chapter 6: up from the inferno: magma and igneous rocks earth: portrait of a planet, 3 rd edition,...

Chapter 6: Up from the Inferno: Magma and Igneous Rocks Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 6Up from the Inferno: Magma and Igneous Rocks

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

Up from the Inferno: Magma and Igneous Rocks Up from the Inferno: Magma and Igneous Rocks

Prepared by

Ron Parker Earlham College Department of Geosciences

Richmond, Indiana

Prepared by

Ron Parker Earlham College Department of Geosciences

Richmond, Indiana


Igneous RocksIgneous Rocks Solidified molten rock (which freezes at high temp).Solidified molten rock (which freezes at high temp).

1,100°C to 650°C.1,100°C to 650°C. Depends on composition.Depends on composition.

Earth is mostly igneous rock. Earth is mostly igneous rock. Magma – Subsurface melt. Magma – Subsurface melt. Lava – Melt at the surface.Lava – Melt at the surface.

Magma erupts via volcanoes.Magma erupts via volcanoes.


Igneous RocksIgneous Rocks Melted rock can cool above or below ground. Melted rock can cool above or below ground.

Intrusive igneous rocks – Cool slowly underground. Intrusive igneous rocks – Cool slowly underground. Extrusive igneous rocks – Cool quickly at the surface. Extrusive igneous rocks – Cool quickly at the surface.

Lava – Cooled liquid.Lava – Cooled liquid.Pyroclastic debris – Cooled fragments.Pyroclastic debris – Cooled fragments.

Ash.Ash. Fragmented lava.Fragmented lava.

There are many varieties There are many varieties

of igneous rocks.of igneous rocks.


Magma FormationMagma Formation Why does magma form? Why does magma form?

Earth is hot inside. Why?Earth is hot inside. Why?Planetesimal and meteorite accretion.Planetesimal and meteorite accretion.Gravitational compression.Gravitational compression.Differentiation. Differentiation. Radioactive mineral decay.Radioactive mineral decay.


Partial melting in Partial melting in

crust / upper mantle.crust / upper mantle. Melting is from…Melting is from…

Pressure release.Pressure release. Volatile addition. Volatile addition. Heat transfer. Heat transfer.

Magma FormationMagma Formation


Magma FormationMagma Formation Geothermal gradient – Earth is hot inside.Geothermal gradient – Earth is hot inside.

Crustal temperature (T) increases 25°C / km with depth.Crustal temperature (T) increases 25°C / km with depth. At the base of the lithosphere T ~ 1,280 °C.At the base of the lithosphere T ~ 1,280 °C.

Geothermal gradient variesGeothermal gradient varies

from place to place.from place to place.


Magma FormationMagma Formation Pressure release.Pressure release.

Base of the crust is hot enough to melt mantle rock. Base of the crust is hot enough to melt mantle rock. Due to high pressure, the rock does not melt. Due to high pressure, the rock does not melt. A drop in pressure initiates “decompressional melting.”A drop in pressure initiates “decompressional melting.”


Heat transfer.Heat transfer. Rising magma Rising magma

carries mantle heat.carries mantle heat. This raises T in This raises T in

crustal rock.crustal rock. Crustal rock melts at Crustal rock melts at

lower T. lower T.

Magma FormationMagma Formation


Addition of VolatilesAddition of Volatiles Volatiles cause rocks to melt at much lower T.Volatiles cause rocks to melt at much lower T.

Water.Water. Carbon dioxide.Carbon dioxide.

Adding volatiles to hot, dry rocks initiates melting.Adding volatiles to hot, dry rocks initiates melting. Important subduction process. Important subduction process.


What Is Magma Made of? What Is Magma Made of? Magmas have 3 components (solid, liquid, and gas).Magmas have 3 components (solid, liquid, and gas).

Solid – Solidified minerals are carried by the liquid.Solid – Solidified minerals are carried by the liquid. Liquid – The melt itself is comprised of mobile ions.Liquid – The melt itself is comprised of mobile ions.

Dominantly Si and O; lesser Ca, Fe, Mg, Al, Na, and K.Dominantly Si and O; lesser Ca, Fe, Mg, Al, Na, and K.Other ions to a lesser extent. Other ions to a lesser extent.

