an introduction to geology: 3 -...

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An Introduction to Geology: 3

Remembered Mnemonics?

Thus

Jogging

Can

Prove

Notably

Queasy

Eating

Crisps

On

Sundays

Doubles

Calorie

Points

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Igneous Rocks

Hand specimens

First impressions?

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Rock-forming minerals

Quartz

(SiO2)

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Rock-forming minerals

Feldspar

(aluminium silicates)

Orthoclase

Plagioclase (Ca-

) feldspars

Alkali (K- or

Na-) feldspars

Rock-forming minerals

Mica

(sheet silicates)

Biotite

Muscovite

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Rock-forming minerals

Calcium carbonate

– CaCO3

Aragonite

Calcite

Rock-forming minerals

Pyroxene / amphibole

(chain silicates)

Hornblende (amphi)

Augite (pyro)

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Rock-forming minerals

Olivine

(Mg-Fe silicate)

Igneous rocks

fire

magma

crystals

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Igneous

UK

Precambrian

Palaeozoic

Palaeogene

Classification

Setting

Cooling depth

Texture

Crystal size

Chemistry

Minerals

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1. Settings

Extrusive

Intrusive

Hypabyssal

Extrusive igneous rocks

Volcanic

Lava / ash

Gassy

Fine-grained

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Extrusive UK

Intrusive igneous rocks

Cooled at depth

Coarse-grained

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Intrusive UK

Hypabyssal igneous rocks

Sills / dykes

Medium-grained

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Hypabyssal UK

2. Textures >5 mm = Coarse-grained

1-5 mm = Medium-grained

<1 mm = Fine-grained

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Textures and temperature

Fine-grained = fast-cooled

Textures and temperature

Coarse-grained = slow-cooled

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Hand specimens

What texture/s are your rocks?

Volcanic temperatures

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3. Chemistry

Felsic – Intermediate – Mafic – Ultramafic

Colours and chemistry

Light coloured (felsic) minerals:

- Quartz

- Feldspar (orthoclase / plagioclase)

- Some micas (e.g. muscovite)

Dark coloured (mafic) minerals:

- Olivine

- Pyroxenes (e.g. augite)

- Amphiboles (e.g. hornblende)

- Other micas (e.g. biotite)

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Silica-based classification

More SiO2 = more viscous magma

(US Geological Survey)

Chemistry of crystals

Fractional crystallization:

1200°C

olivine 800°C

plagioclase

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Hand specimens

What chemistry is your rock?

Common igneous rocks

Granite

Intrusive

Coarse-grained

Felsic

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Common igneous rocks

Basalt

Extrusive

Fine-grained

Mafic

Common igneous rocks

Dolerite

Hypabyssal

Medium-

grained

Mafic

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Common igneous rocks

Gabbro

Intrusive

Coarse-

grained

Mafic

Common igneous rocks

Rhyolite

Extrusive

Fine-grained

Felsic

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Common igneous rocks

Andesite

Extrusive

Fine-grained

Intermediate

Igneous importance

Absolute dating

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Igneous importance

Granite

Dolerite

Igneous importance

Fertile soils

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The Volcano Chaser

Yorkshire Museum talks:

Mon. 28th Oct-Fri. 1st Nov.

Pyroclastic flow, Mt Pelee, 1902

Next week Sand, Mud & Lime:

TUESDAY

OCT. 29th

Room:

D/L/006