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IMPACT CRATER/STRUCTURE GLOSSARY

by: Charles O’Dale

The petrographic and geochemical study of actual rocks from the potential impact structure will bring final confirmation of the presence of an impact structure. In case of a structure that is not exposed on the surface, drill-core samples are essential. Good materials for the recognition of an impact origin are various types of breccia and melt rocks. These rocks often carry unambiguous evidence for the impact origin of a structure in the form of shocked mineral and lithic clasts or a contamination from the extraterrestrial projectile.

SEDIMENTARY ROCK

Rock that has formed through the deposition and solidification of sediment, especially sediment transported by water (rivers, lakes, and oceans), ice (glaciers), and wind. Sedimentary rocks are often deposited in layers, and frequently contain fossils.

 

SELENITE (hydrothermal)

The colorless and transparent variety of gypsum (calcium sulfate: CaSO4.H2O) that shows a pearl like luster and has been described as having a moon-like glow. The word selenite comes from the greek word for Moon and means moon rock. Gypsum is one of the more common minerals in formed sedimentary environments, such as tropical seas.

At Haughton, selenite was formed by hydrothermal activity associated with the impact event. The only hydrothermal systems active today are associated with volcanic regions (e.g., Yellowstone National Park), but it turns out that impact craters can also provide the two most important components of a hydrothermal system: heat and water.

The heat source at Haughton were the pale gray impact melt breccias which were originally at temperatures of >1000°C. As groundwater and rainwater came into contact with these hot rocks, these fluids were heated and circulated through the crater. Some of the target rocks at Haughton contained sedimentary gypsum, which was dissolved by these hot hydrothermal fluids. These fluids then migrated through the crater and re-deposited gypsum or selenite within cavities in the impact melt breccias as they cooled.

 

SHATTER CONE

Shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism. The occurrence of shatter cones is the only accepted meso- to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. The apices of the cones tend to point towards the shock source.

 

Model for shatter cone surface modification: (a) offset shock front, generated due to host rock density variations, causes tearing in the out-of-sequence zone between leading and trailing fronts. The resulting fault transient evolves to a passive fracture as the trailing front passes through; (b) post-shock decompression leads to opening of the fracture.(Gibson, Spray 1998).

Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometres in diameter. ( Baratoux, Reimold 2016)

Yours truly pointing our an in situ shatter cone within the Charlevoix impact crater. The hypothesis that the Charlevoix structure might be the result of a cosmic impact originated here in 1965 when this outcrop was first studied. These “in situ” shatter cones were discovered at this location when Jehan Rondot was in the midst of routine regional mapping in the Charlevoix region (Rondot 1966). He recognized unusual fracture patterns in a well exposed roadside outcrop and later learned from John Murtaugh (who was mapping Manicouagan at the time) that he had discovered shatter cones (Dence 2004).

 

SHOCK COMPRESSION

Most of the structural and phase changes in minerals and rocks are uniquely characteristic of the high pressures (diagnostic shock effects are known for the range from 8 to >50 GPa) and extreme strain rates (up to 108 /s) (for comparison: a bat hitting a baseball generates a strain rate of ~102/s) associated with impact. The products of static compression, as well as those of volcanic or tectonic processes, differ from those of shock metamorphism, because of peak pressures and strain rates that are lower by many orders of magnitude.

 

SHOCK METAMORPHISM:  PRESSURE-TEMPERATURE CONDITIONS

Conditions of endogenic metamorphism and shock metamorphism in the pressure-temperature fields. This comparison diagram exhibits the onset pressures of various irreversible structural changes in the rocks due to shock metamorphism and the relationship between pressure and post-shock temperature for shock metamorphism of granitic rocks (modified after Koeberl 1997). For the formation of total rock melts, shock pressures in excess of roughly 60 GPa (600 kbar) are required.

