1. GEOLOGY 101 Course Website: http://fouch101.asu.edu Today: Chapter 6
“Up from the Inferno: Magma and Igneous Rocks”
Instructor:
Professor Matt Fouch
Office: PSF-540
Phone: x5-9292
TA:
Kara Krelove
Office: PSF-209
Course Website:

2. Igneous Rocks
Devil’s Tower,
Wyoming

3. Igneous Rocks: Why should we care?
Igneous rocks make up the bulk of the Earth’s crust
Earth’s mantle is essentially an enormous igneous rock
Igneous rocks are economically important
Many igneous rocks form striking landscape features

4. Igneous Rocks
Formed from the cooling and consolidation of lava or magma
plutonic (intrusive) — cooled below the surface
volcanic (extrusive) — cooled on the surface

5. Magma Vs. Lava What is the difference?
Magma – molten rock beneath Earth’s surface
Lava – magma that has traveled to the surface

6. Fig. 6.02
W. W. Norton

7. What Makes a Magma?
Usually a silicate melt (liquid) at high temperatures (650 to 1200°C).
Mixture of all the elements that make up minerals plus volatile components:
H2O, CO2, Cl, F, S
These components form gases and will
boil off when pressure is released.

8. Melting Rocks How do we melt rocks? 3 ways: Raise the temperature
“heat-transfer melting”
Lower the pressure
“decompression melting”
Add volatiles
H2O, CO2, etc.

9. Temperature scale digression…
Celsius (centigrade) scale
Fahrenheit scale
Water boils
100oC
212oF
oC = (oF - 32) 5 9
Water freezes
0oC
32oC

10. Fig. 6.04a
W. W. Norton

11. Melting temperature increases with increasing pressure (=depth)
SOLID
LIQUID
Melting temperature curve

12. Fig. 6.04b
W. W. Norton

13. Melting temperature increases with increasing pressure (=depth)
Add water: decreases melting temperature
Temp.
Depth
SOLID
LIQUID
Melting temperature curve
A wet rock
melts more easily

14. From Magma to Igneous Rock
Cools
Solidifies (freezes)
Forms silicate minerals

15. Composition of Magmas
Silicates are divided into groups based on relative amounts of silica (SiO2), and magnesium (Mg) and/or iron (Fe)
4 primary types:
Silicic (70% SiO2; low Mg and Fe)
Intermediate (55% SiO2; low Mg and Fe)
Mafic (< 50% SiO2; high Mg and Fe)
Ultramafic (< 40% SiO2; very high Mg and Fe)

16. Minerals in Mafic and Felsic Rocks
Rock type
Rich in
Poor in
Minerals in Rock
MAFIC
(DARK)
Fe and/or Mg Si
olivine
pyroxene
amphibole
biotite
Silicic
(LIGHT)
Si, K, Na, Ca
Fe and/or Mg
quartz
muscovite
feldspars

17. Crystallization
Ideally, crystallization (freezing) is the opposite of melting.
In fact, the process is more complicated than that because rocks are complex aggregates of many minerals with different melting (crystallization) points.

18. Simple crystallization
Example: Quartz
When melt reaches the crystallization temperature of a mineral, the mineral forms and undergoes no further changes with subsequent cooling.

19. Fractional crystallization
The modification of magma by crystallization and removal of mineral phases.
Because only certain elements will go into a given mineral, this will tend to change the composition of the remaining liquid.

20. Early Crystallization

21. Partial melting Opposite of fractional crystallization
Last minerals to form will melt at lowest temperature
Biggest changes will be for small degrees of melting

22. Fig. 6.05a
W. W. Norton

23. Viscosity: An Important Factor in Magma Movement
Viscosity = Resistance to flow
i.e., honey vs. water
Factors in igneous rocks:
Composition: higher SiO2 -> higher viscosity lower volatiles -> higher viscosity
Temperature: lower temperature -> higher viscosity

24. Tectonic Settings of Igneous Activity

25. Volcanic Island Arc, Indonesia

26. Oceanic Hot Spot
Hawaii

27. Continental Volcanic Arc N. Cascades

28. Types of Igneous Structures

29. Sill
Sill

30. Dike

31. Dike (Grand Tetons, Wyoming)

32. Batholith – Sierra Nevadas

33. Volcanic Necks

34. Volcanic Neck (central France)

35. Classification of Igneous Rocks
We classify igneous rocks using two major properties:
Texture
Is the rock extrusive or intrusive?
Chemical Composition
Relative amounts of Si, O, Mg, Fe, K, Al, Na, Ca

36. Intrusive
Granite
(large crystals)

37. Extrusive
Basalt
(small crystals +
glass)

38. Igneous Textures Glassy no minerals present
Crystalline rocks made of mineral grains
a) Coarse grained
b) Fine grained
c) Mixture of coarse and fine
Vesicular with bubble holes
Pyroclastic fragmental texture

39. Xl Size and Cooling Rate
crystal size
cooling rate
Slow cooling
larger crystals
Fast cooling
small or no crystals

40. Igneous Textures forms far below surface slow cooling intergrown
fast cooling magma/lava
forms at or near surface
sometimes holes present
can’t see individual crystals
forms far
below surface
slow cooling
intergrown
crystals
magma cooled slowly for a while before erupting
minerals crystallized at different temperatures and/or rates
very rapid
cooling
ions unable to
unite in
orderly
crystalline
structure
Fine-grained
Coarse-grained
Mixture of coarse and fine
Glassy

41. Intrusive/Extrusive Igneous Rocks
Granite and rhyolite
Chiefly composed of quartz and feldspar
Same chemical composition – different cooling rates
Melting point
~800 °C
High viscosity
High silica content
~70-75%
SILICIC
Granite Rhyolite

42. Intrusive/Extrusive Igneous Rocks
Diorite and Andesite
Chiefly composed of plagioclase
Same chemical composition – different cooling rates
Melting point
~1000 °C
Medium viscosity
Intermediate silica content (~60%)
INTERMEDIATE
Diorite Andesite

43. Intrusive/Extrusive Igneous Rocks
Gabbro-Peridotite and Basalt
>50% pyroxene and olivine
Same chemical composition – different cooling rates
Melting point
~1200 °C
Low viscosity
Low silica content
~45-50%
MAFIC
Gabbro Basalt