VOLCANOES

Volcanoes around the World

10.1 The Nature of Volcanic Eruptions Factors Affecting Eruptions 10.1 The Nature of Volcanic Eruptions  Factors that determine an eruption • Composition Details on next slide • 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) More silica—high viscosity, thicker 2. Low silica—less viscous, more fluid

10.1 The Nature of Volcanic Eruptions Factors Affecting Eruptions 10.1 The Nature of Volcanic 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 10.1 The Nature of Volcanic Eruptions 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 10.1 The Nature of Volcanic Eruptions • 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 10.1 The Nature of Volcanic Eruptions 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 10.1 The Nature of Volcanic Eruptions 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 10.1 The Nature of Volcanic Eruptions • 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 10.2 Intrusive Igneous Activity 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 10.2 Intrusive Igneous Activity • 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. Store magma. - Laccoliths are lens-shaped masses that arch overlying strata upward. Transport magma. Sill

10.2 Intrusive Igneous Activity Plutons 10.2 Intrusive Igneous Activity • 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 10.2 Intrusive Igneous Activity 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 10.3 Plate Tectonics and Igneous Activity 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 10.3 Plate Tectonics and Igneous Activity 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 10.3 Plate Tectonics and 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