Igneous Rocks and Intrusions

1
Igneous Rocks and Intrusions
2
Fig. 1-12, p. 20
3
Igneous Rocks

• Formed from melts, either magma or lava
• Magma molten rock below the surface ? plutons
• Lava magma that is erupted onto the surface ?
  lava flows
• Igneous rocks comprise crystallized melts (magma
  or lava) also consolidated pyroclastic material
• loose particles blasted into the air by volcanic
  eruptions
• Igneous rocks are either
• Intrusive magma solidifies beneath the earths
  surface ? big crystals. Plutonic rocks.
• Extrusive magma solidifies on the earths
  surface or consolidated pyroclastic material?
  small or no crystals. Volcanic rocks.

4
Extrusive vs. Intrusive
5
Igneous Chemistry

• Recall that most minerals in the crust are
  silicates (Si, Al, O, Na, K, etc), especially
  those in continental crust.
• Mantle silicates are more ferromagnesian as are
  oceanic igneous rocks.
• Chemistry of a melt depends on its origin

6
Fig. 1-11, p. 18
7
Igneous Chemistry

• Felsic gt 65 SiO2
• lots of Na, K, Al but little Mg, Fe, Ca
• e.g. continental crust granite
• Intermediate 53 - 65 SiO2
• e.g. subduction zone volcanic rocks andesite
• Mafic 45 - 52 SiO2
• lots of Mg, Fe, Ca but silica poor
• e.g. oceanic crust basalt
• Ultramafic lt 45 SiO2
• e.g. mantle

8
Table 4-1, p. 89
9
How Hot?

• Temperature of hottest erupting mafic (basalt)
  lavas between 1000C and 1350C. Magma is
  hotter, but we cant measure it.
• Felsic lavas are about 650-900C .
• Takes a long time for melt to cool
• Mafic lavas/magmas tend to be hotter
• Melting point of basalt is about 1250C
• Melting point of granite is about 700C.

10
Fig. 4-2, p. 90
11
How Runny or Gooey?

• Viscosity resistance to flow within a liquid.
  Higher viscosity means it doesnt flow as easy.
• Highly temperature dependent. Most liquids
  become more viscous when cold and less viscous
  when hot.
• Magma/lava viscosity is also determined by silica
  content. Tetrahedra tend to bind together, even
  in the liquid state
• ? Mafic magmas/lavas tend to be runnier allow
  gas to escape. Felsic lavas are thicker trap
  gas.

12
(No Transcript)
13
How do Magmas Form?

• Minerals melt at different temperatures. The
  fraction of rock that has melted at a given
  temperature is the partial melt.
• Pressure equal compositions melt at a higher
  temperature if the pressure is higher.
• Water water tends to make rocks melt at lower
  temperatures. Opposite of pressure effect.

14
Fig. 4-4, p. 92
15
Where do Magmas Form?

• The density of magma is less than solid rock. So
  it rises and collects in large cavities called
  magma chambers. Can be at a range of depths
  depending on tectonic setting. Can be large or
  small in volume. Some never makes it to the
  surface.
• Plate tectonic environments where magma is
  produced
• Mid-ocean ridges.
• Subduction zones.
• Mantle plumes.

16
Fig. 1-11, p. 18
17
Igneous Compositions

• The type of rock that melts and/or the
  composition of the parent magma determines the
  chemistry of the rock that crystallizes.
• Other relevant processes
• Crystallization sequence of minerals
• Crystal settling
• Assimilation
• Magma mixing

18
Fig. 4-3, p. 91
19
Bowens Reaction Series

• Helps understand how you can get intermediate and
  mafic rocks from mafic magmas.
• When a melt cools, different minerals crystallize
  at different temperatures!
• Minerals that cool at nearly the same
  temperatures tend to occur together in rocks.
  Only certain minerals can occur together.
• If you remove the minerals that have crystallized
  at a certain temperature, you change the overall
  chemistry of the system

20
Bowens Reaction Series

• Discontinuous Series
• Only ferromagnesian.
• One mineral changes to next as temperature slowly
  drops.
• Each change is a chemical reaction between solids
  (crystals) and fluids (melt, water, gas) present
  at the time.
• Reactions not always complete.

21
Bowens Reaction Series

• Continuous Series
• Nonferromagnesian plagioclase feldspar (Ca to Na)
• As magma cools, Ca-rich plagioclase reacts with
  melt and proportionally more Na-rich plagioclase
  crystallizes
• Continues until all Ca and Na is used up.
• Plagioclase is often, therefore, zoned with Ca
  rich core and Na rich rims.

22
Bowens Reaction Series

• Felsic Minerals
• Not really part of series.
• As Mg, Fe, Ca, and Na are used up, left over
  magma has more and more SiO2, K, Al, water, and
  other exotic stuff (U).
• Form K feldspars, muscovite, quartz, accessory
  minerals.

