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Tektite
Tektites (from Ancient Greek τηκτός (tēktós) 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz Eduard Suess (1867–1941), son of Eduard Suess. They generally range in size from millimetres to centimetres. Millimetre-scale tektites are known as microtektites.
Tektites are characterized by:
Although tektites are superficially similar to some terrestrial volcanic glasses (obsidians), they have unusual distinctive physical characteristics that distinguish them from such glasses. First, they are completely glassy and lack any microlites or phenocrysts, unlike terrestrial volcanic glasses. Second, although high in silica (>65 wt%), the bulk chemical and isotopic composition of tektites is closer to those of shales and similar sedimentary rocks and quite different from the bulk chemical and isotopic composition of terrestrial volcanic glasses. Third, tektites contain virtually no water (<0.02 wt%), unlike terrestrial volcanic glasses. Fourth, the flow-banding within tektites often contains particles and bands of lechatelierite, which are not found in terrestrial volcanic glasses. Finally, a few tektites contain partly melted inclusions of shocked and unshocked mineral grains, i.e. quartz, apatite, and zircon, as well as coesite.
The difference in water content can be used to distinguish tektites from terrestrial volcanic glasses. When heated to their melting point, terrestrial volcanic glasses turn into a foamy glass because of their content of water and other volatiles. Unlike terrestrial volcanic glass, a tektite produces only a few bubbles at most when heated to its melting point, because of its much lower water and other volatiles content.
On the basis of morphology and physical characteristics, tektites have traditionally been divided into four groups. Those found on land have traditionally been subdivided into three groups: (1) splash-form (normal) tektites, (2) aerodynamically shaped tektites, and (3) Muong Nong-type (layered) tektites. Splash-form and aerodynamically shaped tektites are only differentiated on the basis of their appearance and some of their physical characteristics. Splash-form tektites are centimeter-sized tektites that are shaped like spheres, ellipsoids, teardrops, dumbbells, and other forms characteristic of isolated molten bodies. They are regarded as having formed from the solidification of rotating liquids, and not atmospheric ablation. Aerodynamically shaped tektites, which are mainly part of the Australasian strewn field, are splash-form tektites (buttons) which display a secondary ring or flange. The secondary ring or flange is argued as having been produced during the high-speed re-entry and ablation of a solidified splash-form tektite into the atmosphere. Muong Nong tektites are typically larger, greater than 10 cm in size and 24 kg in weight, irregular, and layered tektites. They have a chunky, blocky appearance, exhibit a layered structure with abundant vesicles, and contain mineral inclusions, such as zircon, baddeleyite, chromite, rutile, corundum, cristobalite, and coesite.
Microtektites, the fourth group of tektites, are less than 1 mm in size. They exhibit a variety of shapes ranging from spherical to dumbbell, disc, oval, and teardrop. Their colors range from colorless and transparent to yellowish and pale brown. They frequently contain bubbles and lechatelierite inclusions. Microtektites are typically found in deep-sea sediments that are of the same ages as those of the four known strewn fields. Microtektites of the Australasian strewn field have also been found on land within Chinese loess deposits, and in sediment-filled joints and decimeter-sized weathering pits developed within glacially eroded granite outcrops of the Victoria Land Transantarctic Mountains, Antarctica.
Most tektites have been found within four geographically extensive strewn fields: the Australasian, Central European, Ivory Coast, and North American.[needs update?] As summarized by Koeberl, the tektites within each strewn field are related to each other with respect to the criteria of petrological, physical, and chemical properties, as well as their age. In addition, three of the four strewn fields have been clearly linked with impact craters using those same criteria. Recognized types of tektites, grouped according to their known strewn fields, their associated craters, and ages are:
Comparing the number of known impact craters versus the number of known strewn fields, Natalia Artemieva considered essential factors such as the crater must exceed a certain diameter to produce distal ejecta, and that the event must be relatively recent. Limiting to diameters 10 km or more and younger than 50 Ma, the study yielded a list of 13 candidate craters, of which the youngest eight are given below.
