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Volcanoes

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Volcanoes are generally not preserved in the geologic rock record as they are usually eroded away. However, the various materials erupted from volcanoes are often found preserved in the rock record. From what you have learned about the different types of volcanoes, how could you infer what type of volcano erupted in a given area based on the type of volcanic deposits now found as layers of rock? Give specific examples, and briefly discuss how some materials may be linked to different types of volcanoes.

Volcanoes generally erode away, they are not often preserved in the geologic record, but we can infer what types of volcanoes were present by the volcanic deposits left behind. The large, violent volcanoes, called stratovolcanoes, often produce pyroclastic flows. Flows of water and ash and tephra, called lahars, flow rapidly down the steep-sided stratovolcanoes and form thick deposits. After the 1980 eruption of Mount St. Helens, pyroclastic flow deposits like this were created. When the stratovolcano Mt. Vesuvius erupted, it covered the city of Pompeii with thick ash deposits. Both the pyroclastic flow deposits and the ash would be indicative of the stratovolcano. Stratovolcanoes do not usually produce large lava flows. Shield volcanoes, however, like those found in the Hawaiian islands, do produce lava and not much ash. These shield volcanoes produce layers of smooth basalt. Cinder cones produce blocky basalt deposits that tend to be interbedded with ash and cinder. Another type of volcanic eruption comes from large fissures and produces flood basalts. These eruptions are not explosive. The very large and thick basalt deposits in the Columbia River Plateau are an example of this type of volcanic activity. Some volcanoes, like Mt. Katmai in Alaska, produce rhyolitic flood eruptions instead of basalt. In summary, stratovolcanoes produce pyroclastic flow deposits and ash deposits, shield volcanoes produce smooth basalt deposits, cinder cones produce blocky...