1. CHAPTER 5: Igneous Rock

2. Rock of Ages granite quarry, Barre, Vermont
What important natural resources are found in igneous rock?

3. Stawamus Chief, Squamish BC
Mafic dyke (~5 m across)
Massive granite
Steven Earle

4. Igneous Rock
Igneous rock is formed when molten, or partially molten, rock solidifies.

5. Igneous Rocks (two types)
Extrusive
Lava and Pyroclastic Debris
Extruded at surface or at very
shallow levels and cooled quickly
Intrusive
Magma crystallized slowly within the crust.
No exposure to the cool atmosphere.
Plutonic – intrusive igneous rock cooled slowly at great depth within crust or mantle.
Basalt is Extrusive
Granite is Intrusive
What is the key textural difference between these two rocks?

6. Why Study Igneous Rocks?
All rocks on Earth have evolved from the first igneous rocks through Igneous Evolution
Provide information about the composition of the mantle
Absolute age can be obtained through Radiometric Dating
Features are characteristic of a specific tectonic environment
Host important mineral deposits (metal ores)
Numerous other critical economic uses
Millions of people are endangered by volcanoes

7. Decompression melting
Convection brings hot rock to shallower depths
Rifting lowers pressure
Lower pressure causes a lower melting point
Igneous rock is formed through a process of crystallization and magma differentiation
Why does a lower pressure lower the melting point?

8. Partial Melting Partial Melting Magma Differentiation
Silica-rich compounds melt before other compounds.
Magma Differentiation
Four types:
Magma mixing
Crystal settling
Wall-rock assimilation
Magma migration

9. Bowen’s Reaction Series
Describes the order and process of magma crystallization
Bowen’s Reaction Series Animation

10. How Are Igneous Rocks Named?
Texture
Texture depends on crystal size
Phaneritic:
Slow cooling (or in the presence of water), larger crystals
Coarse texture
Intrusive or Plutonic
Aphanitic:
Rapid cooling , smaller crystals
Finer texture
Extrusive or Volcanic
Other textures are Glassy, Pyroclastic, Vesicular, and Porphyritic
Composition
Minerals at the top of Bowen’s
Dark in color
Mafic to Ultramafic
Iron and Magnesium are dark.
Minerals at the bottom of Bowen’s
Light in color
Felsic
Sodium plagioclase, potassium plagioclase, and quartz are light
Intermediate composition between these

11. How is Texture Estimated?
Texture is estimated using visual grain size
(depends on crystallization history)
What has to happen for a porphyritic texture to form in an igneous rock?

12. How is Composition Estimated?
Composition is estimated using visual color
Can be misleading

13. Mafic vs. Felsic
Mafic minerals crystallize early and felsic minerals crystallize late in magma
Minerals at the top of Bowen’s Reaction Series:
Dark in color
Mafic to ultramafic
Iron- and magnesium-rich and relatively heavy
Minerals at the bottom of Bowen’s Reaction Series:
Light in color
Intermediate to Felsic
Iron- and magnesium-poor and relatively light in weight

14. Bowen’s Reaction Series
What does olivine react with to produce pyroxene?
What is the significance of the word “reaction” in the Bowen Reaction Series?

15. Igneous Rock Naming
Igneous rocks are named on the basis of their texture and composition

16. Igneous Rock Naming (continued)
The igneous rock diagram shows the range of mineral proportions of the various types of igneous rocks
What are the approximate mineral proportions (in %) of the rocks indicated by the two dashed lines?

17. Classification System for Igneous Rocks
The classification system for igneous rocks is based on the following:
Intrusive and extrusive igneous rocks with the same chemical composition contain the same minerals
Silica content decreases as iron and magnesium content increases
Potassium and sodium content decreases as silica content decreases

18. Seven Common Types of Igneous Rock
Felsic Composition
1) Granite
intrusive
2) Rhyolite
extrusive

19. Seven Common Types of Igneous Rock
Intermediate Composition
3) Diorite
intrusive
4) Andesite
extrusive

20. Seven Common Types of Igneous Rock
Mafic Composition
5) Gabbro
intrusive
6) Basalt
extrusive

21. Seven Common Types of Igneous Rock
Ultramafic Composition
7) Peridotite
intrusive

22. Igneous Evolution
All rocks on Earth have evolved from the first igneous rocks
All Igneous Rocks Result from Magma Differentiation

23. Tectonic Processes
Igneous rock evolves as a product of tectonic processes and is a major component of Earth’s crust
What is the mechanism that leads to partial melting above a subduction zone?

24. Identify the magma source for each type of volcano
Identify the magma source for each type of volcano. How is the magma source responsible for the rock composition?

25. Basalt Formation (Spreading Centers)
High heat flow creates Hydrothermal (hot water) Vents (black smokers)
Oceanic crust consists of layers of igneous rock
Gabbro crystallizes at the base
Sheeted dikes of basalt extend above the gabbro
Pillow basalt, glassy fragments and metallic sulfide deposits (at black smokers) form at the sea floor

26. Basalt Formation (Hot Spots)
Intraplate sites of active Plutonism and Volcanism
Large Igneous Provinces of Flood Basalts form above mantle hot spots
Plumes of anomalously hot mantle rock underly active hot spots. Mantle plumes, which are solid rock, not magma, rise slowly through the mantle (but faster than the rate of mantle convection) due to their positive buoyancy with respect to the surrounding rock.
Basalt is also present at many subduction-related volcanoes, but typically only in minor amounts.

27. Modern sea-floor pillow basalt
40 mya pillow basalt on Vancouver Island
Steven Earle

28. Igneous Intrusions
Igneous intrusions occur in a variety of sizes and shapes.

29. Subduction Zones and Spreading Centre
Subduction zones and spreading centers are areas in which magma is formed as a result of partial melting, plutonism, and volcanism.
What igneous environment is not represented in this image?

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