Different mixes of elements yield different magmas.Different mixes of elements yield different magmas.


What Is Magma Made of? What Is Magma Made of? Gas – Volatiles dissolved in the melt.Gas – Volatiles dissolved in the melt.

Dry magma – No volatiles.Dry magma – No volatiles.Wet magma – To 15% volatiles.Wet magma – To 15% volatiles.

Water vapor (HWater vapor (H22O)O)

Carbon dioxide (COCarbon dioxide (CO22))

Sulfur dioxide (SOSulfur dioxide (SO22))


Magma CompositionsMagma Compositions 4 major magma types based on % silica (SiO4 major magma types based on % silica (SiO22).).

Felsic (Feldspar and silica)Felsic (Feldspar and silica) 66 to 76% silica.66 to 76% silica. IntermediateIntermediate 52 to 66% silica.52 to 66% silica. Mafic (Mg and Fe-rich)Mafic (Mg and Fe-rich) 45 to 52% silica.45 to 52% silica. Ultramafic Ultramafic 38 to 45% silica.38 to 45% silica.


Magma CompositionsMagma Compositions Composition controls density, T, and viscosity.Composition controls density, T, and viscosity.

Most important is the content of silica (SiOMost important is the content of silica (SiO22).).Silica-rich magmas are thick and viscous.Silica-rich magmas are thick and viscous.Silica-poor magmas and thin and “runny.”Silica-poor magmas and thin and “runny.”

These characteristics govern eruptive style.These characteristics govern eruptive style.

Type Density Temperature Viscosity

FelsicFelsic Very lowVery low Very low (600 to 850Very low (600 to 850°°C)C) Very High: Explosive eruptions.Very High: Explosive eruptions.

IntermediateIntermediate LowLow LowLow High: Explosive eruptions.High: Explosive eruptions.

MaficMafic HighHigh HighHigh Low: Thin, hot runny eruptions.Low: Thin, hot runny eruptions.

UltramaficUltramafic Very highVery high Very high (up to 1,300Very high (up to 1,300°°C)C) Very lowVery low


Magma VariationMagma Variation Why are there different magma compositions?Why are there different magma compositions? Magmas vary chemically due to… Magmas vary chemically due to…

Initial source rock compositions.Initial source rock compositions. Partial melting.Partial melting. Assimilation.Assimilation. Fractional crystallization.Fractional crystallization.


Magma VariationMagma Variation Source rock dictates initial magma composition.Source rock dictates initial magma composition.

Mantle source – Ultramafic and mafic magmas. Mantle source – Ultramafic and mafic magmas. Crustal source – Mafic, intermediate, and felsic magmas.Crustal source – Mafic, intermediate, and felsic magmas.


Partial MeltingPartial Melting Upon heating, silica-rich minerals melt first.Upon heating, silica-rich minerals melt first. Partial melting, then, yields a silica-rich magma.Partial melting, then, yields a silica-rich magma. Removing a partial melt from its source creates:Removing a partial melt from its source creates:

Felsic magma.Felsic magma. Mafic residue.Mafic residue.


Magma melts the country Magma melts the country rock it passes through.rock it passes through.

Assimilated materials Assimilated materials change magma change magma composition.composition.

AssimilationAssimilation


Magma MixingMagma Mixing Different magmas may blend in a magma chamber. Different magmas may blend in a magma chamber. The result combines the characteristics of the two. The result combines the characteristics of the two. Often magma mixing is incomplete, resulting in Often magma mixing is incomplete, resulting in

blobs of one rock type suspended within the other.blobs of one rock type suspended within the other.


Fractional CrystallizationFractional Crystallization As magma cools, early crystals settle by gravity.As magma cools, early crystals settle by gravity. Melt composition changes as a result.Melt composition changes as a result.

Fe, Mg, and Ca is removed in earlsettled solids.Fe, Mg, and Ca is removed in earlsettled solids. Si, Al, Na, and K remain in melt and increase.Si, Al, Na, and K remain in melt and increase.


Fractional CrystallizationFractional Crystallization

Felsic magma can Felsic magma can evolve from mafic evolve from mafic magma. magma.

Progressive removal of Progressive removal of mafic minerals.mafic minerals.


