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Volcanoes, Plutons, and Igneous Rocks

Volcanoes, Plutons, and Igneous Rocks. In Lab 2 , plate tectonics taught us a bit about how volcanoes form. In Lab 3 we learned about common rock-forming minerals.

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Volcanoes, Plutons, and Igneous Rocks

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  1. Volcanoes, Plutons, and Igneous Rocks In Lab 2, plate tectonics taught us a bit about how volcanoes form. In Lab 3 we learned about common rock-forming minerals. Today, we’ll study igneous rocks and the minerals that form them. Then we’ll learn some relationships between magma temperature, composition, and tectonic setting. Andesite (pg 124) lava dome at Mt St Helens, USA, 9 May 2006 Basalt (pg 124) lava lake at Erta ‘Ale, Ethiopia, 14 Feb 2002

  2. How Tectonic Setting Affects Volcanism At hotspots and divergent margins, partially melted mantle material rises through weakened crust, creating mafic magma (pg. 40). Magma found in these tectonic settings erupts or intrudes at relatively high temperatures, so minerals like Olivine (shown below) will crystallize as the magma cools. Mafic magma generally has a low viscosity (pg. 47), allowing trapped volatiles (pg. 47) to escape; as a result, it erupts in effusive lava flows, as seen above. Explosive eruptions of mafic magma are rare!

  3. How Tectonic Setting Affects Volcanism At a convergent margin, cold material on the subducting plate releases water into the overlying mantle, causing partial melt in the plate above it. The partial melt then rises to the surface. This type of magma is called felsic. Granite, a coarse-grained (pg. 44), intrusive (pg. 39) igneous rock, forms when felsic magma cools in underground dikes or magma chambers. Granite can contain Quartz, Orthoclase, and Biotite. Felsic magma has a high viscosity and is rich in volatiles. As a result, eruptions of felsic magma tend to be very explosive, such as the 1991 eruption at Mt. Pinatubo, Philippines (above).

  4. Identifying Volcanic Rocks Volcanic rocks are classified by composition and texture. Let’s begin with composition (page 40-41). The porphyritic basalt shown at right is a mafic rock. It is dark in color and appears to contain amphibole and plagioclase, which you studied last week. Below is rhyolite, a felsic rock. Note the light color; it probably contains orthoclase, muscovite and quartz. Color is not always the best indicator, as the sample at left shows!

  5. Identifying Volcanic Rocks By knowing just composition and texture (p. 43-45), we can identify most of the common igneous rocks. The granite shown at right is coarse-grained (pg. 44). This just means that the grains can be seen with the naked eye. (Grains larger than 0.062 mm). Below it is rhyolite. Rhyolite and granite have nearly identical compositions, but rhyolite is fine-grained. Can you see individual mineral grains? They’re smaller than 0.062 mm; good luck with that! Finally, compare the fragmental texture (p. 45) of tuff(left) to the fine-grained rhyolite (right). What’s different? How does each form, and how might this affect their physical properties?

  6. Why You Should Care Remember, the Cascades are a convergent margin. If you go hiking in western Washington, you’ll see granite, diorite, and andesite. We know that these come from high viscosity felsic magma, which erupts explosively. What does that tell you about the area in which you live?

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