Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 3104

VOLCANOES 567 Central volcanoes of substantial dimensions may develop large circular cavities, calderas, focused on their summits: these may be circular, or of rather irregular annular shape; they may even be multiple, one inside or superimposed on the other The Suswa caldera in Kenya is annular, a central plateau being preserved Calderas may be formed either by subsidence, due to withdrawal of magma/lava support, or volcanic explosion Calderas are evident on Mars and Io There are three types of central volcanic pile, all of which may develop calderas: i shield volcanoes: all lava flows ii ash cones: all pyroclastic iii composite or strato-volocanoes: alternating lava and pyroclastic (Figure 3) The material that flows out as lava is termed effusive Extrusive includes both lavas and pyroclastic rocks There are seven types of volcano, classified according to the nature of the eruption Four of the names stem from southern Italy, a field of classical study which can be termed the ‘Birthplace of Vulcanology’; and which has lately been described by Guest et al in modern terms Iceland, Hawaii, and Martinique supply the remaining names The list is given in Table This scheme is a very useful subdivision, but most volcanoes show some departure from these classical definitions derived from a handful of famous volcanoes No other volcano, for example, matches Stromboli, where explosions occur every 10 minutes and activity has been continuous for 2500 years Other types can be long dormant, Mt Pele´ e in Martinique, lay dormant for centuries, only, in 1902, to send off sudden blasts lasting a few minutes separated by weeks of relative inactivity while the plug in the vent built up into a spine (Figure 5) A very useful table of explosivity of volcanoes was published in 1982 (Table 2): Products of Volcanoes Lavas Figure Diagram of a composite volcano: lava (black) and tephra (stippled) interbedded (from Green and Short (1971)) Magma erupted from volcanoes as lava consists of molten rock, crystals, and gas – carried as bubbles, mainly water, and carbon dioxide The less siliceous lavas such as basalt flow freely and build up extensive shield volcanoes as in Hawaii Other lava types may flow freely – a phonolite flow forming the Yatta Plateau in Kenya followed a valley for >300 km from its source Less fluid flows such as trachyte Figure The 13 km diameter basaltic Ambrym volcano, Vanuatu (A) Plaster model (by Jon Stephenson) (B) Air photo showing the caldera with two active craters nested within (Benbow and Marum)