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Today:
Chapter 5: Igneous rocks
1) Classification of igneous rocks
(texture, composition)
2) Naming of igneous rocks
3) Origin of magma
4) Igneous diversity
5) Magma recipes
1) Igneous textures
As magma cools, atoms arrange in an
orderly crystal structure
 crystallization
1) Igneous textures
Note: different crystal sizes
1) Igneous textures
Close up of crystals
 Note the variety of crystal sizes
1) Igneous textures
Texture
a. size
b. shape
c. arrangement
of interlocking crystals
1) Igneous textures
–Factors affecting crystal size
–Rate of cooling
–Amount of silica (SiO2) present
–Amount of dissolved gases
1) Igneous textures
cooling rate
crystal size
Slow cooling
larger crystals
Fast cooling
small or no crystals
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
1) Igneous compositions
Magma
cools
solidifies
forms silicate minerals
Two major silicate mineral groups:
DARK silicates
LIGHT silicates
1) Igneous compositions
Silicate group
DARK
Rich in
Fe and/or Mg
Poor in
Si
Examples
olivine
pyroxene
“ferromagnesian”
amphibole
biotite
LIGHT
Si, K, Na, Ca
Fe and/or Mg
quartz
“non-
muscovite
ferromagnesian”
Feldspars
(40% of most
See Table 5.1
igneous rocks)
1) Igneous compositions
The classification
model of igneous
rock.
Fig. 5.4, see Table 5.2
2) Naming igneous rocks
See Fig. 5.3
rhyolite
volcanic
granite
plutonic
zooming in
•Obsidian
obsidian
–Dark colored
–Glassy texture
volcanic
2) Naming igneous rocks
See Fig. 5.3
pumice
Volcanic
Glassy texture
Frothy
appearance with
numerous voids
volcanic
•Pumice
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…)
2) Naming igneous rocks
pyroclastic rock
Ash and pumice layers
Common in western US
which had high volcanic
Activity millions of years
Ago
2) Naming igneous rocks
See Fig. 5.3
andesite
volcanic
diorite
plutonic
zooming in
gabbro
volcanic
basalt
zooming in
plutonic
2) Naming igneous rocks
See Fig. 5.3
3) Origin of magma
Recall
igneous rock =
cooled & solidified magma or lava
magma =
from melting solid rock in crust
and/or upper mantle
…but how do we melt rock???
1st, let’s define
“melting temperature”
= the temperature at which something melts
Depends on:
 composition
 pressure
 water content
Temperature scale digression…
Celsius (centigrade) scale
Water boils
100oC
oC
0oC
Fahrenheit scale
212oF
oF - 32)
= 5
(
9
Water freezes
32oF
Melting temperature increases at higher
pressure.
Melting temperature decreases with increasing
water content.
Pressure
3 ways to melt rock
Inside Earth examples:
 raise the temperature
Rocks in the lower crust and
upper mantle are near their
melting points. Rising magma
heats rocks further
 lower the pressure
Rock ascends to shallower
depth in convective flow.
When confining pressures drop,
decompression melting occurs
without additional heat
 add volatiles (water)
Water percolates up from
subducting oceanic
lithosphere into overlying
mantle rocks
See Fig. 5.15
Hot mantle rock
Decompression melting
ascends and moves
into zones of lower
pressure. This drop
in confining pressure
may trigger 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.
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
4) Igneous diversity
Mechanisms to account for the great
diversity of igneous rock
magmatic differentiation
assimilation
magma mixing
New…
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
4) Igneous diversity
assimilation
(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
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
5. Magma recipes
“mafic”
 basaltic
andesitic
 granitic
“felsic”
3 main types
Partial
melt of:
mantle rock:
peridotite
Mantle peridotites
+ felsic crustal rocks
Si-rich continental
crust rocks
How/
process:
buoyant rock
ascends/reduction
in pressure, large
outpourings
basalt ascends,
melts/assimilates
crustal rocks
Basalt heats &
melts crust from
below (assoc. w/
subduction)
It is more viscous
And stays subsurface.