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Production of Igneous Production of Igneous Rocks:Rocks:Convergent Plate Convergent Plate BoundariesBoundaries
SubductionSubduction at at convergent plate convergent plate boundaries causes boundaries causes partial melting to partial melting to form magma and form magma and resulting igneous resulting igneous rocks.rocks.
Production of Igneous Production of Igneous Rocks:Rocks:Convergent Plate Convergent Plate BoundariesBoundaries
Magma rises from Magma rises from the mantle at the mantle at divergent plate divergent plate boundaries.boundaries.
Production of Igneous Production of Igneous Rocks:Rocks: Divergent Plate Boundaries Divergent Plate Boundaries
Origin of MagmaOrigin of Magma
Where does the Where does the heat come from heat come from that melts rocks?that melts rocks? Formation of EarthFormation of Earth Heat from the Heat from the
decay of decay of radioactive radioactive elementselements
Factors that Affect Magma Factors that Affect Magma FormationFormation
TemperatureTemperature Increases with depthIncreases with depth
PressurePressure Increases with depthIncreases with depth
Water ContentWater Content Decreases melting Decreases melting
pointpoint
Mineral CompositionMineral Composition Different minerals, Different minerals,
different melting points different melting points
Characteristics of MagmaCharacteristics of Magma
Slushy Mix of molten Slushy Mix of molten rock, gases, and rock, gases, and mineral crystalsmineral crystals
Common ElementsCommon Elements
Oxygen (O)Oxygen (O) Silicon (Si)Silicon (Si) Aluminum (Al)Aluminum (Al) Iron (Fe)Iron (Fe) Magnesium (Mg)Magnesium (Mg) Calcium (Ca)Calcium (Ca) Potassium (K)Potassium (K) Sodium (Na)Sodium (Na)
Compounds in MagmaCompounds in Magma Silica ( Si0Silica ( Si022))
Most abundantMost abundant Greatest effect on Greatest effect on
Magma CharacteristicsMagma Characteristics Effects melting tempEffects melting temp Viscosity of MagmaViscosity of Magma
Types of MagmaTypes of Magma Based on amount of SilicaBased on amount of Silica
BasalticBasaltic AndesiticAndesitic RhyoliticRhyolitic
Type of MagmaType of Magma SiOSiO2 2 ContentContent
RhyoliticRhyolitic 70%70%
AndesiticAndesitic 60%60%
BasalticBasaltic 50%50%
FLUIDITY OF MAGMA
Fluidity or Viscosity of magma depends on content (%) of Silica
Silica Rich-known as Acidic magma-More viscous, so do not spreads and piles up at one place
Silica poor-Known as Basic magma-Less viscous, moves faster and occupies larger area
However, the viscosity of magma is considerably influenced by temperature too. When temperature is less-more viscous and whenHigh temperature- less viscous.
