The document discusses the formation and characteristics of igneous rocks, explaining the differences between intrusive and extrusive types based on magma and lava behavior. It details the composition, textures, and the processes involved in magma evolution, including conditions leading to the formation of various magma types. Additionally, it covers the role of plate tectonics in the distribution and differentiation of igneous rocks.

1. Igneous
Rocks

2. Sawtooth Mtns. near Stanley, ID
-example of a intrusive igneous rocks….not to mention
a very cool place to do fieldwork……

3. Lava flow in Hawaii
[magma @ the Earth’s
surface]
Pyroclastic Flow
Montserrat Volcano

4. What is a rock? [Yes, you really do care.]

6. The Nature of Igneous Rocks
• Form from Magma [Greek=“paste”]
– Hot, partially molten mixture of solid
liquid and gas
– Gases: H2O, CO2, etc.
– less dense than
solid rock
– solidify upon
cooling

7. “The beer I had for breakfast wasn’t bad
so I had one more for desert.”
-Sunday Morning Coming Down
by Kris Kristofferson

8. • Magma vs. Lava
– Magma: molten rock beneath the surface
– Lava: molten rock that has reached the
surface
– Magma: form intrusive igneous rocks
– Lava: form extrusive igneous rocks

9. • Composition varies widely
– Oxygen plus major elements
– Generally a silica (SiO2) melt
– Silica and water content control viscosity
– Silica content used in classification

10. Mafic Magmas
• Silica content ~ 50%
• High Fe, Mg and Ca
• High temperature molten magma
– 1000o to 1200oC
• Major minerals:
– Olivine - Ca Plagioclase
– Pyroxene

13. Silicic Magma
• Silica content: 65-77%
• High Al, Na and K
• Lower temperature magmas
– Less than 850oC
• Major minerals:
– Feldspars - Micas
– Quartz

16. Magma Viscosity
-resistance to flow

17. • Controlled by silica and water content, and
temperature
• As magma cools-silica tetrahedron form links
• Linkages control
viscosity:
•High Silica=high viscosity
•Low Silica=low viscosity

18. Magma Viscosity
• High Silica=high viscosity/Low Silica=low
viscosity
• Cooler Temperatures=higher viscosity/Higher
Temperatures=lower viscosity
• Water and volatiles break linkages=lower
viscosity

19. Occurrence of Igneous Rocks
• Found globally
• Formed in discrete geologic settings
– Convergent plate margins
– Divergent plate margins
– Mantle plumes

21. Igneous Textures
• Texture - the size, shape and relationship of minerals in the
rock
• Cooling history of the magma or lava
• Crystal size increases as rate of cooling slows
• Texture - the size, shape, and arrangement of interlocking
minerals
• Factors affecting crystal size
• Rate of cooling
Slow rate = fewer but larger crystals
Fast rate = many small crystals
Very fast rate forms glass

22. IGNEOUS TEXTURES
 Types of igneous textures
Aphanitic (fine-grained) texture
 Rapid rate of cooling
 Microscopic crystals
 May contain vesicles (holes from gas bubbles)
Phaneritic (coarse-grained) texture
 Slow cooling
 Large, visible crystals
 Kinds of igneous textures
 Porphyritic texture
 Glassy texture
 Pyroclastic texture
 Pegmatitic texture
 Exceptionally coarse grained
 Form in late stages of crystallization of granitic magmas

23. APHANITIC TEXTURE

24. PHANERITIC TEXTURE

25. Glassy Texture
• Very rapid cooling - quenched
– Volcanic glass
– Conchoidal fracture
– Rock is called obsidian
• No apparent crystals
Lava flow quenching
in ocean water.

26. GLASSY TEXTURE

27. Glassy texture in obsidian

29. Crystalline Textures
• Crystal growth requires time for ions to
migrate - form minerals
• Slow rate of cooling=time for crystal
growth
• Crystals grow until melt is quenched or
completely solidified

30. Aphanitic Texture
• Fine grained texture
• Few crystals visible in hand specimen
• Relatively rapid rate of cooling

31. Aphanitic texture in rhyolite

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

33. Phaneritic texture in granite

34. Pegmatites from the
Sawtooth Mtns.
-unusually large
crystals

35. Porphyritic Texture
 Minerals form at different temperatures
 Large crystals (phenocrysts) are embedded in a matrix
of smaller crystals (groundmass)
• Well formed crystals (phenocrysts)
• Fine grained matrix (groundmass)
• Complex cooling history
– Initial stage of slow cooling
– Later stage of rapid cooling

36. PORPHYRITIC TEXTURE

38. Pyroclastic Texture
• Fragmental appearance produced by
violent volcanic eruptions
• Explosive volcanic eruptions
• Appear porphyritic with visible crystals
– Crystals show breakage or distortion
• Matrix dominated by glassy fragments
– Hot fragments may “weld” together

