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STARTER 12-1-14 1. Describe how Hawaii was formed by Hot Spots. 2. Describe how Oceanic-Oceanic convergent boundaries create volcanism. 3. Describe how a Continental Volcanic Arc is formed. 10 The Nature of Volcanoes Objectives P. 78 Today you will be able to describe: • • • • What determines the type of volcanic eruption? What materials are ejected from volcanoes? What are the three main types of volcanoes? What other landforms are associated with volcanic eruptions? 10 The Nature of Volcanoes • Volcanic eruptions are windows to Earth’s interior. −Provide opportunities to observe processes deep beneath Earth’s surface •Why are some volcanic eruptions explosive, while others are quiet? 10 The Nature of Volcanoes Factors that determine the violence of an eruption • Composition of the magma • Temperature of the magma • Dissolved gases in the magma List-Group-Label: Volcanoes 10 The Nature of Volcanoes Viscosity • Viscosity is a material's resistance to flow. • Ie. Syrup, water, oil • Factors affecting viscosity - Temperature (hotter magmas are less viscous) - Composition (silica content) 1. High silica—high viscosity (e.g., flows slowly) 2. Low silica—low viscosity(e.g., flows quick) 10 The Nature of Volcanoes Dissolved gases • Mainly water vapor and carbon dioxide • Gases expand near the surface: • trapped in magma and provide the force to extrude molten rock at surface • A vent is an opening in the surface through which molten rock and gases are released. 10 The Nature of Volcanoes Dissolved gases • Violence of an eruption is related to how easily gases escape from magma -Gases escape easily from fluid magma. - Quiet eruption -Viscous (thick) magma slows the upward movement - more violent eruption. 10 The Nature of Volcanoes Lava Flows • Varying flow rates • Flow rate – speed at which it moves •Dependent upon composition of material 10 The Nature of Volcanoes Gases •Volcanic gases contribute greatly to our atmosphere •Volcanic eruptions are: •70% water vapor •15% CO2 •5% N •5% S •Trace amounts of Cl, H, Ar (Ropy) Lava Flow Slow-Moving Aa Flow (jagged edges) 10 The Nature of Volcanoes Pyroclastic Materials • Pyroclastic materials name given to particles produced in volcanic eruptions. • The fragments ejected during eruptions range in size from very fine dust and volcanic ash to very large. 10 The Nature of Volcanoes Pyroclastic Materials • Types of pyroclastic material •Small Particles - Ash and dust—fine, glassy fragments - Cinders—pea-sized particles • Larger Particles - Blocks—hardened lava - Bombs—ejected as hot lava 10 Types of Volcanoes Objectives P. 80 Today you will be able to describe: • What are the three main types of volcanoes? • What other landforms are associated with volcanic eruptions? Starter 12/2/14 1. Volcanic eruptions can lead to changes in the local climate. • Describe how escaping gases can cause a change in climate. • How can it cause air pollution. 2. When viewing magma/lava it gives us an observation as to what is taking place in the mantle. • Give your description how this is possible. 10 Types of Volcanoes Types of Volcanoes The three main volcanic types have their own volcanic history. •More than 26 different types of volcanoes. 10 Types of Volcanoes Anatomy of a Volcano • Begin as a fissure or crack in the crust • Magma forces it way to surface under pressure • A volcano is a mountain formed of lava and/or pyroclastic material. • A crater is the depression at the summit of a volcano • A conduit, or pipe, carries gas-rich magma to the surface. Anatomy of a “Typical” Volcano 10 Types of Volcanoes Shield Volcanoes • Shield volcanoes are broad, gently sloping volcanoes built from fluid lavas. • Grow from the ocean floor to form islands Shield Volcanoes 10 Types of Volcanoes Cinder Cones • Cinder cones are small volcanoes built primarily of pyroclastic material ejected from a single vent. • May last weeks to years • Once stops erupting never erupts again - Steep slope angle - Rather small in size - Frequently occur in groups Cinder Cones Composite Cones 10 Types of Volcanoes Composite Cones • Composite cones(stratovolcanoes) composed of both lava flows and pyroclastic material. - Most are adjacent to the Pacific Ocean - Most active region is Northern and Western Pacific - Large size - Most violent type of activity Mount St. Helens Before and After the May 18, 1980, Eruption 10 Types of Volcanoes Calderas • Calderas are large depressions in volcanoes. • Nearly circular • Formed by collapse 10 Types of Volcanoes Necks and Pipes •Volcanoes are fed through a conduit called a pipe. •Pipe is connected to the magma chamber •Cinder cone volcanoes are easily eroded •Sometimes leaving crystallized magma behind known as a neck 10 Types of Volcanoes Lava Plateaus • low viscosity lava flows covering a wide area. 10 Types of Volcanoes Plutons are intrusive igneous structures that result from the cooling and hardening of magma. • are generally classified according to their shape, size, and relationship to the surrounding rock layers. 