GEOLOGY 101

1
GEOLOGY 101

• Today Chapter 6
• Up from the Inferno Magma and Igneous Rocks

Instructor Professor Matt Fouch Email
fouch101_at_asu.edu Office PSF-540 Phone x5-9292
TA Kara Krelove Email kara.krelove_at_asu.edu Offic
e PSF-209
Course Website http//fouch101.asu.edu
2
Igneous Rocks
Devils Tower, Wyoming
3
Igneous RocksWhy should we care?

• Igneous rocks make up the bulk of the Earths
  crust
• Earths 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 Earths 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 1200C). 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
Water freezes
0oC
32oC
10
Fig. 6.04a

• W. W. Norton

11
Melting temperature increases with increasing
pressure (depth)
12
Fig. 6.04b

• W. W. Norton

13
Melting temperature increases with increasing
pressure (depth)
Add water decreases melting temperature
A wet rock melts more easily
14
From Magma to Igneous Rock

• Magma
• 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 (lt 50 SiO2 high Mg and Fe)
• Ultramafic (lt 40 SiO2 very high Mg and Fe)

16
Minerals in Mafic and Felsic Rocks
Poor in
Rock type
Minerals in Rock
Rich in
MAFIC (DARK)
Si
olivine pyroxene amphibole biotite
Fe and/or Mg
Fe and/or Mg
Silicic (LIGHT)
Si, K, Na, Ca
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
• Compositionhigher SiO2 -gt higher
  viscositylower volatiles -gt higher viscosity
• Temperature lower temperature -gt higher
  viscosity

24
Tectonic Settings of Igneous Activity
25
Volcanic Island Arc, Indonesia
26
Oceanic Hot Spot
Hawaii
27
Continental Volcanic ArcN. 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
Fine-grained
Coarse-grained
Glassy
Mixture of coarse and fine
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
• gt50 pyroxene and olivine

• Same chemical composition different cooling
  rates
• Melting point
• 1200 C
• Low viscosity
• Low silica content
• 45-50
• MAFIC

Gabbro Basalt