters. Typical samples are similar in composition to plagioclase feldspar, and revert to that mineral when melted and recrystallized. It was named after British peopl ...

; formation of dark melt veins *S6: very strongly shocked (75-90

); olivine recrystallizes, with local alteration to a mineral called

ringwoodite Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a ...

and shock melting of plagioclase to a glass Greater shock pressures will melt the rock, producing what is referred to as an "

impact melt Impact may refer to: * Impact (mechanics), a large force or mechanical shock over a short period of time * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Imp ...

". These are seldom found on Earth, so they are very much sought after by collectors.

References

{{Meteorites Meteorite mineralogy and petrology

Shock grade

See also

References