A rock is a naturally formed, consolidated material usually composed of grains of one or more minerals The rock cycle shows how one type of rocky material gets transformed into another The Rock Cycle Representation of how rocks are formed, broken down, and processed in response to changing conditions Processes may involve interactions of geosphere with hydrosphere, atmosphere and/or biosphere Arrows indicate possible process paths within the cycle
Imagine the first rock and the cycles that it has been through.
Igneous Rocks Form from Magma Hot, partially molten mixture of solid liquid and gas Mineral crystals form in the magma making a crystal slush Gases - H 2 O, CO 2, etc. - are dissolved in the magma Magma is less dense than solid rock
Magma vs. Lava Igneous Rocks Magma is molten rock beneath the surface Lava is molten rock that has reached the surface Magma solidifies to form intrusive igneous rocks Lava solidifies to form extrusive igneous rocks
Igneous Rocks Composition varies widely Oxygen plus major elements Generally a silica (SiO 2 ) melt Silica and water content control viscosity Silica content is used in classification
Mafic Magmas Silica content of ~ 50% High concentrations of Fe, Mg and Ca High temperature of molten magma 1000 o to 1200 o C Major minerals Olivine - Ca Plagioclase Pyroxene
Silicic Magma Silica content of 65-77% High concentrations of Al, Na and K Lower temperature magmas Less than 850 o C Major minerals Feldspars - Micas Quartz
Magma Viscosity Controlled by silica and water and temperature As magma cools, silica tetrahedron form links Similar to polymers - e.g., nylon Increasing linkages Higher silica & lower temp Linkages increase viscosity Note: this is just like oils, fats and other organic compounds used in the household
Magma Viscosity Water and volatiles influence viscosity like a big malted milkshake H 2 O and CO 2 make up 90+% of dissolved gases in magmas Typical range of dissolved gases is 0.1 to 5% Up to 15% is possible High H 2 O content prevents silica linkages High volatile content may cause explosive eruptions what s true for humans is true for Mother Nature
Occurrence of Igneous Rocks Found globally Formed in discrete geologic settings Convergent plate margins Intrusions and overlying volcanoes Divergent plate margins Mid-ocean ridge basalts and continental rifts Mantle plumes (very mafic)
Figure 4.2. Distribution of igneous rocks in North America
Igneous Textures Texture - the size, shape and relationship of mineral crystals in the rock Reflects cooling history of the magma or lava Slow cooling rate >> Big crystals Fast cooling rate >> Small crystals Very fast cooling rate >> glass
Glassy Texture Very rapid cooling - quenched Volcanic glass (obsidian) Conchoidal fracture No apparent crystals embryonic crystals may be present or devitrification may have begun Dark color from low concentrations of Fe (a little goes a long way) and size of crystalline unit - generally silicic composition
Figure 4.3A. Glassy texture in obsidian
Crystalline Textures Crystal growth requires time for ions to migrate - form minerals Slow rate of cooling provides time for crystal growth Crystals grow until melt is quenched or is completely solidified
Aphanitic Texture Fine grained texture Few crystals visible in hand specimen Relatively rapid rate of cooling Vesicles may be formed by gases trapped in cooling magma
Figure 4.3B. Aphanitic texture in rhyolite
Phaneritic Texture Coarse grained texture Relatively slow rate of cooling Equigranular, interlocking crystals Slow cooling = crystallization at depth Pegmatites - very coarse grained texture
Figure 4.3C. Phaneritic texture in granite
Porphyritic Texture Well formed crystals (phenocrysts) Fine grained matrix (groundmass) Complex cooling history Initial stage of slow cooling Large, well formed crystals form Later stage of rapid cooling Remaining magma crystallizes more rapidly
Pyroclastic Texture Produced by explosive volcanic eruptions May appear porphyritic with visible crystals Crystals show breakage or distortion Matrix may be dominated by glassy fragments Fragments also show distortion Hot fragments may weld together
Figure 4.3D. Pyroclastic texture
Classification of Igneous Rocks Texture Aphanitic Phanaritic Composition Silicic Intermediate Mafic Ultramafic Combination of Texture and Composition produces rock name
Figure 4.4. Classification of common igneous rocks
Extrusive Rock Bodies Volcanic Form of extrusive bodies influenced by magma properties Composition Silica content Viscosity Volatile content Temperature
Basaltic Eruptions Low Silica + High T = Low Viscosity Produce Lava Flows - Pahoehoe or Aa Flood basalts Fissure eruptions Spatter cones Shield Volcanoes Pillow lavas
Aa flow Pahoehoe flow Figures 4.6 A & B
Devil s Tower; a volcanic neck, a feeder pipe
Shiprock, New Mexico; a volcanic neck
Rhumski, Cameroon; a volcanic neck
Sill; parallels layers in the country rock
Dike; cuts across layers in the country rock
Half Dome; part of the Sierra Nevada batholith
Beginnings of a spatter cone Large cinder cone
Flood basalts with several thick and thin layers. Each layer represents a separate eruption.
