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Classification of Igneous Rocks

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1 Classification of Igneous Rocks
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 Classification of Igneous Rocks 1. Chemical Composition 2. Colour 3. Texture 4. Mineralogy Olivine Quartz Feldspar Muscovite Mica Biotite mica Augite Hornblende

2 Chemical Composition 1. Acidic >65% SiO2 2. Intermediate 65% - 52% SiO2 3. Basic 52% - 45% SiO2 4. Ultrabasic <45% SiO2

3 Colour 1. Felsic - light 2. Intermediate - medium 3. Mafic - dark 4. Ultramafic - very dark

4 Texture Crystalline, interlocking crystals & randomly orientated Grain size Equigranular or porphyritic Vesicular, flow banding, amygdaloidal

5 Mineralogy SiO2 - Quartz (K, Na) AlSiO - Orthoclase feldspar Na AlSiO to CaAlSiO - Plagioclase feldspar K AlSiO - Muscovite mica Felsic Minerals

6 Mineralogy (Mg, Fe) SiO - Olivine (Mg, Fe, Ca) SiO - Augite Ca (Mg, Fe) SiO - Hornblende K (Mg, Fe) AlSiO - Biotite mica Mafic Minerals

7 How Do Igneous Rocks Form?
Step 1: MELTING rocks to form MAGMA COOL magma so CRYSTALLISATION can take place to form solid rock Step 2:

8 What causes melting? Partially molten Solid Partially molten Liquid
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

9 What causes melting? 1075°C 1200°C
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

10 What causes melting? Melting This is known as Decompression 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.

11 What causes melting? Liquid 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? What causes melting? 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

12 How Do Igneous Rocks Form?
Step 1: MELTING rocks to form MAGMA temperature pressure water vapour content COOL magma so CRYSTALLISATION can take place to form solid rock Step 2: contact with air (extrusive rocks) contact with surrounding country rock (intrusive rocks) contact with water (extrusive rocks)

13 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).

14 1. Rift Valleys.

15 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.

16 2. Mid-Oceanic Ridges.

17 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.

18 3. Hot Spots.

19 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.

20

21 4. Subduction Zones.

22 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.

23 How Does Magma Migrate Upwards? > 1,000,000 years
2. If NO weakness available, needs > 30% melt to force its way upwards 1. If melt > 5%, more buoyant liquid rises IF weakness available SOLID SOLID LIQUID Melting begins PARTIAL MELTING

24 How Does Magma Migrate Upwards?
PLUTON BATHOLITH PLUTON PLUTON SOLID LIQUID Andean Batholith Belt

25 How Does Magma Migrate Upwards?

26 How Does Magma Migrate Upwards?

27 How Does Magma Migrate Upwards?
Earth surface 5km Upper Crust cold, brittle Lower Crust hot, ductile 20km 20km 15km 5km

28 How Does Magma Migrate Upwards?
1. Diapiric Emplacement 2. Dyke Ascent 3. Magmatic Stoping 20km 15km

29 How Do Magmas of Different Composition Form?
K 45% 52% 65% SiO2

30 How Do Magmas of Different Composition Form?
Partial Melting Fractional Crystallisation Assimilation Mixing of Magmas Underplating Thickening of the Continental Crust

31 Bowen’s Reaction Series
1. Partial Melting Bowen’s Reaction Series ~1200°C Olivine Plagioclase feldspar (Mg Fe) SiO2 Ca Al SiO2 Augite Ca Mg Fe SiO2 Plagioclase feldspar Na Al SiO2 Hornblende Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar K Al SiO2 Muscovite mica K Al SiO2 ~600°C Quartz SiO2

32 2. Fractional Crystallisation
M M M

33 2. Fractional Crystallisation
Olivine Plagioclase feldspar (Mg Fe) SiO2 Ca Al SiO2 Augite Ca Mg Fe SiO2 Plagioclase feldspar Na Al SiO2 Hornblende Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar K Al SiO2 Muscovite mica K Al SiO2 ~600°C Quartz SiO2

34 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

35 3. Assimilation

36 4. Magma Mixing

37 4. Magma Mixing

38

39 Mt St Helens Iceland Hawaii Mt Pinatubo

40

41

42 2. Fractional Crystallisation
Olivine Plagioclase feldspar (Mg Fe) SiO2 Ca Al SiO2 Augite Ca Mg Fe SiO2 Plagioclase feldspar Na Al SiO2 Hornblende Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar K Al SiO2 Muscovite mica K Al SiO2 ~600°C Quartz SiO2

43

44 Bowen’s Reaction Series
1. Partial Melting Bowen’s Reaction Series ~1200°C Olivine Plagioclase feldspar (Mg Fe) SiO2 Ca Al SiO2 Augite Ca Mg Fe SiO2 Plagioclase feldspar Na Al SiO2 Hornblende Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar K Al SiO2 Muscovite mica K Al SiO2 ~600°C Quartz SiO2

45

46 Mineral Zoning NaAlSi3O8 to CaAl2Si2O8 Plagioclase crystal Ca-rich Na-rich

47 Mineral Zoning Olivine crystals Fe Mg Zoned olivine (Mg,Fe)2SiO4

48 Reaction Rim or Corona Structure
Olivine Augite Hornblende

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