Transcript Volcanoes
Volcanoes Earth’s Structure Where Volcanoes Occur • Volcanoes occur most frequently at plate boundaries. • Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called hot spots. • The greatest number of volcanoes occur on the ocean floor along spreading ridges spreading ridges. • Over 80% of those on land occur at edges of continents, or subduction zones, where one plate dives, or subducts, under another plate. Why Volcanoes Occur • Temperatures in the mantle are hot enough to melt rock into magma. • Less dense than the solid rock around it, magma rises and some of it collects in magma chambers. • As the magma rises, pressure decreases allowing trapped gases to expand and propel the magma through openings in the Earth’s surface causing an eruption. • •Erupted magma is called lava. How Volcanoes Erupt • Eruptions are described as explosive or effusive (loosely flowing). • How explosive an eruption is depends on the magma’s chemical composition and gas content, which affect the magma’s stickiness, or viscosity. • Magma’s low in silica are low in viscosity (mafic) • Magma’s high in silica are high in viscosity (felsic) • •If magma is fluid, gases can escape rapidly and lava flows; if magma is viscous the gases can not escape and pressure builds inside the magma until the gases escape, sometimes violently. Magma vs. Lava Magma vs. Lava • Magma is molten rock beneath the surface. • Lava is erupted magma. There are 2 types of lava: • A a (ah ah) is largely solidified rock that gets pushed forward. • – Pahoehoe (pahhoyhoy) is flowing “liquid” with a ropy, billowy surface. Types of Lava Flows • Lava flows are superheated streams of molten rock that flow at 1 –50 mph. • Pyroclastic flows are avalanches of hot ash, rock fragments, and gases that flow at speeds greater than 100 mph. • Landslides are avalanches of rock, snow and ice on slopes of volcanoes (loosened and tumbling due to seismic activity). • Lahars(mud flows) are a mixture of volcanic ash and water (like wet concrete) Volcano Terms • A vent is an opening through which eruptions take place. • A crater is a basin like depression over a vent, at the summit of a volcano • A caldera is a depression larger than the original crater (>1km. Diameter) that forms when the summit is blown off, or when the volcano collapses into the Types of Volcanoes • Repeated volcanic eruptions build volcanic mountains of three basic types three basic types, or shapes, depending on the composition of the materials deposited by the eruption. • Shield volcanoes • Stratovolcanoes • Cinder cones Shield Volcanoes • Shield volcanoes are broad gently sloping volcanic mountains slowly formed by layer over layer of solidified lava. • Shield volcanoes are formed by effusive eruptions of fluid lava. • These can become very large as the low viscosity lava spreads widely and thickly. • Examples:Kilauea, Hawaii and Mt. Etna, Italy Stratovolcanoes (Composite) • • Stratovolcanoes formed from both explosive and effusive eruptions. • •Layers of tephra alternating with layers of viscous lava flows create steep sided,often symetrical cones that can be very large. • •Formed over long spans of time as periods of 100,000+ yrs. separate periods of a few years of intense activity. • Examples: Aconcagua, Andes (22,825’) and Mt. St. Helens Cinder Cone Volcanoes • • Cinder cones are the smallest volcanoes (< 500’), formed by explosive eruptions of formed by explosive eruptions of lava. • Blown violently into the air, the erupting lava breaks apart into fragments called cinders that fall and accumulate around the vent. • • Cinder cones are temporary geologic features as they are easily eroded. They have short life spans as gas causing violent eruptions is quickly depleted. • Example: Paricutin, Mexico Monitoring and Predicting Eruptions • Volcanic activity is monitored using several observations: • Land deformation: Rock structures bend outward as gas build up. • Gas Emissions: Higher release of Carbon Dioxide and Sulfur Dioxide gases. • Tremors- As magma moves under the volcano and gas pressure builds, mild earthquakes will occur in the area. – Rock Fall: Due to movement of land, loose rocks will begin to slide.