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eruptive_style - 114 Endogenic processes and landforms...

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Unformatted text preview: 114 Endogenic processes and landforms Figure adopted from Summerfield (1991) Viscous rhyolitic Fig. 5.7 Schematic representation of three major forms of eruptive activity related to magma type and associated mode of gas expansion. In (A) the andesitic magma in zone I may be either saturated or undersaturated with volatiles, but as it rises it enters a zone of lower pressure (2) and bubbles of gas begin to form (3). The viscosity of andesitic magma is sufficient to restrain gas expansion up to the explosion zone (4) where the gas pressure is able to overcome the resistance of the overburden. In (B) magma of basaltic composition is fluid enough to allow the unrestricted expansion of gas bubbles and their relatively quiescent exhalation at the surface. In (C) the rhyolitic magma is so viscous that the bursting of gas bubbles does not occur until very near the surface. Violent lateral explosions producing ignimbritic eruptions consisting of dense mixtures of gases, liquids and pyroclasts are caused by the periodic blocking of the main vent by a viscous lava plug. (After C. M. Clapperton ( I 977) Progress in Physical Geography Fig. 7, p. 394.) ...
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