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The volcanic history of the Northern Cordilleran Volcanic Province presents a record of volcanic activity in northwestern British Columbia, central Yukon and the U.S. state of easternmost Alaska. The volcanic activity lies in the northern part of the Western Cordillera of the Pacific Northwest region of North America. Extensional cracking of the North American Plate in this part of North America has existed for millions of years. Continuation of this continental rifting has fed scores of volcanoes throughout the Northern Cordilleran Volcanic Province over at least the past 20 million years (see Geology of the Pacific Northwest) and occasionally continued into geologically recent times. Eruptive activity in the Northern Cordilleran Volcanic Province throughout its 20 million year history has been mainly the production of alkaline lavas, including alkaline basalts. A range of alkaline rock types not commonly found in the Western Cordillera are regionally widespread in the Northern Cordilleran Volcanic Province. These include nephelinite, basanite and peralkaline phonolite, trachyte and comendite lavas. The trachyte and comendite lavas are understood to have been created by fractionation of mainly alkali basalt magma in crustal reservoirs. An area of continental rifting, such as the Northern Cordilleran Volcanic Province, would aid the formation of high-level reservoirs of capable size and thermal activity to maintain long-lived fractionation. In the past 15 million years, at least four large volcanoes have formed their way through dense igneous and metamorphic composed bedrock of this part of North America. This includes Hoodoo Mountain, the Mount Edziza volcanic complex, Level Mountain and Heart Peaks, which are primarily located in northwestern British Columbia. Most notable of these is the 7.5 million year old Mount Edziza volcanic complex, which has had more than 20 eruptions in the past 10,000 years. The only activity present in the Northern Cordilleran Volcanic Province has been occasional earthquakes and constant boiling of hot springs. However, a high potential exists for renewed eruptive activity that could threaten life and property in the volcanic zone.〔 ==Production and rates in volcanism== More than 100 eruptions have occurred in the past 20 million years with a broad range of eruptive styles.〔 These volcanic processes have created a range of different volcanic landforms, including stratovolcanoes, shield volcanoes, lava domes and cinder cones,〔 along with a few isolated examples of rarer volcanic forms such as tuyas. Large persistent volcanoes of the Northern Cordilleran Volcanic Province can remain dormant for hundreds or thousands of years between eruptions and therefore the greatest risk caused by volcanic activity is not always readily apparent. Volcanics older than 14 million years are mainly found in the northern portion of the volcanic province while volcanics ranging from nine to four million years old exist only in the middle of the volcanic province.〔 At least three types of volcanic zones are present in the Northern Cordilleran Volcanic Province, including large persistent lava plateaus like those found at the Mount Edziza volcanic complex, polygenetic volcanoes such as Hoodoo Mountain and monogenetic volcanoes like the basaltic cinder cones found throughout the volcanic province.〔 When Northern Cordilleran volcanoes do erupt, pyroclastic flows, lava flows and landslides can devastate areas away and mudflows of volcanic ash and debris can inundate valleys downstream. Falling ash from explosive eruptions can disrupt human activities hundreds of kilometres downwind, and drifting clouds of fine volcanic ash can cause severe damage to jet aircraft even hundreds of kilometres away. Volcanic deposits in the Northern Cordilleran Volcanic Province include lava flows, welded and unwelded pyroclastic deposits, hydroclastic deposits and other ice-contact volcanic deposits.〔 The variety of different volcanic deposits is partly due to changes in eruption characteristics from mainly subaerial eruptions to broadly subglacial eruptions throughout the history of the Northern Cordilleran Volcanic Province.〔 There is evidence of a decline in volcanic activity over the past few million years. This decline in volcanic activity can be grouped into two phases. From eight to four million years ago, volcanism rates were higher than they are at present. Magma production during this volcanic phase was most active from seven to five million years ago and was related to a period of rifting along the Pacific and North American Plate boundary.〔 Between four and three million years ago in the Pliocene epoch, a pause in volcanic activity began to happen.〔 The most recent magmatic phase ranging from two million years ago to present resulted from nearby areas of rifting during a period of compression between the Pacific and North American plates.〔 Volcanism rates during this volcanic phase was most active from two to one million years ago with the construction of 25 volcanic zones then decreased one million years ago with the construction of 11 volcanic zones.〔 To date, the most recent volcanic phase has produced of volcanic material whereas the first phase produced of volcanic material.〔 Even though the rate in volcanism throughout the Northern Cordilleran Volcanic Province has changed considerably, there is no correlation between the rate in magma production and the number of active volcanoes during any interval of time.〔 The present day volcanism rate for the Northern Cordilleran Volcanic Province is considerably lower than the Cascade Volcanic Arc and Hawaiian volcanism rates.〔 However, geologists are aware the temporal volcanic patterns known for the Northern Cordilleran Volcanic Province should be looked at carefully because volcanics that pre-date the last glacial period have been eroded by glacial ice and many of the volcanics have not been directly dated or have not been dated in significant detail to identify more individual temporal patterns.〔 Lava fountains can occur in the Northern Cordilleran Volcanic Province roughly every 100 years.〔 Large basaltic shield volcanoes throughout the central Northern Cordilleran Volcanic Province form large lava plateaus and are the largest volcanoes of the volcanic zone.〔 These extensive volcanoes are generally larger and more long-lived than volcanoes of adjacent volcanic zones, including the stratovolcanoes of the Cascade and Aleutian arcs. Mount Shasta, the largest stratovolcano in the Cascade Arc and Mount Veniaminof, one of the largest and most active volcanoes in the Aleutian Arc, have volumes of at least , less than half of that of the Level Mountain and Edziza shield complexes. The massive Level Mountain shield, occupying an area of and a volume of more than , is the most voluminous and most long-lived volcano of the Northern Cordilleran Volcanic Province. Further south, the Mount Edziza volcanic complex resides as the seconed most voluminous volcano with an area of and a volume of .〔 Just west of Level Mountain lies Heart Peaks, the third most voluminous volcano of the Northern Cordilleran Volcanic Province with an area of .〔 However, remains of a shield volcano that once covered an area of more than are present in the western Cassiar Mountains as Maitland Volcano.〔〔 All of the known most recent eruptions have occurred in British Columbia, although there is evidence for volcanic activity in Yukon in the past thousand years.〔 The two most recent were lava flow eruptions at Tseax Cone in the 18th century and at The Volcano in 1904. Reports of an eruption south of Gladys Lake in northern British Columbia were made by placer miners at the end of the 19th century, but no evidence for this eruption has been found, leading researchers to speculate the eruption as uncertain. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Volcanic history of the Northern Cordilleran Volcanic Province」の詳細全文を読む スポンサード リンク
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