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Lecture

Chapter 7-8.docx

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Department
Geophysics
Course
GOPH 375
Professor
All Professors
Semester
Winter

Description
Chapter 7 Volcanoes  Volcanoes are influenced by plate tectonics, and magma viscosity, chemical composition, gas content, and volume  Rifting process releases magma to create new oceanic crust  Volcanoes mostly exist at: o (oceanic) spreading centers (release more magma than subduction zones; but are more subtle/peaceful. Ideal for volcanoes because they sit above the hot athenosphere that has low SiO2 %, and ocean plates pull apart causing hot rock t rise with lower pressure& convert to magma (basalt magma - peaceful) o Subduction zones: (less magma than spreading centers): Down going plate caries water saturated sediment into rising magma which promotes melting by lowering temperature. These volcanoes are more explosive/dangerous (rhyolite magma). Mantle convention brings hot rock near the planetary surface  Convection  Pressure relief melting; influenced by pressure – when you get 10-15% melting by volume, that`s enough melting to get it to move up to the surface because it`s less dense (buoyancy results in upward moving)  1,100 degrees C basaltic melt  Crust density = 2.67 g/cm^3  Basalt = 3 g/cm^3 – melt is 9% less approx; amounts of volatiles affect density o Hot spots: shallow magmas/plumes of slowly rising mantle rock that creates volcanoes on earth  Transform faults & continent-continent collisions have no volcano eruption, subduction zones have plenty  Figure 7.6 shows the different boundaries, faults and subduction zones  Africa is stationary relative to the drifting mantle underneath – America doesn`t have  Subduction zones: cool rock going downwards at T approx 400 degrees C – sediments and hydrates in minerals release water + carbon dioxide. Acts as a flux (something that will alter the melting point of a substance)  Figure 7.7: volatile rich magma ascending through the crust, nothing to do with friction – water is the flux  Eruption styles (3 v’s) o Viscosity o Volatiles: water, carbon dioxide, sulphur dioxide (SO2), hydrochloric acid (HCl), hydrofluoric acid (HF) are volatiles that come out of magma o Volume  Different compositions of magma: silicon and oxygen bond as a basic structure, and then different positively charged atoms bond onto that compound in the crust. oceanic crusts differ from continental crusts o Basalt – associated with hot spots and spreading centers (45-50%)  viscosity is lowest (10^3 poise) (mantle is 10^22)  Volatile content is less in basalt than in the others (.1-1%) least explosive  Crystallizing first in cooling magma – highest temperature/lowest SiO2 content  80% of magma reaching earth’s surface is basaltic because it flows fastest. During process of rising, magma becomes more andesite and rhyloite  Contains minimal dissolved water making it safer when erupting (dissolved water forms gas bubbles that burst [explode] to escape-rhyolite; unsafe) o Andesite - associated with subduction zones (55-65%)  Volatile 2-3%. Crystallizes Middle o Rhyolite – associated with subduction zones (65-75%)  Viscosity is highest (10^11 poise) Feels like a solid – last to crystallize  Volatile 4-6% most explosive – explodes when it gets to the surface of the earth  Lowest temperature/highest Sio2 content  Contain lots of dissolved water (most dangerous)  Plutonic rock: magma cools and solidifies beneath the surface  Volcanic rock: magma reaches the surface  Magma is more viscous at: o Higher temperatures o More silicon-oxygen bonds o Increasing mineral crystal content  Magma contains dissolved gasses volatiles – solubility increases as pressure increases or temperature decreases (bottle of pop)  Rocks melt by lowering pressure, raising temperature, increasing water content. When superheated rocks rise to levels with low pressure, they melt into liquid magma (expanding- higher velocity) causing overlying rock to fracture, which allows more material to rise, creating gas bubbles  Decompression melting (lowering pressure) is the biggest reason rocks melt, rocks will melt without increasing temperature  Most magma is from athenosphere. Water (volatiles) concentration first, then viscosity determines explosive vs. peaceful volcano  Table 7.7 and 7.8 shows recent volcanic eruptions table7.4/7.6  Pyroclastic debris fire fragments. Elements in the crust that get embedded in magma, + magma chemicals make excellent fertilizer once settled in the ground  Glass is formed when magma solidifies before crystallization takes place. Obsidian (cooled volcanic glass)  Pumice: porous material, lava that had gas bubbles escape so quickly it solidified with holes in it  Fissure eruptions: lava pours out of linear fractures up to 25km long  Non-explosiveexplosive classification: Icelandic, Hawaiian, strobolian, vulcanian, plinian  Figure 7.14 Anatomy of eruptions: plinian; magma charged with volatiles. Volatiles want to get out, under pressure. Magma rises until the overlying/confining pressure is less that the volatile pressure. Volatiles begin pushing hot rock fragments along with them) Once it gets high enough the gas pressure exceeds confining pressure and “cap” blows off. o Hot gas cloud comes out as well, flowing down mountain (pyroclastic flows)  Table 7.9: largest eruptions; Tambora  Prediction of eruptions: o Swelling of volcanoes; obvious - by mete
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