Magma MigrationMagma Migration Magma is less dense than rock; it moves upward. Magma is less dense than rock; it moves upward. Magma moves by…Magma moves by…

Injection into cracks. Injection into cracks. Melting overlying rocks.Melting overlying rocks. Squeezed by overburden.Squeezed by overburden.

Pressure decrease with upward migration releases Pressure decrease with upward migration releases volatiles (bubbles), thereby decreasing density. volatiles (bubbles), thereby decreasing density.

Viscosity controls migration ease. Viscosity controls migration ease.


Magma MigrationMagma Migration Viscosity depends on temp, volatiles, and silica.Viscosity depends on temp, volatiles, and silica.

Temperature: Temperature: Hotter - Lower viscosityHotter - Lower viscosityCooler – Higher viscosity.Cooler – Higher viscosity.

Volatile content: Volatile content: More volatiles – Lower viscosity.More volatiles – Lower viscosity.Less volatiles – Higher viscosity.Less volatiles – Higher viscosity.

Silica (SiOSilica (SiO22) content:) content:

Less SiOLess SiO22 (Mafic) – Lower viscosity. (Mafic) – Lower viscosity.

More SiOMore SiO22 (Felsic) – Higher viscosity. (Felsic) – Higher viscosity.


Igneous EnvironmentsIgneous Environments Two major categories - Based on cooling site.Two major categories - Based on cooling site.

Extrusive settings – Cool at or near the surface.Extrusive settings – Cool at or near the surface.Cool rapidly.Cool rapidly.Chill too fast to grow big crystals.Chill too fast to grow big crystals.

Intrusive settings – Cool at depth. Intrusive settings – Cool at depth. Lose heat slowly.Lose heat slowly.Crystals grow large.Crystals grow large.

Most mafic magmas extrude. Most mafic magmas extrude. Most felsic magmas do not.Most felsic magmas do not.


Extrusive CharacteristicsExtrusive Characteristics Lava flows – Sheets of cooled lava.Lava flows – Sheets of cooled lava. Lava flows exit volcanic vents and flow outward.Lava flows exit volcanic vents and flow outward. Lava cools as it flows, eventually solidifying.Lava cools as it flows, eventually solidifying. Low-viscosity lava (basalt) can flow long distances.Low-viscosity lava (basalt) can flow long distances.


Extrusive CharacteristicsExtrusive Characteristics Explosive ash eruptions. Explosive ash eruptions.

High-viscosity felsic magma builds volcanic pressure.High-viscosity felsic magma builds volcanic pressure. Violent eruptions yield huge volumes of volcanic ash. Violent eruptions yield huge volumes of volcanic ash. Ash can cover large regions.Ash can cover large regions.


Intrusive CharacteristicsIntrusive Characteristics Intrusive rocks cool at depth, they don’t surface. Intrusive rocks cool at depth, they don’t surface. Magma invading colder country rock initiates…Magma invading colder country rock initiates…

Thermal (heat) metamorphism and melting. Thermal (heat) metamorphism and melting. Inflation of fractures pushing the rock aside.Inflation of fractures pushing the rock aside. Incorporation of country rock fragments (xenoliths).Incorporation of country rock fragments (xenoliths). Hydrothermal (hot water) alteration. Hydrothermal (hot water) alteration.


Intrusive CharacteristicsIntrusive Characteristics Intrusive contacts preserve evidence of high heat.Intrusive contacts preserve evidence of high heat.

Baked zone – Rim of heat altered country rock.Baked zone – Rim of heat altered country rock. Chill margin – Magma at contact that cooled rapidly.Chill margin – Magma at contact that cooled rapidly.

Xenolith - Country rock fragment in magma.Xenolith - Country rock fragment in magma. Thermally altered.Thermally altered. Magma cooled before zenolith could be assimilated.Magma cooled before zenolith could be assimilated.


Intrusive ActivityIntrusive Activity Magma intrudes into other rocks in 2 ways.Magma intrudes into other rocks in 2 ways.

As planar, tabular bodies (dikes and sills), and As planar, tabular bodies (dikes and sills), and As balloon-shaped blobs (plutons).As balloon-shaped blobs (plutons).

Size varies widely; plutons can be massive.Size varies widely; plutons can be massive.


