B. Distribution of Composite Volcanoes
Composite volcanoes are found globally in nearly all regions of volcanism, although their abundance relative to classic shield volcanoes, lava fields and domes, calderas, and cinder cones varies considerably. At convergent plate margins, composite volcanoes are arguably the type edifices (Fig. 2).
FIGURE 2 Examples of different composite volcano forms from a convergent plate margin setting, the North Island of New Zealand: (a) Mount Taranaki, a "classic" cone shape; (b) Mount Ruapehu, a compound volcano; (c) Tongariro, a compound volcano with numerous Holocene vents (the youngest being the symmetrical cone of Mount Ngauruhoe with a single active vent). Photograph d is an aerial view of Tongariro (N . Ngauruhoe) looking south to Ruapehu (R) in the distance, for comparison. The variation from (a) to (c) reflects an increasing tendency for vent location to migrate over time. Although they are cluster or composite edifices (Table II), Ruapehu and Tongariro might be considered twin volcanoes (to each other) in a broad sense.
Indeed subduction zones are perhaps most dramatically characterized by classic cone-shaped basaltic andesite to andesite volcanoes such as Mount Fuji in Japan. In fact, most well-known historic eruptions have occurred at composite cones located along convergent plate margins (Table I). Although some convergent margin environments where extension occurs in intra-arc grabens or backarc rifts are dominated by cinder cone fields, the magmatic expression of a convergent plate margin is generally a chain of composite volcanoes, subequally spaced, comprising a volcanic arc. The arc is subparallel to the trench and constructed on the upper plate approximately 100-150 km above the upper surface (known as the Wadati-Benioff zonesee "Plate Tectonics and Volcanism") of the subducted slab. The spacing of the volcanoes along the arc is subregular, varying from 30 to 100 km among different arcs, although most arcs may be characterized by an average spacing. The control on volcano spacing is unclear and may reflect the spacing of Rayleigh-Taylor diapiric instabilities in a melt zone above the subducted slab or periodic variations in the stress state of the upper plate lithosphere that impede or allow melt ascent.
TABLE I Selection of Notable Eruptions from Composite Volcanoes Since A.D. 1500 Involving 1000 or More Fatalitiesª
ªModified from Report by the Task Group for the International Decade of Natural Disaster Reduction. (1990). Bull. Volcan. Soc. Jpn. Ser. 2 35, 80-95. Numbers refer to fatalities, listed under column for cause of deaths.|
Composite volcanoes are not common at divergent plate margins, although examples can be found. Along oceanic divergent margins (midocean ridges) they are effectively absent, although on Iceland, where the midocean ridge lies over a mantle plume, composite cones are present, such as Hekla and Askja. In the early stages of divergent margin development (continental rifting), composite volcanoes do occur, although they are typically off-axis. Along the East African Rift, for instance, huge composite volcanoes (e.g., Mount Kilimanjaro) are found at the rift shoulders. Intraplate volcanism, perhaps by virtue of the high heat (and therefore magma) flux over a protracted time period, typically gives rise to large composite volcanic edifices. Many, particularly in the ocean basins, are dominated by basaltic effusion and are classic shield volcanoes. Composite intraplate volcanoes may, however, be found in both oceanic and continental environments. They are effectively indistinguishable morphologically from those found at convergent plate margins (Fig. 3), differing only in terms of the rock chemistry and petrography.
FIGURE 3 Large composite volcanoes from intra-plate settings: (a) Hasandag volcano, Cappadocia, Turkey; (b) Teide volcano, Tenerife; (c) Mount Damavand in northern Iran (5671m). Both continental (a, c) andoceanic (b) crustal substrates are represented.
What factors determine the surface expression of magmatism, that is, the construction of composite volcanoes rather than other volcano landforms? Two factors at least promote the formation of composite volcanoes: (1) the composition of magma erupted and (2) the style of eruption. Magma compositions at convergent plate margins are largely the result of differentiation and volatile concentration. These factors are, of course, not unconnected, as volatile concentration is typically a consequence of differentiation (cf. "Volatiles in Magmas"). Styles of eruption are controlled more by the physical structure and stress environment of the lithosphere through which magmas ascend (although this also influences the extent of magma differentiation). In order to produce compound volcanoes, eruptions at a given vent are frequent and small to moderate in size.
Differentiation increases SiO2 content and increases magma viscosity, leading to high aspect ratio lavas and impeding lateral distribution. Thus there is a distinction between shield volcanoes, at which low-viscosity basalts can flow large distances from the vent, and cones, at which lava flows are shorter and construct a steeper sloped edifice around the vent. The combined increase in volatile content and magma viscosity also increases the propensity for explosive eruption and the production of pyroclastic material. Convergent margin magmas are characterized by high H2O contents, and, at least by the time they reach the surface, they tend to be differentiated (basaltic andesites and andesites). Because magma ascent is fundamentally dictated by the buoyancy contrast between magma and the lithosphere, ascent through thick, low-density continental crust will be impeded unless extensive differentiation occurs to reduce the magma density. Protracted differentiation can ultimately produce large volumes of rhyolite in continental regions, which, in turn, may be erupted in caldera-forming events (cf. "Calderas").
The association of calderas with precursor composite volcanoes is common. Smaller, less differentiated caldera systems such as Crater Lake in Oregon may represent the cataclysmic evacuation of a silicic magma chamber from beneath a simple composite cone. Larger caldera-forming eruptions (such as at Long Valley in California and Valles in New Mexico) are typically preceded, albeit by a few million years, by magmatism expressed as clusters of composite volcanoes concentrated in the region of the future caldera. Composite volcanoes and calderas may also be contemporaneous, varying only slightly in location along and across zones of magmatism, such as the Taupo Volcanic Zone of New Zealand. These associations suggest a further complex balance between magma/heat flux and the rheological state of the crust in determining the morphological expression of volcanism.
In summary then, relative to shield volcanoes, composite volcanoes are characterized by the eruption of more differentiated, silica- and volatile-rich magma. Compared with caldera systems, composite volcanoes erupt smaller volumes more frequently and less explosively, which likely inhibits the long-term extreme differentiation, which typifies caldera-forming magmas.
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