How volcanoes are formed

The top part of Earth's mantle (called the asthenosphere) contains concentrations of heavy, radioactive elements such as uranium and thorium. These isotopes decay over geologic time into lighter, more stable elements. With this transformation, a large amount of energy is released to the surroundings, raising the temperature. For billions of years, this heat energy has been gradually moving from this zone to the cooler crust. The general transportation mechanism of this heat includes super-heated water, gas venting, and liquid rock and ash extrusion through the surface, termed volcanism. The openings in the Earth's crust where this occurs are called volcanoes, and their characteristics vary, depending on size, type, location, and activity. Active volcanoes today exist in the cold Antarctic, under the surface of the Pacific Ocean, on tropical islands, and in desert wastelands. Some emit steam and searing-hot noxious gases, some eject choking and blinding, sunlight-blocking ash, while others expel liquid rock called lava. Many exhibit a combination of these. Composite volcanoes are notorious for lying dormant for hundreds of years and then exploding suddenly and violently, only to fall silent again for generations. Cinder cones (which typically range only a few hundred meters in diameter and height) occasionally belch forth boulders or dust called tephra. Meanwhile, the huge, gently-sloping shield volcanoes (which can be thousands of meters high and a hundred kilometers wide) steadily exude free-flowing lava. Volcanoes are typically cone-shaped, but can be irregular depending on the composition and kind of lava beneath them. Shown here is the Mount Pinatubo, Philippines volcanic eruption of 1991, which sent a towering ash cloud toward the stratosphere.


Credit: Photo by United States Geological Survey

For more info: http://vulcan.wr.usgs.gov/Volcanoes/Philippines/Pinatubo/images.html