ptember 27, 2011 Agenda: SILENCE YOUR CELL PHONE SILENCE YOUR CELL PHONE • Exams complete – hand back Exams complete – hand back • Hand in Igneous rock QR Hand in Igneous rock QR • Volcanoes, weathering, sedimentary rock Volcanoes, weathering, sedimentary rock QR’s online QR’s online
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September 27, 2011
Agenda: SILENCE YOUR CELL PHONESILENCE YOUR CELL PHONE• Exams complete – hand backExams complete – hand back• Hand in Igneous rock QRHand in Igneous rock QR• Volcanoes, weathering, sedimentary rocksVolcanoes, weathering, sedimentary rocks QR’s onlineQR’s online
Geology – Chapter 3 – Igneous Rocks
Magma, Igneous Rocks,Magma, Igneous Rocks,
and and
Intrusive ActivityIntrusive Activity
Magma, Igneous Rocks,Magma, Igneous Rocks,
and and
Intrusive ActivityIntrusive Activity
IgneousIgneousRocksRocks
Solid rockSolid rock
PressurePressure 150 mi below earth’s surface150 mi below earth’s surface partial melting of solid rock partial melting of solid rock
Solid RockSolid Rock Solid RockSolid Rock Solid Rock Solid Rock
Droplets rise throughDroplets rise throughthe mantle and collectthe mantle and collectto form magma poolsto form magma pools
Magma poolMagma pool
SurfaceSurface
Cooled solid rockCooled solid rock
Extrusive Ig-rocksLava
Intrusive Ig-rocks
Magma: The parent material for all rocksMagma: The parent material for all rocksThere are three distinct components of magma:There are three distinct components of magma:
Liquid portion Liquid portion referred to as referred to as “melt”“melt”• “ “liquid rock” – low densityliquid rock” – low density• composed of: Si, O, Al, Fe, Ca, Mg, Na, Kcomposed of: Si, O, Al, Fe, Ca, Mg, Na, K
Solid portion • crystallized portions of the melt as magma cools – precipitating various silicate minerals.
Gaseous portionGaseous portion• “ “volatiles” created from vaporizingvolatiles” created from vaporizing materials at the Earth’s surfacematerials at the Earth’s surface (H(H22O, COO, CO22 and SO and SO22))
MagmaMagmaTypical temperature depending on chemical composition:Typical temperature depending on chemical composition:
Melting solid rock into magmaMelting solid rock into magma
Rising temperatures within a solid (tightly packed Rising temperatures within a solid (tightly packed atoms)atoms)creates “vibrating ions” eventually breaking thecreates “vibrating ions” eventually breaking thechemical bonds and causing ions to roll over eachchemical bonds and causing ions to roll over eachother ----- producing a liquid.other ----- producing a liquid.
How would the process of crystallization work?How would the process of crystallization work?
Intrusive RocksIntrusive Rocks
Extrusive RocksExtrusive Rocks
Intrusive Rocks
•Solidified below the Earth’s surface
• Plutonic rocks – “the God of lower world”
• magma cooled slowly allowing xl growth
• coarse-grained textures (can see grains)
• large granite mountains • Sierra Nevada Mountains•Mt Rushmore
Extrusive RocksExtrusive Rocks
• Solidified above the Earth’s surfaceSolidified above the Earth’s surface
• Volcanic rocks – “the roman God of fire”Volcanic rocks – “the roman God of fire”
• magma cooled fast allowing microscopicmagma cooled fast allowing microscopic xl growth (can’t see grains)xl growth (can’t see grains)
• fine-grained textures (can’t see grains)fine-grained textures (can’t see grains)
• Hawaiian Volcanoes, Cascade RangeHawaiian Volcanoes, Cascade Range
visible visible grainsgrains
non-visible grains
When observing “frozen magma” (an igneous rock), how does a geologist know if the rock is an Extrusive or Intrusive igneous rock????????When observing “frozen magma” (an igneous rock), how does a geologist know if the rock is an Extrusive or Intrusive igneous rock????????
Checking the crystal size - does the rock contain visible crystals or are crystals non-existent?Checking the crystal size - does the rock contain visible crystals or are crystals non-existent?
(4) Factors that affect crystal size in an igneous rockThe rate at which magma cools
Classification of Igneous RocksIgneous rocks are classified based on TEXTURE and
COMPOSTION
TE
XT
UR
E
ph
ane
ritica
ph
an
itic
COMPOSITIONFelsic (light color)
High silicaMafic (dark color)
Low silicaIntermediate color
Granite Diorite Gabbro
Rhyolite Andesite Basalt
Ig rocks have the same chemistry but different textural characteristicsdue to the type of geologic environment the rock forms
The Origin of Magma
•How can magma form from “solid rock ?”How can magma form from “solid rock ?”
•Why is one magma Why is one magma maficmafic and others and others felsicfelsic??
•Why do volcanoes erupt basaltic lava, and Why do volcanoes erupt basaltic lava, and continent volcanoes erupt andesitic/rhyolitic lava ?continent volcanoes erupt andesitic/rhyolitic lava ?
• most magma originates in the upper mantle• greatest quantities form at divergent boundaries• small amounts of magma at subduction zones
Questions:Questions:
Increasing Temperature:Increasing Temperature:
• increasing the temperature within the earthincreasing the temperature within the earth begins to melt solid rockbegins to melt solid rock
Temperatures within the Earth increase as a Temperatures within the Earth increase as a function of depth ---- Geothermal Gradientfunction of depth ---- Geothermal Gradient
Temperature inside the earthTemperature inside the earth
00 500500 10001000 15001500 20002000
De
pth
(km
)D
ep
th (
km)
100100
200200
300300
400400
5,0005,000
10,00010,000
15,00015,000
Pre
ssu
re (
mpa
)P
ress
ure
(m
pa)
GeothermalGeothermalgradientgradient
• the rate at whichthe rate at which temperature increasestemperature increases with depthwith depth
Continent gradientContinent gradient
• In thicker crust,In thicker crust, gradient increases.gradient increases.
