1. Lecture 4: Igneous Rocks
GY111 Physical Geology
Lecture 4: Igneous Rocks
Lecture notes on Igneous Rocks for Physical Geology.

2. Types of Rocks Rock: an aggregate of one or more minerals
Igneous Rocks: crystallize from a magma
Sedimentary Rocks
Clastic: formed by the erosion of pre-existing rocks
Chemical/Biochemical: precipitated from chemical reactions
Metamorphic Rocks: formed by exposure to extreme heat & pressure below the melting point
Types of Rocks:
1. Rock: an aggregate of one or more minerals.
2. Igneous Rocks: crystallize from a magma.
3. Sedimentary Rocks:
Clastic: formed by the erosion of pre-existing rocks
Chemical/Biochemical: precipitated from chemical reactions
4. Metamorphic Rocks: formed by exposure to extreme heat & pressure below the melting point.

3. Magma
Magma is generated in the interior earth by heat from radioactive minerals
Volcanic eruptions prove that magma exists near the surface of the earth
Laboratory studies verify that common rocks will melt at the T & P inside the earth
Coarse grained igneous rocks prove that magma must cool slowly, and the only way that that can happen is that the surrounding rocks must be almost as hot as the magma itself
Magma:
1. Magma is generated in the interior earth by heat from radioactive minerals.
2. Volcanic eruptions prove that magma exists near the surface of the earth.
3. Laboratory studies verify that common rocks will melt at the T & P inside the earth.
4. Coarse grained igneous rocks prove that magma must cool slowly, and the only way that that can happen is that the surrounding rocks must be almost as hot as the magma itself.

4. Intrusive Igneous Rocks
Cool slowly at depths > 1 km
Form coarse-grained textures
Surrounding rock is termed “country” rock
May contain portions of the country rock that “fall” into the original magma chamber forming a xenolith
Intrusive Igneous Rocks:
1. Cool slowly at depths > 1 km.
2. Form coarse-grained textures.
3. Surrounding rock is termed “country” rock.
4. May contain portions of the country rock that “fall” into the original magma chamber forming a xenolith.

5. Extrusive Igneous Rocks
Form on the Earth’s surface
Lava: flow of magma onto the Earth’s surface
Pahoehoe: ropy surface (low viscosity)
Aa: fragmental surface (high viscosity)
Pyroclastic rocks: form from the explosive eruption of volcanoes
Ash: particles of glass
Tuff: a rock composed of fragments of pre-existing rock in an ash matrix
Pumice: a rock so full of voids (vesicles) that it can float in water (S.G. < 1.0)
Obsidian: massive volcanic glass
Extrusive Igneous Rocks:
1. Form on the Earth’s surface.
2. Lava: flow of magma onto the Earth’s surface:
Pahoehoe: ropy surface (low viscosity)
Aa: fragmental surface (high viscosity)
3. Pyroclastic rocks: form from the explosive eruption of volcanoes:
Ash: particles of glass
Tuff: a rock composed of fragments of pre-existing rock in an ash matrix
Pumice: a rock so full of voids (vesicles) that it can float in water (S.G. < 1.0)
Obsidian: massive volcanic glass

6. Lava Flow Types Pahoehoe: ropy Aa: fragmented Lava Flow Types:

7. Igneous Textural Terms
Aphanitic: mineral grains in rock are too small to be identified with a hand lens (rock cooled from magma rapidly)
Phaneritic: minerals grains in rock are large enough to be identified with a hand lens (rock cooled relatively slowly)
Phenocrysts: crystals that are distinctly larger than surrounding mineral grains
Porphyritic: a texture where relatively large phenocryst mineral grains are surrounded by smaller grains
Igneous Textural Terms:
1. Aphanitic: mineral grains in rock are too small to be identified with a hand lens (rock cooled from magma rapidly).
2. Phaneritic: minerals grains in rock are large enough to be identified with a hand lens (rock cooled relatively slowly).
3. Phenocrysts: crystals that are distinctly larger than surrounding mineral grains.
4. Porphyritic: a texture where relatively large phenocryst mineral grains are surrounded by smaller grains.

8. View of Textural Types Aphanitic Phaneritic Textural Types:
1. Aphanitic: grains too small to be identified with naked eye.
2. Phaneritic: grains large enough to be identified with naked eye.

9. Composition
Felsic: light colored igneous rock relatively rich in Si, Na and K.
Intermediate: rock made up of equal proportions light and dark minerals
Mafic: dark colored rock relatively rich in Ca, Fe and Mg
Ultramafic: dark colored rock relatively rich in Fe and Mg
Note: red is considered a felsic (light) color; green is considered a mafic (dark) color
Composition:
1. Felsic: light colored igneous rock relatively rich in Si, Na and K.
2. Intermediate: rock made up of equal proportions light and dark minerals.
3. Mafic: dark colored rock relatively rich in Ca, Fe and Mg.
4. Ultramafic: dark colored rock relatively rich in Fe and Mg.
Note: red is considered a felsic (light) color; green is considered a mafic (dark) color

10. Where Different Igneous Textures Form
Aphanitic texture: fast-cooling rate, form on or near surface.
Phaneritic texture: slow-cooling rate, form at > 1 km depth.
Glassy texture: extremely fast cooling rate.

