Download pp chpt 4 Igneous Rocks.pptx

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 - H2O, CO2, 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
– 1000o to 1200oC
•  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 850oC
•  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