• average 7average 7ooC/km rateC/km rate
• temperature increasestemperature increases gentlygently
Oceanic gradientOceanic gradient
• Below the ocean floor,Below the ocean floor, temperature increasestemperature increases rapidly.rapidly.
• average 13average 1300C/kmC/km
So, why is the mantle still considered solid?• Pressure increases with depth – raising the rocks melting points
Decompression Melting:• ascending mantle rock moves into lower pressure zones which lowers rock melting points ------- generating voluminous magma
• Remember: most magma occurs along spreading ridges.
Addition of volatiles (water and gases)• addition of water lowers the rocks melting point
• “wet” rock drives the melting points downward
• mantle rock (peridotite) melting points lower by 1000C/.1% water!!
Magma is generated in three ways:Magma is generated in three ways:• Increase in temperature causing rocks to exceedIncrease in temperature causing rocks to exceed their melting points ---- melting occurstheir melting points ---- melting occurs
• Decrease pressure (decompression melting), rocksDecrease pressure (decompression melting), rocks ascend to low pressure zonesascend to low pressure zones
• Introduction of volatiles (principally water) lowersIntroduction of volatiles (principally water) lowers rock melting point.rock melting point.
Evolving MagmasEvolving Magmas: : • A variety of igneous rocks = wide variety of magmasA variety of igneous rocks = wide variety of magmas
• observations of many igneous compositions fromobservations of many igneous compositions from volcanoesvolcanoes
• magma – the parent material of all igneous rocksmagma – the parent material of all igneous rocks
• investigated by N. L. Bowen (famous geologist)investigated by N. L. Bowen (famous geologist) (1887-1956) --- Bowen’s Reaction Series(1887-1956) --- Bowen’s Reaction Series
Magmatic DifferentiationMagmatic Differentiation
• the formation of many kinds of igneous rocks fromthe formation of many kinds of igneous rocks from a single magmaa single magma
FeFe
Simple exampleSimple example
FeFeFeFe
FeFe MgMgMgMg
MgMg
MgMg
SiOSiO22
SiOSiO22
SiOSiO22
SiOSiO22
SiOSiO22
SiOSiO22
SiOSiO22
SiOSiO22
Liquid magmaLiquid magma Part liquid/solidPart liquid/solid
FeSiOFeSiO22
FeSiOFeSiO22MgSiOMgSiO22
FeSiOFeSiO22 MgSiOMgSiO22
FeSiOFeSiO22MgSiOMgSiO22
solidsolid
liquidliquid
SiOSiO22
SiOSiO22SiOSiO22
SiOSiO22
CoolingCooling
How has the liquid magma changed composition?How has the liquid magma changed composition?
As the liquid magma begins to cool, various minerals precipitate asAs the liquid magma begins to cool, various minerals precipitate assolids and become separated from the liquid melt. This separationsolids and become separated from the liquid melt. This separationof various chemistries changes the composition of the original magma.of various chemistries changes the composition of the original magma.
3333
Changing the composition of magmas:
Magmatic Differentiation:• separating the xlized minerals from the melt changing the overall magma composition
Assimilation and Mixing Magma:• mixing magmas with various compositions
Magmatic Differentiation
MixingMagmas
Changing the magma from a basaltic composition to a graniticcomposition
Composition of magmas – explained by P.T.The Earth- One big chemistry set!
Basaltic composition
Granitic composition
Basalt magma:
Assimilating oceanic plate material (basalt + basalt = basalt magma) Rocks concentrated in mafic minerals – formed high on BRS Low in Silica
Granitic magma:
Assimilating oceanic plate material (basalt + continental = granitic magma)
Rocks concentrated in felsic minerals – formed low on BRS High in Silica
What types of features areWhat types of features areformedformed
when magmawhen magmacools below the surface?cools below the surface?
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Intrusive Igneous Rock ActivityIgneous rocks solidifying below the surface can be classified as:
• tabular or massive : (“table-tops” or irregular”)
• discordant: cuts across other rock bodies
• concordant : parallels other rock bodies
Tabular intrusive bodiesTabular intrusive bodiesforming below the earth’sforming below the earth’ssurfacesurface
DikesDikes
SillSill
TabularTabular
BatholithBatholith
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BatholithBatholithBatholithBatholith
LoccolithLoccolithLoccolithLoccolith
DikeDikeDikeDike
SillSillSillSill
StockStockStockStock
Intrusive BodiesIntrusive Bodies
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Intrusive Bodies:Intrusive Bodies:
Batholith: intrusive body GREATER than 40 mi2
Stock: intrusive body LESS than 40 mi2
Dike: intrusive body cutting across strata (discordant)
Sill: intrusive/extrusive body parallel to strata (concordant)
Laccolith: “mushroom-shaped” intrusive body forming a dome-like structure
Sierra Nevada BatholithSierra Nevada BatholithGranite/DioriteGranite/Diorite
Melting magma rises andMelting magma rises andmixes with continentalmixes with continentalmaterial (high SiOmaterial (high SiO22) and) andsolidifies beneath the solidifies beneath the surface.surface.
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I I I I intrusiveintrusiveintrusiveintrusive1.1.Given the block diagram below, describeGiven the block diagram below, describe the following plutonic (intrusive) typethe following plutonic (intrusive) type features:features: