The Tanat Valley ( Welsh : Dyffryn Tanat ) is a large valley in northern Powys , Wales, formed by the course of the River Tanat and its tributaries. Situated south of the Berwyn range , the valley forms from at Llangynog from the merger of two narrow valleys, the Cwm Pennant and Cwm Rhiwarth. Various historic sites, including the Iron Age hillfort Llwyn Bryn-Dinas and the medieval St Melangell's Church are located along the valley. Largely agrarian, sheep farming has formed a major industry in the region for several centuries. Large-scale slate and lead mining in the region lasted from the 16th to late 20th century, although earlier mines possibly existed during the Iron Age .
30-526: The alluvial valley is primarily composed of Ordovician sedimentary rocks such as mudstone and shale , with smaller amounts of tuff and calcareous rocks . Mineral deposits in the valley include galena , sphalerite , chalcopyrite , barite , and witherite . The soils are podzolic and rich in clay . The tributary streams of the Afon Tanat have their sources in the Aran and Berwyn ranges along
60-648: 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
90-415: 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
120-561: 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
150-595: 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
180-633: 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
210-672: 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
240-722: Is generally between 100–200 metres (330–660 ft) O.D . The Cwm Pennant is a narrow valley following the upper course of the Afon Tanat, prior to its merger with the Afon Eirth at Llangynog . Pennant Melangell , a former village and medieval church, is located in the valley. A small, narrow valley, the Cwm Llêch, emerges from the south of the Cwm Pennant, following the course of the Afon Goch. The Cwm Blowty follows
270-491: Is loose clay , silt , sand , or gravel that has been deposited by running water in a stream bed, on a floodplain , in an alluvial fan or beach , or in similar settings. Alluvium is also sometimes called alluvial deposit . Alluvium is typically geologically young and is not consolidated into solid rock. Sediments deposited underwater, in seas, estuaries , lakes, or ponds, are not described as alluvium. Floodplain alluvium can be highly fertile, and supported some of
300-462: 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
330-547: Is often referred to as "cover" because these sediments obscure the underlying bedrock . Most sedimentary material that fills a basin ("basin fill") that is not lithified is typically lumped together as "alluvial". Alluvium of Pliocene age occurs, for example, in parts of Idaho. Alluvium of late Miocene age occurs, for example, in the valley of the San Joaquin River , California. Rhyolite Rhyolite ( / ˈ r aɪ . ə l aɪ t / RY -ə-lyte )
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#1732793948259360-521: Is the most silica -rich of volcanic rocks . It 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
390-777: The Afon Rhaeadr upstream of Llanrhaeadr-ym-Mochnant . A large waterfall, the Pistyll Rhaeadr , lies at the head of the valley. The earliest evidence of human presence in the Tanat Valley dates to the Upper Palaeolithic ; a barbed point from c. 12,000 BC indicates temporary hunting settlements in the area. Hunter-gatherer seasonal settlements likely existed in the Mesolithic and Neolithic . Permanent settlements were likely established in
420-661: The Late Neolithic or Bronze Age . Although no settlements from the Late Neolithic period are known, several Bronze Age funerary sites are located around the valley, including henges , burial mounds , and cairns . The Rhos-y-Beddau stone circle sits in the hanging valley of the Rhaeadr, upstream of the Pistyll Rhaeadr. Two Iron Age hillforts , Llwyn Bryn-Dinas and Craig Rhiwarth , lie above
450-473: The 16th century, continuing into the 20th century. The 18th and 19th centuries were the period of greatest mining activity, mainly centered at the Llangynog mine. Five smaller mines surrounded Llangynog, alongside two at Cwm Hirnant. Although metal mining largely stopped after the closure of the Llangynog mine in 1899, activity at Cwm Orog continued until 1912, alongside slate and rhyolite quarrying throughout
480-645: The 19th century, the term had come to mean recent sediments deposited by rivers on top of older diluvium , which was similar in character but interpreted as sediments deposited by Noah's flood . With the rejection by geologists of the concept of a primordial universal flood, the term "diluvium" fell into disfavor and was replaced with "older alluvium". At the same time, the term "alluvium" came to mean all sediment deposits due to running water on plains. The definition gradually expanded to include deposits in estuaries, coasts, and young rock of marine and fluvial origin. Alluvium and diluvium were grouped as colluvium in
510-620: 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
540-608: The earliest human civilizations. The present consensus is that "alluvium" refers to loose sediments of all types deposited by running water in floodplains or in alluvial fans or related landforms . However, the meaning of the term has varied considerably since it was first defined in the French dictionary of Antoine Furetière , posthumously published in 1690. Drawing upon concepts from Roman law , Furetière defined alluvion (the French term for alluvium) as new land formed by deposition of sediments along rivers and seas. By
570-495: The early 20th century. Mining operations at Llangynog led to large-scale heavy metal contamination across the whole of the valley. The Tanat Valley Light Railway was opened in 1904 to serve the communities in the region, continuing operations until 1964. 52°48′45″N 3°17′35″W / 52.81250°N 3.29306°W / 52.81250; -3.29306 Alluvium Alluvium (from Latin alluvius , from alluere 'to wash against')
600-508: 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
630-408: The late 19th century. "Colluvium" is now generally understood as sediments produced by gravity-driven transport on steep slopes. At the same time, the definition of "alluvium" has switched back to an emphasis on sediments deposited by river action. There continues to be disagreement over what other sediment deposits should be included under the term "alluvium". Most alluvium is Quaternary in age and
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#1732793948259660-454: The medieval period. Pennant Melangell is associated with Saint Melangell , who supposedly founded a nunnery in the area, and the shrine dedicated to her . Llanrhaeadr was formed as a clas dedicated to Saint Dogfan . Sheep farming has been a major industry in the area since the 16th century. Cattle farming also takes place in the valley. Limestone quarries have long been used in the region. Slate quarries and lead mines also emerged in
690-538: The northwestern borders of Powys . The western extent of the valley is divided into two main valleys: the Cwm Pennant, following the upper Tanat; and the Cwm Rhiwarth, following the Afon Eirth . The two river valleys merge at the village of Llangynog . The Berwyn range lies to the north of the valley, rising up to 500 meters above the valley floor, with the rolling Montgomery Hills to the south. The valley floor
720-434: 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
750-576: 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
780-487: The valley. These may have been a center of early metallurgy , due to abundant iron ore deposits in the region. An Iron Age hut circle has been found at Garnedd Wen, east of Craig Rhiwarth . Mining likely took place at the lead mines of Cwm Orog and Crag y Mwyn during the Roman period . Pennant Melangell and Llanrhaeadr, the principal settlements of the largely rural agrarian valley, were both founded as ecclesiastical sites in
810-450: 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,
840-655: 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
870-657: 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
900-567: 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
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