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Answered by Faith P. Answered by. Composite volcanoes are also called stratovolcanoes because of the alternating layers, or strata , of which they are made Figure 8. The magma that creates stratovolcanoes tends to be more viscous, or thick. Viscous lava creates greater pressure which, in turn, tends to create explosive eruptions. In addition, the viscous lava cannot travel far down the sides of the volcano before it solidifies.
This viscous lava thus creates steep sides on stratovolcanoes. When a stratovolcano erupts, it ejects a great deal of pyroclastic material into the air, which then settles back down on the Earth. After an initial explosion, lava then flows from the volcano creating a second layer of material.
As these layers solidify, they create alternating levels, or strata, of material. Ash from the volcanic eruption is also present between the lava layers along the edge of the volcano. Composite volcanoes are common along the Pacific Ring of Fire and other major tectonic plate boundaries where the presence of water in the magma chamber creates explosive eruptions.
Shield volcanoes get their name from their shape—a huge shield laid on its side. You can see that shield volcanoes do not have the steep mountainous sides of composite volcanoes. They have a very wide base and are much flatter on the top than composite volcano. Although they are not steep, they may be very large. The Mauna Loa Volcano has a diameter of over kilometers 70 miles and forms a significant part of the island of Hawaii. The Mauna Kea Volcano, also in Hawaii, is another shield volcano that is over ten kilometers 6 miles high from its base below sea level to its peak.
Shield volcanoes are more common at spreading centers or volcanic hot spots in the middle of tectonic plates Figure 8.
The magma that creates shield volcanoes is less viscous, so it flows much more easily. For this reason, the eruptions of shield volcanoes are non-explosive. The last explosive eruption left a funnel-shaped crater at the top of the cone.
After the excess gases had largely dissipated, the molten rock quietly poured out on the surrounding surface of the cone and moved downslope as lava flows. This order of events--eruption, formation of cone and crater, lava flow--is a common sequence in the formation of cinder cones. S ome of the Earth's grandest mountains are composite volcanoes--sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8, feet above their bases.
Helens and Mount Rainier in Washington. M ost composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone.
Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone. T he essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc.
Schematic representation of the internal structue of a typical composite volcano. W hen a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit the volcanic plug and fissures the dikes becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano.
Shishaldin Volcano, an imposing composite cone, towers 9, feet above sea level in the Aleutian Islands, Alaska. A n interesting variation of a composite volcano can be seen at Crater Lake in Oregon. This lava is basaltic, but it is also charged with gas.
Composite cones are tall, cone shaped volcanoes that produce explosive eruptions. Shield volcanoes form very large, gently sloped volcanoes with a wide base. Cinder cones are the smallest volcanic landform. They are formed from accumulation of many small fragments of ejected material. Why are shield volcanoes wider than composite volcanoes?
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