The size, shape, and eruptive style of any volcano ultimately depend on the magma composition. We will focus mainly on mafic and felsic magmas as intermediate magmas have properties that are intermediate between these two types, and ignore the ultramafic magma as this type no longer forms (due to a cooler earth’s interior). As mentioned earlier, mafic magmas are lower in silica and are therefore characterized as having a low viscosity. As mafic magma erupts on to the surface through a central vent (see Figure 9.6, and photo in Figure 9.3), the magma (now called lava) will spread out quite easily due to its low viscosity. Mafic lava flows can travel quite far before solidifying completely, and the type of volcano that forms from mafic lava is called a shield volcano (Figure 9.9). Shield volcanoes are very broad at the base and have relatively gentle slopes. There are several locations around the world where large amounts of mafic basalt flows are found, without forming a shield volcano: places such as the Deccan Traps in India, Siberian Traps in Russia, and here in the U.S. (Columbia River Basalt Group). Such vast outpourings of mafic lava are called flood basalts and are believed to be caused by mantle plumes (hotspots), which partially melt the mantle beneath the earth’s crust. The mafic magma that is generated by the mantle plume reaches the earth’s surface through fractures (fissures) instead of one central vent.
Figure 9.9 also shows a much smaller volcano type, called a composite volcano. This volcano also forms from eruptions through a central vent, but the smaller size indicates that any lava that is generated from a central vent did not travel far before solidifying completely, which indicates a more viscous magma, such as an intermediate or felsic magma type. The term “composite” comes from the layers of lava flows, and the accumulation of ash and other volcanic material produced during a more explosive type of eruption due to the dissolved gases present in the magma (Figure 9.10, and photos in Figures 9.1 and 9.2). It is common to have a certain amount of dissolved gases within the magma, and some gases may escape from the magma while still underground, but the most spectacular release of these gases occurs during the eruption. The common gases associated with magma are usually water vapor, carbon dioxide, carbon monoxide, and hydrogen sulfide. Mafic magmas erupting onto the surface may form a lava fountain which spurts the magma (now called lava) into the air, where the height of the lava fountain depends on the gas content. However, gases within felsic magmas are not released as easily due to the magma’s high viscosity; as viscous magmas move closer to the earth’s surface, the dissolved gases are under less pressure and large gas bubbles can form. At some point, the gas pressure becomes so great that an explosive eruption occurs, and fragmented volcanic material is released. This material can travel great distances along air currents, but enough deposits around the central vent to build up the composite volcano structure. Once the gases are released and the explosive eruptions ended, felsic magma may still be extruded onto the surface inside the volcanic crater. This felsic material, now viscous lava, will flow with great difficulty. Furthermore, a large amount of the felsic lava will cool completely in the area around the central vent and may end up “plugging” the vent and forming what is called a lava dome (Figure 9.11).