Nature of Igneous Rocks, Magma, Lava, Textures, Types classification,compositions,Bowen’s Reaction Series, characteristics of magma, Origin of Magmas, Evolution of Magma, Magma Differentiation,Partial Melting,Fractional Crystallization, Plate Tectonic Setting of Igneous Rocks
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
11.
12.
13. Silicic Magma
• Silica content: 65-77%
• High Al, Na and K
• Lower temperature magmas
– Less than 850oC
• Major minerals:
– Feldspars - Micas
– Quartz
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
20.
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
25. Glassy Texture
• Very rapid cooling - quenched
– Volcanic glass
– Conchoidal fracture
– Rock is called obsidian
• No apparent crystals
Lava flow quenching
in ocean water.
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
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
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
39.
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
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
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
49.
50.
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)
52.
53.
54. Classification of Igneous Rocks
–Texture
• Aphanitic
• Phanaritic
–Composition
• Silicic
• Intermediate
• Mafic
• Ultramafic
Combination of Texture and Composition
produces rock name
55.
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
57.
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
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
63.
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
65.
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
68. • Fractional Crystallization
– Individual minerals precipitate from
magma at different T
– Magma is enriched in many elements,
especially SiO2
– Magma may migrate [buoyancy]
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]