Igneous Rocks III
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A second type of volcano is called a stratovolcano. Stratovolcanoes are created by alternating layers of volcanic ash and other broken up bits of igneous rock covered by layers of solid igneous rock formed from lava flows. The lava from stratovolcanoes is thicker and cooler than the lava from shield volcanoes. But don't let that fool you; stratovolcanoes are far more dangerous and more common than shield volcanoes. The reason stratovolcanoes are so dangerous is: when they erupt, they erupt with a violent explosion that hurls thousands of tons of hot igneous rock, ash, and gas into the air. Sometimes these eruptions form columns of a hot gas and rock mixture. When the column cools enough that it can no longer support its own weight it falls back down, flowing down the sides of the volcano in a scorching hot flow called a pyroclastic flow. These flows are usually 1000ºC (1832ºF) and travel at about 700 kph (435mph). It was a pyroclastic flow that destroyed the ancient cities of Pompeii and Herculaneum in 79CE.
Mt. Vesuvius was the name of the volcano that destroyed Pompeii and Herculaneum.
Igneous rocks formed by stratovolcanoes have a lot more silica (mineral quartz) in them. An example of this kind of rock is called rhyolite. Rhyolite is almost the same rock as granite, the only difference is the magma that formed granite was able to cool down slow enough to form large crystals: because rhyolite was exposed to the air, it cooled faster so large mineral crystals were not able to form.
The third and last kind of volcano is called a cinder cone. (This is the kind of volcano you would make for your science fair.) Cinder cone volcanoes are essentially big piles of ash and rocks formed by splattering lava. Pumice and rhyolite are common rocks made by this type of volcano.
Igneous rocks are the most common of the rocks that make up the crust. About 95% of the rocks that make up the crust of the planet are igneous rocks. Igneous rocks are the beginning of the rock cycle. The rock cycle is how the crust of our planet is formed and recycled.
Did You Know?
The same reason a 2L bottle of shaken soda explodes when it is opened is the same reason a stratovolcano explodes. Dissolved gas, usually CO2, builds in pressure until the pressure is so great that the rock above the magma chamber can no longer hold. The rapid release of pressure pulls gas and liquid magma out of the magma chamber and onto the surface. Just like opening a bottle of shaken soda pulls most of the soda out of the bottle and on to the floor.