Misplaced Pages

#37962

42-396: Trachyte ( / ˈ t r eɪ k aɪ t , ˈ t r æ k -/ ) is an extrusive igneous rock composed mostly of alkali feldspar . It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava (or shallow intrusions) enriched with silica and alkali metals . It is the volcanic equivalent of syenite . Trachyte

84-523: A groundmass containing much smaller imperfect sanidine laths. Rhomb porphyry is an example with usually large porphyritic rhomb shaped phenocrysts embedded in a very fine-grained matrix . Some of the best known trachytes, such as the trachyte of Drachenfels on the Rhine, show striking porphyritic character, having large sanidine crystals of tabular form an inch or two in length scattered through their fine-grained groundmass. In many trachytes, however,

126-648: A soil amendment . Rhyolite is an extrusive igneous rock, formed from magma rich in silica that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content of mafic minerals, and it is typically very fine-grained ( aphanitic ) or glassy . An extrusive igneous rock is classified as rhyolite when quartz constitutes 20% to 60% by volume of its total content of quartz, alkali feldspar , and plagioclase ( QAPF ) and alkali feldspar makes up 35% to 90% of its total feldspar content. Feldspathoids are not present. This makes rhyolite

168-415: A composition very close to the water-saturated granite eutectic and with extreme enrichment in most incompatible elements . However, they are highly depleted in strontium , barium , and europium . They are interpreted as products of repeated melting and freezing of granite in the subsurface. HSRs typically erupt in large caldera eruptions. Rhyolite is common along convergent plate boundaries , where

210-512: A dome shape. Domes typically solidify to form the rich in silica extrusive rock obsidian and sometimes dacite domes form the extrusive rock dacite, like in the case of Mount St. Helens . Calderas are volcanic depressions formed after an erupted volcano collapses. Resurgent calderas can refill with an eruption of rhyolitic magma to form the extrusive rock rhyolite like the Yellowstone Caldera . Submarine volcanoes erupt on

252-595: A mosaic or like the tiles on a roof. They often cover the surfaces of the larger feldspars or line the vesicles of the rock, where they may be mingled with amorphous opal or fibrous chalcedony . In the older trachytes, secondary quartz from the recrystallization of tridymite is not rare. Of the mafic minerals present, augite is the most common. It is usually of pale green color, and its small crystals are often very perfect in form. Brown hornblende and biotite occur also, and are usually surrounded by black corrosion borders composed of magnetite and pyroxene ; sometimes

294-561: A natural glass or vitrophyre, also called obsidian . Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations , spherulitic , nodular , and lithophysal structures. Some rhyolite is highly vesicular pumice . Peralkaline rhyolites (rhyolites unusually rich in alkali metals) include comendite and pantellerite . Peralkalinity has significant effects on lava flow morphology and mineralogy , such that peralkaline rhyolites can be 10–30 times more fluid than typical calc-alkaline rhyolites. As

336-595: A result of their increased fluidity, they are able to form small-scale flow folds, lava tubes and thin dikes. Peralkaline rhyolites erupt at relatively high temperatures of more than 1,200 °C (2,190 °F). They comprise bimodal shield volcanoes at hotspots and rifts (e.g. Rainbow Range , Ilgachuz Range and Level Mountain in British Columbia , Canada). Eruptions of rhyolite lava are relatively rare compared to eruptions of less felsic lavas. Only four eruptions of rhyolite have been recorded since

378-633: A result, many eruptions of rhyolite are highly explosive, and rhyolite occurs more frequently as pyroclastic rock than as lava flows . Rhyolitic ash flow tuffs are the only volcanic product with volumes rivaling those of flood basalts . Rhyolites also occur as breccias or in lava domes , volcanic plugs , and dikes . Rhyolitic lavas erupt at a relatively low temperature of 800 to 1,000 °C (1,470 to 1,830 °F), significantly cooler than basaltic lavas, which typically erupt at temperatures of 1,100 to 1,200 °C (2,010 to 2,190 °F). Rhyolites that cool too quickly to grow crystals form

420-503: A silica content of 60 to 65% and an alkali oxide content of over 7%. This gives it less SiO 2 than rhyolite and more (Na 2 O plus K 2 O) than dacite . These chemical differences are consistent with the position of trachyte in the TAS classification , and they account for the feldspar-rich mineralogy of the rock type. Trachydacite occupies the same field in the TAS diagram as trachyte, but