Increasing Si content
23
Fig. 4-5, p. 93
24
Fig. 4-4, p. 92
25
Crystallization

• Formation of crystals from a melt.
• Liquids have lots of entropy (disorder) due to
  temperature ? kinetic energy (movement) of atoms
  and molecules.
• Solids, especially, crystals have less entropy
  because of their ordered, fixed internal
  structures.
• Entropy lost as temperature is decreased.
• Crystallization begins when atoms, by chance,
  bond together to form nuclei. Other atoms will
  follow

26
(No Transcript)
27
Differentiation of Magma

• Magma chemistry isnt static or homogenous.
  Processes upset the normal Bowen sequence.
• Crystal Settling
• Assimilation
• Magma Mixing
• Partial Melting
• We need these processes (and others) to get
  felsic rocks from mafic magmas

28
Differentiation of Magma

• Crystal settling (fractional crystallization) is
  the physical separation of the minerals from the
  melt. Minerals crystallize and sink to the
  bottom of the magma chamber. Examples layered
  intrusions.
• Separate the melt from solid ? change the
  chemistry of the melt (more Si rich). Ok!
• But not good enoughFor one unit of granite, need
  10 of the mafic melt! Not enough mafic intrusive
  rocks in continental crust
• Also it takes a long time for crystals to
  settletoo long.

29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
Differentiation of Magma

• Assimilation is the process wherein magma reacts
  and/or incorporates surrounding, pre-existing
  rock (country rock). Examples xenoliths.
• Wall rocks get hot! Hot enough to melt and get
  added to the magma. If it happens enough, it can
  change the magma chemistry (make it more Si
  rich). Ok!
• But not good enoughAssimilation cools the magma.
  If it happened on the scale necessary, the magma
  would freeze too soon.

33
Fig. 4-6a, p. 94
34
(No Transcript)
35
Fig. 4-6b, p. 94
36
Differentiation of Magma

• Magma mixing occurs when different composition
  magmas interact, changing the overall composition
  as they do.
• Happens in a single magma chamber.
• Happens as magmas move through crust.

37
Fig. 4-7, p. 95
38
Differentiation of Magma

• Partial melting minerals melt at different
  temperatures, in the opposite order as the Bowen
  series.
• The first melt fraction is high in Si. If you
  remove that melt from the rest of the system, you
  can get a Si-rich magma
• Same idea as crystal settling, but in reverse.
  The basic idea is fractionation.

39
Igneous Textures

• Generally igneous rocks have interlocking
  crystals.
• Details of texture controlled by how they
  crystallized e.g. the cooling rate

40
Igneous Textures

• Extrusive rocks usually cool quickly ? small
  crystals (aphanitic) or no crystals (glass)
• Intrusive rocks usually cool slowly ? big
  crystals have time to form (phaneritic).

41
Fig. 4-8a, p. 96
42
Fig. 4-8b, p. 96
43
Fig. 4-8c, p. 96
44
Fig. 4-8d, p. 96
45
aphanitic
phaneritic
46
Igneous Textures

• Porphyritic textures are a mix. 2 populations of
  mineral grains
• Some very large phenocrysts within
• A fine grained groundmass.

47
Fig. 4-8e, p. 96
48
Fig. 4-8f, p. 96
49
Igneous Textures

• If a magma/lava is quenched very rapidly, there
  is no time to form crystals. Instead, you get a
  semi-ordered, amorphous solid (glass).

50
Fig. 4-8g, p. 96
51
Igneous Textures

• Vesicular textures have holes, pores, or cavities
  resulting from the expansion of gas dissolved in
  the melt.
• Vesicular basalt is common. Pumice and scoria
  are also common types of vesicular rocks (frothy
  glass).

52
Fig. 4-8h, p. 96
53
Igneous Textures

• Fragmental textures characterize pyroclastic
  igneous rocks. Made of numerous grains or
  fragments (glass, ash, pumice, etc.) that have
  been welded together by the heat of volcanic
  eruption.

54
Fig. 4-8i, p. 96
55
Classification

• Based on composition and texture.
• For each compositional type, there are two names,
  one for the extrusive (aphanitic) and one for the
  intrusive (phaneritic) equivalent.

56
(No Transcript)
57
Compositional Classifications

• Ultramafic largely ferromagnesian, lt 45 Si
• Mafic 45 - 52 Si
• Intermediate 53 - 65 Si
• Felsic largely nonferromagnesian, gt 65 Si

58
(No Transcript)
59
Ultramafic Rocks

• Intrusive peridotite
• Mostly olivine, some pyroxene, a little Ca
  plagioclase
• Intrusive pyroxenite
• Mostly pyroxene
• Dark green rocks in the upper mantle.
• Rare extrusive ultramafics
• Komatiites were erupted only in the Precambrian
  (2.5 Ga).
• Very mafic, very low viscosity (like water!),
  very hot (1600C)

60
Fig. 4-10, p. 97
61
(No Transcript)
62
Mafic Rocks

• Intrusive gabbro
• Extrusive basalt
• Ca-rich plagioclase pyroxene some olivine
  some amphibole
• Dark colored
• Basalt is most common extrusive igneous rock.
  Oceanic crust is made of basalt and gabbro.

63
Fig. 4-11a, p. 98
64
Fig. 4-11b, p. 98
65
Intermediate Rocks

• Intrusive diorite
• Extrusive andesite
• Plagioclase amphibole biotite
• Medium to dark gray (andesite) salt and pepper
  (diorite)
• Common in volcanoes at subduction zones (mix of
  oceanic and continental melts).

