FIGURE 9.10 scoria and pumice are volcanic rocks that exhibit a vesicular texture. vesicles are small holes left by escaping qas bubbles. A. scoria is usually a product of mafic(basaltic) magma. B.pumice forms during explosive eruptions of viscous magmas having an intermediate (andesitic) or felsic (rhyolitic) composition. (photos by E.J. Tarbuck)
conduit, or pipe, thatterminates at a surface opening called a vent (Figure 9.11). Successive eruptions of lava,pyroclastic material, or frequently a combination of both, often separated by iong period of inactivity, eventaully buildthe cone-shapedstructure we call volcano.
Located at the summit of most volcanoes is a somewhat funnel-shaped bepression,called a crater (crater=a bowl). volcanoes that are built primarily of pyroclastic materials typically have craters that form by gradual accumulation of volcanic debris on the surrounding rim.Other craters form during explosive eruptions as the rapidlyejectedparticles erode the crater walls.Craters also form when the summit area of a volcano collapses following an eruption (Figure9.12). Some volcanoes have very large circular
FIGURE 9.11 Anatomy of a "typical" composite cone (see also Figures 9.13 and 9.16) for a com parison with a shield and cinider cone,respectively).