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Transcript of Igneousrocks geo
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Chapter 6
Magma and Igneous Rocks
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Chapter 6
Outline• Igneous rocks
- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)
• Magma composition-4 major types of magma (based on silica content)
- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism
Chapter 6
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Chapter 6
Igneous Rocks
• Defined: rock solidified from the melt• Freezes at temperatures of 1,100degreesC
• Composition dependent
• Earth is mostly igneous rock• Magma – subsurface melt• Lava – melt at the surface• Magma erupts via volcanoes
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Chapter 6
Igneous Rocks• Melted rock can cool above or below ground.
• Intrusive igneous rocks – Cool slowly underground• E.g. granite - yosemite
• Extrusive igneous rocks – Cool quickly at the surface
-E.g. basalt – hawaii
-lava
-flowing, cooling molten rock
Pyroclastic debris
-cooled rock frgaments
-e.g. ash, fragmented lava
Many types of igneous rocks!
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Chapter 6
Outline• Igneous rocks
- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)
• Magma composition-4 major types of magma (based on silica content)
- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism
Chapter 6
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Chapter 6
Magma Formation
• Why does magma form?
• -earth is hot inside• Why?• 1. Earth formation• -planetesimal and
meteorite accretion• -differentiation• 2. Existence of
radioactive decay
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Chapter 6
• Melting (partial) in
crust/upper mantle.
• Melting mechanisms:• 1 pressure release• 2 heat transfer• 3 volatile addition
Magma Formation
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Chapter 6
Magma Formation• Earth is hot inside, thus a geothermal gradient.• -crustal temperature (T) increases 25degreesC/km depth• -base of crust T is 1,280degreesC• GEOTHERMAL GRADIENT VARIES FROM PLACE TO PLACE
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Chapter 6
Magma Formation• A mechanism for melting: pressure release.
• Base of crust is hot enough to melt mantle rock• But due to high pressure, the rock does not• A drop in pressure initiates “decompression melting”• A B:
-Big change in pressure, little in
Temperature
Vocab:
-Solidus melting begins
-Liquidus No solid left
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Chapter 6
• A mechanism for melting: Heat transfer.
Magma Formation
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Chapter 6
Addition of Volatiles• A mechanism for melting: introduction of volatiles • -volatiles decrease rock melting temp• -water• -carbon dioxide• Adding volatiles to hot, dry rocks initiates melting• Important in subduction process
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Chapter 6
What Is Magma Made of?
• Magmas have 3 components (solid, liquid, and gas).• Solid – solidified minerals carried by the liquid• Liquid – melt itself comprised of mobile ions
Mostly si and o; some ca, fe ,mg, al, na , k
Other ions to a lesser extent
Different mixes of elements
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Chapter 6
What Is Magma Made of?
• Gas – volatiles dissolved in the melt.• Dry magma – no volatiles• Wet magma – up to 15% volatiles:
-water vapor (h2o)
-carbon dioxide (co2)
-sulfur dioxide (so2)
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Chapter 6
Outline• Igneous rocks
- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)
• Magma composition-4 major types of magma (based on silica content)
- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism
Chapter 6
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Chapter 6
Types of Magma (composition)
• 4 major types based on % silica (SiO2).• Felsic (Feldspar and silica) 66 to 76% silica.• Intermediate 52 to 66% silica.• Mafic (Mg and Fe-rich) 45 to 52% silica.• Ultramafic 38 to 45% silica.
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Chapter 6
Magma Composition -> Eruptive Style
• Composition controls density, T, and viscosity.• Most imp - is the silica (SiO2) content.
• Silica-rich magmas thick and viscous• Silica-poor magmas thin and flow easily (less viscous)
These characteristics govern eruptive style:Type Density Temperature Viscosity
Felsic Very low Very low (600 to 850°C) Very High: Explosive eruptions.
Intermediate Low Low High: Explosive eruptions.
Mafic High High Low: Thin, hot runny eruptions.
Ultramafic Very high Very high (up to 1,300°C) Very low
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Chapter 6
Magma Composition Variation• Why various magma compositions?• Due to:
-intital source rock composition
-partial melting
-assimilation
-frctional crystallization
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Chapter 6
Magma Composition Variation• Source rock dictates initial magma composition.
• Mantle source – ultramafic and mafic magmas• Crustal source – mafic, intermdiate, felsic magmas
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Chapter 6
Partial Melting• Upon heating, silica-rich minerals melt first.• Thus, partial melting yields a silica-rich magma• Removing a partial melt from its source creates:• -felsic magma• -mafic residue left behind
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Chapter 6
Assimilation• Magma melts the country rock it passes through• Assimilated materials change maga composition
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Chapter 6
Magma Mixing• Different magmas may blend in a magma chamber
-result combines characteristics of both
-mixing often incomplete, resulting in blobs of one type suspended w the other
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Chapter 6
Fractional Crystallization
• As magma cools, early formed crystals settle by gravity.• Melt compostion changes as a result
-fe, mg, ca is removed in early formed solids
-si, al, na, and k remain in melt
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Chapter 6
Fractional Crystallization
• Felsic magma can evolve from mafic magma.
• By progressive removal of mafic minerals.
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Chapter 6
Outline• Igneous rocks
- What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma-Why does it form?-Melting mechanisms (pressure release, heat transfer, volatiles)-Components (solids, melts, gas)
• Magma composition-4 major types of magma (based on silica content)
- Controls eruptive style (explosive vs. non-explosive)- Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….-Magma migration, extrusive vs. intrusive environments-Intrusive forms: sills, dikes, plutons, influence on landscape-Igneous textures, classification, global distribution of magmatism
Chapter 6
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Chapter 6
Magma Migration• Magma is less dense than rock, so it rises. • Magma moves by…
-injection into cracks
-melting overlying rocks
-pressure decrease with upward migration releases volatiles (bubbles), thereby decreasing density
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Chapter 6
Magma Migration
• Viscosity depends on temp, volatiles, and silica.
