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Magma • The ash that some volcanoes can form billowy clouds that travel around the world before raining back down to Earth. • In the last 10 000 years, more than 1500 different volcanoes have erupted, providing evidence that Earth is indeed geologically active. http://science.howstuffworks.com/nature/natural-disasters/volcano3.htm Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases that fuels all volcanoes. Magma forms when temperatures are high enough to melt the rocks involved, usually between 800°C and 1200°C. Such temperatures exist at the base of the lithosphere and in the asthenosphere. How Magma Forms Pressure Pressure (increases with depth) is one factor that determines whether rocks will melt to form magma. As pressure increases, the temperature a substance melts also increases. Most of the rocks in Earth’s lower crust and upper mantle do not melt to form magma. Water – Water also influences whether a rock will melt. – A wet mineral or rock will melt at a lower temperature than the same under dry conditions. Types of Magma http://library.thinkquest.org/TQ0311 160/vollava.htm – Basaltic magma - same composition as basalt and fuels the volcanoes that make up the Hawaiian Islands and Surtsey (Iceland). – Andesitic magma - same composition as andesite and fuels Mount St. Helens in Washington State and Tambora in Indonesia. – Rhyolitic magma - same composition as granite and fueled the dormant volcanoes in Yellowstone National Park. Magma Composition – A number of factors determine the composition of magma. – Viscosity is the internal resistance to flow; the higher the viscosity, the thicker the magma. Basaltic Magma (magnetic) – Basaltic magma forms when rocks in the upper mantle melt. – Most basaltic magma rises rapidly to Earth’s surface and reacts very little because of its low viscosity. – The volcanoes erupt relatively quietly. Types of Magma Andesitic Magma – Andesitic magma is found along continental margins - oceanic subduction into Earth’s mantle, and is formed from oceanic crust or oceanic sediments. – Andesitic magma contains about 60 percent silica (intermediate viscosity). – The volcanoes - intermediate eruptions. Types of Magma Rhyolitic Magma – Rhyolitic magma - molten material rises and mixes with silica- and water-rich continental crust. – Rhyolitic magma - high viscosity and very explosive volcanoes. Types of Magma Viscosity The viscosity of magma and lava depends on both temperature and composition. – The hotter the magma or lava, the lower the viscosity. – Magmas - high in silica have higher viscosities . Magma 1. Match the magma types with their characteristics. A. intermediate viscosity B basaltic ___ A andesitic ___ ___ C rhyolitic content, forms from oceanic crust and oceanic sediments B. low viscosity and gas content, forms from rocks in the upper mantle C. high viscosity, forms from continental crust materials Section Assessment Magma 2. What would be the likely effect if the volcano at Yellowstone National Park were to erupt? Why? It would most likely be a devastating eruption because it would be fueled by rhyolitic magma, which has a very high viscosity and gas content. Section Assessment Magma Section Assessment 3. Identify whether the following statements are true or false. ______ true It is unlikely that Mount Kilauea in Hawaii will explosively erupt. ______ true Wet granite will melt at a lower temperature than dry granite. ______ false A liquid with a high viscosity will also have a high flow rate. ______ true Major eruptions of Mount St. Helens in Washington state and Mount Fuji in Japan would probably be similar in nature. Intruding magma can affect the crust: A. Magma can force the overlying rock apart and enter the newly formed fissures. B. Magma can also cause blocks of rock to break off and sink into the magma, where the rocks may eventually melt. C. Magma can melt the rock into which it intrudes. Intrusive Activity Plutons are (cooled) intrusive igneous rock bodies that can be exposed at Earth’s surface as a result of uplift and erosion and are classified based on their size, shape, and relationship to surrounding rocks. – Batholiths, the largest plutons, are irregularly shaped masses of coarse-grained igneous rocks and take millions of years to form. – Stocks are irregularly shaped plutons that are similar to batholiths but smaller in size. – Both cut across older rocks and generally form 10– 30 km beneath Earth’s surface. – A laccolith is a mushroom-shaped pluton with a round top and flat bottom resulting from a Magma intrusion into parallel rock layers close to Earth’s surface. – Compared to batholiths and stocks, laccoliths are relatively small; at most, up to 16 km wide. Plutons – A sill is a pluton, ranging from only a few centimeters to hundreds of meters in thickness, that forms when magma intrudes parallel to layers of rock. – A dike is a pluton, ranging from a few centimeters to several meters wide and up to tens of kilometers long, that cuts across preexisting rocks. – While the textures of sills and dikes vary, many are coarse grained, which indicates they formed deep in Earth’s crust and cooled slowly. Plutons Batholiths – form from continental-continental and oceanic-oceanic convergence. The plutons that form deep beneath Earth’s surface represent the majority of igneous activity on our planet. Plutons and Tectonics Plutons are formed from mountain-building. 