Classification of Igneous Rocks 1. Chemical Composition 2. Colour 3. Texture 4. Mineralogy Felsic Basic Amygdaloidal Mafic Undersaturated/oversaturated

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

Colour

1. Felsic

2. Intermediate

3. Mafic

4. Ultramafic

- light

- medium

- dark

- very dark

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).

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?

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


SOLID

LIQUID

PLUTONBATHOLITH

PLUTONPLUTON

Andean Batholith Belt




20km

15km 5km

Earth surface

Upper Crust

Lower Crust

cold, brittle

hot, ductile

5km

20km


20km

15km

1. Diapiric Emplacement

2. Dyke Ascent

3. Magmatic Stoping

How Do Magmas of Different Composition Form?

SiO2

45% 52% 65%

K

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


Ca Al SiO2Olivine

(Mg Fe) SiO2

~600°C

~1200°C

Bowen’s Reaction Series


M

M

M

Quartz SiO2






Na Al SiO2



Ca Al SiO2Olivine

(Mg Fe) SiO2

~600°C

~1200°C



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

Hawaii

IcelandMt St Helens

Mt Pinatubo

Quartz SiO2






Na Al SiO2



Ca Al SiO2Olivine

(Mg Fe) SiO2

~600°C

~1200°C


1. Partial Melting

Quartz SiO2






Na Al SiO2



Ca Al SiO2Olivine

(Mg Fe) SiO2

~600°C

~1200°C

Bowen’s Reaction Series

Mineral Zoning

Plagioclase crystal

Ca-rich Na-rich

NaAlSi3O8 to CaAl2Si2O8

Mineral Zoning

Olivine crystals

Zoned olivine

(Mg,Fe)2SiO4

Mg

Fe

Reaction Rim or Corona Structure

Olivine

Augite

Hornblende

Documents

Classification of Igneous Rocks 1. Chemical Composition 2. Colour 3. Texture 4. Mineralogy Felsic Basic Amygdaloidal Mafic Undersaturated/oversaturated