66
Fig. 4-12a, p. 98
67
Fig. 4-12b, p. 98
68
Felsic Rocks

• Intrusive granite
• Extrusive rhyolite (usually pyroclastics, not
  flows)
• K-feldspar Na-plagioclase quartz some
  biotite some amphibole some muscovite
  accessories.
• Light colored (pink, white).
• Essentially the composition of the continents.

69
Fig. 4-13a, p. 99
70
Fig. 4-13b, p. 99
71
Pegmatite

• Very coarse intrusives (usually granitic).
• Interaction of water and gas phases with melt.
• Last dregs of a magma.
• Get very interesting (and valuable)
  mineralization.

72
Fig. 4-14b, p. 100
73
Fig. 4-15, p. 100
74
Fig. 4-16a, p. 101
75
Fig. 4-16b, p. 101
76
Fig. 4-16c, p. 101
77
Fig. 4-16d, p. 101
78
Fig. 4-9, p. 97
79
(No Transcript)
80
Intrusions

• Pluton is the general term for (large) igneous
  bodies that form when magma intrudes, cools, and
  crystallizes within the crust ? Igneous
  intrusions.
• Plutons are classified based on geometry, size,
  and relationship to country rocks

81
Fig. 4-1a, p. 88
82
Intrusions

• Size kilometer to centimeter size
• Geometry massive, irregular, tabular,
  cylindrical, mushroom, etc.
• Concordant boundries parallel to layering in the
  country rock
• Discordant boundaries cut across country rock

83
(No Transcript)
84
Fig. 4-14a, p. 100
85
Types of Igneous Intrusions
86
(No Transcript)
87
Dikes

• Discordant, small (1 cm to 100 m thick),
  shallow intrusions.
• May occur as isolated bodies or as swarms
  emanating from a large intrusion.

88
(No Transcript)
89
Sills

• Concordant, small (1 m to 100 m thick) shallow
  intrusions.
• Usually fed by dikes, but may not be exposed in
  the field. 

90
(No Transcript)
91
Dikes and Sills

• Dikes are emplaced within fractures or where
  fluid pressure is enough to fracture country rock
  ? zones of weakness.
• Sills are emplaced where fluid pressure is enough
  to lift overlying rocks to make a gap ? typically
  only occur at shallow depth.
• Both can occur over very long distances or be
  very localized ? wide range of scales.

92
Laccoliths

• Medium-sized, concordant intrusions.
• Mushroom shape w/ flat floor and domed upper
  part.
• Result in uplift and folding of the preexisting
  rocks above the intrusion ? domes.

93
Concept Art, p. 103
94
Intrusives Related to Volcanoes

• Volcanic pipe conduit connecting the vent of a
  volcano with the subterranean magma chamber.
• Volcanic neck exposed volcanic pipe.
• Often associated with dikes.

95
(No Transcript)
96
Fig. 1, p. 105
97
Fig. 2a, p. 105
98
Fig. 2b, p. 105
99
(No Transcript)
100
Batholiths Stocks

• Batholiths very large intrusions, so large that
  the bottoms are rarely exposed.  Sometimes they
  comprise several smaller intrusions.
• Stocks smaller bodies that fed from deeper
  batholiths.  They may have been feeders for
  volcanic eruptions, but the associated volcanic
  rocks are rarely exposed.

101
Fig. 4-1b, p. 88
102
Fig. 4-CO, p. 86
103
Concept Art, p. 102
104
Intrusive Processes

• Unresolved questions
• 1. How do we make a granitic magma?
• 2. How do we overcome the space problem?

105
Intrusive Processes Making Granite

• Wet melting of continental crust. Water lowers
  the melting temperature of continental source
  rocks.
• Granitic (rhyolitic) magma erupts explosively
  from volcanoes, indicating high gas content.
• Solidified granite contains hornblende, biotite,
  and muscovite, all hydrous minerals.
• Heat source? Basaltic magma at the bottom of the
  crust releases heat into the surrounding crust.
• Assimilation, mixing, and fractionation
  (settling, partial melting) processes also very
  important

106
basalt ? heat
water ? easier to melt
107
Intrusive Processes The Space Problem

• Large intrusive bodies are not formed in place.
• The melt moves in from elsewhere and collects to
  form large plutons.
• But what happens to all the rock that was there
  before the pluton formed?

108
Intrusive Processes

• Deformation pushing or moving rock out of the
  way by folding and faulting.
• Magma is less dense than rock ? moves up.
  Gravity-driven deformation like salt domes

109
Fig. 4-17b, p. 107
110
Fig. 4-17c, p. 107
111
Fig. 4-17d, p. 107
112
Intrusive Processes

• Stoping dropping blocks into an intrusion.
  Blocks settle to bottom, allowing room for magma
  to go up.
• Shallow depths where rocks will fail by
  fracturing.

113
Intrusive Processes

• Assimilation melting the rock around the
  intruding magma and incorporating it.

114
(No Transcript)
115
(No Transcript)
116
(No Transcript)