462-672: A slab of oceanic lithosphere is being subducted into the Earth's mantle beneath overriding oceanic or continental lithosphere . It can sometimes be the predominant igneous rock type in these settings. Rhyolite is more common when the overriding lithosphere is continental rather than oceanic. The thicker continental crust gives the rising magma more opportunity to differentiate and assimilate crustal rock. Rhyolite has been found on islands far from land, but such oceanic occurrences are rare. The tholeiitic magmas erupted at volcanic ocean islands, such as Iceland , can sometimes differentiate all

SECTION 10

#1732791477038

504-409: Is characterized by fine-grained crystals indistinguishable to the human eye, described as aphantic . Crystals in aphantic rocks are small in size due to their rapid formation during eruption. Any larger crystals visible to the human eye, called phenocrysts , form earlier while slowly cooling in the magma reservoir. When igneous rocks contain two distinct grain sizes, the texture is porphyritic , and

546-577: Is common wherever alkali magma is erupted, including in late stages of ocean island volcanism and in continental rift valleys , above mantle plumes , and in areas of back-arc extension. Trachyte has also been found in Gale crater on Mars. Trachyte has been used as decorative building stone and was extensively used as dimension stone in the Roman Empire and the Republic of Venice . Trachyte has

588-477: Is distinguished from trachyte by a normative quartz content over 20%. Trachydacite is not a recognized rock type in the QAPF classification, where rocks rich in alkali feldspar and with quartz over 20% would be classified as rhyolites. The mineral assemblage of trachytes consists of essential alkali feldspar. Relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present. This

630-471: Is generally glassy or fine-grained ( aphanitic ) in texture , but may be porphyritic , containing larger mineral crystals ( phenocrysts ) in an otherwise fine-grained groundmass . The mineral assemblage is predominantly quartz , sanidine , and plagioclase . It is the extrusive equivalent of granite . Its high silica content makes rhyolitic magma extremely viscous . This favors explosive eruptions over effusive eruptions , so this type of magma

672-462: Is more often erupted as pyroclastic rock than as lava flows . Rhyolitic ash-flow tuffs are among the most voluminous of continental igneous rock formations. Rhyolitic tuff has been used extensively for construction. Obsidian , which is rhyolitic volcanic glass , has been used for tools from prehistoric times to the present day because it can be shaped to an extremely sharp edge. Rhyolitic pumice finds use as an abrasive , in concrete , and as

714-621: Is one known voluminous flow from Puʻu Waʻawaʻa on the north flank of Hualālai in Hawaiʻi. Here the trachyte is glassy and black in color. In Iceland, the Azores , Tenerife and Ascension there are recent trachytic lavas, and rocks of this kind occur also in New South Wales ( Cambewarra Range ), Queensland ( Main Range ), East Africa, Madagascar , Yemen and in many other districts. Among

756-645: Is reflected in the position of the trachyte fields in the QAPF diagram . Biotite , clinopyroxene and olivine are common accessory minerals. The plagioclase is typically sodium-rich oligoclase . The alkali feldspar is typically also sodium-rich sanidine ( anorthoclase ) and is often cryptoperthitic , with alternating microscopic bands of sodium feldspar ( albite ) and potassium feldspar (sanidine). Trachytes are typically fine-grained and light-colored, but can be black if they consist mostly of glass. They are often porphyritic, with large well-shaped crystals of sanidine in

798-579: Is that the magma can cool much more quickly in the open air or under seawater , and there is little time for the growth of crystals . Sometimes, a residual portion of the matrix fails to crystallize at all, instead becoming a natural glass like obsidian . If the magma contains abundant volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice , scoria , or vesicular basalt . Other examples of extrusive rocks are rhyolite and andesite . The texture of extrusive rocks

840-440: Is the rock type called keratophyre , which is the sodium-rich-plagioclase equivalent of trachyte. [REDACTED]   This article incorporates text from a publication now in the public domain :  Flett, John S. (1911). " Trachyte ". In Chisholm, Hugh (ed.). Encyclopædia Britannica . Vol. 27 (11th ed.). Cambridge University Press. pp. 116–117. Extrusive The main effect of extrusion

882-833: The Eifel , also in Auvergne, Bohemia and the Euganean Hills . In the neighborhood of Rome, Naples and the island of Ischia trachytic lavas and tuffs are of common occurrence. Trachytes are also found on the island of Pantelleria . In the United States, trachytes crop out extensively in the Davis Mountains , Chisos Mountains , and Big Bend Ranch State Park in the Big Bend (Texas) region, as well as southern Nevada and South Dakota ( Black Hills ). There

