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Just What You’ve Been Waiting For!!!!!!! Another Terhaar Power Point…… Things just don’t get any better than this…huh?. Volcano!!!!. Volcano…What is it?. eruptions of hot magma from inside the earth. The magma comes from magma chambers. How do they happen?.
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Just What You’ve Been Waiting For!!!!!!!Another Terhaar Power Point……Things just don’t get any better than this…huh?
Volcano…What is it? • eruptions of hot magma from inside the earth. The magma comes from magma chambers
How do they happen? • Deep within the Earth it is so hot that rocks slowly melt. This forms magma. It’s less dense than the surrounding rock so it begins to rise and collects in “magma chambers”. Eventually it pushes through vents and fissures and a volcano erupts.
A Few Facts • Magma in magma chambers is usually gas-rich. • Magma becomes lava when it reaches the earth’s surface. • Mafic flows: (basaltic) magmas are generated directly from the mantle, either within the asthenosphere or within the overlying mantle lithosphere
Magma: Where does it come from? • Magma originates from solid rock located in the crust and mantle. • Granite with silica melts at 750oC • Basaltic melts above 1000o C
Radioactive decay If oceanic crust (basalt) subducts and melts, comes into contact w/granite crust and melt it. Introduction of substances such as water can lower the Mp Zones of low pressure as descend can change the melting point Partial melting results in higher silica content than the parent rock because it melts at lower temps. What are the possible sources of heat? -
Where do volcanoes occur? • Hot spots
Where volcanoes occur, continued….. • Divergent plate boundaries
more places…… • Convergent plate boundaries
and……….. Unstable, mountainous belts
Fun Volcano Facts! -More than 500 volcanoes are known to have erupted on the earth’s surface since historic times many more have erupted on the ocean floor unobserved by humans. -Fifty volcanoes have erupted in the United States, which ranks third, behind Indonesia and Japan, in the number of historically active volcanoes. -Of the world’s active volcanoes, more than half are found around the perimeter of the Pacific (Ring of Fire) about a third on midoceanic islands and in an arc along the south of the Indonesian islands, and about a tenth in the Mediterranean area, Africa, and Asia Minor. Volcanoes Around The World
What comes out of volcanoes via eruptions? • Lava flows • Gases (primarily H2O vapors) • Pyroclastic materials: rock and lava fragments: ash; pumice; lapilli (little stones); cinders; bombs
Factors that determine the nature of volcanic eruptions • Magma’s Temperature • Magma’s composition • Magma’s amount of dissolved gases
Magma’s Temperature Viscosity, as it cools the viscosity increases and flows more slowly. Flow of lava depends upon:
Magma’s Composition Mafic flows(high in Mg and Fe) are less viscous and flows faster. (ex: pahoehoe and aa lava…pahoehoe is the fastest flowing) Felsic flows (rich in Feldspars and Silca) are more viscous. This can cause more explosive eruptions Flow of lava depends upon….continued
Aa and pahoehoe FlowsBoth Mafic • Aa lava: basaltic lava. Surface is jagged with sharp edges.Cool and thick. Flows 5 to 50 meters/h • Pahoehoe: basaltic. Faster flowing than aa. Relatively smooth.10-300meters/h
Amount of Dissolved Gas • In general, the more the dissolved gas, the more violent the eruption. They provide the force to propel the molten rock out of the volcano.
Volcanic Rocks Textures due to: • Rate of cooling Faster they cool, the smaller the crystals • Amounts of gas The more gas, more holes from gas escaping
More Fun Volcano Facts!!! • More than 80 percent of the earth's surface is volcanic in origin. • The sea floor and some mountains were formed by countless volcanic eruptions. • An erupting volcano can trigger: tsunamis, flashfloods, earthquakes, mudflows and rockfalls. • Gaseous emissions from volcanoes formed the earth's atmosphere.
Types of Volcanoes • Shield cones • Cinder cones • Composite (aka Strato)
Shield Cone Volcanoes • Gently sloping in the shape of a flattened dome. Built almost exclusively of mafic originated from the mantle (low viscosity basalt) flows (ex. Hawaiian Islands) • Large • Produce a large volume of lava • Usually doesn’t erupt violently
Cinder Cone Volcanoes • Built from ejected lava fragments out of a single vent • Basaltic flows • Can have pyroclastic activity • As the gas-filled lava is blown violently in to the air, it breaks into smaller pieces/fragments. They solidify and form a circular or oval cone
More cinder cone…….(just can’t get enough) • small • steep slope • occur in groups • ex: Paricutin, west of Mexico City • Flagstaff, Arizona
Composite (aka Strato) Volcano • Largest • Most violent • Steep sided, built of alternating layers of lava flows, volcanic ash; cinders; blocks and bombs. • Ex: Mt St. Helens erupted most recently in the Cascade Mtn (1980) Mt Vesuvius
Composite/Strato cont’d • Can get tall enough to collapse—form calderas • Andesite magma is common—more brittle than basaltic due to higher viscosity • Often produce nuee ardente (fiery cloud), hot gases infused w/ash
Volcano “Parts” The ones you need to know: (definitions coming up!) Craters Caldera Batholith Sill Dike Pluton
Parts defined….. • Craters: steep walled depressions at the summit of a volcano • Caldera: when a crater of a volcano gets very large due to the top of the volcano collapsing/exploding. Exceeds 1 km in diameter
Parts defined, cont’d • Batholith: a large mass of igneous rock formed when magma crystallizes. Can become exposed due to erosion. Covers an area of 100 km2 or more
Yes…more parts • Pluton: igneous rocks formed by solidifying magma inside the crust • Sill: a tabular igneous body (a pluton) that was intruded parallel to the layering of pre-existing rock, concordant • Dike: small pluton that are tabular in shape. It cuts through pre-existing rock, discordant
Create Landforms Iceland Hawaii
Montserrat • Stratovolcano • Began erupting in 1995; became severe in 1997 • 10 killed • Buried its capital city of Plymouth
