Classification of Igneous Rocks 1. Chemical Composition 2. Colour 3. Texture 4. Mineralogy Felsic...
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Transcript of Classification of Igneous Rocks 1. Chemical Composition 2. Colour 3. Texture 4. Mineralogy Felsic...
Classification of Igneous Rocks
1. Chemical Composition
2. Colour
3. Texture
4. Mineralogy
• Felsic
• Basic
• Amygdaloidal
• Mafic
• Undersaturated/oversaturated
• Intermediate
• Volcanic
• Ultrabasic
• Hypabyssal
• Ultramafic
• Plutonic
• Acidic
• Vesicular
• Saturated
• Porphyritic
• Equigranular
• Flow banding
• Crystalline
• Interlocking crystals
• Randomly orientated
• Euhedral/Anhedral/ Subhedral
Olivine Quartz Feldspar
Muscovite Mica Biotite mica
Augite Hornblende
Chemical Composition
1. Acidic
2. Intermediate
3. Basic
4. Ultrabasic
>65% SiO2
65% - 52% SiO2
52% - 45% SiO2
<45% SiO2
Texture
Crystalline, interlocking crystals & randomly orientated
Grain size
Equigranular or porphyritic
Vesicular, flow banding, amygdaloidal
Mineralogy
SiO2
(K, Na) AlSiO
Na AlSiO to CaAlSiO
K AlSiO
- Quartz
- Orthoclase feldspar
- Plagioclase feldspar
- Muscovite mica
Felsic Minerals
Mineralogy
(Mg, Fe) SiO
(Mg, Fe, Ca) SiO
Ca (Mg, Fe) SiO
K (Mg, Fe) AlSiO
- Olivine
- Augite
- Hornblende
- Biotite mica
Mafic Minerals
How Do Igneous Rocks Form?
Step 1:
Step 2:
MELTING rocks to form MAGMA
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
• 1000 Mpa & 1100°C
• 400 Mpa & 1150°C
• 200 Mpa & 1250°C
Partially molten
Solid
Partially molten
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?
• at what temperature does it become completely molten?
1075°C
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?
What affect does the presence of water have on the melting point of mafic mantle material?
Water lowers the melting temperatures.
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
This is known as Hydration Melting
How Do Igneous Rocks Form?
Step 1:
Step 2:
MELTING rocks to form MAGMA
COOL magma so CRYSTALLISATION can take place to form solid rock
• temperature
• pressure
• water vapour content
• 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).
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.
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.
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.
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?
Melting begins
1. If melt > 5%, more buoyant liquid rises IF weakness availableSOLID
PARTIAL MELTING
SOLID
LIQUID
2. If NO weakness available, needs > 30% melt to force its way upwards
< 1,000 years
> 1,000,000 years
How Does Magma Migrate Upwards?
20km
15km 5km
Earth surface
Upper Crust
Lower Crust
cold, brittle
hot, ductile
5km
20km
How Does Magma Migrate Upwards?
20km
15km
1. Diapiric Emplacement
2. Dyke Ascent
3. Magmatic Stoping
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
Quartz SiO2
Muscovite mica K Al SiO2
Orthoclase feldspar K Al SiO2
Biotite mica Fe Mg K Al SiO2
Hornblende Ca Mg SiO2
Plagioclase feldspar
Na Al SiO2
Augite Ca Mg Fe SiO2
Plagioclase feldspar
Ca Al SiO2Olivine
(Mg Fe) SiO2
~600°C
~1200°C
Bowen’s Reaction Series
Quartz SiO2
Muscovite mica K Al SiO2
Orthoclase feldspar K Al SiO2
Biotite mica Fe Mg K Al SiO2
Hornblende Ca Mg SiO2
Plagioclase feldspar
Na Al SiO2
Augite Ca Mg Fe SiO2
Plagioclase feldspar
Ca Al SiO2Olivine
(Mg Fe) SiO2
~600°C
~1200°C
2. Fractional Crystallisation
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
Quartz SiO2
Muscovite mica K Al SiO2
Orthoclase feldspar K Al SiO2
Biotite mica Fe Mg K Al SiO2
Hornblende Ca Mg SiO2
Plagioclase feldspar
Na Al SiO2
Augite Ca Mg Fe SiO2
Plagioclase feldspar
Ca Al SiO2Olivine
(Mg Fe) SiO2
~600°C
~1200°C
2. Fractional Crystallisation
1. Partial Melting
Quartz SiO2
Muscovite mica K Al SiO2
Orthoclase feldspar K Al SiO2
Biotite mica Fe Mg K Al SiO2
Hornblende Ca Mg SiO2
Plagioclase feldspar
Na Al SiO2
Augite Ca Mg Fe SiO2
Plagioclase feldspar
Ca Al SiO2Olivine
(Mg Fe) SiO2
~600°C
~1200°C
Bowen’s Reaction Series