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The San Quintín Volcanic Field is a collection of ten or eleven volcanic cinder cones situated along the Pacific coast of the Baja California peninsula in Mexico. The field formed by repeated eruptions beginning in the Pleistocene and ending about 3000 years ago.〔Gorsline, D. S. and R. A. Stewart, Benthic marine exploration of Bahía de San Quintín, Baja California, 1960-61. Marine and Quaternary geology. ''Pac. Nat., 3'', 282-319, 1962.〕〔Woodford, A. O., (The San Quintín volcanic field, lower California ). ''American Journal of Science'' 15:337-345, 1928.〕 It is one of several known Quaternary period volcanic fields in Baja. The lava shields appear to have first grown as subaqueous volcanoes that emerged as islands.〔Luhr, J. F., Aranda-Gómez, J. J., and Housh, T. B. San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry. ''Journal of Geophysical Research'' 100:10353–10380, 1995. 〕 Currently located in a shallow bay, the cones are found in two groups and two isolated islands. The Northern group is composed of two cones, formed 126,000-90,000 years ago; the Southern group is larger, with two cones in the lagoon itself, and formed over a much larger and undetermined span of time. Most volcanic complexes in the field have a well-preserved scoria cone and lava apron, dotted with eruptive vents and lava flows. 42 eruptive units can be seen on the ground today.〔 The San Quintín field is the only known location of intraplate-type mafic alkalic volcanism on the Baja California peninsula. The oldest cones mainly erupted primitive magmas with occasional small xenoliths. As the field evolved, differentiated magma became more common, although extremely pure primitive magmas – virtually devoid of xenoliths and unusually rich in olivine phenocrysts – still dominated at the young cones.〔 These primitive magmas originated from progressive partial melting of spinel lherzolite at unusually shallow levels in the mantle.〔 The ultramafic xenoliths found are mostly spinel lherzolite, with the final 20% harzburgites, dunites, and pyroxenites. The San Quintín field is also the only source of peridotitic and granulitic xenoliths on the peninsula.〔 The largest and most abundant xenoliths are found in differentiated magmas. ==Description of the volcanic field== The San Quintín Volcanic Field is found about 260 km south of the U.S. border and 200 km south of Ensenada, Baja California, at a latitude of ~30.5°N. Today, the cones rest on over a kilometer of unconsolidated Plio-Quaternary sediment overlaid on the volcanic basement of the lower Cretaceous Alistos Formation.〔Espindola-Cardeña, J. M., J. M. Romo-Jones, and M. Almeida-Vega, Gravimetria y estructura del Valle de San Quintín, B. C. ''GEOS Bol. Inf., 11'', pp 10-15, Union Geofis. Mex. Mexico City, 1991.〕 The ten volcanic complexes recognized in the San Quintín field include Media Luna and Woodford in a northern group; a southern group consisting of Basu, Riveroll, Kenton, Picacho Vizcaino, Sudoeste, and Ceniza; and the isolated complexes Monte Mazo and Isla San Martín. A seacliff north of Basu exposes what may be an eleventh complex.〔 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「San Quintín Volcanic Field」の詳細全文を読む スポンサード リンク
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