Volcanoes! Chapter 10. Origin of Magma Magma originates when solid rock, located in the crust and...
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![Page 1: Volcanoes! Chapter 10. Origin of Magma Magma originates when solid rock, located in the crust and upper mantle, melts. Factors that influence the.](https://reader030.fdocuments.us/reader030/viewer/2022013101/56649f425503460f94c627de/html5/thumbnails/1.jpg)
Volcanoes!
Chapter 10
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Origin of Magma Magma originates when solid rock,
located in the crust and upper mantle, melts.
Factors that influence the formation of magma include…1. Heat2. Pressure3. Water Content
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Origin of Magma Factors that influence the formation of
magma• Heat
• Additional heat is generated by • Friction in subduction zones• Crustal rocks heated during subduction • Rising, hot mantle rocks
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Origin of Magma Factors that influence the formation of
magma• Pressure
• Increase in confining pressure causes an increase in melting temperature
• Drop in confining pressure can cause decompression melting
• Lowers the melting temperature • Occurs when rock ascends
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Origin of Magma Factors that influence the formation of
magma• Water Content
• Cause rock to melt at a lower temperature • Plays an important role in subducting ocean
plates
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Volcanoes and Plate BoundariesGlobal distribution of igneous activity is
not random!• Most volcanoes form along divergent and
convergent plate boundaries.• Some volcanoes form far from plate
boundaries above “hot spots”.
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Locations of some of Earth’s major volcanoes
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Volcanoes and Plate Boundaries Divergent Boundary Volcanism
• The greatest volume of volcanic rock is produced along the oceanic ridge system
• Lithosphere pulls apart • Mantle rock rises to fill the gap between
the plates. • Decompression melting occurs forming
magma.
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Volcanoes and Plate Boundaries Convergent Boundary Volcanism
• Descending plate partially melts • Magma slowly rises upward • Rising magma can form…
• Volcanic island arcs in an ocean (Aleutian Islands)
• Continental volcanic arcs (Andes Mountains)
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Volcanoes and Plate Boundaries Intraplate Volcanism
• Activity within a rigid plate NOT near a plate boundary
• Plumes of hot mantle material rise• Form localized volcanic regions called hot
spots • Examples include…
• Hawaiian Islands (middle of the Pacific Plate)• Yellowstone National Park (Middle of the N.
American Plate)
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The Nature of Volcanic Eruptions
Factors Affecting Eruptions1. Viscosity of the magma
• Viscosity is a measure of a material’s resistance to flow
• Maple syrup vs. water
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The Nature of Volcanic Eruptions Factors Affecting Eruptions
Viscosity of magma• Temperature (hotter magma is less viscous) • Composition (silica content)
• High silica content – high viscosity (e.g., rhyolitic lava) this creates an explosive eruption.
• Low silica content – more fluidlike (e.g., basaltic lava) quiet eruption.
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The Nature of Volcanic Eruptions
Factors affecting Eruptions 2. Dissolved Gases
• Gases trapped in the magma • Violence of an eruption is related to how
easily gases escape from magma • Easy escape from fluid magma • Viscous magma produces an explosive
eruption
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Volcanic MaterialLava flows
• Basaltic lavas are more fluid (low silica content)
• Types of lava • Pahoehoe lava (resembles braids in ropes) • Aa lava (rough, jagged blocks)
Gases • One to five percent of magma by weight• Mainly water vapor and carbon dioxide
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A pahoehoe lava flow
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A typical aa flow
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Volcanic Material Pyroclastic materials “Fire fragments”
• Types of pyroclastic material• Ash and dust – fine, glassy fragments • Pumice – from “frothy” lava• Lapilli – “walnut” size • Cinders – “pea-sized” • Particles larger than lapilli
• Blocks – hardened lava • Bombs – ejected as hot lava
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Volcanic bombs on Kilauea volcano in Hawaii
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Volcano Formation 1. Begins when a fissure or crack
develops in the crust as magma is forced to the surface.
2. Repeated eruptions of lava or pyroclastic material eventually build a mountain called a volcano.
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Three Main Types of Volcanoes
1. Shield Volcanoes2. Cinder Cones3. Composite Cones
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Types of Volcanoes
Shield volcano• Broad, slightly domed (like a warriors shield)• Primarily made of basaltic (fluid) lava • Generally large in size • e.g., Mauna Loa and Mauna Kea in Hawaii
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Shield volcano
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Types of Volcanoes
Cinder cone • Built from ejected lava fragments that
harden in the air• Steep slope angle • Small in size
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A cinder cone near Flagstaff, Arizona
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Types of Volcanoes Composite cone (or stratovolcano)
• Most are adjacent to the Pacific Ocean along the ‘Ring of Fire’ (e.g., Mt. Rainier)
• Magma is viscous with a high silica content • Large size• Composed of layers of lavas and
pyroclastic material • Most dangerous and explosive volcano
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Composite volcano
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Mount St. Helens – a typical composite volcano
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Mount St. Helens following the 1980 eruption
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A size comparison of the three types of volcanoes
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Other Volcanic Landforms Calderas
• Steep-walled depression at the summit • Formed by collapse • Nearly circular • Size exceeds 1 kilometer in diameter
Lava plateaus • Fluid basaltic lava erupts from crustal
fractures called fissures • e.g., Columbia Plateau
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Crater Lake, Oregon, is a good example of a caldera
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The Columbia Plateau
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Other volcanic landforms
Volcanic pipes and necks • Volcanic necks (e.g., Ship Rock, New
Mexico) Landform made of magma that hardened in a volcano’s pipe and later exposed by erosion.
• Pipes are short conduits that connect a magma chamber to the surface
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Formation of a volcanic neck
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Volcanic HazardsTypes of volcanoes
• Composite cone (or stratovolcano)• Often produce
• Fiery pyroclastic flow made of hot gases infused with ash
• Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour
• May produce a lahar – volcanic mudflow
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Pyroclastic flow Mount St. Helens
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A lahar along the Toutle River near Mount St. Helens
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Intrusive igneous activity Most magma crystallizes within Earth’s crust An underground igneous body is called a
pluton *Pluto is Roman God of the underworld*
Types of Plutons• Sills - Magma flows between sedimentary layers• Laccoliths - ‘lens shaped’ Magma pushed the overlying
rock layers upward.• Dikes - Magma moves into fractures and cuts across
rock layers.
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Intrusive igneous structures exposed by erosion
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A sill in the Salt River Canyon, Arizona
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Intrusive igneous activity • Batholith
• Largest intrusive body• Much larger than a pluton• Surface exposure 100+ square
kilometers• Frequently form the cores of mountain
ranges
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A batholith exposed by erosion