Valcan-O's

Volcanoes Zones of volcanism Volcanism deescribes all the processes associated with the discharge of magma hot fluids & gasses. Most volcanoes form at plate boundaries. the majority form at convergent boundaries and divergent boundaries. Convergent volcanism in an oceanic continental subduction zone, the denser oceanic plate slides under the continental plate into the hot mantle. parts of the plate melt & magma rises eventually leading to the formation of a volcano most volcanoes located on land result from oceanic continental subduction. these volcanoes are characterized by explosive eruptions. 2 major belts the volcanoes associated with convergent plate boundaries form 2 major belts. the larger belt, the Circum-Pacific belt, is also called the pacific Ring of Fire. the outline of the belt corresponds to the outline of the Pacific Plate. The smaller belt is the Mediterranean Belt. its general outlines correspond to the boundaries between the Eurasian, African, and Arabian plates. Divergent Volcanism eruptions at divergent boundaries tend to be non-explosive. at the divergent boundary on the ocean floor, eruptions often form huge piles of lava called pillow lava. Hot spots Some volcanoes from far from plate boundaries over hot spots. A "Hot Spot" is an unusually hot area in earths mantle where high temperature plumes of mantle material rise toward the surface. The Hawaiian islands are located over a plume of magma. the hot spot formed by the magma plume remained stationary while the Pacific Plate slowly moved Northwest. the volcanoes on the oldest Hawaiian island, Kauai are inactive because the island no longer sits above the stationary hot spot. the worlds most active volcano, Kilauea on the big island of Hawaii is currently located over the hot spot. Chains of volcanoes that form over stationary hot spots provide information about plate motions. the rate & direction of plate motion can be calculated from the positions of these volcanoes. the Hawaiian islands are at one end of the Hawaiian emporer volcanic chain the oldest seamount, Meiji, is at the other end of the chain and is about 80 million years old. Flood basalts from when lava flows out of long cracks in earth's crust these cracks are called fissures The Columbia river basalts, located in the northwestern United States were formed this way About 65 ya in India, a huge flood basalt eruption created an enormous plateau called the Deccan Traps. The Volume of basalt in Deccan Traps is estimated to be about 512,000 Km3 Anatomy of a volcano Lava reaches the surface by traveling through a tubelike structure called a Conduit. the lava then emerges through an opening called a vent. over time layers of solidified lava can accumulate to form a mountain known as a volcano. at the top of a volcano, around the vent, is a bowl shaped depression called a crater. find out more of the unique carbon-based lava that erupts from this volcano. Volcanic craters are usually less than 1 KM in diameter. Larger depressions called calderas can be up to 50 KM in diameter. the appearance of a volcano depends on 2 factors: the type of material that forms the volcano and the type of eruptions that occur. Shield Volcanoes A shield volcano is a mountain with broad gently sloping sides and a nearly circular base. shield volcanoes form when layers of lava accumulate during non-explosive eruptions. they are the largest type of volcano. Cinder cones When eruptions eject small pieces of magma into the air Cinder Cones form as this material called tephra falls back to earth and piles up around the vent. cinder cones have steep sides and are the smallest type of volcano. Composite Volcanoes are formed of layers of hardened chunks of lava from violent eruptions alternating with layers of lava that oozed down slope before solidifying. these volcanoes are generally cone-shaped with concave slopes. Making Magma The activity of a volcano depends on the composition of the magma lava from an eruption can be thin and runny or thick and lumpy. Temperature Depending on their composition, most rocks begin to melt at temperatures between 800C and 1200C in addition to temperature, pressure and the presence of water also affect the formation of magma. Pressure Pressure increases with depth because of the weight of overlaying rocks. as pressure increases, the temperature at which a substance melts also increases, which explains why most the rocks in Earth's lower crust and upper mantle do not melt. Composition of Magma The composition of magma determines a volcano's explosively, which is how it erupts and how its lava flows. Understanding the factors that determine the ehavior of magma can aid scientists in predicting the explosively of volcanic eruptions. Dissolved gases in general, as the amount f gases in magma increases, the magma's explosivity also increases Important gases in magma include water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide. Rhyolitic magma when molten material rises and mixes with the overlaying continental crust rich in silica and water, it forms Rhyolitic magma. Rhyoltic magma contains more than 60 percent silica. High viscosity, along with the large volume of gas trapped within rhyolitic magma, makes the volcanoes fueled by this magma very explosive. Visualizing Eruptions as magma rises due to plate tectonics and hot spots, it mixes with Earth's crust. this mixing causes deferences in the temperature, silica content, and gas content of magma as it reaches Earth's surface. These properties of magma determine how volcanoes erupt. Explosive Eruptions When lava is too viscous to flow freely from the vent, pressure builds up in the lava and rock into the air. the erupted materials are called tephra. Tephra are classified by size. the smallest fragments, with diameters less than 2mm, are called ash. the Largest tephra thrown from volcano are called blocks. Large explosive eruptions can disperse tephra over much of the planet In 1991, the eruption of Mount Pinatubo in the Phillippines sent so much ash into the stratosphere that it lowered global tmeperatures for two years. Pyroclastic Flows Violent volcanic eruptions can send clouds of ash and other tepra down a slpoe at speeds of nearly 200 km/h. Rapidly moving clouds of tephra mixed with hot, suffocating gases are called Pyrclastic flows in 1902, a pyroclastic flow from Mount Pelee on the island of Martinique in the Caribbean Sea was so powerful that it destroyed the entire ton of St. Pierre in only a few minutes. Plutons Most of Earth's Volcanism hapens bewlow the suface because not all magma emerges at the surface. Before it gets to the surface, rising magma can interact with the crust in several ways. Magma can force the overlaying rock apart and enter the newly formed fisures. Magma can cause blocks of rock to break off and sink into magma. it can melt its way through the rock into which it inntrudes. Plutons are intrusive ingneous rock odies, formed through mountain-building processes and oceanic-oceanic collisions. They can be Exposes at Earth's surface due to uplift and erosion and are classified based on their size, shape, and relationship to surrounding rocks. Batholths and stocks Batholiths, the largest plutons, are irregularly shaped masses of coarse-gained igneous rocks that cover at least 100km2 and take millions of years to from Batholiths are common in the interior of mountains Sills a sill forms when magma inturdes parallel to layers of rock. Because it takes great amounts of force to lift entire layers of rock, most sills form relatively close to surface Dikes it is a pluton that cuts across preexisting rocks and often forms when magma invades cracks in surrounding rock bodies. A volcanic neck occurs when the magma in a volcano conduit solidifies. Dikes are often associated with the conduit but do not always form the neck. The coarse-grained texture of most sills and dikes suggests that they formed deep in the earth’s crust, where magma cooled slowly enough for large mineral grains to develop. Dikes and sills with a fine-grained texture formed closer to the surface where many crystals began growing at the same time. Plutons and tectonics many plutons form as the result of mountain-building processes. In fact, batholiths are found at the cores of many of earth’s mountain ranges. Scientists think that some of the collisions along continental-continental convergent plate boundaries might have forced continental crust down into the upper mantle where it melted, intruded into the overlying rocks, and eventually cooled to form batholiths. Plutons are also thought to form as a result of oceanic plate convergence. When an oceanic plate converges with another plate, water from the sub ducted plate causes the overlying mantle to melt. Plutons often form when the melted material rises but down not erupt at the surface.