![]() ![]() ![]() Whereas stratovolcanoes and lava domes are the product of highly viscous flows, and cinder cones are constructed of explosively eruptive tephra, shield volcanoes are the product of gentle effusive eruptions of highly fluid lavas that produce, over time, a broad, gently sloped eponymous "shield". Of these four forms, shield volcanoes erupt the least viscous lavas. Shield volcanoes are distinguished from the three other major volcanic types- stratovolcanoes, lava domes, and cinder cones-by their structural form, a consequence of their particular magmatic composition. Geology Structure ĭiagram of the common structural features of a shield volcano The term 'shield volcano' is taken from the German term Schildvulkan, coined by the Austrian geologist Eduard Suess in 1888 and which had been calqued into English by 1910. Giant shield volcanoes are found on other planets of the Solar System, including Olympus Mons on Mars and Sapas Mons on Venus. They include the largest volcanoes on earth, such as Tamu Massif and Mauna Loa. However, they are most characteristic of ocean island volcanism associated with hot spots or with continental rift volcanism. Shield volcanoes are found wherever fluid, low- silica lava reaches the surface of a rocky planet. Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form. ![]() It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava erupted from a stratovolcano. Having these demonstrations in a movie format is especially good for large classes, where it might be difficult for students in the back to see what the instructor is doing on the bench top.Low-profile volcano usually formed almost entirely of fluid lava flows Mauna Loa, a shield volcano in Hawaii An Ancient Greek warrior's shield-its circular shape and gently sloping surface, with a central raised area, is a shape shared by many shield volcanoes.Ī shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. However, if you don't feel like you have the time to spend with all the "interactive" elements, like small-group discussions and iClicker questions, you can just import the movies into your own Powerpoint and use them however you like. The Powerpoint should be pretty self explanatory in terms of the progression of ideas I take the students through. My ultimate goal is to get them to the point that they can pick up a piece of basalt, identify it, and make up a fairly complete story about how it originated, referring back to basic physical and chemical principles, when appropriate. This illustrates why runny lavas make shield volcanoes and flood basalts, but more viscous magmas make tall, pointy cones. The toothpaste comes out and piles up on itself, rather than running down the sides very far. The water runs out and spreads out because it isn't very viscous. The second demo is a movie of me shooting water up out of an inverted funnel, and then shooting out toothpaste. The crystal structure demos illustrate the idea that more silica = more polymerization. They also have a harder time making it to the surface and don't spread out as much once they are erupted. More viscous magmas build up gas pressure more readily, and so are more explosive. Likewise, silica molecules in magmas and minerals like to polymerize, so the more silica there is, the more viscous the magma, and when it's hotter, it becomes less viscous. They stick together less readily when heated. The point is that sugar molecules make the liquid more viscous, because they like to stick together. The first demo is a movie of me blowing air into a beaker of water, then a beaker of honey, and then a beaker of heated honey. (Why are some volcanoes more explosive than others? Why are particular surface features associated with particular kinds of volcanoes? Why does basaltic magma have an easier time making it to the surface?) It's this kind of connected knowledge that students will have a better chance of retaining and being able to use after the class is over. With this activity, I try to help them see that all the stuff they learned earlier about molecules, phases of matter, silicate crystal structures, and igneous rock classification can help them understand the different types of volcanoes. It's just a mass of disconnected facts that they have to memorize. ![]() One of the main problems I face is that many of my students treat material from different chapters in the textbook as completely separate information. ![]()
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