Tabular IntrusionsTabular Intrusions Tend to have a uniform thickness. Tend to have a uniform thickness. Can be traced laterally.Can be traced laterally. Two major subdivisions.Two major subdivisions.

Sill – Parallels rock fabric. Sill – Parallels rock fabric. Dike – Crosscuts rock fabric.Dike – Crosscuts rock fabric.


Tabular IntrusionsTabular Intrusions Dikes and sills modify invaded country rock.Dikes and sills modify invaded country rock.

They cause the rock to expand and inflate.They cause the rock to expand and inflate. They thermally alter the country rock.They thermally alter the country rock.

Dikes… Dikes… Spread rocks sideways.Spread rocks sideways. Dominate in extensional settings.Dominate in extensional settings.


Tabular IntrusionsTabular Intrusions SillsSills

Lift entire landscapes skyward.Lift entire landscapes skyward. Usually intruded near the surface.Usually intruded near the surface.

Both dikes and sills exhibit wide variability.Both dikes and sills exhibit wide variability. Size.Size. Thickness (or width).Thickness (or width). Lateral continuity.Lateral continuity.


Large SillLarge Sill


Plutonic ActivityPlutonic Activity Most magma is emplaced at depth in the Earth.Most magma is emplaced at depth in the Earth.

A large, deep igneous body is called a pluton.A large, deep igneous body is called a pluton.

Plutonic intrusions modify the crust. Plutonic intrusions modify the crust.


Plutonic ActivityPlutonic Activity Plutons may coalesce to Plutons may coalesce to

form a larger batholith.form a larger batholith. Plutons are created at Plutons are created at

subduction zones.subduction zones. Magma generation may Magma generation may

occur of over 10s of Ma.occur of over 10s of Ma. Long subduction history Long subduction history

linked to large batholiths.linked to large batholiths.


Intrusive and ExtrusiveIntrusive and Extrusive Intrusive and extrusive rocks commonly co-occur. Intrusive and extrusive rocks commonly co-occur. Magma chambers feed overlying volcanoes.Magma chambers feed overlying volcanoes. Magma chambers may cool to become plutons.Magma chambers may cool to become plutons. Many igneous geometries are possible. Many igneous geometries are possible.

Dikes.Dikes. Sills.Sills. Laccoliths.Laccoliths.


Influence on LandscapeInfluence on Landscape Deeper features are exposed by uplift and erosion.Deeper features are exposed by uplift and erosion.

Intrusive rocks are more resistant to erosion.Intrusive rocks are more resistant to erosion. Intrusive rocks often stand above the landscape. Intrusive rocks often stand above the landscape.

““Unroofing” takes long periods of geologic time.Unroofing” takes long periods of geologic time.


Cooling RatesCooling Rates Cooling rate – How fast Cooling rate – How fast

is heat lost? is heat lost? Depth: Deep is hot, Depth: Deep is hot,

shallow is cool.shallow is cool.Deep plutons cool slowly.Deep plutons cool slowly.Shallow flows cool Shallow flows cool

rapidly.rapidly. Shape: Surface to volume Shape: Surface to volume

ratio.ratio.Spherical bodies cool Spherical bodies cool

slowly. slowly. Tabular bodies cool Tabular bodies cool

faster. faster. Ground water. Ground water.

Ground water removes Ground water removes heat.heat.


Igneous Textures Igneous Textures The size, shape, and arrangement of the minerals. The size, shape, and arrangement of the minerals.

Glassy – Made of solid glass or glass shards.Glassy – Made of solid glass or glass shards. Interlocking crystals – Minerals that fit like jigsaw pieces.Interlocking crystals – Minerals that fit like jigsaw pieces. Fragmental – Pieces of pre-existing rocks. Fragmental – Pieces of pre-existing rocks.

Texture directly reflects magma history.Texture directly reflects magma history.


Glassy TexturesGlassy Textures Form by very rapid cooling of lava in water or air.Form by very rapid cooling of lava in water or air.

Quenching forms obsidian (volcanic glass).Quenching forms obsidian (volcanic glass). Basalts may quench into blobs of lava called pillows.Basalts may quench into blobs of lava called pillows. Glasses may fragment explosively.Glasses may fragment explosively.


