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Classification of Igneous Rocks
• Felsic
• Basic
• Amygdaloidal
• Mafic
• Undersaturated/oversaturated
1. Chemical Composition
• Intermediate
• Volcanic
• Ultrabasic
• Hypabyssal
2. Colour
• Ultramafic
• Plutonic
• Acidic
3. Texture
• Vesicular
• Saturated
4. Mineralogy
• Porphyritic
• Equigranular
• Flow banding
Olivine Quartz
Feldspar
Muscovite Mica
Biotite mica
Augite
Hornblende
• Crystalline
• Interlocking crystals
• Randomly orientated
• Euhedral/Anhedral/ Subhedral
Chemical Composition
1. Acidic
>65% SiO2
2. Intermediate
3. Basic
4. Ultrabasic
65% - 52% SiO2
52% - 45% SiO2
<45% SiO2
Colour
1. Felsic
- light
2. Intermediate
3. Mafic
4. Ultramafic
- medium
- dark
- very dark
Texture
Crystalline, interlocking crystals & randomly orientated
Grain size
Equigranular or porphyritic
Vesicular, flow banding, amygdaloidal
Mineralogy
- Quartz
SiO2
(K, Na) AlSiO
- Orthoclase feldspar
Na AlSiO to CaAlSiO
K AlSiO
- Plagioclase feldspar
- Muscovite mica
Felsic Minerals
Mineralogy
(Mg, Fe) SiO
- Olivine
(Mg, Fe, Ca) SiO
- Augite
Ca (Mg, Fe) SiO
- Hornblende
K (Mg, Fe) AlSiO
- Biotite mica
Mafic Minerals
How Do Igneous Rocks Form?
Step 1:
MELTING rocks to form MAGMA
Step 2:
COOL magma so CRYSTALLISATION can
take place to form solid rock
What causes melting?
1. What state is material of
mafic composition under
following conditions:
• O.1 Mpa & 1100°C
Partially molten
• 1000 Mpa & 1100°C
Solid
• 400 Mpa & 1150°C
Partially molten
• 200 Mpa & 1250°C
Liquid
What causes melting?
2. Keeping the pressure
constant at atmospheric
pressure (0.1 Mpa):
• at what temperature does
mafic mantle material begin
to melt?
1075°C
• at what temperature does
it become completely
molten?
1200°C
What causes melting?
3. What would happen if you took
a piece of mafic mantle material
at 1000 Mpa & 1100°C &
gradually decreased the
pressure to 0.1 Mpa without
changing the temperature?
Melting
This is known as
Decompression Melting.
What causes melting?
1. What would happen if
you took a dry piece of
mafic mantle material at
1000 Mpa & 1100°C and
(without changing P or T)
added sufficient water
to make conditions
saturated?
Liquid
What affect does the presence
of water have on the melting
point of mafic mantle material?
Water lowers the melting temperatures.
This is known as Hydration Melting
How Do Igneous Rocks Form?
Step 1:
Step 2:
MELTING rocks to form MAGMA
•
temperature
•
pressure
•
water vapour content
COOL magma so CRYSTALLISATION can
take place to form solid rock
•
contact with air (extrusive rocks)
•
contact with surrounding country rock
(intrusive rocks)
•
contact with water (extrusive rocks)
Where is magma formed?
Normal situation around the globe. Geotherm is lower than
the solidus curve so rocks do not begin to melt (i.e. no
partial melting occurs).
1. Rift Valleys.
At constructive plate margins the lithosphere is being pulled apart,
causing it to stretch and thin. The ductile and mobile (but solid)
asthenosphere can then rise to fill the gap. As the asthenosphere is
now nearer to the surface it is under less pressure and partially melt
due to decompression melting. The solid peridotite partially melts to
form basaltic magma which erupts at the surface in an effusive
manner.
2. Mid-Oceanic Ridges.
At constructive plate margins the lithosphere is being pulled apart,
causing it to stretch and thin. The ductile and mobile (but solid)
asthenosphere can then rise to fill the gap. As the asthenosphere is
now nearer to the surface it is under less pressure and partially melt
due to decompression melting. The solid peridotite partially melts to
form basaltic magma which erupts at the surface in an effusive
manner.
3. Hot Spots.
At intra plate locations (away from plate boundaries) the lithosphere
is moving slowly (2-10cm/year) over the top of a mantle plume. A
mantle plume is an area of extra high heat flow (up to 300°C) rising up
through the mantle. It is NOT magma, but hot rocks which rise as
they are less dense than the surrounding rocks. As the plume nears
the surface it is under less pressure and partially melts due to
decompression melting. The solid peridotite partially melts to form
basaltic magma which erupts at the surface in an effusive manner.
