At the beginning, mostly basaltic magma erupts. This change in the mineralogy of the magma leads to changes in the type of eruption and the morphology of volcano itself. Many subduction zone stratovolcanoes go through a life cycle spanning millions of years in which the composition of their magma changes over time. Stratovolcanoes are more likely to erupt explosively and emit ash and gas. Stratovolcanoes are shaped the way they are because they are formed by alternating layers of lava and pyroclastic material, both of which have a higher viscosity, and thus a greater resistance to flow than the basalt-only shields. In fact, its actual elevation above sea level (4392 m) is higher than Mauna Loa's, but remember that the base of Mauna Loa is really about 5 km below sea level! From the photo, Rainier looks like a bigger mountain than Mauna Loa. Rainier shown here and compare it to the photo of Mauna Loa above. If you asked a child to draw a volcano, the shape they are most likely to come up with is the conical, steep-sided shape of a stratovolcano. The Cascade Range volcanoes are all stratovolcanoes, for example. They most often form on the continental overriding plate in a subduction zone. Stratovolcanoes are sometimes also called "composite" volcanoes. Gradually, after many repeated eruptions from a central fissure, a mountain will be formed-one that is much wider at its base than it is tall, but one that has the potential to be quite tall, depending on how long the magma source remains active. Because basalt has such a low viscosity, it spreads out in thin sheets and can travel a long way before freezing. Shield volcanoes are formed by low-viscosity basaltic lava flows. Only the top 4km rises above the water, though. The photo almost makes Mauna Loa look like a small, insignificant mountain, doesn't it? Actually, if you start measuring at the ocean floor, which is the real bottom of the mountain, Mauna Loa is 9 km tall, easily taller than Mt. See the photo of Mauna Loa shown here and note the shape of the mountain. The most obvious examples of shield volcanoes are the Hawaiian Islands. Continental shields, such as the Canadian shield, and the Kaapvaal craton in South Africa, represent the oldest still-intact blocks of crust from the early history of the Earth.) The two don't have anything in common except for their morphology. (Side note here: Don't confuse shield volcanoes with continental shields, which are also named for their shield-like shape. Their widths are much broader than their heights and they have gentle slopes. "Shield" volcanoes are so named because their shape is reminiscent of a shield lying on the ground with its convex side pointing up.
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