Four main groups of igneous rocks Four main groups of igneous rocks based on magma type/mineral based on magma type/mineral compositioncomposition
Felsic: Felsic: high silica content, light colored, from thick & slow moving high silica content, light colored, from thick & slow moving magma, contains low amounts of Ca, Fe, and Mg dominant magma, contains low amounts of Ca, Fe, and Mg dominant minerals quartz, potassium rich feldsparminerals quartz, potassium rich feldspar Ex: granite, pumice, rhyoliteEx: granite, pumice, rhyolite Magma Type:Magma Type:
Intermediate: Intermediate: moderate amount of silica, mixture of colors moderate amount of silica, mixture of colors dominant minerals: sodium and calcium rich feldspardominant minerals: sodium and calcium rich feldspar Ex: andesite, dioriteEx: andesite, diorite Magma Type:Magma Type:
Mafic: Mafic: low silica content, dark colored, high levels of Fe & Mg low silica content, dark colored, high levels of Fe & Mg formed from thinner, more fluid, & hotter magma than Felsic formed from thinner, more fluid, & hotter magma than Felsic rocks dominant minerals hornblende, calcium rich feldsparrocks dominant minerals hornblende, calcium rich feldspar Ex: basalt, gabbroEx: basalt, gabbro Magma Type:Magma Type:
Ultramafic: Ultramafic: very low silica content, dark colors, high levels of Fe & very low silica content, dark colors, high levels of Fe & Mg dominant minerals: olivine, pyroxeneMg dominant minerals: olivine, pyroxene Ex: peridotite, duniteEx: peridotite, dunite Magma Type:Magma Type:
Scheme For Igneous RocksScheme For Igneous Rocks
Characteristics of magmaCharacteristics of magma
Igneous rocks form as molten rock cools Igneous rocks form as molten rock cools and solidifiesand solidifies
General Characteristic of General Characteristic of magmamagma Parent material of igneous rocksParent material of igneous rocks Forms from partial melting of rocks inside Forms from partial melting of rocks inside
the Earththe Earth Magma that reaches the surface is called Magma that reaches the surface is called
lavalava
Characteristics of magmaCharacteristics of magma
General Characteristic of magmaGeneral Characteristic of magma Rocks formed from lava at the surface are Rocks formed from lava at the surface are
classified as classified as extrusive,extrusive, or or volcanicvolcanic rocks rocks Rocks formed from magma that crystallizes Rocks formed from magma that crystallizes
at depth are termed at depth are termed intrusiveintrusive, or , or plutonicplutonic rocksrocks
Characteristics of magmaCharacteristics of magma
The nature of magmaThe nature of magma Consists of three components:Consists of three components:
A liquid portion, called A liquid portion, called meltmelt, that is composed , that is composed of mobile ionsof mobile ions
SolidsSolids, if any, are silicate minerals that have , if any, are silicate minerals that have already crystallized from the meltalready crystallized from the melt
VolatilesVolatiles, which are gases dissolved in the melt, , which are gases dissolved in the melt, including water vapor (Hincluding water vapor (H22O), carbon dioxide O), carbon dioxide
(CO(CO22), and sulfur dioxide (SO), and sulfur dioxide (SO22))
Characteristics of magmaCharacteristics of magma
Crystallization of magmaCrystallization of magma Texture in igneous rocks is determined by Texture in igneous rocks is determined by
the size and arrangement of mineral grainsthe size and arrangement of mineral grains Igneous rocks are typically classified byIgneous rocks are typically classified by
TextureTexture Mineral compositionMineral composition
Origin of MagmaOrigin of Magma
Highly debated topicHighly debated topic Generating magma from solid rockGenerating magma from solid rock
Produced from partial melting of rocks in Produced from partial melting of rocks in the crust and upper mantlethe crust and upper mantle
Role of heatRole of heat Temperature increases within Earth’s upper Temperature increases within Earth’s upper
crust (called the crust (called the geothermal gradientgeothermal gradient) average ) average between 20between 20ooC to 30C to 30ooC per kilometerC per kilometer
Estimated temperatures in Estimated temperatures in the the crust and mantlecrust and mantle
Origin of MagmaOrigin of Magma
Role of heatRole of heat Rocks in the lower crust and upper mantle are Rocks in the lower crust and upper mantle are
near their melting pointsnear their melting points Any additional heat (from rocks descending Any additional heat (from rocks descending
into the mantle or rising heat from the mantle) into the mantle or rising heat from the mantle) may induce melting may induce melting
Origin of MagmaOrigin of Magma
Role of pressureRole of pressure An increase in confining pressure causes an An increase in confining pressure causes an
increase in a rock’s melting temperature or increase in a rock’s melting temperature or conversely, reducing the pressure lowers the conversely, reducing the pressure lowers the melting temperaturemelting temperature
When confining pressures drop, decom-When confining pressures drop, decom-pression melting occurspression melting occurs
