4.3 varieties of Volcanoes

There are numerous varieties of volcanoes or volcano sources; some of the much more common ones room summarized in Table 4.1.

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TypeTectonic SettingSize and ShapeMagma and also Eruption CharacteristicsExample
Cinder coneVarious; some type on the flanks of bigger volcanoesSmall (10s come 100s that m) and also steep (>20°)Most are mafic and form from the gas-rich early stages that a shield- or rift-associated eruptionEve Cone, northern B.C.
Composite volcanoAlmost all room at subduction zonesMedium size (1000s of m) and moderate steepness (10° come 30°)Magma composition varies from felsic to mafic, and also from explosive come effusiveMt. St. Helens
Shield volcanoMost space at mantle plumes; some space on dispersing ridgesLarge (up to numerous 1,000 m high and 200 km across), no steep (typically 2° to 10°)Magma is nearly always mafic, and also eruptions are typically effusive, although cinder hat are typical on the flanks that shield volcanoesKilauea, Hawaii
Large igneous provincesAssociated with “super” mantle plumesEnormous (up to millions of km2) and also 100s of m thickMagma is always mafic and also individual flows have the right to be 10s of m thickColumbia river basalts
Sea-floor volcanismGenerally associated with dispersing ridges but additionally with mantle plumesLarge locations of the sea floor linked with dispersing ridgesAt common eruption rates, pillows form; at faster rates, lava operation developJuan de Fuca ridge
KimberliteUpper-mantle sourcedThe remnants are generally 10s come 100s that m acrossMost appear to have had actually explosive eruptions creating cinder cones; the youngest one is end 10 ka old, and all others room over 30 Ma old.Lac de Gras Kimberlite Field, N.W.T.

Table 4.1 A summary of the important varieties of volcanism

The sizes and also shapes of typical shield, composite, and also cinder-cone volcanoes are compared in figure 4.9, although, to it is in fair, Mauna Loa is the largest shield volcano top top Earth; all others space smaller. Mauna Loa rises native the surrounding level sea floor, and its diameter is in the bespeak of 200 km. Its elevation is 4,169 m above sea level. Mt. St. Helens, a composite volcano, rises over the surrounding hills of the Cascade Range. Its diameter is about 6 km, and its elevation is 2,550 m above sea level. Cinder hat are much smaller. Top top this drawing, even a large cinder cone is just a dot.

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Figure 4.9 file of Mauna Loa shield volcano, Mt. St. Helens composite volcano, and a huge cinder cone

Cinder Cones

Cinder cones, favor Eve Cone in north B.C. (Figure 4.10), are frequently only a couple of hundred metres in diameter, and few are much more than 200 m high. Many are comprised of fragments of vesicular mafic absent (scoria) that were expelled together the magma boiled once it approached the surface, developing fire fountains. In countless cases, this later became effusive (lava flows) as soon as the gases to be depleted. Most cinder cones room monogenetic, meaning that lock formed during a solitary eruptive step that might have lasted weeks or months. Because cinder cones room made up almost exclusively of loosened fragments, they have actually very tiny strength. They deserve to be easily, and reasonably quickly, eroded away.

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Figure 4.10 night Cone, situated close to to Mt. Edziza in northern B.C., formed approximately 700 years ago Composite Volcanoes

Composite volcanoes, like Mt. St. Helens in Washington State (Figure 4.11), are practically all linked with subduction at convergent plate limits — one of two people ocean-continent or ocean-ocean boundaries (Figure 4.4b). Lock can expand up to number of thousand metres indigenous the surrounding terrain, and, with slopes ranging up to 30˚, are frequently up come 10 km across. At countless such volcanoes, magma is save in a magma chamber in the upper component of the crust. Because that example, at Mt. St. Helens, there is proof of a magma chamber the is approximately 1 km broad and extends from around 6 km to 14 km listed below the surface (Figure 4.12). Methodical variations in the composition of volcanism end the past several thousand year at Mt. St. Helens indicate that the magma chamber is zoned, from much more felsic at the top to an ext mafic at the bottom.

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Figure 4.11 The north next of Mt. St. Helens in southwestern Washington State, 2003 . The big 1980 eruption lessened the height of the volcano by 400 m, and also a ar collapse eliminated a large part of the north flank. In between 1980 and also 1986 the slow eruption of an ext mafic and also less viscous lava led to building of a dome within the crater.

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Figure 4.12 A cross-section v the upper component of the crust at Mt. St. Helens reflecting the zoned magma chamber.

Mafic eruptions (and part intermediate eruptions), top top the various other hand, create lava flows; the one shown in number 4.13b is thick sufficient (about 10 m in total) to have actually cooled in a columnar jointing sample (Figure 4.14). Lava flows both flatten the profile of the volcano (because the lava frequently flows farther than pyroclastic debris falls) and also protect the fragmental deposits native erosion. Also so, composite volcanoes tend to erode quickly. Patrick Pringle, a volcanologist through the Washington State room of herbal Resources, defines Mt. St. Helens as a “pile of junk.” The rock that renders up Mt. St. Helens varieties in composition from rhyolite (Figure 4.13a) come basalt (Figure 4.13b); this indicates that the varieties of past eruptions have varied commonly in character. As currently noted, felsic magma doesn’t flow easily and doesn’t permit gases to escape easily. Under this circumstances, pressure builds up till a conduit opens, and then an explosive eruption results from the gas-rich upper part of the magma chamber, producing pyroclastic debris, as displayed on number 4.13a. This type of eruption can also lead to fast melting the ice and also snow top top a volcano, which typically triggers big mudflows recognized as lahars (Figure 4.13a). Hot, fast-moving pyroclastic flows and lahars room the 2 main causes of casualties in volcanic eruptions. Pyroclastic operation killed about 30,000 world during the 1902 eruption that Mt. Pelée ~ above the Caribbean island the Martinique. Many were incinerated in your homes. In 1985 a substantial lahar, prompted by the eruption that Nevado del Ruiz, killed 23,000 civilization in the Colombian town of Armero, about 50 kilometres from the volcano.

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In a geology context, composite volcanoes tend to kind relatively quickly and do not last an extremely long. Mt. St. Helens, for example, is consisted of of rock that is every younger than 40,000 years; most of it is younger than 3,000 years. If that volcanic task ceases, it might erode far within a few tens of thousands of years. This is largely due to the fact that of the visibility of pyroclastic eruptive material, which is no strong.

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Figure 4.13 Mt. St. Helens volcano deposits: (a) lahar deposits (L) and also felsic pyroclastic store (P) and also (b) a columnar basalt lava flow. The 2 photos were taken at places only about 500 m apart.