Pseudotachylite
Encyclopedia
Pseudotachylite is a fault rock that has the appearance of the basaltic glass
, tachylyte
. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It may contain clasts of the country rock and occasionally crystals with quench
textures that began to crystallize from the melt.
to a breccia
, or as veins
injected into the walls of the fault. In most cases there is good evidence that the pseudotachylite formed by frictional melting of the wall rocks during rapid fault movement associated with a seismic event. This has caused them to be termed "fossil earthquakes". The thickness of the pseudotachylite zone also gives geologists a general idea of the magnitude of the associated displacement and the general magnitude of the paleoseismic event. Some pseudotachylites have been interpreted as forming by comminution
rather than melting. They have a similar occurrence to melt-derived pseudotachylites but lack clear indications of a melt origin.
in the U.S. state of Wyoming to its present location, the largest known landslide in history on land.
s such as that which formed the Vredefort crater
, South Africa. In an impact event, the melting forms part of the shock metamorphic
effects. The pseudotachylite veins associated with impacts are much larger than those associated with faults and are thought to have formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. The most extensive examples of impact related pseudotachylites come from impact structures that have been deeply eroded to expose the floor of the crater, such as Vredefort crater
, South Africa and the Sudbury Basin
, Canada.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, tachylyte
Tachylyte
Tachylite is a vitreous form of basaltic volcanic glass. This "glass" is formed naturally by the rapid cooling of molten basalt. It is a basic type of igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster...
. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It may contain clasts of the country rock and occasionally crystals with quench
Quench
In materials science, quenching is the rapid cooling of a workpiece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and...
textures that began to crystallize from the melt.
Seismic faulting
It is generally found either along fault surfaces, often as the matrixMatrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
to a breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
, or as veins
Vein (geology)
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation...
injected into the walls of the fault. In most cases there is good evidence that the pseudotachylite formed by frictional melting of the wall rocks during rapid fault movement associated with a seismic event. This has caused them to be termed "fossil earthquakes". The thickness of the pseudotachylite zone also gives geologists a general idea of the magnitude of the associated displacement and the general magnitude of the paleoseismic event. Some pseudotachylites have been interpreted as forming by comminution
Comminution
Comminution is the process in which solid materials are reduced in size, by crushing, grinding and other processes. It occurs naturally during faulting in the upper part of the crust and is an important operation in mineral processing, ceramics, electronics and other fields. Within industrial uses,...
rather than melting. They have a similar occurrence to melt-derived pseudotachylites but lack clear indications of a melt origin.
Landslides
Pseudotachylite has been found at the base of some large landslides involving the movement of large coherent blocks, such as the one that moved Heart MountainHeart Mountain (Wyoming)
Heart Mountain is an 8,123-foot klippe just north of Cody in the U.S. state of Wyoming, sticking up from the floor of the Bighorn Basin...
in the U.S. state of Wyoming to its present location, the largest known landslide in history on land.
Impact structures
Pseudotachylite is also associated with impact structureImpact structure
The term impact structure is closely related to the terms impact crater or meteorite impact crater, and is used in cases where erosion or burial have destroyed or masked the original topographic feature with which we normally associate the term crater...
s such as that which formed the Vredefort crater
Vredefort crater
Vredefort crater is the largest verified impact crater on Earth. It is located in the Free State Province of South Africa and named after the town of Vredefort, which is situated near its centre. The site is also known as the Vredefort dome or Vredefort impact structure...
, South Africa. In an impact event, the melting forms part of the shock metamorphic
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 metamorphic effects unequivocally associated with explosions...
effects. The pseudotachylite veins associated with impacts are much larger than those associated with faults and are thought to have formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. The most extensive examples of impact related pseudotachylites come from impact structures that have been deeply eroded to expose the floor of the crater, such as Vredefort crater
Vredefort crater
Vredefort crater is the largest verified impact crater on Earth. It is located in the Free State Province of South Africa and named after the town of Vredefort, which is situated near its centre. The site is also known as the Vredefort dome or Vredefort impact structure...
, South Africa and the Sudbury Basin
Sudbury Basin
The Sudbury Basin, also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geologic structure in Ontario, Canada. It is the second-largest known impact crater or astrobleme on Earth, as well as one of the oldest....
, Canada.