2. Igneous Rocks
Ignis = “fire”
Form from cooling & crystallization
of magma (molten rock)
Lava = molten rock at surface
3. Igneous Rocks
Classified by texture &
composition
Texture = crystal size of minerals
Composition = minerals present
4. Changes of State of Matter
Add heat (melting):
Rx Magma (atoms faster)
Remove heat (crystallize):
Rx (atoms slower) Magma
5. Texture of Igneous Rocks
Depends on magma’s cooling rate
1) Cools slowly
Atoms can move through liquid
- crystals grow (large, small)
6. Magma’s Cooling Rate
2) Cools quickly
Forms a solid faster than atoms
can move around
- crystals are (large, small)
7. Texture (Crystal Size)
Two environments where magma
cools:
1) At surface or shallow burial
2) Deep underground
8. Types of Texture
1) Aphanitic
- crystals invisible to naked eye
- magma cooled ___________
- extrusive/volcanic
9. Types of Texture
2) Vesicular
- aphanitic rock w/many holes
(vesicles) in it
- gas trapped as magma cools
very quick
10. Types of Texture
3) Phaneritic
- crystals easily visible
- magma cooled ___________
- intrusive/plutonic
11. Types of Texture
4) Porphyritic
- two size crystals in same rock
Large crystals = phenocrysts
Small crystals = groundmass
12. Porphyritic Texture
2-stage cooling process:
1st stage = slow cooling at depth
_______________ are formed
2nd stage = rapid cooling of
remaining magma
_______________ are formed
14. 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
15. Types of Texture
7) Pyroclastic
“pyro” = fire; “clastic” = pieces
Consolidated rock fragments
ejected during violent volcanic
eruptions
17. Igneous Rock Composition
The most abundant elements of
Earth’s crust are _____ &
______
Magma is mostly silica with
varying amounts of other six
elements
18. Composition
Other elements collect in water at
top of magma chamber
Creates hydrothermal solution
- cools in fractures as highly
concentrated ore deposits
19. Igneous Compositions
Mafic
- 45-50% silica (“silica-poor”)
- rich in magnesium, iron (Fe), &
calcium
- dark-colored: black, dark gray,
brownish-red
24. Results:
** Same sequence of minerals
crystallized from magma
Referred to as Bowen’s Reaction
Series (Fig. 3.16)
25. Bowen’s Reaction Series
Discontinuous series (left) vs.
continuous series (right)
Discontinuous series:
- each mineral has a unique
silicate structure
29. 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?
30. Phenocryst Mineral:
a) Quartz
b) Potassium feldspar
c) Biotite
d) Calcium-rich plagioclase f-spar
e) Sodium-rich plagioclase f-spar
31. Felsic magma
Felsic minerals melt at cooler
temp’s
- magma not as hot as mafic
magma
- felsic magma not as buoyant
35. Important Concepts
If a mineral is put under different
environmental conditions, it
becomes unstable
- mineral has to react to new
environment by changing