GLOSSARY – S

GLOSSARY – S

by: Charles O’Dale

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, CaSO4.H2O (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.
The heat source at the Haughton Impact Crater 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.

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.

SHATTER CONE
A distinctively striated conical fragment of rock along which fracturing has occurred, ranging in length from less than a centimeter to several meters, generally found in nested or composite groups in the rocks of crypto-explosion structures, and generally believed to have been formed by shock waves generated by meteorite impact. Formed at >8 GPa, 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. 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)

 “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).
[see – SHOCK METAMORPHISM – shattercone]

SHOCK COMPRESSION – Contact and Compression Stage
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.
[see –  CRATER FORMATION – Contact & Compression]

SHOCK FRONT
Front of a shock pulse from hypervelocity impact propagating into the target rocks (and into the impactor). In the shock front, rock ambient pressures and temperatures are rapidly raised to shock pressures and temperatures which may attain to several 100 Gigapascal (GPa) and several 10,000 Kelvin (K) near the impact point.

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.

[see – SHOCK METAMORPHISM]

SHOCK PRESSURES

Shock pressures and their effects (after French, 1998: 33).

SHOCKED GNEISS
Gneiss (pronounced “nice”) is normally a dark dense rock, but at Haughton Impact Crater, 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.
[see – SHOCK METAMORPHISM]

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).
[see – SHOCK METAMORPHISM]

SHOCK WAVE
In a material, a shock wave is a deformation, a non-elastic wave that moves at a velocity exceeding the sound velocity of that material. The sound or seismic velocity is defined by the propagation of elastic waves. Shock waves are produced in hypervelocity impact and are the cause of shock metamorphism in rocks and minerals.

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.
[see – CRATER CLASSIFICATION]
[see –  CRATER FORMATION]
[see – CRATER IDENTIFICATION]

SLICKENSIDE
Slickenside is a smoothly polished surface caused by frictional movement between rocks along the two sides of a fault. Slickensides are naturally polished rock surfaces that occur when the rocks along a fault rub against each other, making their surfaces smoothed, lineated, and grooved. Slickensides are characterized by a diagnostic unidirectional step-like pattern that actually allows investigation of the sense of movement on fractures (Passchier and Trouw 1996).

STISHOVITESiO2 (IMPACT)
Stishovite is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide with a mass density of 4.287 g/cm3. 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. 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.
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 at the Chesapeake Bay impact crater were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.
[see – SHOCK METAMORPHISM – PRESSURE-TEMPERATURE CONDITIONS]

SUEVITE (IMPACT)
Defined as an impact-derived, polymict breccia containing a mixture of shocked and unshocked, lithic and melt fragments and generally considered to possess clastic matrices [von Engelhardt, W. and Graup, G. 1984. Geologische Rundschau 73:2:447–481.].
A grayish or yellowish breccia that is associated with meteorite impact craters and that contains both shock-metamorphosed rock fragments and glassy inclusions that occur typically as aerodynamically-shaped bombs. It closely resembles a tuff breccia or pumiceous tuff but is of non-volcanic orgin and can be distinguished by the presence of shock-metamorphic effects.
[see – SHOCK METAMORPHISM – PRESSURE-TEMPERATURE CONDITIONS]