Types of Volcanoes Help
Scoria Cone Volcanoes
Scoria cone volcanoes , also called cinder cones , are the most common type of volcano. Reaching heights commonly less than 300 meters, cinder cones make up the smallest type of volcano.
Scoria cones have straight sides with steep slopes (as much as 35 degrees) and a very wide, often symmetrical peak crater. These volcanoes generally have Strombolian type eruptions. They can be seen as single volcanoes in basaltic lava fields or as “piggy-back” cones built up by eruptions along the sides of shield volcanoes and stratovolcanoes. Scoria cones are made up mostly of ejected basaltic tephra . This is generally of lapilli size, but can have bomb-size fragments and splatter lava as well. The cinder look of the rock comes from the high amount of gas bubbles ( vesicles ) in the tephra fragments. The shape of a Scoria-type volcano is shown in Figure 11-3.
Shield volcanoes are wide, flat volcanoes with diameters of a few kilometers to over 100 kilometers. They are generally short and squat with heights roughly 1/20th of their width. So a shield volcano that is 100km across would only be 5km tall. They have gently sloping lower slopes (2–3 degrees) that build to medium height slopes of about 10 degrees and then flatten at the peak. The spreading shield area grows from the over layering of many hundreds or thousands of fluid lava flows, one upon another, that gurgle and bubble out from the same point of release. Figure 11-4 illustrates the unimpressive low shape and vertical height of shield volcanoes compared to other types.
Fig. 11-4. Shield volcanoes are short and squat compared to other types.
Shield volcanoes are the largest volcanoes on the planet. Hawaii’s volcanoes (like Kilauea and Mauna Kea) are shield volcanoes. In fact, the island of Hawaii is made up of five overlapping shield volcanoes. The Mauna Loa volcano on the big island of Hawaii is the world’s largest shield volcano. Shield volcanoes are also said to have Hawaiian eruptions , the large spreading volume of lava (thousands of cubic kilometers) that comes from the summit or fissures.
When the flow is almost perfectly symmetrical and of a low volume (10–15km 3 ), the eruption produces an Icelandic shield . These small volume flows are commonly centered at the summit compared to high-volume linear fissure eruptions typical of the Hawaiian shields. Galapagos shields are symmetrical with steep middle slopes (about 10 degrees) and flat tops with large, deep calderas. These shields are thought to be formed by ring - fracture eruptions which encircle the caldera and show the site of the caldera collapse.
Shield volcanoes are almost completely basalt, the lava type that is very liquid when it erupts. For this reason, the lava runs out like water from a spilled glass, instead of piling up in a pyramid shape.
Eruptions of shield volcanoes are usually mild and of low-explosive power. The bubbling matter that forms cinder cones and spatter cones around the vent is 90% lava instead of pyroclastic matter. The shield forms around the opening where the volcano spits out a long flowing supply of magma. The hot flowing lava is little changed since it was formed deep in the earth.
Shield volcanoes are common sights in areas of hot spot volcanism. They are also found along subduction arcs or as lone volcanic sentinels in the middle of the ocean.
Large, long-lived volcanoes of andesitic , dacitic , and rhyolitic composition release a combination of lava flows and pyroclastic debris. These volcanoes, found mostly on the continents, are known as composite or stratovolcanoes . They usually emit a liquid lava flow, a pyroclastic blast and build up steep conical mounds of alternating layers of pyroclastic material and lava flows. Lava that has hardened within fissures form strengthening, rod-like supports that fortify the cone.
Often the slope of a composite volcano is about 30°, like the angle near the peak of a pyroclastic cone. The base of a composite volcano flattens to about 8–10° angles. This type of volcano is the “classic” volcano shape of the Oregon and Washington volcanoes (Mts. Hood, Baker, Rainier) in the United States and Mount Fugiyama in Japan. Figure 11-5 shows the steep sides and narrow peak crater of a composite volcano.
Practice problems of this concept can be found at: Volcano Practice Test
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