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    Igneousrocks geo Igneousrocks geo Presentation Transcript

    • Magma and Igneous Rocks 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
    • 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 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 lavaMany types of igneous rocks! 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
    • Magma Formation• Why does magma form?• -earth is hot inside• Why?• 1. Earth formation• -planetesimal and meteorite accretion• -differentiation• 2. Existence of radioactive decay Chapter 6
    • Magma Formation• Melting (partial) in crust/upper mantle.• Melting mechanisms:• 1 pressure release• 2 heat transfer• 3 volatile addition 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 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 Chapter 6
    • Magma Formation• A mechanism for melting: Heat transfer. 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 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 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) 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
    • 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. 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)  TypeThese characteristics govern eruptive Density Temperature style: ViscosityFelsic 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 Chapter 6
    • Magma Composition Variation• Why various magma compositions?• Due to:-intital source rock composition-partial melting-assimilation-frctional crystallization Chapter 6
    • Magma Composition Variation• Source rock dictates initial magma composition. • Mantle source – ultramafic and mafic magmas • Crustal source – mafic, intermdiate, felsic magmas 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 Chapter 6
    • Assimilation• Magma melts the country rock it passes through• Assimilated materials change maga composition 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 typesuspended w the other 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 Chapter 6
    • Fractional Crystallization• Felsic magma can evolve from mafic magma.• By progressive removal of mafic minerals. 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
    • 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 releasesvolatiles (bubbles), thereby decreasing density 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 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 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 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 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 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 rapidlyXenolith: country rock fragment in magma-thermally altered-magma cooled before xenolith• Xenolith - Country rock fragment in magma. 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 Chapter 6
    • Tabular Intrusions• Tend to have a uniform thickness.• Can be traced laterally.2 subdivisions:1. Sill – parallels rock fabric2. Dike – crosscuts rock fabric Chapter 6
    • Example Large Sill Chapter 6
    • Plutons• Most magma is emplaced at depth within Earth.-a large deep igneous body (blob) is called a plutonPlutonic intrusions modify the crust Chapter 6
    • Plutons  Batholith• Plutons may coalesce to form a larger batholith-plutons are created atsubduction zones-magma generation may occurfor 10s of myrs.-long subduction history linkedto large batholiths 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 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 landscapeExposing intrusive rocks by erosion takes a long time 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. 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. Chapter 6
    • Glassy Textures• Form by very rapid cooling of lava in water or air.-basalts may quench into blobs of lava called pillows inwater Chapter 6
    • Crystalline Textures• Texture reveals cooling history. • Aphanitic (small crystals – too hard to see). -rapid cooling: extrusive -crystals: no time to grow 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 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 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 GraniteIntermediate Andesite DioriteMafic Basalt Gabbro Chapter 6
    • Igneous Rock Classification• Composition.• Texture.• Grain size. Chapter 6
    • Igneous Activity Distribution• Igneous activity tracks tectonic plate boundaries.• Also in plate interiors – hot spots (Hawaii) 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 Chapter 6