40. IGNEOUS COMPOSITIONS
 Granitic versus basaltic compositions
Granitic [felsic] composition
 Light-colored silicates
 Termed felsic (feldspar and silica) in composition
 High silica (SiO2) content
 Major constituent of continental crust
Basaltic [mafic] composition
 Dark silicates and Ca-rich feldspar
 Termed mafic (magnesium and ferrum, for iron) in
composition
 Higher density than granitic rocks
 Comprise the ocean floor and many volcanic islands

41. GRANITE

42. RHYOLITE

43. BASALT

44. GABBRO

45. IGNEOUS COMPOSITIONS
 Other compositional groups
Intermediate (or andesitic) composition
 Contain 25% or more dark silicate minerals
 Associated with explosive volcanic activity
Ultramafic composition
 Rare composition that is high in magnesium and iron
 Composed entirely of ferromagnesian silicates

46. ANDESITE

47. DIORITE

48. IGNEOUS COMPOSITIONS
 Silica content as an indicator of composition
Crustal rocks exhibit a considerable range
 45% to 70%
 Silica content influences magma behavior
Granitic magmas = high silica content and
viscous
Basaltic magmas = much lower silica content
and more fluid-like behavior

51. GENERAL CHARACTERISTICS
OF MAGMA
 Igneous rocks form as molten rock cools and solidifies
 General characteristics of magma
 Parent material of igneous rocks
 Forms from partial melting of rocks
 Magma at surface is called lava
 Rocks formed from lava = extrusive, or volcanic rocks
 Rocks formed from magma at depth = intrusive, or plutonic
rocks
 The nature of magma
 Consists of three components:
 Liquid portion = melt
 Solids, if any, are silicate minerals
 Volatiles = dissolved gases in the melt, including water vapor (H2O),
carbon dioxide (CO2), and sulfur dioxide (SO2)

54. Classification of Igneous Rocks
–Texture
• Aphanitic
• Phanaritic
–Composition
• Silicic
• Intermediate
• Mafic
• Ultramafic
Combination of Texture and Composition
produces rock name

56. Origin of Magmas
Generating magma from solid rock
• Role of heat
 Temperature increases in the upper crust (geothermal gradient) average
between 20oC to 30oC per kilometer
 Rocks in the lower crust and upper mantle are near their melting points
 Additional heat may induce melting
• Role of pressure
 Increases in confining pressure increases a rock’s melting temperature
 When confining pressures drop, decompression melting occurs
• Role of volatiles
 Volatiles (primarily water) cause melting at lower temperatures
 Important factor where oceanic lithosphere descends into the mantle
• Solid rock is at equilibrium with its surrounding
• Changes in the surroundings cause solid rock magma [melting]
• Lowering P
 Mantle convection moves deep mantle rocks upwards
• Raising T
 Hot mafic magma intrudes into the crust
• Changing composition
 Adding small amounts of water

58. How do rocks melt?
Heat: Geothermal Gradient
Temperature of most
Silicic magmas [600-800oC]
-about the base of crust
Temperature of Mafic
Magmas [1100-1200oC]
-upper mantle conditions

59. EVOLUTION OF MAGMAS
 A single volcano may extrude lavas exhibiting
very different compositions
 Bowen’s reaction series
Minerals crystallize in a systematic fashion
based on their melting points
During crystallization, the composition of the
liquid portion of the magma continually changes

60. BOWEN’S REACTION SERIES

62. EVOLUTION OF MAGMAS
 Processes responsible for changing a magma’s
composition
Magmatic differentiation
 Separation of a melt from earlier formed crystals
Assimilation
 Changing a magma’s composition by incorporating
surrounding rock bodies into a magma
Magma mixing
 Two chemically distinct magmas may produce a
composition quite different from either original magma

64. 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

66. • Partial Melting
– Individual minerals in source rock melt at
different T
– Magma is enriched in many elements,
especially SiO2
How do we know this?
Norman Bowen [1887-1956]
-Experimental Petrologist
-developed Bowen’s Reaction Series

67. Bowen’s reaction series

68. • Fractional Crystallization
– Individual minerals precipitate from
magma at different T
– Magma is enriched in many elements,
especially SiO2
– Magma may migrate [buoyancy]

70. Bowen’s reaction series

71. Plate Tectonic Setting of Igneous Rocks
• Divergent Plate Boundaries
– Partial melting of mantle produces basaltic magma
• Convergent Plate Boundaries
– Subduction produces partial melting of basalt, sediments, parts of mantle
– Andesitic and rhyolitic magma
– Ascending magma assimilates lower crustal material
• Mantle Plumes
– Partial melting of plumes of mantle material
– Basaltic magma is produced
– Rising magma produce
• Intraplate island chains
• Flood basalt [Columbia River Basalts]

73. DECOMPRESSION MELTING

74. FIGURE 4.21