FIGURE 9.10 scoria and pumice are v

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.