Decompression meltingDecompression melting
Origin of MagmaOrigin of Magma
Role of volatilesRole of volatiles Volatiles (primarily water) cause rocks to melt Volatiles (primarily water) cause rocks to melt
at lower temperaturesat lower temperatures This is particularly important where oceanic This is particularly important where oceanic
lithosphere descends into the mantlelithosphere descends into the mantle
Evolution of magmasEvolution of magmas
A single volcano may extrude lavas A single volcano may extrude lavas exhibiting very different compositionsexhibiting very different compositions
Bowen’s reaction seriesBowen’s reaction series and the and the composition of igneous rockscomposition of igneous rocks
N.L. Bowen demonstrated that as a magma N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic cools, minerals crystallize in a systematic fashion based on their melting pointsfashion based on their melting points
Evolution of magmasEvolution of magmas
Bowen’s reaction seriesBowen’s reaction series During crystallization, the composition of During crystallization, the composition of
the liquid portion of the magma continually the liquid portion of the magma continually changeschanges Composition changes due to removal of Composition changes due to removal of
elements by earlier-forming mineralselements by earlier-forming minerals The silica component of the melt becomes The silica component of the melt becomes
enriched as crystallization proceedsenriched as crystallization proceeds Minerals in the melt can chemically react and Minerals in the melt can chemically react and
changechange
Evolution of magmasEvolution of magmas
Processes responsible for changing a Processes responsible for changing a magma’s compositionmagma’s composition
Magmatic differentiationMagmatic differentiation Separation of a melt from earlier formed Separation of a melt from earlier formed
crystals to form a different composition of crystals to form a different composition of magmamagma
AssimilationAssimilation Changing a magma’s composition by the Changing a magma’s composition by the
incorporation of foreign matter (surrounding incorporation of foreign matter (surrounding rock bodies) into a magmarock bodies) into a magma
Assimilation and magmatic Assimilation and magmatic differentiationdifferentiation
Evolution of magmasEvolution of magmas
Processes responsible for changing a Processes responsible for changing a magma’s compositionmagma’s composition
Magma mixingMagma mixing Involves two bodies of magma intruding one Involves two bodies of magma intruding one
anotheranother Two chemically distinct magmas may produce Two chemically distinct magmas may produce
a composition quite different from either a composition quite different from either original magmaoriginal magma
Evolution of magmasEvolution of magmas
Partial melting and magma formationPartial melting and magma formation Incomplete melting of rocks is known as Incomplete melting of rocks is known as
partial meltingpartial melting Formation of basaltic magmasFormation of basaltic magmas
Most originate from partial melting of Most originate from partial melting of ultramafic rock in the mantleultramafic rock in the mantle
Basaltic magmas form at mid-ocean ridges by Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zonesdecompression melting or at subduction zones
Evolution of magmasEvolution of magmas
Partial melting and magma formationPartial melting and magma formation Formation of basaltic magmasFormation of basaltic magmas
As basaltic magmas migrate upward, confining As basaltic magmas migrate upward, confining pressure decreases which reduces the melting pressure decreases which reduces the melting temperaturetemperature
Large outpourings of basaltic magma are Large outpourings of basaltic magma are common at Earth’s surfacecommon at Earth’s surface
Evolution of magmasEvolution of magmas
Partial melting and magma formationPartial melting and magma formation Formation of andesitic magmasFormation of andesitic magmas
Interactions between mantle-derived basaltic Interactions between mantle-derived basaltic magmas and more silica-rich rocks in the crust magmas and more silica-rich rocks in the crust generate magma of andesitic compositiongenerate magma of andesitic composition
Andesitic magma may also evolve by magmatic Andesitic magma may also evolve by magmatic differentiationdifferentiation
Evolution of magmasEvolution of magmas
Partial melting and magma formationPartial melting and magma formation Formation of granitic magmasFormation of granitic magmas
Most likely form as the end product of Most likely form as the end product of crystallization of andesitic magmacrystallization of andesitic magma
Granitic magmas are higher in silica and Granitic magmas are higher in silica and therefore more viscous than other magmastherefore more viscous than other magmas
Because of their viscosity, they lose their Because of their viscosity, they lose their mobility before reaching the surfacemobility before reaching the surface
Tend to produce large plutonic structuresTend to produce large plutonic structures