11. Common Igneous Minerals
Felsic: Quartz, K-feldspar, Na-Plagioclase, Muscovite
Intermediate: Amphibole, Na-Plagioclase, Biotite
Mafic: Olivine, Pyroxene, Ca-Plagioclase
Ultramafic: Pyroxene, Olivine

12. Classification of Igneous Rocks
Based on Mineral Content & Texture
1. Classification is based on composition and texture.

13. Magma Formation
Magma formation is favored by increasing temperature and decreasing pressure
Magma formation is favored by increasing H2O content because it effectively lowers the melting point of minerals in rocks
Several tectonic environments favor magma formation:
Divergent boundaries, Hot Spots: pressure reduction in upwelling mantle (Decompression melting)
Convergent boundaries: increasing temperature and water content in subducting slab; frictional heating
Magma Formation:
1. Magma formation is favored by increasing temperature and decreasing pressure.
2. Magma formation is favored by increasing H2O content because it effectively lowers the melting point of minerals in rocks.
3. Several tectonic environments favor magma formation:
Divergent boundaries, Hot Spots: pressure reduction in upwelling mantle (decompression melting).
Convergent boundaries: increasing temperature and water content in subducting slab; frictional heating.

14. Granite Melting Curves
Experimental results with actual granite rock displays effect of pressure and water
Divergent
decompression 10
35 km
solid
melt
Dry melting
curve 8 P
Kbar
Convergent
Subduction
Granite Melting Curves:
Dry melting: increased pressure correlates to higher melting point (Divergent).
Wet melting: increased pressure correlates to lower melting point (Convergent).
20 km 6
Wet (H2O) melting
curve
solid
melt 4
600
T Deg. C
500
800

15. Fractional Crystallization
Controlled by Bowen’s Reaction Series
Bowen’s Reaction Series
Discontinuous: Olivine > Pyroxene > Amphibole > Biotite > Quartz > Muscovite > Orthoclase.
Continuous: Ca-Plagioclase > Na-Plagioclase.
Discontinuous
Series
Continuous
Series

16. Palisades Sill: Example of Fractional Crystallization
Early high-temp crystals settle to the base of the magma chamber
1. Layering within sill follows Bowen’s reaction series.

17. Palisades Sill cont.
The end result is a layered intrusion- different layers have different compositions
Note the chill margin is the initial magma that was “chilled” by being in contact with cold wall rock. Because the margin was cooled fast the chill margin is basalt.
The sill crystallized from the bottom to the top.

18. Forms of Magma Intrusions
Batholith: discordant; >= 100 km2
Stock: discordant; >= 1 and < 100 km2
Pluton: discordant; < 1 km2
Dike: discordant; tabular
Sill: concordant; tabular
Laccolith: concordant; shield shaped
Lopolith: concordant; saucer shaped
Forms of Magma Intrusions:
1. Discordant
a. Batholith: discordant; >= 100 km2
b. Stock: discordant; >= 1 and < 100 km2
c. Pluton: discordant; < 1 km2
d. Dike: discordant; tabular
2. Concordant
a. Sill: concordant; tabular
b. Laccolith: concordant; shield shaped
c. Lopolith: concordant; saucer shape

19. Intrusive Forms
Note: laccoliths and lopoliths are not shown in this schematic
1. Intrusive rocks will normally have phaneritic texture.

20. Plate Boundary Associations: Divergent
Divergent Boundaries: production of ophiolite sequences
Ultramafic mantle partially melts to form basalt and gabbro (mafic rocks)
While in contact with ocean water the ocean crust is hydrated and altered chemically (seawater alteration)
Plate Boundary Associations: Divergent
1. Divergent Boundaries: production of ophiolite sequences.
2. Ultramafic mantle partially melts to form basalt and gabbro (mafic rocks).
3. While in contact with ocean water the ocean crust is hydrated and altered chemically (seawater alteration).

21. Plate Boundary Associations: Convergent
Subducted ocean lithosphere partially melts to produce intermediate and felsic magma
The hydration of the ocean lithosphere dramatically lowers its melting point leading to abundant felsic to intermediate magma generation
Plate Boundary Associations: Convergent
1. Subducted ocean lithosphere partially melts to produce intermediate and felsic magma.
2. The hydration of the ocean lithosphere dramatically lowers its melting point leading to abundant felsic to intermediate magma generation.

