![]() Even so, composite volcanoes tend to erode quickly. Lava flows both flatten the profile of the volcano (because the lava typically flows farther than pyroclastic debris falls) and protect the fragmental deposits from erosion. Mafic eruptions (and some intermediate eruptions), on the other hand, produce lava flows the one shown in Figure 4.3.5b is thick enough (about 10 m in total) to have cooled in a columnar jointing pattern (Figure 4.3.7). Figure 4.3.4 A cross-section through the upper part of the crust at Mount St. Between 19 the slow eruption of more mafic and less viscous lava led to construction of a dome inside the crater. The large 1980 eruption reduced the height of the volcano by 400 m, and a sector collapse removed a large part of the northern flank. Helens in southwestern Washington State, 2003. Helens imply that the magma chamber is zoned, from more felsic at the top to more mafic at the bottom. Systematic variations in the composition of volcanism over the past several thousand years at Mount St. Helens, there is evidence of a magma chamber that is approximately 1 kilometre wide and extends from about 6 km to 14 km below the surface (Figure 4.3.4). At many such volcanoes, magma is stored in a magma chamber in the upper part of the crust. They can extend up to several thousand metres from the surrounding terrain, and, with slopes ranging up to 30˚ They can be up to about 20 km across. ![]() Helens in Washington State (Figure 4.3.3), are almost all associated with subduction at convergent plate boundaries-either ocean-continent or ocean-ocean boundaries (Figure 4.1.2b). ![]() Composite VolcanoesĬomposite volcanoes, like Mount St. Figure 4.3.2 Eve Cone, situated near to Mount Edziza in northern B.C., formed approximately 700 years ago. They can be easily, and relatively quickly, eroded away. Because cinder cones are made up almost exclusively of loose fragments, they have very little strength. Most cinder cones are monogenetic, meaning that they formed during a single eruptive phase that might have lasted weeks or months. In many cases, these later became the sites of effusive lava flows when the gases were depleted. Most are made up of fragments of vesicular mafic rock (scoria) that were expelled as the magma boiled when it approached the surface, creating fire fountains. (Figure 4.3.2), are typically only a few hundred metres in diameter, and few are more than 200 m high. Cinder ConesĬinder cones, like Eve Cone in northern B.C. Helens composite volcano, and a large cinder cone. Figure 4.3.1 Profiles of Mauna Loa shield volcano, Mount St. On this drawing, even a large cinder cone is just a dot. Its diameter is about 6 km, and its height is 2,550 m above sea level. Helens, a composite volcano of average size, rises above the surrounding hills of the Cascade Range. Its elevation is 4,169 m above sea level. Mauna Loa rises from the surrounding flat sea floor, and its diameter is in the order of 200 km. The sizes and shapes of typical shield, composite, and cinder-cone volcanoes are compared in Figure 4.3.1, although, to be fair, Mauna Loa is the largest shield volcano on Earth all others are smaller. Most appear to have had explosive eruptions forming cinder cones the youngest one is dated at about 10 ka, and all others are at least 30 Ma The remnants are typically 10s to 100s of metres across Pillows form at typical eruption rates lava flows develop if the rare of flow is faster Large areas of the sea floor associated with spreading ridges Generally associated with spreading ridges but also with mantle plumes Magma is always mafic and individual flows can be 10s of metres thick Magma is almost always mafic, and eruptions are typically effusive, although cinder cones are common on the flanks of shield volcanoesĮnormous (up to millions of square kiometres) and 100s of metres thick ![]() Large (up to several 1,000 metres high and up to 200 kilometres across), not steep (typically 2° to 10°) Most are at mantle plumes some are on spreading ridges Magma composition varies from felsic to mafic, and from explosive to effusive Medium size (1000s of metres high and up to 20 km across) and moderate steepness (10° to 30°) Most are mafic and form from the gas-rich early stages of a shield- or rift-associated eruption Small (10s to 100s of metres) and steep (Greater than 20°) Various some form on the flanks of larger volcanoes Table 4.1 A summary of the important types of volcanism There are numerous types of volcanoes or volcanic sources some of the more common ones are summarized in Table 4.1.
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