SECTION 20

#1732791477038

924-820: The Rhine valley, but their augite and biotite are often replaced by chlorite and other secondary products. Permian trachytes occur also in Thuringia and the Saar district in Germany. Alkaline rocks such as trachyte are rare in the Archean , but become common in the Proterozoic . Alkaline rocks with an age close to 570 million years are common around the perimeters of many continental shields and are evidence of worldwide rifting at that time. Closely allied to trachyte

966-598: The alkaline magma series , in which alkaline basaltic magma experiences fractional crystallization while still underground. This process removes calcium, magnesium, and iron from the magma to give it a composition close to that of alkali feldspar. As a result, trachyte is common wherever alkali magma is erupted, including late eruptions of ocean islands and in continental rift valleys and mantle plumes . Only rarely does magmatic differentiation proceed beyond trachyte to phonolite or even more evolved alkaline magmas. Trachyte also occurs in areas of back-arc extension, such as

1008-409: The feldspathoid group, such as nepheline , sodalite and leucite , are present in trachytes, and rocks of this kind are known as foid-bearing trachytes. The sodium-bearing amphiboles and pyroxenes so characteristic of the phonolites may also be found in some trachytes; thus aegirine or aegirine augite forms outgrowths on diopside crystals, and riebeckite may be present in spongy growths among

1050-572: The R field of the TAS diagram . The alkali feldspar in rhyolites is sanidine or, less commonly, orthoclase . It is rarely anorthoclase . These feldspar minerals sometimes are present as phenocrysts. The plagioclase is usually sodium -rich ( oligoclase or andesine ). Cristobalite and trydimite are sometimes present along with the quartz. Biotite , augite , fayalite , and hornblende are common accessory minerals. Due to their high content of silica and low iron and magnesium contents, rhyolitic magmas form highly viscous lavas . As

1092-508: The extrusive equivalent of granite. However, while the IUGS recommends classifying volcanic rocks on the basis of their mineral composition whenever possible, volcanic rocks are often glassy or so fine-grained that mineral identification is impractical. The rock must then be classified chemically based on its content of silica and alkali metal oxides ( K 2 O plus Na 2 O ). Rhyolite is high in silica and total alkali metal oxides, placing it in

1134-440: The extrusive rock basalt. Composite or stratovolcanoes often have andesitic magma and typically form the extrusive rock andesite. Andesitic magma is composed of many gases and melted mantle rocks. Cinder or scoria cones violently expel lava with high gas content, and due to the vapor bubbles in this mafic lava, the extrusive basalt scoria is formed. Lava domes are formed by high viscosity lava that piles up, forming

1176-518: The feldspars of the groundmass (as in the trachyte of Berkum on the Rhine ). Glassy forms of trachyte ( obsidian ) occur, as in Iceland , and pumiceous varieties are known (in Tenerife and elsewhere), but these rocks as contrasted with the rhyolites have a remarkably strong tendency to crystallize, and are rarely to any considerable extent vitreous. Trachyte is the usual silica-rich end member of

1218-520: The finer crystals are called the groundmass . The extrusive rocks scoria and pumice have a vesicular, bubble-like, texture due to the presence of vapor bubbles trapped in the magma. Shield volcanoes are large, slow forming volcanoes that erupt fluid basaltic magma that cools to form the extrusive rock basalt . Basalt is composed of minerals readily available in the planet's crust, including feldspars and pyroxenes . Fissure volcanoes pour out low viscosity basaltic magma from fissure vents to form

1260-626: The northern Aegean Sea and the Aeolian arc of Italy. The Aeolian back-arc includes the Campi Flegrei volcanic field, where trachytes have been erupted. Trachytes are well represented among the Cenozoic volcanic rocks of Europe. In Britain they occur in Skye as lava flows and as dikes or intrusions, but they are much more common on the continent of Europe, as in the Rhine district and

1302-427: The ocean floor and produce the extrusive rock pumice. Pumice is a light-weight glass with a vesicular texture that differs from scoria in its silicic composition and therefore floats. This article about igneous petrology is a stub . You can help Misplaced Pages by expanding it . Liparite Rhyolite ( / ˈ r aɪ . ə l aɪ t / RY -ə-lyte ) is the most silica -rich of volcanic rocks . It

