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VOLCANOES MICHELE HOLLER MAIDEN HIGH SCHOOL MOUNT SAINT HELENS • Erupted May 18, 1980 to create the largest volcanic eruption in North America • The blast blew the entire north flank of the volcano leaving a gaping hole • It ejected nearly a cubic kilometer of ash and rock debris FACTORS AFFECTING ERUPTIONS • Magma composition, magma temperature and the amount of dissolved gases determine whether an eruption is violent or quiet VISCOSITY AND MAGMA’S TEMPERATURE • A substance’s resistance to flow. (viscosity) • The cooler magma is, the more its viscosity increases. The warmer the easier it flows • Is directly related to the composition. The more silica that is found in the magma, the more viscosity that magma has DISSOLVED GASES • • • • • During explosive eruptions, the gases trapped in the magma provide the force to eject molten rock from the vent, which is an opening to the surface Most gases are water vapor and carbon dioxide The reduced pressure near the surface allows the dissolved gases to be released suddenly Basaltic magmas allow the gases to escape easily and have relatively quiet eruptions Highly viscous magmas, slow the upward movement and the gas bubbles increase in size until they eject molten rock from the volcano VOLCANIC MATERIAL Pahoehoe flow • • Evidence of a a flow LAVA FLOWS basaltic lavas are very fluid. When basaltic lava hardens, it forms a relatively smooth skin as the still-molten subsurface lava continues to move, this is called pahoehoe. It resembles twisted braids of rope. Basaltic lava can also have a a flow. It has a rough jagged surface with dangerously sharp spiny edges GASES • The gaseous portion is only about 1-6% of the total weight • Composition is important. They have contributed greatly to the gases that make up the atmosphere • Hawaiian eruptions have a gas makeup of 70% water vapor, 15% carbon dioxide, 5% sulfur, 5% nitrogen and lesser amount of chlorine, argon and hydrogen Water vapor Carbon dioxide PYROCLASTIC MATERIALS • Name given to particles produced by eruption. They range in size from very fine dust to pieces that weigh several tons • Lapilli- range from small beads to walnuts (2-64 mm). Also called cinders • Blocks- anything larger than 64 mm and are made of hardened lava and bombs (which are pieces of semi-molten lava hurled in the air. Form elongated rocks) ANATOMY OF A VOLCANO • Volcanic activity begins when there is a crack or fissure that develops in the crust • The gas rich magma moves up the fissure through a circular pipe that ends at the vent • Repeated eruptions eventually build a volcano • Located at the summit of the volcano is a steep walled depression called SHIELD VOLCANOES • Produced by the accumulation of fluid basaltic magma • Have the shape of a broad, slightly domed structure • Most have grown up from the ocean floor to form islands • Examples include the Hawaiian Islands and Iceland CINDER CONE VOLCANO • Formed from ejected lava fragments the size of cinders • Product of relatively gas-rich basaltic magma • Have steep sided slopes that are maintained by loose pyroclastic materials as they come to rest • After the eruption, which can last days or weeks, the pipe solidifies • Have a short life span, so they are typically small • An example is: Mount Etna COMPOSITE VOLCANO • • • • • • Most beautiful and potentially dangerous volcanoes Most located in the Pacific Ring of Fire which includes the Andes and the Cascades (Mount St Helens, Mount Rainier) Stretches from the Aleutian Islands to Japan Nearly symmetrical with layers of both lava and pyroclastic materials Magma has andesitic composition Have a high silica content and very explosive eruptions Composite volcano in New Zealand DANGERS FROM COMPOSITE VOLCANOES • • • • • Pyroclastic flows are the most dangerous. They consist of hot gases, glowing ash, and larger rock fragments Pyroclastic flows can race down at speeds of 200km per hour Can blast sides of the mountain out Can also generate mudflows that are called lahars. These occur when the volcanic debris becomes saturated with water and rapidly moves down slope lahars can be triggered by heavy rain OTHER VOLCANIC LANDFORMS CALDERAS • Large depression in a volcano • Form: 1. After a collapse of the top of a composite volcano after an explosive eruption 2. Top of a shield volcano collapses • Example is Crater Lake in Oregon OTHER VOLCANIC LANDFORMS CONTINUED NECKS AND PIPES • Most volcanoes are fed through conduits called pipes • Cinder cone volcanoes erode easily and leave behind the hardened pipe that is now called a volcanic neck • Best known pipes are the diamond bearing pipes in South Africa OTHER VOLCANIC LANDFORMS CONT. LAVA PLATEAUS • Greatest volume of volcanic material is extruded from fissures • These extrusions cover a large area and form plateaus • Example is the Columbia Plateau PLUTONS • The structures that result from the cooling and hardening of magma • They all form beneath the surface and can only be studied after uplift and erosion • Are generally classified according to their size, shape and relationship to the surrounding rock layers SILLS AND LACCOLITHS • Form when magma intrudes close to the surface SILL • Formed when magma is injected along sedimentary bedding surfaces, parallel to the bedding planes • Sills only form at shallow depths where pressure is low sill laccoliths • Occur close to the surface • Magma that generates laccoliths is more viscous • Collects as a lens shaped mass that pushes the overlying strata upward DIKES • Formed when magma is injected into fractures that cut across preexisting rock • Are sheet like structures that are a few meters thick and extend a few kilometers • Form when magma from a magma chamber invades the surrounding rock BATHOLITHS • Largest intrusive igneous body • Must have a surface exposure of greater than 110 square kilometers • Smaller plutons called stocks are parts of a batholith Yosemite National Park ORIGIN OF MAGMA • Originates when essentially solid rock in the crust and upper mantle partially melts. • Heat plays a major role • The change in temperature with depth is called geothermal gradient • Heat can be generated at subduction zones with the friction that is generated • Crustal rocks are heated during subduction ORIGIN OF MAGMA CONT. • Pressure also plays a role • Pressure also increases with depth • Melting occurs at higher temperatures because of the greater confining pressure • Reducing the confining pressure reduces the rock’s melting point. This is called decompression melting • Decompression melting has generated magma beneath Hawaii ORIGIN OF MAGMA CONT. • THE ROLE OF WATER • Water causes rocks to melt at lower temperatures • Wet rock has a much lower melting point than dry rock CONVERGENT PLATE BOUNDARIES • • • • • Plate tectonics and volcanism have one connection and that is plate motion At convergent boundaries, oceanic crust is pushed into the mantle The magma that is formed, slowly migrates upward forming volcanoes like Mount Saint Helens Ocean-ocean-have a chain of volcanoes on the floor called volcanic island arcs Ocean-continental-produces a continental volcanic arc. Magma changed composition due to the high silica content of the continental crust DIVERGENT PLATE BOUNDARIES • Most magma is produced along the oceanic ridges • As rock rises the confining pressure decreases and decompression melting occurs • The new magma is less dense and rises to the surface forming rift valleys INTRAPLATE IGNEOUS ACTIVITY • Intraplate volcanism occurs within a plate • Occurs where a mass of hotter than normal mantle material called a mantle plume, rises to the surface and creates hot spots • Causes decompression melting of basaltic magma • Mantle plumes also have created lava plateaus