1. Igneous Rocks
Text ref. Ch5 (pg. 98)

2. Contents
What are igneous rocks?
Classifying igneous rocks.

3. What are igneous rocks?
Text ref. ch.5.1 (pg )

4. Objectives Compare and contrast intrusive and extrusive igneous rocks
Describe the composition of magma
Discuss the factors that effect how rocks melt and crystallize

5. Igneous Rocks Formed from the crystallization of magma
Lava is magma the flows out onto the Earth’s surface.
Extrusive igneous rocks are fine grained and cool quickly on Earth’s surface (magma)
Intrusive igneous rocks are coarse grained and cool slowly beneath the Earth’s surface. (lava flows)

7. Composition of Magma
Magma is often a slushy mix of molten rock, gases, and mineral crystals.
Magma is classified on its SiO2 content
Group
SiO2 content
Rhyolitic
70 percent
Andesitic
60 percent
Basaltic
50 percent

8. Factors that Affect Magma Formation
Temperature
Pressure
Water content
Mineral composition

9. Temperature
Temperature gradient compares the internal temperature of the Earth with depth.

10. Pressure Pressure also increases with depth.
In general, as pressure on a rock increases, so does its melting temperature.
Ex. Rock sample may melt at 1000°C at Earth’s surface, but 1400°C at a depth of 100m.

11. Water content Also affects magma formation.
As water content increases, melting decreases

12. Mineral content
Magma containing minerals such as olivine, calcium feldspar, and pyroxene melt at higher temperatures
In general oceanic crust tends to melt at higher temperatures than continental crust, due to its Fe content.

13. How Rocks Melt Rocks melt depending on their composition
Partial Melting
Fractional Crystallization

14. Partial Melting
The process whereby some minerals melt at low temperatures while other minerals remain solid is called partial melting.

15. Fractional Crystallization
The process wherein different minerals form at different temperatures is called fractional crystallization.

16. Fe-Mg minerals

17. Iron-rich minerals
Left branch of Bowen’s reaction series represents iron-rich minerals.
Undergo abrupt changes during fractional crystallization.

18. Layered Intrusions
Minerals sometimes form in layers according to Bowen’s reaction series.
Known a layered intrusion
Important formations in mining: largest sources of platinum in U.S. found in layered intrusions in Montana

20. Classifying Igneous Rocks
Text ref. Ch.5.2,

21. Objectives Classify different types and textures of igneous rocks.
Recognize the effects of cooling rates on the grain sizes of igneous rocks.
Describe some uses of igneous rocks.

22. Mineral Composition
Felsic rocks such as granite are light-colored, have high silica contents, and contain quartz and the feldspars orthoclase and plagioclase.

23. Mineral Composition
Intermediate rocks such as diorite contain moderate amounts of biotite, amphibole, and pyroxene

24. Mineral Composition
Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium.

25. Mineral Composition
Ultramafic rocks are unusual, containing peridotite, dunite, have low silica contents and very high levels of iron and magnesium.

27. Grain Size
Igneous rocks differ in the size of the grains that compose them
Cooling rates will affect grain size
Fast cooling rocks will form smaller grains
Slow cooling rocks will form larger grains

29. Texture
Distinct crystal shapes caused by fractional crystallization can be used to indentify igneous rocks
Porphyritic texture is characterized by large crystals surrounded by smaller crystals of the same or other minerals

30. Igneous Rocks as Resources
Used as building materials
Granite

31. Ore Deposits
Veins of gold, platinum, silver, and other precious metals can be found in igneous rock deposits.

32. Pegmatites are veins that contain extremely large-grained crystals
Pegmatite containing Amethyst

33. Kimberlites
Diamond is found naturally occurring ultramafic rocks known as kimberlites.
Named after Kimberly, South Africa where they were first discovered
Form km; intense pressures required for kimberlites to form