Trachyte - Misplaced Pages Continue

1344-577: The older volcanic rocks trachytes also are not scarce, though they have often been described under the names orthophyre and orthoclase-porphyry, while trachyte was reserved for Tertiary and recent rocks of similar composition. In England there are Permian trachytes in the Exeter district, and Carboniferous trachytes are found in many parts of the central valley of Scotland. The latter differ in no essential respect from their modern representatives in Italy and

1386-412: The phenocrysts are few and small, and the groundmass comparatively coarse. The ferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in hand-sized specimens of these rocks. Two types of groundmass are generally recognized: the trachytic, composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consisting of small squarish or rectangular prisms of

1428-736: The replacement is complete and no hornblende or biotite is left, though the outlines of the cluster of magnetite and augite may clearly indicate from which of these minerals it was derived. Olivine is unusual, though found in some trachytes, for example those of the Arso in Ischia . Basic varieties of plagioclase, such as labradorite , are known also as phenocrysts in some Italian trachytes. Dark brown varieties of augite and rhombic pyroxene ( hypersthene or bronzite ) have been observed but are not common. Apatite , zircon and magnetite are practically always present as accessory minerals. Occasionally minerals of

1470-434: The rhyolite appears to be a product of melting of crustal sedimentary rock. Water vapor plays an important role in lowering the melting point of silicic rock, and some rhyolitic magmas may have a water content as high as 7–8 weight percent. High-silica rhyolite (HSR), with a silica content of 75 to 77·8% SiO 2 , forms a distinctive subgroup within the rhyolites. HSRs are the most evolved of all igneous rocks, with

1512-419: The same mineral. Sometimes granular augite or spongy riebeckite occurs in the groundmass, but as a rule this part of the rock is highly feldspathic. Trachytes very often have minute irregular vesicles which make the broken surfaces of specimens of these rocks rough and irregular, and it is from this distinctive texture that they received their name. It was first given to rocks of this class from Auvergne , and

1554-576: The start of the 20th century: at the St. Andrew Strait volcano in Papua New Guinea and Novarupta volcano in Alaska as well as at Chaitén and Cordón Caulle volcanoes in southern Chile . The eruption of Novarupta in 1912 was the largest volcanic eruption of the 20th century, and began with explosive volcanism that later transitioned to effusive volcanism and the formation of a rhyolite dome in

1596-450: The vent. Rhyolite magmas can be produced by igneous differentiation of a more mafic (silica-poor) magma, through fractional crystallization or by assimilation of melted crustal rock ( anatexis ). Associations of andesites , dacites , and rhyolites in similar tectonic settings and with similar chemistry suggests that the rhyolite members were formed by differentiation of mantle-derived basaltic magmas at shallow depths. In other cases,

1638-655: The way to rhyolite, and about 8% of the volcanic rock in Iceland is rhyolite. However, this is unusual, and the Hawaiian Islands (for example) have no known occurrences of rhyolite. The alkaline magmas of volcanic ocean islands will very occasionally differentiate all the way to peralkaline rhyolites, but differentiation usually ends with trachyte . Small volumes of rhyolite are sometimes erupted in association with flood basalts , late in their history and where central volcanic complexes develop. The name rhyolite

1680-657: Was introduced into geology in 1860 by the German traveler and geologist Ferdinand von Richthofen from the Greek word rhýax ("a stream of lava") and the rock name suffix "-lite". In North American pre-historic times , rhyolite was quarried extensively in what is now eastern Pennsylvania . Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site in Adams County . Rhyolite

1722-511: Was long used in a much wider sense than that defined above, so that it included quartz-trachytes (now known as liparites and rhyolites ) and oligoclase -trachytes, which are now classified as andesites . Quartz is rare in trachyte, but tridymite (which likewise consists of silica ) is not uncommon. It is rarely in crystals large enough to be visible without the aid of the microscope , but in thin sections it may appear as small hexagonal plates, which overlap and form dense aggregates , like

Trachyte - Misplaced Pages Continue

1764-567: Was mined there starting 11,500 years ago. Tons of rhyolite were traded across the Delmarva Peninsula , because the rhyolite kept a sharp point when knapped and was used to make spear points and arrowheads. Obsidian is usually of rhyolitic composition, and it has been used for tools since prehistoric times. Obsidian scalpels have been investigated for use in delicate surgery. Pumice, also typically of rhyolitic composition, finds important uses as an abrasive , in concrete , and as

#37962