Transcript volcano
Volcanoes around the World 10.1 The Nature of Volcanic Eruptions Factors Affecting Eruptions Factors that determine an eruption • Composition • Temperature • Dissolved gases Viscosity • Viscosity is the measure of a material's resistance to flow. Comparison of ash plumes Mt. St Helens was a relatively small eruption compared to prehistoric eruptions 10.1 The Nature of Volcanic Eruptions Factors Affecting Viscosity - Temperature (hotter magmas are less viscous) - Composition (silica content) 1. More silica—high viscosity, thicker 2. Low silica—less viscous, more fluid 10.1 The Nature of Volcanic Eruptions Factors Affecting Eruptions - Viscous (thicker) magma produces a more violent eruption. 10.1 The Nature of Volcanic Eruptions Dissolved gases • Mainly water vapor and carbon dioxide More trapped gas = Greater Eruption • A vent is an opening in the surface of Earth through which molten rock and gases are released. • Provide the force to extrude lava 10.1 The Nature of Volcanic Eruptions Volcanic Material Lava Flows • Basaltic lavas are more fluid. • Types of lava -Pahoehoe lava (resembles braids in ropes) -Aa lava (rough, jagged blocks) Mayon Volcano, Philippines 10.1 The Nature of Volcanic Eruptions Volcanic Material • Pyroclastic materials-fragments ejected during eruptions Includes dust, ash, and rock • Range in size from very fine dust and ash to pieces that weigh several tons. - Yes – write this down- names only!!! Ash and dust — fine, glassy fragments Pumice — frothy, air-filled lava Lapilli — walnut-sized particles Cinders — pea-sized particles Blocks — hardened lava (large) Bombs — ejected as hot lava (large) 10.1 The Nature of Volcanic Eruptions Types of Volcanoes Anatomy of a Volcano • A volcano is a mountain formed of lava and/or pyroclastic material. • A crater is the depression at the summit/top of a volcano • A conduit carries gas-rich magma to the surface. 10.1 The Nature of Volcanic Eruptions Types of Volcanoes – 3 main Types Shield Volcanoes • Broad, gently sloping volcanoes, usually the largest built from basaltic lavas, shaped like a shield • Magma - Very fluid & the lava can flow great distances – rich in Fe & Mg • Eruptions are usually mild & can occur several times • Ex. Mauna Loa, HI Cinder Cones Cinder Cones - Ex. Paricutin, Mexico • Small steep sloped volcanoes built primarily of pyroclastic material ejected from a single vent. -Magma: gas rich in basaltic lava, ejects lava fragments – mostly cinders - Eruption: typically a single eruption lasting a few weeks to a few years 10.1 The Nature of Volcanic Eruptions Composite Cones (a.k.a. Stratovolcano) • Steep sides w/ alternating layers of ash, cinder, and lava - Magma: Thick magma contains trapped gases, very viscous, silica rich - Eruptions alternate between flows and explosions (cinder & ash) & pyroclastics Most violent Ex. Mt. Shasta, CA Mt. St. Helens, WA Mount St. Helens Before and After the May 18, 1980, Eruption Size Comparisons of Volcanoes Different types of volcanoes 10.1 The Nature of Volcanic Eruptions Other Volcanic Landforms • Calderas are large depressions in volcanoes. • Nearly circular • Formed by collapse of the vent Lava Plateaus • Fluid basaltic lava extruded from crustal fractures called fissures. 10.2 Intrusive Igneous Activity Plutons Plutons are intrusive igneous structures that result from the cooling and hardening of magma beneath the surface of Earth. 10.2 Intrusive Igneous Activity Plutons • Sills and laccoliths are plutons that form when magma is intruded close to the surface. - Sills resemble buried lava - Laccoliths are lensflows and may exhibit columnar shaped masses that arch joints. Store magma. overlying strata upward. Transport magma. Sill 10.2 Intrusive Igneous Activity Plutons • Batholiths are large masses of igneous rock that formed when magma intruded at depth, became crystallized, and subsequently was exposed by erosion. Ex) Mount Rushmore • An intrusive igneous body must have a surface exposure greater than 100 square kilometers to be considered a batholith. 10.2 Intrusive Igneous Activity Origin of Magma Magma originates when rock located in the lithosphere partially melts. 10.2 Intrusive Igneous Activity • The geothermal gradient — Earth’s natural temperature is not sufficient to melt rock in lithosphere • Additional heat is generated by: - friction in subduction zones - rocks heated during subduction - rising, hot mantle rocks 10.3 Plate Tectonics and Igneous Activity Lava Where Do Volcanoes Occur? • Mostly occur at plate boundaries • Convergent plate boundaries: 75% of active volcanoes – Ex: Ring of Fire • Divergent plate boundaries: as plates move apart, magma rises to fill the space - Ex: Iceland, Mid-Atlantic Ridge 10.3 Plate Tectonics and Igneous Activity Convergent Plate Boundaries Ocean-Ocean • Forms volcanic island arcs in an ocean Ex. Aleutian Islands Ocean-Continent • Forms continental volcanic arcs Ex. Andes Mountains 10.3 Plate Tectonics and Igneous Activity Divergent Plate Boundaries The greatest volume of volcanic rock is produced along the oceanic ridge system. Ex. Mid-Atlantic ridge • Lithosphere pulls apart. • Partial melting occurs • Oceanic Ridges form 10.3 Plate Tectonics and Igneous Activity Intraplate Igneous Activity Intraplate volcanism occurs within a tectonic plate away from plate boundaries. • Occurs when hotter than normal mantle material called a mantle plume rises to the surface. • Forms localized volcanic regions called hot spots. • Ex. Hawaiian Islands (Kilauea) & Columbia Plateau. 10 Most Active Volcanoes