1. Match the following terms with their definitions. A. a pluton that forms when D batholith ___ ___ B stock ___ E laccolith ___ A sill ___ C dike Section Assessment magma intrudes parallel to layers of rock B. an irregularly shaped pluton that is similar to a batholith but smaller in size C. a pluton that cuts across preexisting rocks D. an irregularly shaped pluton that covers at least 100 km2 E. a mushroom-shaped pluton with a round top and flat bottom 2. How do sills and laccoliths differ? Sills and laccoliths both result from magma intrusions that are parallel to existing rock. Laccoliths push the overlying layers upward, creating a distinct mushroom-shape when they cool and solidify. Sills are generally thinner and do not cause a noticeable bump in the surface. Section Assessment 3. What surface feature are batholiths most associated with? Batholiths are found at the cores of many of Earth’s mountain ranges. Section Assessment Anatomy of a Volcano A vent is where lava erupts through an opening in the crust. As lava flows out onto the surface, it cools and solidifies around the vent to form a mountain known as a volcano. A crater is a bowl-shaped depression, connected to the magma chamber by a vent, less than 1 km in diameter. Calderas are large depressions up to 50 km in diameter that can form when the summit or the side of a volcano collapses Types of Volcanoes Volcano depends on 2 factors: – The type of material that forms the volcano – The type of eruptions that occur 3 major types of volcanoes have been identified: Shield volcanoes Cinder-cone volcanoes Composite volcanoes Volcanoes Sizes and Shape – Shield volcanoes – largest but shield volcanoes have the gentlest slopes. – Cinder-cone – smallest, but steepest slopes – Cinder-cone and composite volcanoes are concave, and shield volcanoes are straight. Types of Volcanoes Volcanoes – A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base that forms when layer upon layer of basaltic lava accumulates during nonexplosive eruptions. Types of Volcanoes Volcanoes – A cinder-cone volcano - small, steep-sided forms when material ejected high into the air falls back to Earth and piles up around the vent. – The magma contains more water and silica than shield volcanoes, more explosive in nature. Types of Volcanoes Volcanoes – Composite volcanoes are large volcanoes that form with layers of volcanic fragments alternate with lava. – The magma contains large amounts of silica, water, and gases, making these volcanoes violently explosive. Types of Volcanoes Tephra rock fragments from a volcanic eruption. • Tephra are classified by size, the smallest being dust (less than 0.25 mm) and ash (0.25–2 mm). • Somewhat larger fragments are called lapilli, or “little stones” (2–64 mm in diameter). • The largest tephra thrown from a volcano include angular volcanic blocks and rounded or streamlined volcanic bombs, both of which can be the size of a house or larger. Volcanic Material http://videos.howstuffworks.com/discovery/ 7153-volcanoes-pyroclastic-flow-video.htm – Some tephra cause tremendous damage and kill thousands of people. – A pyroclastic flow is a cloud of volcanic gas, dust, and other tephra traveling at speeds of nearly 200 km/h. – The temperature at the center of a pyroclastic flow can exceed 700°C. Volcanic Material Where do volcanoes occur? – About 80 % convergent boundaries. – About 15 % divergent boundaries. – Only about 5 % of extrusive igneous activity occurs far from plate boundaries. Volcanoes Where do volcanoes occur? Convergent Volcanism – Convergence involving oceanic plates creates subduction zones, and the magma generated is forced upward through the overlying plate and forms volcanoes when it reaches the surface. – The volcanoes associated with convergent plate boundaries form two major belts: • The larger belt, the Circum-Pacific Belt, is also called the Pacific Ring of Fire. • The smaller belt is called the Mediterranean Belt. Volcanoes Where do volcanoes occur? Volcanoes Divergent Volcanism – At divergent plate boundaries, magma is forced upward into the fractures and faults that form as the plates separate. – These areas of major faults and fractures are called rift zones. – Most of the world’s rift volcanism occurs under water along ocean ridges. Where do volcanoes occur? Volcanoes Where do volcanoes occur? Hot Spots – Some volcanoes are located far from plate boundaries and form as the result of hot spots. – Hot spots are unusually hot regions of Earth’s mantle where high-temperature plumes of mantle material rise toward the surface. – A plume does not move laterally,(sideways) resulting in a trail of older volcanoes that formed as a plate moved over a hot spot. Volcanoes Hot Spots – The Hawaiian Islands continue to rise above the ocean floor as the Pacific Plate moves slowly over a hot spot. – The rate and direction of plate motion can be calculated from the positions of volcanoes in a chain that has formed over a hot spot. Where do volcanoes occur? Volcanoes Hot Spots – Hot spots can result in the formation of flood basalts. – Flood basalts erupt from fissures (cracks) rather than a central vent and form flat plains or plateaus. – Volcanic activity is proof that Earth is a dynamic planet. Where do volcanoes occur? Volcanoes 1. Match the following terms with their definitions. A. an opening in the crust from A ___ vent C ___ crater D tephra ___ ___ E pyroclastic flow ___ B caldera which lava flows B. depression caused by a collapsed magma chamber C. a bowl-shaped depression around an opening in the crust D. volcanic materials that are thrown into the air during a volcanic eruption E. a cloud of rapidly moving, extremely hot volcanic material Section Assessment Volcanoes 2. How can chains of volcanoes that form over a hot spot track plate movement? The hot spot is in a fixed location. All of the volcanoes in the chain were over the hot spot when they formed. The volcanoes’ movement and the direction of the chain’s alignment indicates the movement of the plate. Section Assessment Volcanoes 3. Identify whether the following statements are true or false. true ______ Many cinder-cone volcanoes are less than 500 m high. false ______ Volcanism is more common along divergent boundaries as compared to convergent boundaries. false ______ The Mediterranean Belt is also known as the “Ring of Fire”. true ______ It can be 700ºC in the center of a pyroclastic flow. Section Assessment