• Temp: • Hot low viscosity (flows well)• Cooler high viscosity (flows poor)
• Volatile content: • More low vsc• Less high visc
• Silica (SiO2) content:
• Less (Mafic) low visc• More (Felsic) high visc
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Chapter 6
Igneous Environments
• 2 major categories - based on cooling site.1. Extrusive settings – Cool at or near the surface.
-cool rapidity
-chill too fast-only small crystals form
2. Intrusive settings – Cool at depth.
-cool slowly
-crystals grow large
-most mafic magmas extrude
-most felsic magmas do not
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Chapter 6
Extrusive Characteristics• 1. lava flows – sheets of cooled lava• 2. lava flows exit volcanic vents and flow outward• 3. lava cools as it flows, eventually solidifies• 4. low viscosity lava (basalt) can flow long distances
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Chapter 6
Extrusive Characteristics• Explosive ash eruptions.
-high viscosity felsic magma builds up pressure
-violent eruptions yield huge volumes of volcanic ash
-ash can cover large regions
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Chapter 6
Intrusive Characteristics• Intrusive rocks cool at depth, they don’t surface. • Magma invading colder country rock initiates…
-thermal heat metamorphism and partial melting.
-Inflates fractures, pushing rock aside
-incorporation of country rock fragments (xenoliths)
-hydrothermal (hot water) alteration
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Chapter 6
Intrusive Characteristics• Intrusive contacts preserve evidence of high heat.
-baked zone: rim of heat altered country rock
-chill margin: magma at contact that cooled rapidly
Xenolith: country rock fragment in magma
-thermally altered
-magma cooled before xenolith
• Xenolith - Country rock fragment in magma.
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Chapter 6
Intrusive Activity• Magma intrudes into rocks in 2 main ways:
-as planar, tabular bodies (dikes, sills)
-as balloon-shaped blobs (plutons)
Size varies widely: plutons can be massive
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Chapter 6
Tabular Intrusions• Tend to have a uniform thickness. • Can be traced laterally.
2 subdivisions:
1. Sill – parallels rock fabric
2. Dike – crosscuts rock fabric
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Chapter 6
Example Large Sill
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Chapter 6
Plutons• Most magma is emplaced at depth within Earth.
-a large deep igneous body (blob) is called a pluton
Plutonic intrusions modify the crust
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Chapter 6
Plutons Batholith• Plutons may coalesce to form
a larger batholith
-plutons are created at subduction zones
-magma generation may occur for 10s of myrs.
-long subduction history linked to large batholiths
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Chapter 6
Intrusive and Extrusive• Intrusive & extrusive rocks commonly co-occur.• Magma chambers feed overlying volcanoes• Magma chambers can cool – become plutons• Many igneous geometries are possible • Dikes• Sills• Laccoliths• Plutons
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Chapter 6
Influence on Landscape
• Deeper features are exposed by uplift and erosion.
-intrusive rocks are resistant to erosion
-intrusive rocks often stand above the landscape
Exposing intrusive rocks by erosion takes a long time
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Chapter 6
Rate of Cooling
• How fast is heat lost? • Depth: Deep is hot, shallow
is cool.-deep plutons cool slowly-shallow flows cool rapidly
Shape: Surface to volume ratio.-spherical bodies cool slowly-tabular bodies cool faster
• Ground water.
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Chapter 6
Igneous Textures • Size, shape, and arrangement of the minerals.
-glassy: solid glass or glass shards
-interlocking crystals: minerals that fit like jigsaw pieces
-fragmental: pieces of pre-existing rocks
-texture directly reflects magma history
Texture directly reflects magma history.
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Chapter 6
Glassy Textures• Form by very rapid cooling of lava in water or air.
-basalts may quench into blobs of lava called pillows in water
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Chapter 6
Crystalline Textures• Texture reveals cooling history.
• Aphanitic (small crystals – too hard to see).
-rapid cooling: extrusive
-crystals: no time to grow
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Chapter 6
Crystalline Textures• Texture reveals cooling history.• Apharitic (small crystals – too hard to see)
-rapid cooling: extrusive
-crystals: no time to grow
• Phaneritic (large cryst• als – easy to see).• Slow cooling: intrusive• Crystals have a long• Time to grow
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Chapter 6
Crystalline Textures• Texture reveals cooling history.
• Porphyritic – A mixture of coarse and fine crystals.
-indicates a 2-stage history
-initial slow cooling creates large phenocrysts
-subsequent eruption
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Chapter 6
Igneous Rock Classification
• Based on composition and texture.• Composition (silica) – Felsic, intermediate, mafic, ultramafic.• Texture - Fine (aphanitic), coarse (phaneritic).
Type Aphanitic (fine) Phaneritic (coarse)
Felsic Rhyolite Granite
Intermediate Andesite Diorite
Mafic Basalt Gabbro
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Chapter 6
• Composition.• Texture.• Grain size.
Igneous Rock Classification
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Chapter 6
Igneous Activity Distribution• Igneous activity tracks tectonic plate boundaries.• Also in plate interiors – hot spots (Hawaii)
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Chapter 6
Igneous Activity Distribution
• Igneous activity tracks tectonic plate boundaries.
-convergent boundaries: felsic igneous activity
-divergent boundaries: mafic igneous activity
-hot spots: mafic volcanic activity
-NOT at continental transform boundaries