The Nature of 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
The Nature of Igneous Rocks Magma vs. Lava 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
The Nature of Igneous Rocks Composition varies widely Silica and water content control viscosity 2 end members are: Mafic magmas Silicic magmas
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, pyroxene, Ca plagioclase Fluid flow (low viscosity)
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, quartz, micas Viscous, thicker than mafic magmas
Figure 4.2. Distribution of igneous rocks in North America
Igneous Textures Texture - the size, shape and relationship of minerals in the rock Relates the cooling history of the magma or lava fast vs. slow Large crystals slow cooling; Small/microscopic crystals fast cooling
Glassy Texture Very rapid cooling - quenched Volcanic glass Conchoidal fracture No apparent crystals embryonic crystals may be present Dark color from low concentrations of Fe - 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
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
Porphyritic andesite
Porphyritic olivine basalt
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 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; cinder cones (v. small) Shield Volcanoes (v. large) Pillow lavas
Aa flow Pahoehoe flow Figures 4.6 A & B
Beginnings of a spatter cone (Fig 4.6F) Large cinder cone (Fig 4.8)
Fig 4.7. Flood basalts with several thick and thin layers. Each layer represents a separate eruption.
Formation of pillow lavas (Fig 4.12)
Intermediate & Silicic Eruptions Higher Silica + Lower T = Higher Viscosity Produce Lava (Rhyolite) Domes - small Composite volcanos - medium Ash Flow Calderas - large
Formation of Volcanic Domes (Fig. 4.13 A & B)
Fig 4.14. Mt. St. Helen's prior to 1980 eruption, a classic composite volcano
Process of formation of ash flow caldera - e.g., Crater Lake, OR (Fig 4.15)
Fig. 4.9. Size comparison of various volcanic features
Intrusive Rock Bodies 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
Figure 4.2. Distribution of igneous rocks in North America
Plate Tectonic Setting of Igneous Rocks Divergent Plate Boundaries mid-ocean ridges and continental rifts Partial melting of mantle produces basaltic magma Convergent Plate Boundaries Subduction and partial melting of basalt, sediments and the surrounding mantle forms overlying volcanoes Andesitic and rhyolitic magma generated
Plate Tectonic Setting of Igneous Rocks Mantle Plumes aka Hot Spots Partial melting of rising plumes of solid mantle material If located in oceanic crust then basaltic magmas ex. Hawaiian Islands If located in continental crust then either rhyolite calderas (Yellowstone Nat l Park) or flood basalts (Snake River/Columbia Plateau)
Igneous Rocks and Plate Tectonics Convergent margins (cont. & oceanic)
Igneous Rocks and Plate Tectonics Divergent (oceanic crust)
End of Chapter 4