classification of igneous rocks (texture, composition) 2) naming of igneous rocks
DESCRIPTION
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. 1) Igneous textures. As magma cools, atoms arrange in an orderly crystal structure crystallization. - PowerPoint PPT PresentationTRANSCRIPT
1) Classification of igneous rocks
(texture, composition)
2) Naming of igneous rocks
3) Origin of magma
4) Igneous diversity
5) Magma recipes
Chapter 5: Chapter 5: Igneous rocksIgneous rocks
Today:
1) Igneous textures
As magma cools, atoms arrange in anorderly crystal structure
crystallizationcrystallization
1) Igneous textures
Note: different crystal sizes
1) Igneous textures
Close up of crystals Note the variety of crystal sizes
Texture
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
crystal sizecrystal size
Slow cooling
cooling ratecooling rate
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
DARK“ferromagnesian”
LIGHTLIGHT“non-
ferromagnesian”
Rich in
Fe and/or Mg
Si, K, Na, CaSi, K, Na, Ca
Poor in
Si
Fe and/or MgFe and/or Mg
Silicate group
olivine
pyroxene
amphibole
biotite
quartzquartz
muscovitemuscovite
FeldsparsFeldspars
(40% of most (40% of most
igneous rocks)igneous rocks)
Examples
See Table 5.1
1) Igneous compositions
Fig. 5.4, see Table 5.2
The classification model of igneous rock.
2) Naming igneous rocks
granitegranite
rhyoliterhyolite
zooming inzooming in
f
e l s
i c
f
e l s
i c
plu
ton
icvolc
an
ic
See Fig. 5.3
2) Naming igneous rocks
obsidianobsidian
pumicepumice
f
e l s
i c
f
e l s
i c
volc
an
icvolc
an
ic
•Obsidian–Dark colored–Glassy texture
•PumiceVolcanicGlassy textureFrothy appearance with numerous voids
See Fig. 5.3
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
andesiteandesite
dioritediorite
i n
t e
r m
e d
i a
t e
i n
t e
r m
e d
i a
t e
zooming inzooming in
volc
an
icp
luto
nic
See Fig. 5.3
2) Naming igneous rocks
basaltbasalt
gabbrogabbro
m
a f
i c
m
a f
i c
zooming inzooming in
volc
an
icp
luto
nic
See Fig. 5.3
RecallRecall
igneousigneous rock =
cooled & solidified magma or lava
magmamagma =from melting solid rock in crust and/or upper mantle
…but how do we melt rock???
3) Origin of magma
Depends on: composition
pressure
water content
““melting temperature”melting temperature”
= the temperature at which something melts
1st, let’s define
Temperature scale digression…Temperature scale digression…
Celsius (centigrade) scale Fahrenheit scale
Water freezesWater freezes
Water boilsWater boils
0oC
100oC
32oF
212oF
oC = (oF - 32)59
Pressure
Tem
p.
SOLIDSOLID
LIQUIDLIQUID
Melting temperature curve
Melting temperature decreases with increasing
waterwater content.
SOLIDSOLID
LIQUIDLIQUID
Melting temperature curve
Melting temperature increases at higher pressure.
3 ways to melt rock3 ways to melt rock3 ways to melt rock3 ways to melt rock
raise the temperature
lower the pressure
add volatiles (water)
Inside Earth examples:
Rocks in the lower crust and Rocks in the lower crust and upper mantle are near their upper mantle are near their melting pointsmelting points.. Rising magma Rising magma heats rocks furtherheats rocks further
Rocks in the lower crust and Rocks in the lower crust and upper mantle are near their upper mantle are near their melting pointsmelting points.. Rising magma Rising magma heats rocks furtherheats rocks further
Rock ascends to shallowerRock ascends to shallowerdepth in convective flow.depth in convective flow.When confining pressures drop, When confining pressures drop, decompression melting occursdecompression melting occurswithout additional heatwithout additional heat
Rock ascends to shallowerRock ascends to shallowerdepth in convective flow.depth in convective flow.When confining pressures drop, When confining pressures drop, decompression melting occursdecompression melting occurswithout additional heatwithout additional heat
Water percolates up fromWater percolates up fromsubducting oceanic subducting oceanic lithosphere into overlyinglithosphere into overlyingmantle rocksmantle rocks
Water percolates up fromWater percolates up fromsubducting oceanic subducting oceanic lithosphere into overlyinglithosphere into overlyingmantle rocksmantle rocks
See Fig. 5.15
Decompression meltingHot mantle rock
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.
Water vapor mixed with molten sedimentary rocks from subducting plate rise and lower the melting temperature of mantle and lithospheric rock.
= fluid-induced melting
Effect of water in melting mantle rock:
5.15
4) Igneous diversity
magmatic differentiationmagmatic differentiation
New…
Mechanisms to account for the great
diversity of igneous rock
assimilationassimilation
magma mixingmagma mixing
4) Igneous diversity
magmatic differentiationmagmatic 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
Mechanisms to account for the great diversity of igneous rock
See Figure story 5.5
4) Igneous diversity
assimilationassimilation
As magma migrates upward:•may incorporate surrounding host rock
•In deeper layers magma may melt surrounding host rock
magma mixingmagma mixing
If magma body collides into another:
•magmas can mix
(see Fig 5.6)
4) Igneous diversity
magmatic differentiationmagmatic differentiation
Mechanisms to account for the great
diversity of igneous rock
assimilationassimilation
magma mixingmagma mixing
Fig. 5.6Mixing of two magmas=andesitic magma
basaltic
andesitic
granitic
“felsic”
“mafic”
mantle rock:peridotite
Mantle peridotites+ felsic crustal rocks
Si-rich continentalcrust rocks
Partialmelt of:
buoyant rockascends/reductionin pressure, largeoutpourings
basalt ascends,melts/assimilatescrustal rocks
Basalt heats &melts crust frombelow (assoc. w/subduction)It is more viscous And stays subsurface.
How/process:
“felsic”“felsic”
5. Magma recipes5. Magma recipes 3 main types