Unique deformation effects occurred as changes in minerals such as mineral deformations and melting under the extreme high pressure and temperature (e.g., the shock pressure may be than 60 GPa and post-shock temperature may be 2000◦C). Quartz begins to convert to coesite (another polymorph or atomic-structural form of silica) at about 20 kilobars (that pressure is reached in the solid Earth at about a depth of 70 km (43 miles). Solids begin to convert to glass at 400 kb, to melt at ~500 kb, and to vapor (gas) at a megabar. Thus, the range of shock metamorphism is from 0.02 to 1 megabar – these pressures are known to occur in the Earth only in its mantle and core but the rock types affected by shock metamorphism are dominantly those of the crust where the natural pressure gradient achieves values less than those affecting shocked rocks.

Metastable preservation of coesite and stishovite requires rapid cooling prior to amorphization. Stishovite is unstable above about 300-600°C, whereas coesite is stable up to about 1100°C, suggesting that the quartz grains studied here were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.

Shock Metamorphism of Terrestrial Impact Structures and its Application in the Earth and Planetary Sciences[A. Gucsik (ed.), Cathodoluminescence and its Application in the Planetary Sciences, 23 c Springer-Verlag Berlin Heidelberg 2009]

SHOCKED GNEISS

Gneiss (pronounced “nice”) is normally a dark dense rock, but at Haughton, the gneiss resembles pumice stone – it is ash-white, porous and very lightweight. In fact, some of these fragments float in water! The reason why this gneiss is so light is due to the air spaces or bubbles, which formed as the gneiss was compressed by the shock wave, and then released. Certain minerals in the rock were also vaporized, leaving behind a porous ghost of the gneiss it originally was.

A foliated or banded metamorphic rock, which forms when igneous or sedimentary rocks are buried to deep levels in the Earth’s crust (up to several kilometers deep!) where they are changed by extreme heat and pressure.

[see – SHOCK METAMORPHISM:  PRESSURE-TEMPERATURE CONDITIONS.]

 

SHOCKED TARGET ROCK

Is an informal term describing a rock created or modified by the impact of a meteorite. The term encompasses shock-metamorphosed target rocks, melts, breccias, suevites and mixtures, as well as sedimentary rocks with significant impact-derived components (shocked mineral grains, tektites, anomalous geochemical signatures, etc).

 

TOP: The shocked Coconino Sandstone is weakly shocked sandstone (<10 GPa) that lacks remnant porosity and contains abundant grain comminution and fracturing. Note the "rock flour" on the shocked sample. BOTTOM: The unshocked Coconino Sandstone consists of a fine to medium-grained, moderately well-sorted, rounded quartz arenite with ~ 20 vol% porosity. Coconino sandstone layers are typically buff to white in color. It consists primarily of sand deposited by eolian processes (wind-deposited) approximately 260 million years ago.
TOP: The shocked Coconino Sandstone is weakly shocked sandstone (<10 GPa) that lacks remnant porosity and contains abundant grain comminution and fracturing. Note the “rock flour” on the shocked sample. BOTTOM: The unshocked Coconino Sandstone consists of a fine to medium-grained, moderately well-sorted, rounded quartz arenite with ~ 20 vol% porosity. Coconino sandstone layers are typically buff to white in color. It consists primarily of sand deposited by eolian processes (wind-deposited) approximately 260 million years ago.

