Volcanoes are generally not preserved in the geologic rock record because they a

Question

Volcanoes are generally not preserved in the geologic rock record because 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 principle 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.

Response should be at least 200 words in length.

Explanation / Answer

Volcanoes generally erode away, and because of this they are not usually preserved in the geologic rock record. Field by lava flows, pyroclastic flows, and lahars causes great damage and generally destroys everything on the ground, thick falls of volcanic ash are a nearly perfect preservative. Layers of rock are formed when the cooling of lava happens. This leaves behind igneous rocks like basalt, andesite and rhyolite.

The form of a volcano is governed by the composition of erupting magma and type of erupted products (pyroclastic and autoclastic fragments or effusive lava). Their shapes are determined mainly by the explosivity of eruptions, and volume of water that interacts with magma.

Stratovolcanoes:-The large, violent volcanoes, called stratovolcanoes, always produce pyroclastic flows. Flows of water, 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:-These are huge, gently sloping volcanoes built of very thin lava spreading out in all directions from a central vent. These volcanoes, like those found in the Hawaiian islands, do produce lava and not much ash. These shield volcanoes produce layers of smooth basalt.

Cinder cone:-This type occur as single volcanoes or as secondary volcanoes on the sides of stratovolcanoes or shield volcanoes. Airborne fragments of lava, called tephra, are ejected from a single vent. 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 conclusion, stratovolcanoes produce pyroclastic flow deposits and ash deposits, shield volcanoes produce smooth basalt deposits, cinder cones produce blocky basalt deposits that tend to be interbedded with ash and cinder.

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