Volcanoes : Volcanoes Volcanoes : Volcanoes About 500 active volcanoes are known on Earth, not counting those that lie beneath the sea. Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire". Volcanic eruptions occur only in certain places and do not occur randomly. That's because the Earth's outermost shell -- the lithosphere -- is broken into a series of slabs known as lithospheric or tectonic plates. There are 16 major plates. : Volcanic eruptions occur only in certain places and do not occur randomly. That's because the Earth's outermost shell -- the lithosphere -- is broken into a series of slabs known as lithospheric or tectonic plates. There are 16 major plates. Slide 4: Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 feet above sea level, its top being over 28,000 feet above the deep ocean floor. From its base below sea level to its summit, Mauna Loa is taller than Mount Everest How are they formed? : How are they formed? Volcanoes are generally found where tectonic plates are pulled apart or come together. Because of the faults and tension in Earth’s crust magma is able to pressure it’s way out to the surface forming a volcano.
The structure and behavior of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has escaped to the surface, and ash) and gases (mainly steam and gases) can be located anywhere on the landform. Slide 7: Shield volcano Stratovolcano
Lake Toba caldera Volcanic fissure Slide 8: 1. Magma chamber2. Bedrock3. Conduit (pipe)4. Base5. Sill6. Branch pipe7. Layers of ash emitted by the volcano8. Flank9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15. Ash cloud Typical stratovolcano Slide 9: By way of its color, incandescent rock called lava gives a crude estimate of temperature. For example, orange-to-yellow colors are emitted when rocks (or melt) are hotter than about 900 degrees Celsius (1,650 degrees Fahrenheit). Dark-to-bright cherry red is characteristic as material cools to 630 degrees Celsius (1,165 degrees Fahrenheit). Faint red glow persists down to about 480 degrees Celsius (895 degrees Fahrenheit). For comparison, a pizza oven is operated at temperatures ranging from 260 to 315 degrees Celsius (500 to 600 degrees Fahrenheit). Slide 11: Pyroclastics, magma also contains steam and other gases under pressure, with variation again a function of its mineral content. The acidic, silica-rich magmas tend to contain more gases, and when they erupt as lavas, these gases often escape explosively. Slide 12: Pyroclastic flows destroy everything on their way with rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than 80 km per hour. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C, can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses. Slide 13: Lahar is an Indonesian word describing mudflows and debris flows that originate from the slopes of a volcano and usually follow the river beds, Lahars have the consistency of concrete: fluid when moving, then solid when stopped . Tsunami : Tsunami Sometimes underwater volcano eruptions cause giant waves called Tsunami that destroy much of the nearby coastlines and cause considerable damage.
Listen to what Tsunami sounds like