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Chapter 7-8.docx

Course Code
GOPH 375

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Chapter 7
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
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
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
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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-explosiveexplosive 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
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