Crystalline TexturesCrystalline Textures Texture immediately reveals cooling history.Texture immediately reveals cooling history.

Aphanitic (finely crystalline).Aphanitic (finely crystalline).Rapid cooling – extrusive. Rapid cooling – extrusive. Crystals do not have time to grow.Crystals do not have time to grow.

Phaneritic – (coarsely crystalline).Phaneritic – (coarsely crystalline).Slow cooling – Intrusive.Slow cooling – Intrusive.Crystals have a long time to grow.Crystals have a long time to grow.


Crystalline TexturesCrystalline Textures Texture immediately reveals cooling history.Texture immediately reveals cooling history.

Porphyritic – A mixture of coarse and fine crystals.Porphyritic – A mixture of coarse and fine crystals. Indicates a 2-stage history.Indicates a 2-stage history. Initial slow cooling creates large phenocrysts.Initial slow cooling creates large phenocrysts.Subsequent eruption cools remaining magma quickly.Subsequent eruption cools remaining magma quickly.


Other TexturesOther Textures Pegmatitic - Exceptionally coarse mineral crystals.Pegmatitic - Exceptionally coarse mineral crystals.

From late stage crystallization of granitic magmasFrom late stage crystallization of granitic magmas Many unusual minerals are found in pegmatites. Many unusual minerals are found in pegmatites.

Made from ions that don’t easily fit into crystals.Made from ions that don’t easily fit into crystals. Some pegmatites are rich in prized minerals.Some pegmatites are rich in prized minerals.


Igneous ClassificationIgneous Classification Based upon composition and texture.Based upon composition and texture.

Composition – Felsic, intermediate, mafic, ultramafic.Composition – Felsic, intermediate, mafic, ultramafic. Texture - Fine (aphanitic), coarse (phaneritic).Texture - Fine (aphanitic), coarse (phaneritic).

TypeType Aphanitic (fine)Aphanitic (fine) Phaneritic (coarse)Phaneritic (coarse)

FelsicFelsic RhyoliteRhyolite GraniteGranite

IntermediateIntermediate AndesiteAndesite DioriteDiorite

MaficMafic BasaltBasalt GabbroGabbro

UltramaficUltramafic Very highVery high Very high (up to 1300Very high (up to 1300°°C)C)


Glassy ClassificationGlassy Classification Glassy igneous rocks.Glassy igneous rocks.

Obsidian – Volcanic glass from rapidly cooled lava.Obsidian – Volcanic glass from rapidly cooled lava.Quenching – Lava flowing into water.Quenching – Lava flowing into water.High-silica lavas – These can make glass without quenching.High-silica lavas – These can make glass without quenching.

Pumice – Frothy felsic rock full of vesicles; it floats. Pumice – Frothy felsic rock full of vesicles; it floats. Scoria – Glassy, vesicular mafic rock.Scoria – Glassy, vesicular mafic rock.


Composition.Composition. Texture.Texture.

Crystalline ClassificationCrystalline Classification


Pyroclastic ClassificationPyroclastic Classification Pyroclastic – Fragments of violent eruptions. Pyroclastic – Fragments of violent eruptions.

Tuff – Volcanic ash that has fallen on land.Tuff – Volcanic ash that has fallen on land. Volcanic breccia – Made of larger volcanic fragments.Volcanic breccia – Made of larger volcanic fragments. Hyaloclasite – Fragments of lava exploded into water. Hyaloclasite – Fragments of lava exploded into water.


Igneous Activity DistributionIgneous Activity Distribution Igneous activity tracks tectonic plate boundaries.Igneous activity tracks tectonic plate boundaries.


Igneous Activity DistributionIgneous Activity Distribution Igneous activity tracks tectonic plate boundaries.Igneous activity tracks tectonic plate boundaries.

Hot spots – Mafic volcanic activity.Hot spots – Mafic volcanic activity. Convergent boundaries – Felsic igneous activity.Convergent boundaries – Felsic igneous activity. Divergent boundaries – Mafic igneous activity.Divergent boundaries – Mafic igneous activity.


Chapter 6Up from the Inferno: Magma and Igneous Rocks

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

This concludes the

chapter 6: up from the inferno: magma and igneous rocks earth: portrait of a planet, 3 rd edition,...

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