10 Types of Volcanoes Role of Heat • The geothermal gradient—Earth’s natural temperature increases with depth but is not sufficient to melt rock in the lower crust and upper mantle • Additional heat is generated by - friction in subduction zones - crustal rocks heated during subduction - rising, hot mantle rocks • http://www.tulane.edu/~sanelson/Natural_ Disasters/volclandforms.htm 10.2 Intrusive Igneous Activity Sills and Laccoliths • Sills and laccoliths are plutons that form when magma is intruded close to the surface. - Sills resemble buried lava flows and may exhibit columnar joints. - Laccoliths are lens-shaped masses that arch overlying strata upward. Sills Sill 10.2 Intrusive Igneous Activity Dikes • Dikes are tabular-shaped intrusive igneous features that cut across preexisting rock layers. • Many dikes form when magma from a large magma chamber invades fractures in the surrounding rocks. 10.2 Intrusive Igneous Activity Batholiths • Batholiths are large masses of igneous rock that formed when magma intruded at depth, became crystallized, and subsequently was exposed by erosion. • An intrusive igneous body must have a surface exposure greater than 100 square kilometers to be considered a batholith. Batholiths Types of Igneous Plutons 10.2 Intrusive Igneous Activity Geologists conclude that magma originates when essentially solid rock, located in the crust and upper mantle, partially melts. The most obvious way to generate magma from solid rock is to raise the temperature above the level at which the rock begins to melt. 10.2 Intrusive Igneous Activity Role of Water • Causes rock to melt at a lower temperature • Plays an important role in subducting ocean plates Basaltic Magma at the Surface STARTER 03/24/11 • Answer the reading checkpoint questions on page 290 and 291. 10.2 Review 1. Is the following sentence true or false? Plutons can be studied on Earth’s surface as they form. 2. What three characteristics are used to classify intrusive igneous bodies? 3. when magma from a large magma chamber invades fractures in the surrounding rocks 4. when a large intrusive igneous body of greater than 100 km2 accumulates and becomes exposed 5. when magma is injected between sedimentary layers close to Earth’s surface and collects as a lens-shaped mass 6. when magma is injected along sedimentary bedding surfaces close to Earth’s surface a. Sill b. Laccolith c. Batholith d. dike 7. Is the following sentence true or false? Magma forms when solid rock in the crust and upper mantle partially melts. 8. Circle the letter of one way magma is generated. a. The confining pressure of rocks is increased. b. The water content of rocks is reduced. c. The temperature of rocks is lowered below their melting points. d. The temperature of rocks is raised above their melting points. 9. The rate at which temperature changes with depth below Earth’s surface is called the _______________. 10. How is decompression melting of rocks triggered? 11. ______________rock buried at depth has a much lower melting temperature than does ______________ rock of the same composition and under the same pressure. 10.3 Plate Tectonics and Igneous Activity The basic connection between plate tectonics and volcanism is that plate motions provide the mechanisms by which mantle rocks melt to generate magma. Ocean-Ocean • Rising magma can form volcanic island arcs in an ocean (Aleutian Islands). Ocean-Continent • Rising magma can form continental volcanic arcs (Andes Mountains). Convergent Boundary Volcano 10.3 Plate Tectonics and Igneous Activity The greatest volume of volcanic rock is produced along the oceanic ridge system. • • • • Lithosphere pulls apart. Less pressure on underlying rocks Partial melting occurs Large quantities of fluid basaltic magma are produced. 10.3 Plate Tectonics and Igneous Activity Intraplate volcanism is igneous activity that occurs within a tectonic plate away from plate boundaries. • Most intraplate volcanism occurs where a mass of hotter than normal mantle material called a mantle plume rises toward the surface. • The activity forms localized volcanic regions called hot spots. • Examples include the Hawaiian Islands and the Columbia Plateau. Kilauea, an Intraplate Volcano STARTER • Complete Map Master Skills Activiy on page 296