Presentation on theme: "The Nature of Volcanic Eruptions"— Presentation transcript:
1The Nature of Volcanic Eruptions Earth Science10-2The Nature of Volcanic Eruptions
2Factors Affecting Eruptions The Primary factors that determine whether a volcano erupts explosively or quietly include characteristics of the magma and the amount of dissolved gases in the magma.Mount Saint Helens before and after eruption
3Viscosity Viscosity Is a substance’s resistance to flow Viscosity is strongly affected by temp.If lava cools and begins to harden, its Viscosity increases
4Continued… Chemical composition Chemical composition on magma has a more important effect on the type of eruptionBecause of high silica content, rhyolitic lavas are very viscous and erupt explosivelyLess silica has a calmer eruption
5Dissolved Gases Vent An opening to the surface During explosive eruptions, the gases trapped in magma provide the force to propel molten rock out of the vent.These gases are mostly water vapor and carbon dioxide
6Volcanic Material Lava Flows Silica content and the temperature affect the characteristics of lava flows.Temperature differences produce 2 types of basaltic lava: pahoehoe and aa2. PahoehoeIs hotter, fast moving basaltic lavaPahoehoe forms relatively smooth skin that wrinkles as the still-molten lava continues to flow3. AaIs cooler, slower-moving basaltic lavaAa forms a surface of rough, jagged blocks with sharp, spiny projections
75. Pyroclastic Materials 4. GasesMagmas contain varied amounts of dissolved gases held under pressure in the molten rockThe gaseous portion of the magma is a small portion about 1 to 6%5. Pyroclastic MaterialsParticles produced in volcanic eruptions are called pyroclastic materialsThe fragments ejected range in size from very fine dust and volcanic ash to pieces that weigh several tons
8Types of volcanoesAnatomy of a volcanoVolcanic activity usually begins when a fissure, or crack, develops in the crustVolcano- repeated eruptions of lava or pyroclastic material eventually build up a mountainCrater- steep-walled depressionThe three main volcanic types of volcanoes are shield volcanoes, cinder cones, and composite cones.
10Shield volcanoesAre produced by the accumulation of fluid basaltic lavas.Shield volcanoes have a shape of a broad, slightly domed structure that resembles a warrior’s shield.
112. Cinder conesEjected lava fragments that harden in the air build a cinder coneThe shape of a cinder cone is determined by the steep-sided slope that forms as loose pyroclastic material builds up around the vent
123. Composite conesIs a large, nearly symmetrical volcano mountain composed of layers of both lava and pyroclastic depositsMost composite cones are located in a relatively narrow zone that rims the Pacific Ocean, the Ring of Fire
13How many of these Volcanoes are located in the U.S.
14Other Volcanoc Landforms Volcanic landforms include calderas, volcanic necks, and lava plateausLong after eruptions have ended, these landforms can provide evidence of volcanic activity
15Calderas A caldera is a depression in a volcanic mountain. Most calderas form in two ways:By the collapse of the top of a composite volcano after an explosive eruptionFrom the collapse of the top of a shield volcano after the magma chamber drained
16Volcanic necksIs a landform made of magma that hardened in a volcanoes pipe and later was exposed by erosionWeathering and erosion work constantly to wear away the volcanoesWhen a volcanoes activity ends, magma remaining in the pipe hardens to form igneous rock
17Lave plateausIs a volcanic landform produced by repeated eruptions of very fluid, basaltic lavaInstead of building a cone the lava spreads out over a wide areaThe lava that forms the lava plateaus erupts through long cracks called fissures
18Volcanic HazardsVolcano hazards include lava flows, volcanic ash, pyroclasitc flows, and mudflows.LaharOccurs when water-soaked volcanic ash and rock slide rapidly downhillComposite volcanoes are among the most dangerous. They can cause a laharAn explosive eruption can also create a pyroclastic flowVolcanoes usually give some warnings that an eruption is nearUsually most people settle near a volcano because of the fertile land from the ash