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CH. 3 – IGNEOUS ROCKS
Igneous Rocks

Ignis = “fire”
Form from cooling & crystallization
  of magma (molten rock)
Lava = molten rock at surface
Igneous Rocks

Classified by texture &
 composition
Texture = crystal size of minerals
Composition = minerals present
Changes of State of Matter

Add heat (melting):
Rx       Magma (atoms faster)
Remove heat (crystallize):
Rx (atoms slower)        Magma
Texture of Igneous Rocks

Depends on magma’s cooling rate
1) Cools slowly
Atoms can move through liquid
   - crystals grow (large, small)
Magma’s Cooling Rate

2) Cools quickly
Forms a solid faster than atoms
 can move around
 - crystals are (large, small)
Texture (Crystal Size)

Two environments where magma
 cools:
1) At surface or shallow burial
2) Deep underground
Types of Texture

1) Aphanitic
   - crystals invisible to naked eye
   - magma cooled ___________
   - extrusive/volcanic
Types of Texture

2) Vesicular
- aphanitic rock w/many holes
  (vesicles) in it
- gas trapped as magma cools
  very quick
Types of Texture

3) Phaneritic
 - crystals easily visible
 - magma cooled ___________
 - intrusive/plutonic
Types of Texture

4) Porphyritic
 - two size crystals in same rock
Large crystals = phenocrysts
Small crystals = groundmass
Porphyritic Texture

2-stage cooling process:
1st stage = slow cooling at depth
_______________ are formed
2nd stage = rapid cooling of
  remaining magma
_______________ are formed
Types of Texture

5) Glassy
 - no crystalline structure

Ex: Obsidian; pumice
Types of Texture

6) Pegmatic
All minerals larger than 1 cm
Forms during later stage of
  crystallization
- large percentage of liquid allows
  ions to migrate easier
Types of Texture

7) Pyroclastic
“pyro” = fire; “clastic” = pieces
Consolidated rock fragments
  ejected during violent volcanic
  eruptions
Pyroclastic texture

Fragments can be volcanic ash,
  blobs of magma, or angular
  blocks
- not composed of interlocking
  crystals!
Igneous Rock Composition

The most abundant elements of
  Earth’s crust are _____ &
  ______
Magma is mostly silica with
  varying amounts of other six
  elements
Composition

Other elements collect in water at
  top of magma chamber
Creates hydrothermal solution
- cools in fractures as highly
  concentrated ore deposits
Igneous Compositions

Mafic
- 45-50% silica (“silica-poor”)
- rich in magnesium, iron (Fe), &
  calcium
- dark-colored: black, dark gray,
  brownish-red
Igneous Compositions

Felsic (feldspar & silica)
- 60-75% silica (“silica rich”)
- rich in aluminum, potassium, &
  sodium
- light-colored: light gray, pinkish
Igneous Compositions

Intermediate
Aphanitic: medium gray
Phaneritic: black & white crystals
Importance of Igneous Rocks

Compose 90% of Earth’s crust
Two types of crust:
 oceanic crust = basalt
 continental crust = granite
Question:

Why different igneous rock
 compositions if only 8 elements?
N.L. Bowen experimented with
 mafic magmas
Results:

** Same sequence of minerals
  crystallized from magma
Referred to as Bowen’s Reaction
  Series (Fig. 3.16)
Bowen’s Reaction Series

Discontinuous series (left) vs.
  continuous series (right)
Discontinuous series:
- each mineral has a unique
  silicate structure
Continuous series

Different compositions of
  plagioclase feldspars
- ions of Ca2+ and Na+ can
  interchange within crystalline
  structure
Bowen’s Reaction Series

As magma crystallizes:
First mineral to form = mafic
Composition of remaining magma
 changes
Bowen’s Reaction Series

Biotite uses last mafic elements in
  magma
- remaining magma becomes
  more “silica rich”
Bowen’s Reaction Series

Minerals that form in same temp.
 conditions are found together in
 same igneous rock
QUESTION: Which mineral would
 you expect to form phenocrysts
 in a porphyritic basalt?
Phenocryst Mineral:

a) Quartz
b) Potassium feldspar
c) Biotite
d) Calcium-rich plagioclase f-spar
e) Sodium-rich plagioclase f-spar
Felsic magma

Felsic minerals melt at cooler
  temp’s
- magma not as hot as mafic
  magma
- felsic magma not as buoyant
Felsic magma

Magma doesn’t reach surface
Ex: Continental crust = granite
Important Concepts

A mineral is stable if the
 environmental conditions are
 similar to where it formed
Ex: temperature, pressure, water-
  content
Example:

Mafic minerals are stable in ____
 temperatures
Felsic minerals are stable in ____
 temperatures
Important Concepts

If a mineral is put under different
  environmental conditions, it
  becomes unstable
- mineral has to react to new
  environment by changing
New Environmental
Conditions:

If at the surface, weathering
  occurs
If within the crust, metamorphism
  occurs

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