Edziza in northern B.C., formed approximately 700 years ago Ĭomposite volcanoes, like Mt. Figure 4.10 Eve Cone, situated near to Mt. 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 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 4.10), are typically only a few hundred metres in diameter, and few are more than 200 m high. Helens composite volcano, and a large cinder cone Ĭinder cones, like Eve Cone in northern B.C. Figure 4.9 Profiles of Mauna Loa shield volcano, Mt. 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, 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 4.9, although, to be fair, Mauna Loa is the largest shield volcano on Earth all others are smaller. Table 4.1 A summary of the important types of volcanism Most appear to have had explosive eruptions forming cinder cones the youngest one is over 10 ka old, and all others are over 30 Ma old. The remnants are typically 10s to 100s of m across Large areas of the sea floor associated with spreading ridgesĪt typical eruption rates, pillows form at faster rates, lava flows develop Generally associated with spreading ridges but also with mantle plumes Magma is always mafic and individual flows can be 10s of m 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 km 2) and 100s of m thick Large (up to several 1,000 m high and 200 km 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 m) 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 m) and steep (>20°) Various some form on the flanks of larger volcanoes 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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