Shock metamorphism of the Coconino Sandstone at Meteor Crater, Arizona
Susan Werner Kieffer
ABSTRACT
A study of the shocked Coconino sandstone from Meteor Crater, Arizona, was undertaken to examine the role of porosity in the compression of rocks and in the formation of highpressure phases. A suite of shocked Coconino specimens collected at the crater is divided into five classes, arranged in order of decreasing quartz content. The amounts of coesite, stishovite (measured by quantitative X-ray diffraction), and glass vary systematically with decreasing quartz content. Coesite may comprise 1/3 by weight of some rocks, whereas the stishovite content does not exceed 1%. The five classes of rocks have distinct petrographic properties, correlated with the presence of regions containing coesite, stishovite, or fused silica. Very few occurrences of diaplectic glass are observed. In the lowest stages of shock metamorphism (class 1), the quartz grains are fractured, and the voids in the rock are filled with myriads of small chips derived from neighboring grains. The fracture patterns in the individual quartz grains are controlled by the details of the initial morphology of the colliding grains. In one weakly shocked rock, it was possible to map the general direction of shock passage by recording the apparent direction of collision of individual grains. The principal mechanism of energy deposition by a shock wave in a porous material is the reverberation of shock and rarefaction waves through grains due to collisions with other grains. A one-dimensional model of the impact process can predict the average pressure, volume, and temperature of the rock if no phase changes occur but cannot predict the observed nonuniformity of energy deposition. In all rocks shocked to higher pressure than was necessary to close the voids, high-pressure and/or high-temperature phases are present. Locally high pressures enduring for microseconds and high temperatures enduring for milliseconds controlled the phases of SiO2 that formed in the rock. Collapsing pore walls became local hot spots into which initial deposition of energy was focused. Microcrystalline coesite in class 2 rocks occurs in symplektic regions on quartz grain boundaries that were regions of initial stress and energy concentration, or in sheared zones within the grains. The occurrence and morphology of the coesite-rich regions can be explained only if the transformation from quartz to coesite proceeds slowly in the shock wave. In class 3 rocks, microcrystalline coesite occurs in opaque regions that surround nearly isotropic cores of cryptocrystalline coesite. The cores are interpreted to be the products of the inversion of stishovite (or a glass with Si in sixfold coordination) that initially formed in the shock front in regions of grains shocked to pressures near 300 kb. Stishovite is preserved only in the opaque regions, which are believed to have been cooler than the cores. In class 4 rocks vesicular glass occurs in core regions surrounded by opaque regions containing coesite. The relation of the glass to the coesite and quartz suggests that the glass was formed by inversion of stishovite formed above 350 kb on release to lower pressure. Class 5 rocks are composed almost entirely of glass, with vesicles uniformly distributed in the glass. These vesicles were probably formed by exsolution of water that had been dissolved in melted SiO2 during passage of the shock.

Journal of Geophysical Research 10 August 1971
[see – SHOCK METAMORPHISM:  PRESSURE-TEMPERATURE CONDITIONS.]

 

SIDEROPHILE ELEMENTS

Literally, “iron-loving” elements, such as iridium, osmium, platinum, and palladium, that, in chemically segregated asteroids and planets, are found in the metal-rich interiors. Consequently, these elements are extremely rare on Earth’s surface.

 

SIMPLE IMPACT CRATER/STRUCTURE

A bowl-shaped crater having undergone only slight modifications of its transient crater.

Pingualuit Impact Crater, a 3.44 km diameter simple crater in northern Quebec.

[see – CRATER CLASSIFICATIONS]

[see –  CRATER FORMATION]

 

STISHOVITE (IMPACT)

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 was first found in nature and named after Sergey M. Stishov, a renowned Russian high-pressure physicist who first synthesized this mineral. It is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide (SiO2). At normal temperature and pressure, stishovite is metastable, as it will eventually decay to quartz; however, this phase change is slow enough that it has never been observed.

Until recently, the only known occurrences of stishovite in nature formed at the very high shock pressures (>100 kbar or 10 GPa) and temperatures (> 1200 °C) present during hypervelocity meteorite impact into quartz-bearing rock

Recently (2007), minute amounts of stishovite has been found within diamonds, and post-stishovite phases were identified within ultra-high pressure mantle rocks.

With a mass density of 4.287 g/cm3,  stishovite is the heaviest polymorph of silica.

[see – SHOCK METAMORPHISM:  PRESSURE-TEMPERATURE CONDITIONS.]

 

SUEVITE (IMPACT)

Defined as a polymict breccia with a particulate matrix, containing lithic and mineral clasts in all stages of shock metamorphism, including microscopic impact melt particles.