22. Volcanic Landforms Central Vent Eruptions 1. Central Vent Eruptions:
Shield Volcanoes: low viscosity lava flows
Volcanic domes: viscous lava extruded as a dome after major pyroclastic eruption
Cinder cones: small low viscosity eruptions that spatter small fragments of lava that solidify as cinders
Stratovolcanoes: high viscosity pyroclastic eruptions build a steep-sided cone
Craters/Calderas: explosive eruptions will blast a small crater at the summit of a volcano, or a large caldera for more violent eruptions
Diatremes: rapid intrusion of a very low viscosity carbonate-rich magma. Diamond bearing diatremes are termed “Kimberlites”
Volcanic Landforms:
1. Central Vent Eruptions:
Shield Volcanoes: low viscosity lava flows.
Volcanic domes: viscous lava extruded as a dome after major pyroclastic eruption.
Cinder cones: small low viscosity eruptions that spatter small fragments of lava that solidify as cinders.
Stratovolcanoes: high viscosity pyroclastic eruptions build a steep-sided cone.
Craters/Calderas: explosive eruptions will blast a small crater at the summit of a volcano, or a large caldera for more violent eruptions.
Diatremes: rapid intrusion of a very low viscosity carbonate-rich magma. Diamond bearing diatremes are termed “Kimberlites”.

23. Volcanic Landforms cont.
Central vent eruptions
Shield
Lava dome
Cinder cone
Stratovolcano (Composite)
Caldera

24. Caldera Formation
Result from very large pyroclastic eruptions (Super Eruptions)
The Yellowstone Caldera is one example
Caldera Formation:
1. Result from very large pyroclastic eruptions (Super Eruptions).
2. The Yellowstone Caldera is one example.

25. Fissure Eruptions
Flood Basalts: large outpourings of low viscosity basaltic lava fills in low areas
Ash Flow deposits: result from the fissure eruption of felsic magma to produce extremely large pyroclastic flows (Yellowstone)
Fissure Eruptions:
1. Flood Basalts: large outpourings of low viscosity basaltic lava fills in low areas.
2. Ash Flow deposits: result from the fissure eruption of felsic magma to produce extremely large pyroclastic flows (Yellowstone).

26. Columbia River Flood Basalts
An example of a fissure eruption of mafic lava
Columbia River Flood Basalts:
1. An example of a fissure eruption of mafic lava. Magma originates from a hot spot.

27. Hydrothermal Vents Water-rich liquid at high temperature
Under high pressure water may have a temperature of over 400 deg. C and still be a liquid phase
Geysers: interaction between groundwater and a volcanic magma chamber
Hydrothermal veins: important economic mineral sources; boil off from magma during fractional crystallization
Hydrothermal Vents:
1. Water-rich liquid at high temperature = hydrothermal.
2. Under high pressure water may have a temperature of over 400 deg. C and still be a liquid phase.
3. Geysers: interaction between groundwater and a volcanic magma chamber.
4. Hydrothermal veins: important economic mineral sources; boil off from magma during fractional crystallization.

28. Global Patterns of Volcanism
Divergent: low viscosity mafic magma with little or no H2O; generate shield volcanoes (Iceland)
Convergent: high viscosity intermediate and felsic magma with abundant H2O; generate stratovolcanoes (Cascade Range)
Hot Spot: low viscosity dry mafic magma produces shield volcanoes under ocean lithosphere (Hawaii); high viscosity wet felsic magma under continental lithosphere (Yellowstone)
Global Patterns of Volcanism:
1. Divergent: low viscosity mafic magma with little or no H2O; generate shield volcanoes (Iceland).
2. Convergent: high viscosity intermediate and felsic magma with abundant H2O; generate stratovolcanoes (Cascade Range).
3. Hot Spot: low viscosity dry mafic magma produces shield volcanoes under ocean lithosphere (Hawaii); high viscosity wet felsic magma under continental lithosphere (Yellowstone Caldera).

29. Exam Summary Know intrusive geometry classes
Know textural terms (aphanitic, phaneritic, etc.)
Know common rock-forming silicates in felsic, intermediate, etc., compositions
Know the characteristics of Shield versus Composite Cone volcanoes.
Be able to diagram Bowen’s Reaction Series and describe the Palisades Sill as an example or fractional crystallization.
Be able to describe the conditions that lead to the formation of aa, pahoehoe, pumice, obsidian, welded tuff, scoria.
Be able to explain why some volcanoes extrude low-viscosity lava whereas others tend to erupt explosively. Relate low- versus high-viscosity magma to types of plate tectonic boundaries.
Exam Summary:
1. Know intrusive geometry classes.
2. Know textural terms (aphanitic, phaneritic, etc.).
3. Know common rock-forming silicates in felsic, intermediate, etc., compositions.
4. Know the characteristics of Shield versus Composite Cone volcanoes.
5. Be able to diagram Bowen’s Reaction Series and describe the Pallisades Sill as an example or fractional crystallization.
6. Be able to describe the conditions that lead to the formation of aa, pahoehoe, pumice, obsidian, welded tuff, scoria.
7. Be able to explain why some volcanoes extrude low-viscosity lava whereas others tend to erupt explosively. Relate low- versus high-viscosity magma to types of plate tectonic boundaries.