1. Igneous Rocks and Classifying Igneous Rocks
Chapter 5

2. Igneous Rocks
Igneous rocks are formed from the crystallization of magma.

3. Igneous Rocks
Extrusive igneous rocks are fine-grained rocks that cool quickly on Earth’s surface.

4. Igneous rocks
Basalt is an extrusive igneous rock that is very dark in color. It is the most common type of rock in the Earth's crust and it makes up most of the ocean floor.

5. Igneous Rocks
Intrusive igneous rocks are coarse-grained and cool slowly beneath Earth’s surface.

6. Igneous Rocks Granite The most common intrusive igneous rock
Many granite deposits cross-cut into other rock formations
This cross-cutting is evidence that granite was intruded into existing rocks

7. Igneous Rocks Review of magma:
A slushy mix of molten rock, gases, and mineral crystals.
Silica (SiO2) is the most abundant compound in magma and has the greatest effect on its characteristics.
Basaltic: 50% silica, Andesitic: 60% silica, Rhyolitic: 70% silica
Silica affects melting temp. and viscosity

8. Igneous Rocks Factors that affect magma formation Temperature Pressure
Temperature generally increases with depth in Earth’s crust.
Pressure
Pressure also increases with depth
As the pressure on a rock increases, its melting point also increases

9. Igneous Rocks Water content Mineral content
As water content increases, melting pt. decreases
Mineral content
Different minerals have different melting points
In general, oceanic crust melts at higher temperatures than continental crust
Rocks melt only under certain conditions – the right combination of temperature, pressure, and composition

10. Igneous Rocks How rocks melt
Different parts of a rock may melt at different temperatures due to the different minerals present in the rock
Partial melting: the process whereby some minerals melt at low temperatures while other minerals remain solid

11. Igneous Rocks How rocks melt
As each group of minerals melts, different elements are added to the magma “stew,” thereby changing its composition
If temperatures are not great enough to melt the entire rock, the resulting magma will have a different chemistry from that of the original rock.

12. Igneous Rocks How rocks melt Fractional crystallization
The process wherein different minerals form at different temperatures
When magma cools, it crystallizes in the reverse order of partial melting (the first minerals to crystallize from magma are the last minerals to melt during partial melting)

13. Igneous Rocks Feldspars
Feldspar minerals undergo a continuous change of composition
As magma cools, the first feldspars to form are rich in calcium
As cooling continues, these feldspars react with magma, and their calcium-rich compositions change to sodium-rich compositions

14. Igneous Rocks Feldspars
In come instances, as when magma cools rapidly, the calcium-rich cores are unable to react completely with the magma.
The result is a zoned crystal that has sodium-rich outer layers and calcium-rich cores

15. Igneous Rocks Iron-rich minerals
These minerals undergo abrupt changes during fractional crystallization.
As minerals form, elements are removed from the remaining magma
Silica and oxygen are left over
When the remaining magma finally crystallizes, quartz is formed.

16. Igneous Rocks Crystal separation Layered intrusions
Crystal separation can occur when:
Crystals settle to the bottom of the magma body
Liquid magma is squeezed from the crystal mush to form two distinct bodies with different compositions.
Layered intrusions
Formed when minerals form into distinct bands

17. Intermission – Part II next class

18. Classifying Igneous Rocks
Mineral composition
Felsic
Light-colored, have high silica contents
Contain quartz and feldspars orthoclase, and plagioclase
Example: Granite

19. Classifying Igneous Rocks
Mineral composition
Mafic
Dark-colored, have lower silica contents, rich in iron and magnesium
Contain plagioclase, biotite, amphibole, pyroxene, and olivine.
Example: Diorite

20. Classifying Igneous Rocks
Ultramafic
Low silica content and very rich in iron and magnesium
Theory: formed by the fractional crystallization of olivine and pyroxene
The minerals may have separated from magma and did not convert to another mineral upon reaching a particular temperature

21. Classifying Igneous Rocks
Grain size
Fine-grained vs. coarse-grained
Cooling rates
When lava cools on Earth’s surface, there is not enough time for large crystals to form.
Thus, extrusive igneous rocks have no visible mineral grains
When magma cools beneath the surface, large crystals form.
Thus, intrusive igneous rocks may have crystals larger than 1cm.

22. Classifying Igneous Rocks
Texture
Porphyritic texture: when a rock has grains of two different sizes.
Large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.
Porphyritic textures indicate a complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly.

23. Classifying Igneous Rocks
Ore deposits
Veins
The fluid left over during fractional crystallization contains any leftover elements that were not incorporated into the common igneous minerals
They include: gold, silver, lead, and copper.
These elements are released at the end of magma crystallization in a hot, mineral-rich fluid that fills cracks and voids in the surrounding rock
This fluid solidifies to form metal-rich quartz veins.

24. Classifying Igneous Rocks
Pegmatites
Veins of extremely large-grained minerals are called pegmatites.
Ores of rare elements such as lithium and beryllium are found in pegmatites
Pegmatites can also produce beautiful crystals
Because these veins fill cavities and fractures in rock, minerals grow into voids and retain their shapes.

25. Classifying Igneous Rocks
Kimberlites
Rare, ultramafic rocks where minerals such as diamonds are found
Kimberlites are a variety of periodite
They likely form deep in the crust at depths of km or in the mantle because diamond and other minerals found in kimberlites can form only under very high pressures.