4. Subduction Zones.
At destructive plate margins the lithosphere is being dragged down
into the mantle at a subduction zone. The descending oceanic
lithosphere takes down water with it, which is released into the
overlying mantle at about 100km in depth. This water lowers the
melting point of the asthenosphere which partially melts due to
hydration melting. The solid peridotite partially melts to form basaltic
magma which as it rises to the surface changes its composition to
andesitic magma due to assimilation and fractional crystallisation.
This magma erupts at the surface in an explosive manner.
How Does Magma Migrate Upwards?
> 1,000,000 years
< 1,000 years
SOLID
1. If melt > 5%,
more buoyant
liquid rises IF
weakness available
2. If NO
weakness
available, needs
> 30% melt to
force its way
upwards
SOLID
LIQUID
Melting begins
PARTIAL MELTING
How Does Magma Migrate Upwards?
PLUTON
PLUTON
PLUTON
BATHOLITH
SOLID
LIQUID
Andean Batholith Belt
How Does Magma Migrate Upwards?
How Does Magma Migrate Upwards?
How Does Magma Migrate Upwards?
Earth surface
5km
20km
Upper Crust
cold, brittle
Lower Crust
hot, ductile
20km
15km
5km
How Does Magma Migrate Upwards?
1. Diapiric Emplacement
2. Dyke Ascent
3. Magmatic Stoping
20km
15km
How Do Magmas of Different Composition Form?
K
45%
52%
SiO2
65%
How Do Magmas of Different Composition Form?
1. Partial Melting
2. Fractional Crystallisation
3. Assimilation
4. Mixing of Magmas
5. Underplating
6. Thickening of the Continental Crust
1. Partial Melting
~1200°C
Olivine
Augite
Bowen’s Reaction Series
Plagioclase
feldspar
(Mg Fe) SiO2
Ca Mg Fe SiO2
Hornblende
Plagioclase
feldspar
Ca Mg SiO2
Biotite mica
Fe Mg K Al SiO2
Orthoclase feldspar
Muscovite mica
~600°C
Ca Al SiO2
Quartz
SiO2
K Al SiO2
K Al SiO2
Na Al SiO2
2. Fractional Crystallisation
M
M
M
2. Fractional Crystallisation
~1200°C
Olivine
Augite
Plagioclase
feldspar
(Mg Fe) SiO2
Ca Mg Fe SiO2
Hornblende
Plagioclase
feldspar
Ca Mg SiO2
Biotite mica
Fe Mg K Al SiO2
Orthoclase feldspar
Muscovite mica
~600°C
Ca Al SiO2
Quartz
SiO2
K Al SiO2
K Al SiO2
Na Al SiO2
2. Fractional Crystallisation
Physical separation of solid part and liquid part
of rising pluton. How?
1. Filter pressing
2. Differentiation/Gravity settling
3. Convection within a magma chamber
3. Assimilation
4. Magma Mixing
4. Magma Mixing
Mt St Helens
Hawaii
Mt Pinatubo
Iceland
2. Fractional Crystallisation
~1200°C
Olivine
Augite
Plagioclase
feldspar
(Mg Fe) SiO2
Ca Mg Fe SiO2
Hornblende
Plagioclase
feldspar
Ca Mg SiO2
Biotite mica
Fe Mg K Al SiO2
Orthoclase feldspar
Muscovite mica
~600°C
Ca Al SiO2
Quartz
SiO2
K Al SiO2
K Al SiO2
Na Al SiO2
1. Partial Melting
~1200°C
Olivine
Augite
Bowen’s Reaction Series
Plagioclase
feldspar
(Mg Fe) SiO2
Ca Mg Fe SiO2
Hornblende
Plagioclase
feldspar
Ca Mg SiO2
Biotite mica
Fe Mg K Al SiO2
Orthoclase feldspar
Muscovite mica
~600°C
Ca Al SiO2
Quartz
SiO2
K Al SiO2
K Al SiO2
Na Al SiO2
Mineral Zoning
NaAlSi3O8 to CaAl2Si2O8
Plagioclase crystal
Ca-rich
Na-rich
Mineral Zoning
Olivine crystals
Fe
Mg
Zoned olivine
(Mg,Fe)2SiO4
Reaction Rim or Corona Structure
Olivine
Augite
Hornblende