pillow lavas http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html
Intermediate & Silicic Eruptions Higher Silica + Lower T = Higher Viscosity Composite or Stratovolcanos Lava Domes Ash Flow Calderas
Formation of Volcanic Domes
Mt Fuji: Stratovolcano
Mt. St. Helen's prior to 1980 eruption, a classic stratovolcano http://www.youtube.com/watch?v=bgrnvhbfikq
Process of formation of ash flow caldera - e.g., Crater Lake, Oregon or the super Caldera of Yellowstone
Size comparison of various volcanic features
Intrusive Rock Bodies Plutonic Less dense magmas rise through the crust Rising magmas slowly cool Viscosity increases Density increases Intrusions form as magma solidifies beneath the surface
Intrusive Rock Bodies Intrusions are classified by their size, shape and relative age Large intrusions Batholiths Stocks Small intrusions Dikes Sills Laccoliths
Figure 4.18. Types of magmatic intrusions
Origin of Magmas Solid rock is at equilibrium with its surrounding Changes in the surroundings may cause solid rock magma Raising T Lowering P Changing composition
Lowering P Origin of Magmas Mantle convection moves deep mantle rocks upwards Raising T Hot mafic magma intrudes into the crust Changing composition Adding small amounts of water
Magma Differentiation Magmas, and the resulting igneous rocks, show a wide range of compositions Source Rock variations cause major and minor variations in the magma Magma Mixing Assimilation
Figure 4.21. Processes of magma differentiation
Magma Differentiation Partial Melting Individual minerals within source rock melt at different T Resulting magma is enriched in many elements, especially SiO 2 Resulting magma is less dense than source rock
Magma Differentiation Fractional Crystallization Individual minerals precipitate from magma at different T Bowen s Reaction Series Remaining magma is enriched in many elements, especially SiO 2 Remaining magma may migrate
Bowen s Reaction Series
Figure 4.22. Bowen s reaction series
Classification of Igneous Rocks 1000 C Solidifying Temperature Increasing Grain Size 500 C Volcanic Rocks Silica Content Minerals Present (in order of abundance) Basalt low pyroxene, olivine, feldspar, & amphibole Andesite intermediate feldspar, amphibole, pyroxene, biotite mica Rhyolite high feldspar, quartz, muscovite mica, & amphibole Plutonic Rocks Gabbro Diorite Granite Lighter Color
Plate Tectonic Setting of Igneous Rocks Divergent Plate Boundaries Partial melting of mantle produces basaltic magma Convergent Plate Boundaries Subduction and partial melting of wet basalt, sediments and the surrounding mantle Andesitic and rhyolitic magma generated through fractional crystallization Ascending magma may assimilate lower crustal material
Plate Tectonic Setting of Igneous Rocks Mantle Plumes Partial melting of rising plumes of solid mantle material Distinctive basaltic magma is produced Rising magma may produce Intraplate island chains Flood basalt Basalt plateaus and rhyolitic calderas
Rock types and tectonic setting
Coarse grained igneous rock
Fine grained igneous rock
Pegmatite: Very coarse grained igneous rock
Porphyritic igneous rock: Big xtals in a fine grain matrix