1. Chapter 5: Igneous rocks
Today:
Chapter 5: Igneous rocks
Classification of igneous rocks
(texture, composition)
2) Naming of igneous rocks
3) Origin of magma
4) Igneous diversity
5) Magma recipes

2. As magma cools, atoms arrange in an orderly crystal structure
1) Igneous textures
As magma cools, atoms arrange in an
orderly crystal structure
crystallization

3. 1) Igneous textures
Note: different crystal sizes

4. 1) Igneous textures Close up of crystals
Note the variety of crystal sizes

5. Texture Texture 1) Igneous textures a. size b. shape c. arrangement
of interlocking crystals

6. Factors affecting crystal size
1) Igneous textures
Factors affecting crystal size
Rate of cooling
Amount of silica (SiO2) present
Amount of dissolved gases

7. cooling rate crystal size 1) Igneous textures Slow cooling
larger crystals
Fast cooling
small or no crystals

8. 1) Igneous compositions
mainly silicate minerals
determined by composition of magma from
which it crystallized
magma mainly 8 elements:
Si, O, Al, Ca, Na, K, Mg, Fe
Most abundant constituents

9. LIGHT silicates DARK silicates 1) Igneous compositions Magma cools
solidifies
forms silicate minerals
Two major silicate mineral groups:
DARK silicates
LIGHT silicates

10. 1) Igneous compositions
Silicate group
Rich in
Poor in
Examples
DARK
“ferromagnesian”
Fe and/or Mg Si
olivine
pyroxene
amphibole
biotite
LIGHT
“non-
ferromagnesian”
Si, K, Na, Ca
Fe and/or Mg
quartz
muscovite
Feldspars
(40% of most
igneous rocks)
See Table 5.1

11. 1) Igneous compositions
The classification model of igneous rock.
1) Igneous compositions
Fig. 5.4, see Table 5.2

12. 2) Naming igneous rocks volcanic rhyolite f e l s i c plutonic granite
See Fig. 5.3
2) Naming igneous rocks
zooming in
rhyolite
volcanic
f e l s i c
granite
plutonic

13. 2) Naming igneous rocks Obsidian Pumice volcanic obsidian f e l s i c
See Fig. 5.3
2) Naming igneous rocks
Obsidian
Dark colored
Glassy texture
obsidian
volcanic
f e l s i c
Pumice
Volcanic
Glassy texture
Frothy appearance with numerous voids
pumice
volcanic

14. 2) Naming igneous rocks Another type of igneous rock pyroclastic rock
from fragments ejected during eruption
most common: “tuff” (ash-sized fragments,
cemented together
called volcanic breccia if it mostly contains
fragments larger than ash (rocks from vent
wall,crystals, glass fragments…)

15. pyroclastic rock Ash and pumice layers
2) Naming igneous rocks
pyroclastic rock Ash and pumice layers
Common in western US
which had high volcanic
Activity millions of years
Ago

16. 2) Naming igneous rocks volcanic andesite i n t e r m e d i a t e
See Fig. 5.3
2) Naming igneous rocks
zooming in
andesite
volcanic
i n t e r m e d i a t e
diorite
plutonic

17. 2) Naming igneous rocks basalt volcanic m a f i c gabbro plutonic
See Fig. 5.3
2) Naming igneous rocks
zooming in
basalt
volcanic
m a f i c
gabbro
plutonic

18. igneous rock = magma = Recall 3) Origin of magma
cooled & solidified magma or lava
magma =
from melting solid rock in crust
and/or upper mantle
…but how do we melt rock???

19. “melting temperature”
1st, let’s define
“melting temperature”
= the temperature at which something melts
Depends on:
composition
pressure
water content

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

21. SOLID LIQUID SOLID LIQUID
Melting temperature increases at higher pressure.
Melting temperature decreases with increasing
water content.
SOLID
LIQUID
Melting temperature curve
Pressure
Temp.
SOLID
LIQUID
Melting temperature curve

22. 3 ways to melt rock raise the temperature lower the pressure
Inside Earth examples:
Rocks in the lower crust and
upper mantle are near their
melting points. Rising magma
heats rocks further
raise the temperature
lower the pressure
add volatiles (water)
Rock ascends to shallower
depth in convective flow.
When confining pressures drop,
decompression melting occurs
without additional heat
Water percolates up from
subducting oceanic
lithosphere into overlying
mantle rocks
See Fig. 5.15

23. Decompression melting
Hot mantle rock
ascends and moves
into zones of lower
pressure. This drop
in confining pressure
may trigger melting.
Decompression melting
Ascend rates of magma are 0.3 to 50 m/yr. Magma chambers (several km3 big) form in the cavities of the lithosphere as magma rises.

24. Effect of water in melting mantle rock:
Water vapor mixed with molten sedimentary rocks from subducting plate rise and lower the melting temperature of mantle and lithospheric rock.
= fluid-induced melting
5.15

25. magmatic differentiation
New…
4) Igneous diversity
Mechanisms to account for the great
diversity of igneous rock
magmatic differentiation
assimilation
magma mixing

26. magmatic differentiation
4) Igneous diversity
Mechanisms to account for the great diversity of igneous rock
magmatic differentiation
In a magma body:
crystals form as magma cools
not all at the same time! = fractional crystallization
heavy crystals sink to bottom =crystal settling
more than 1 type of magma from same parent
variety of igneous rocks from same source
See Figure story 5.5

27. assimilation magma mixing 4) Igneous diversity
(see Fig 5.6)
As magma migrates upward:
may incorporate surrounding host rock
In deeper layers magma may melt surrounding host rock
magma mixing
If magma body collides into another:
magmas can mix

28. magmatic differentiation
4) Igneous diversity
Mechanisms to account for the great
diversity of igneous rock
magmatic differentiation
assimilation
magma mixing
Mixing of two magmas=andesitic magma
Fig. 5.6

29. 5. Magma recipes basaltic andesitic granitic 3 main types Partial
melt of:
How/
process:
“mafic”
buoyant rock
ascends/reduction
in pressure, large
outpourings
basaltic
andesitic
granitic
mantle rock:
peridotite
Mantle peridotites
+ felsic crustal rocks
basalt ascends,
melts/assimilates
crustal rocks
Basalt heats &
melts crust from
below (assoc. w/
subduction)
It is more viscous
And stays subsurface.
Si-rich continental
crust rocks
“felsic”
“felsic”