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Tektite
Tektites (from Ancient Greek τηκτός (tēktós) 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz Eduard Suess (1867–1941), son of Eduard Suess. They generally range in size from millimetres to centimetres. Millimetre-scale tektites are known as microtektites.
Tektites are characterized by:
Although tektites are superficially similar to some terrestrial volcanic glasses (obsidians), they have unusual distinctive physical characteristics that distinguish them from such glasses. First, they are completely glassy and lack any microlites or phenocrysts, unlike terrestrial volcanic glasses. Second, although high in silica (>65 wt%), the bulk chemical and isotopic composition of tektites is closer to those of shales and similar sedimentary rocks and quite different from the bulk chemical and isotopic composition of terrestrial volcanic glasses. Third, tektites contain virtually no water (<0.02 wt%), unlike terrestrial volcanic glasses. Fourth, the flow-banding within tektites often contains particles and bands of lechatelierite, which are not found in terrestrial volcanic glasses. Finally, a few tektites contain partly melted inclusions of shocked and unshocked mineral grains, i.e. quartz, apatite, and zircon, as well as coesite.
The difference in water content can be used to distinguish tektites from terrestrial volcanic glasses. When heated to their melting point, terrestrial volcanic glasses turn into a foamy glass because of their content of water and other volatiles. Unlike terrestrial volcanic glass, a tektite produces only a few bubbles at most when heated to its melting point, because of its much lower water and other volatiles content.
On the basis of morphology and physical characteristics, tektites have traditionally been divided into four groups. Those found on land have traditionally been subdivided into three groups: (1) splash-form (normal) tektites, (2) aerodynamically shaped tektites, and (3) Muong Nong-type (layered) tektites. Splash-form and aerodynamically shaped tektites are only differentiated on the basis of their appearance and some of their physical characteristics. Splash-form tektites are centimeter-sized tektites that are shaped like spheres, ellipsoids, teardrops, dumbbells, and other forms characteristic of isolated molten bodies. They are regarded as having formed from the solidification of rotating liquids, and not atmospheric ablation. Aerodynamically shaped tektites, which are mainly part of the Australasian strewn field, are splash-form tektites (buttons) which display a secondary ring or flange. The secondary ring or flange is argued as having been produced during the high-speed re-entry and ablation of a solidified splash-form tektite into the atmosphere. Muong Nong tektites are typically larger, greater than 10 cm in size and 24 kg in weight, irregular, and layered tektites. They have a chunky, blocky appearance, exhibit a layered structure with abundant vesicles, and contain mineral inclusions, such as zircon, baddeleyite, chromite, rutile, corundum, cristobalite, and coesite.
Microtektites, the fourth group of tektites, are less than 1 mm in size. They exhibit a variety of shapes ranging from spherical to dumbbell, disc, oval, and teardrop. Their colors range from colorless and transparent to yellowish and pale brown. They frequently contain bubbles and lechatelierite inclusions. Microtektites are typically found in deep-sea sediments that are of the same ages as those of the four known strewn fields. Microtektites of the Australasian strewn field have also been found on land within Chinese loess deposits, and in sediment-filled joints and decimeter-sized weathering pits developed within glacially eroded granite outcrops of the Victoria Land Transantarctic Mountains, Antarctica.
Most tektites have been found within four geographically extensive strewn fields: the Australasian, Central European, Ivory Coast, and North American.[needs update?] As summarized by Koeberl, the tektites within each strewn field are related to each other with respect to the criteria of petrological, physical, and chemical properties, as well as their age. In addition, three of the four strewn fields have been clearly linked with impact craters using those same criteria. Recognized types of tektites, grouped according to their known strewn fields, their associated craters, and ages are:
Comparing the number of known impact craters versus the number of known strewn fields, Natalia Artemieva considered essential factors such as the crater must exceed a certain diameter to produce distal ejecta, and that the event must be relatively recent. Limiting to diameters 10 km or more and younger than 50 Ma, the study yielded a list of 13 candidate craters, of which the youngest eight are given below.