The Nature of Igneous Rocks

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2 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

3 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

4 The Nature of Igneous Rocks Composition varies widely Silica and water content control viscosity 2 end members are: Mafic magmas Silicic magmas

5 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)

6 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

7 Figure 4.2. Distribution of igneous rocks in North America

8 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

9 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

10 Figure 4.3A. Glassy texture in obsidian

11 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

12 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

13 Figure 4.3B. Aphanitic texture in rhyolite

14 Phaneritic Texture Coarse grained texture Relatively slow rate of cooling Equigranular, interlocking crystals Slow cooling = crystallization at depth

15 Figure 4.3C. Phaneritic texture in granite

16 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

17 Porphyritic andesite

18 Porphyritic olivine basalt

19 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

20 Figure 4.3D. Pyroclastic texture

21 Classification of Igneous Rocks Texture Aphanitic Phanaritic Composition Silicic Intermediate Mafic Ultramafic Combination of Texture and Composition produces rock name

22 Figure 4.4. Classification of common igneous rocks

23 Extrusive Rock Bodies Form of extrusive bodies influenced by magma properties Composition Silica content Viscosity Volatile content Temperature

24 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

25 Aa flow Pahoehoe flow Figures 4.6 A & B

26 Beginnings of a spatter cone (Fig 4.6F) Large cinder cone (Fig 4.8)

27 Fig 4.7. Flood basalts with several thick and thin layers. Each layer represents a separate eruption.

28 Formation of pillow lavas (Fig 4.12)

29 Intermediate & Silicic Eruptions Higher Silica + Lower T = Higher Viscosity Produce Lava (Rhyolite) Domes - small Composite volcanos - medium Ash Flow Calderas - large

30 Formation of Volcanic Domes (Fig A & B)

31 Fig Mt. St. Helen's prior to 1980 eruption, a classic composite volcano

32 Process of formation of ash flow caldera - e.g., Crater Lake, OR (Fig 4.15)

33 Fig Size comparison of various volcanic features

34 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

35 Intrusive Rock Bodies Intrusions are classified by their size, shape and relative age Large intrusions Batholiths Stocks Small intrusions Dikes Sills Laccoliths

36 Figure Types of magmatic intrusions

37 Figure 4.2. Distribution of igneous rocks in North America

38 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

39 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)

40 Igneous Rocks and Plate Tectonics Convergent margins (cont. & oceanic)

41 Igneous Rocks and Plate Tectonics Divergent (oceanic crust)

42 End of Chapter 4