Chapter 7: Volcanoes 8/18/2014. Section 1 (Volcanoes and Plate Tectonics) 8 th Grade. Ring of Fire

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1 Section 1 (Volcanoes and Plate Tectonics) Chapter 7: Volcanoes 8 th Grade Ring of Fire a major belt of es that rims the Pacific Ocean Volcanic belts form along the boundaries of Earth s plates as they diverge or converge, fracturing the crust and letting magma reach the surface. Volcanoes at Diverging Boundaries: As plates pull apart (at mid-ocean ridges or on land), magma comes out to form volcanic mountains. Volcanoes at Converging Boundaries: A denser plate (usually oceanic crust) is subducted under a less dense plate and melts into magma. Magma is less dense than the rock underground that surrounds it so the magma is pushed upward through cracks in the crust. If this process happens on the sea floor, an island arc forms. island arc a string of islands formed by the es along a deep-ocean trench islands of Japan, New Zealand, Philippines, Caribbean islands Continental trench Rift valley magma forms during subduction as crust is re-melted trench Island arc ocean Volcanoes at Hot Spots: Hot spot an area of the mantle that is hotter than usual and the magma melts through the crust above it. Dormant es no longer over hotspot Forming Years of magma melting up through the crust can form a chain of volcanic islands. Plate movement Earth s crust Hot spots may form in the middle of plates, away from any boundaries. Example: Hawaiian Islands mantle Hot spot melts through the crust hotspot as the plate moves over it. 1

2 Section 2 (Properties of Magma) Physical and Chemical Properties of Magma: element compound a substance in which all the atoms are the same that cannot be broken down into any other substance oxygen, hydrogen, carbon, etc. a substance made of two or more elements that have been chemically combined water (H 2 O), carbon dioxide (CO 2 ), table salt (NaCl) The elements and compounds in lava determine its chemical and physical properties. Physical property any characteristic of a substance that can be observed or measured without changing it to another substance. density, hardness, volume, color, texture, shape, mass, melting point, boiling point Chemical property any property of a substance that allows it to change into another substance flammability (ability to burn), light sensitivity, corrosiveness, its ability to react with other substances viscosity a liquid s resistance to flowing (a physical property) A liquid with high viscosity flows very slowly (honey) A liquid with low viscosity flows easily (water) Viscosity of magma depends on: 1. the amount of silica in it. Silica a material in magma that determines its viscosity and is formed from the elements oxygen and silicon. One of the most abundant materials in Earth s crust. The more silica in magma, the higher its viscosity (very thick, sticky, and flows slowly) The less silica, the runnier the magma. Silica-rich magma is light colored and hardens into the rock rhyolite when it cools. Silica-poor magma is dark colored and produces basalt when it cools. 2. the temperature of it. Cooler magma is thicker and flows more slowly (high viscosity) Very hot magma is runnier and flows easily (low viscosity) pahoehoe aa fast moving, hot lava that has a low viscosity and hardens to form smooth wrinkles, or rope-like coils. a cooler, slow-moving type of lava that hardens to form rough, jagged chunks Magma chamber Pipe Vent Section 3 (Volcanic Eruptions) Structure of a Volcano: the pocket beneath a where magma collects a long tube that connects the magma chamber to Earth s surface the opening through which molten rock and gas leaves a Usually one vent out the top, but there may be one or more vents out the sides of the. 2

3 Lava flow Crater the area covered by lava as it flows out of a vent. a bowl-shaped area that forms around a s central opening A Volcanic Eruption: Magma is less dense than the surrounding rock. This causes magma to be pushed upward. As magma moves upward, the pressure on it becomes less. As pressure on the magma decreases, trapped gases begin to expand and form bubbles. Gases expand faster than the magma can move upward, increasing the pressure below ground. If the vents are clogged, magma quits moving upward, but gases continue to expand. When pressure becomes too great, the vents are blown open and the erupts. The force of the expanding gases pushes the magma out the vents. Kinds of Volcanic Eruptions: Quiet eruptions occur with low-silica magma (has a low viscosity and flows easily) The magma oozes out gradually and gases in the magma bubble out gently. Explosive eruptions occur if magma is high in silica (very thick magma) Thick magma can plug the pipe and the gases build up pressure until they explode. break lava into fragments that cool quickly and harden into different sized pieces. ash Explosive eruption particles cinders pebble-sized particles bombs fine dust-like particles pyroclastic flow large particles that range in size from baseball size to the size of a car occurs when an explosive eruption throws out a mixture of ash, cinders, bombs, and gases. Active Dormant Extinct Stages of Volcanic Activity: a that is erupting or shows signs that it may erupt in the near future not currently active, but may become active in the distant future a that is no longer active and is very unlikely to erupt again. Section 4 (Volcanic Landforms) Volcanic Landforms 1. Landforms from Lava and Ash A. shield a wide, gently sloping mountain made of layers of lava and formed by repeated quiet eruptions. B. cinder cone a steep cone-shaped hill or small mountain made of ash, cinders, and bombs piled around a s opening (from explosive eruptions) 3

4 C. composite tall cone-shaped mountain in which layers of lava alternate with layers of ash, cinders, and bombs. (caused by quiet eruptions and explosive eruptions trading off.) D. lava plateau a raised, somewhat flat area made up of many layers of thin, runny lava that has seeped out cracks in the crust, building flat layer upon flat layer E. caldera a huge hole left when the roof of a magma chamber collapses Enormous eruptions may empty a magma chamber faster than it can fill again, causing the roof to fall in. F. soils from lava and ash over time, the hard surface of the lava breaks down to form some of the richest soil on Earth. 2. Landforms from Magma A. volcanic neck forms when magma in a s pipe does not come out and hardens, then the softer rock around the pipe wears away, leaving a tall rock tower B. dikes vertical slabs of hardened magma formed when magma forced itself across rock layers dike C. sills D. batholith horizontal slabs of hardened magma formed when magma squeezed between rock layers sill a mass of rock formed when a large body of magma cools inside the crust (can be hundreds of miles long) E. dome mountains forms when uplifting forces push a batholith towards the surface, bending the rock layers above it into a dome shape (This formed the Black Hills.) batholith 3. Features From Geothermal Activity (the heating of underground water by magma) A. hot springs form when groundwater is heated by nearby magma, rises to the surface and collects in pools B. geyser a fountain of water and steam that shoots from the ground caused by hot springs that do not have an easy access to the surface, so pressure builds up underground. 4

5 geothermal energy Water heated by magma can be used for energy. Icelanders pipe it into their homes for heat. Magma-heated water can be used to produce electricity in power plants. 5