Continental volcanics

Gushchenko, I.I., 1965. The composition of young continental volcanic rocks and the conditions of removal of the iron group. In Vulkanogenno-osadochnyye formatsii i poicznyye iskopayemyye (Volcanogenic-sedimentary Formations and Mineral Deposits). Izd. Nauka, Moscow, pp. 63-69 (in Russian). 

The age of the felsic volcanics at Gallipoli Heights was determined by Faure and Gair (1970) by means of a two-point whole-rock Rb-Sr age determination based on samples collected by S.J. Carryer. The results yielded a Middle-to-Late Devonian date of 370 40 Ma (recalculated to A, = 1.42 x 10 year ) and an initial Sr/ Sr ratio of 0.70548. This value of the initial Sr/ Sr ratio is typical for continental volcanic rocks (e.g., rhyolites of Central America and Mexico Faure 2001), but it is lower than the initial ratio (at360Ma) of most of the 27 specimens of Admiralty Intrusives analyzed by Kreuzer et al. (1987). 

Studies of Sr, Pb and Nd isotopes, coupled with trace-element geochemical studies, have shown that the source regions of many continental alkaline volcanics have been enriched in LIL elements, generally some hundreds of million years prior to the beginning of volcanism, and enrichment has been interpreted as due to mantle metasomatism.It has been suggested that mantle metasomatism is a necessary precursor to alkaline continental volcanism.Alkaline rocks erupted in oceanic environments, and even ridge tholei-ites, sometimes bear evidence of source enrichment, which has been interpreted as a result of mantle metasomatism. ... 

Ocean Basins. Ocean basins are primarily formed from oceanic basalts and maybe interspersed with continental remnants, ridges, seamounts, or volcanic islands rising from the depths. Average water depth is around 4000 m but the most significant mineralization is generally found at 5000 m for manganese nodules, 4000 m for biogenic oozes, and 3000 m for hydrothermal metalliferous sulfides. The area is poorly explored, however. 

The outer shell of the earth, consisting of the upper mantle and the crust (Figure I4. lO), is formed of a number of rigid plates. These plates are 20 in number and are shown in Figure 14.1 I. Of these, six or seven are major plates, as can be seen in the map. The edges of these plates define their boundaries and the arrows indicate the direction of their movement. These plates contain the continents, oceans and mountains. They almost float on the partially molten rock and metal of the mantle. The outer shell, known as the lithosphere, is about 70 to 1,50 km thick. It has already moved great distances below the etirth s surface, ever since the earth was formed and is believed to be in slow and continuous motion all the time. The plates slide on the molten mantle and move about lO to 100 mm a year in the direction shown by the arrows. The movement of plates is believed to be the cause of continental drifts, the formation of ocean basins and mountains and also the consequent earthquakes and volcanic eruptions. 

The movement of Earth s crustal plates and the continents they contain - continental drift -has had enormous effects on climate, sea levels, and the distributions of organisms. Mass extinctions of organisms have usually accompanied major drops in sea levels. The collision of all the continents to form the gigantic landmass called Pangaea about 260 million years ago, triggered massive volcanic eruptions. The volcanoes... 

Bates et al. (1992) global total for volcanic emissions reapportioned as 1/3 continental and 2/3 marine. 

Hammer, C. U., Clausen, H. B., Dansgaard, W. et al. (1987). Dating of Greenland ice cores by flow models, isotopes, volcanic debris, and continental dust. /. Glacial. 20,3-26. 

INTRA-PLATE VOLCANISM BACKARC VOLCANIC CONTINENTAL RIFT SYSTEM BASIN BASIN... 

Shinozuka et al. (1999) analyzed the host volcanic and intrusive rocks in the Minamidani mine district in the Maizuru tectonic Belt and found that these rocks formed in an island arc back-arc system near Laurasia during late Paleozoic. Probably the Yanahara deposits, one of the representative Hitachi subtype deposits, were formed in an island arc back-arc system as same as the Minamidani. Sato and Kase (1996) thought that the Hitachi-subtype deposits formed in back-arc rift or continental rift (Table 2.21). 

In contrast to the southern volcanic zone, Parinacota volcano lies on very thick continental crust (> 70 km) in the central volcanic zone of Chile. Bourdon et al. (2000a) showed that young Parinacota lavas encompass a wide range of U-Th disequilibria. excesses were attributed to fluid addition to the mantle wedge but °Th-excesses in lavas from the same volcano are more difficult to explain. The lavas with °Th-excesses also have low ( °Th/ Th) (< 0.6) characteristic of lower continental crust characterized by low Th/U and in their preferred model. Bourdon et al. (2000a) attributed the °Th-excesses to contamination by partial melts, formed in the presence of residual garnet, of old lower crustal materials. 

The basement complex for the Patagonian desert and arid northeastern Brazil is formed by metamorphosed Precambrian rocks. Landscapes are characterized by level erosion surfaces of different ages. The landscape is dissected by a large number of valleys. Large depressions are filled with marine and continental beds of sedimentary rocks. Rocks in the Andean system, that stretches the entire length of the west side of the continent, vary greatly. Many depressions are filled with sediments. In addition, many active volcanoes are responsible for periodic lava flows and the deposition of volcanic ash. East of the Andes, the land surface is level and slopes towards the Atlantic Ocean. Broad depressions contain saline or sodic soils. 

Mercury is a naturally occurring element. Natural emissions of mercury, e.g. from ore deposits and from volcanic activity, are variously estimated at amounts between 2500 and 5500 tonnes/year and are thus similar in magnitude to anthropogenic emissions, which are currently estimated at some 3600-4100 tonnes/year world-wide. Some 30000 tonnes of mercury are readily available in the environment, i.e. in the atmosphere or in the mixing zone of the oceans, with tens of millions of tonnes in the upper layers of the continental masses and still more in the deep oceans (see Table 2.1). 

Earth s crust is a source of particles produced as a consequence of weathering and volcanic activity. Weathering of continental rocks generates terrigenous particles that are carried into ocean via rivers, glaciers, and winds. As shown in Table 13.2, the most abundant mineral types are quartz, plagioclase, and clay minerals. The most abimdant... 

Weathered fragments of continental crust comprise the bulk of marine sediments. These particles are primarily detrital silicates, with clay minerals being the most abmidant mineral type. Clay minerals are transported into the ocean by river runoff, winds, and ice rafting. Some are authigenic, being produced on and in the seafloor as a consequence of volcanic activity, diagenesis and metagenesis. 

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