Rock Identification

Modified from a PowerPoint presentation prepared by J. Crelling, Southern Illinois University

Characterizing RocksCharacterizing Rocks

There are 3 major types of rocks

IGNEOUS – formed from molten magma

SEDIMENTARY – formed from sediment (mud, sand, shells, etc,)

METAMORPHIC – formed by applying heat and pressure to other existing rocks

The Rock Cycle – a representation of the interrelationship between different types of rocks.

NOTE:How do rocks formed at depth (i.e. all except extrusive igneous) get exposed on the surface???

Characterizing RocksCharacterizing Rocks

The three major characterizing features of The three major characterizing features of rocks are:rocks are:

• • ColourColour

•• Composition (Mineralogy/Chemistry)Composition (Mineralogy/Chemistry)

•• TextureTexture

Note: Even the most sophisticated Note: Even the most sophisticated geological classification schemes are geological classification schemes are based on these featuresbased on these features

Characterising RocksCharacterising Rocks

Classification by ColourClassification by Colour

Colour Index (used mainly for igneous rocks)Colour Index (used mainly for igneous rocks)

• • Leucocratic – light colourLeucocratic – light colour • • Mesocratic – intermediate colourMesocratic – intermediate colour • • Melanocratic – dark colourMelanocratic – dark colour

and/orand/or • • Felsic – rich in light coloured mineralsFelsic – rich in light coloured minerals • • Mafic – rich in dark coloured mineralsMafic – rich in dark coloured minerals

Igneous RocksIgneous Rocks

Textures of Igneous RocksTextures of Igneous Rocks

•• Aphanitic – fine-grained. Individual grains Aphanitic – fine-grained. Individual grains can’t be seen with naked eyecan’t be seen with naked eye

•• PhaneriticPhaneritic - grains easily seen with the - grains easily seen with the naked naked eyeeye

•• Porphyritic – larger grains in finer grainsPorphyritic – larger grains in finer grains •• Inclusions Inclusions

> > XenolithsXenoliths

>> Xenocrysts Xenocrysts

Vesicular & Glassy Textures

Xenoliths

1 2

345

6

7

89

10 1112

1314 15

1

3

2 4

What % Si02??455266

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)?

Grain size / Texture?

Phaneritic (coarse)

Aphanitic (fine)?

Medium?

Mineralogy?

Pink?

i.e. silicic v mafic

Grey vitreous?

Black shiny?

K feldspar

Quartz

Biotitemica

Granite

A quicker way?– pink crystals, coarse grained – granite

(then check for other minerals to be sure)

…plenty of quartz… but no / not much K feldspar…

Granodiorite

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)?

Grain size / Texture?

Phaneritic (coarse)

Aphanitic (fine)?

Medium?

Mineralogy?

Dark?

i.e. silicic v mafic

white?

Mafic (pyroxene or amphibole) Plagioclase

feldspar

GabbroNot easy!! – are these green or grey?

I think pale green!! – so…Olivine

…but if grey Quartz

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)?

Grain size / Texture?

Phaneritic (coarse)

Aphanitic (fine)?

Medium?

i.e. silicic v mafic

Rhyolite So what is it?

Flow banding texture – distinctive rhyolite

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)?

Grain size / Texture?

Phaneritic (coarse)

Aphanitic (fine)?

Medium?

Basalt So what is it?

Larger crystals called??

phenocrysts…white, so… ??Plagioclase feldsparNB – story?... Think Bowen’s Reaction series…

These aren’t crystals (round – bubble-shaped!)

amygdales (infilled vesicles)

What are the holes??Vesicles

Colour?

mesocraticso… ??andesiteBUT… What are the GREEN phenocrysts?

Olivine!! So can’t be andesite

So… Thinking of magma differentiation, what’s the story here, maybe?Mafic magma…Erupted before olivine could be reabsorbed into melt…Melt will be relatively depleted in Mg,Fe so…lighter coloured than usual??

Basalt (vesicular

basalt)

So what is it?

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)?

Grain size / Texture?

Phaneritic (coarse)

Aphanitic (fine)?

Medium?

Dolerite So what is it?

Looks pretty light-coloured – and obviously course-grained

Whereas what’s here???Plagioclase feldspar and mafics…Diorite

So what is it?

So might think ‘granite’… BUT…What minerals would you expect?K feldspar and quartz for a start….

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)? Grain size / Texture?

Phaneritic (coarse)Aphanitic (fine)?

Medium?

Andesite So what is it?

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)? Grain size / Texture?

Phaneritic (coarse)Aphanitic (fine)?

Medium?

Andesite So what is it?

Colour?Leucocratic (light)?

Melanocratic (dark)?

Mesocratic (medium)? Grain size / Texture?

Phaneritic (coarse)Aphanitic (fine)?

Medium?

Andesite So what is it?

Yep, they’re all

andesite!!

Glassy texture, black coloured… so…

obsidian

Super-cooled – so no time for crystals to

form – usually when erupted into water

How formed?

Volcanic glass is chemically unstable – out of equilibrium with surface temperatures – so begins to

crystallise (de-vitrify) – snowflake obsidian

If you see this rock, just note the large green crystals ??olivine

…so it’s peridotite (ultramafic) – not dark black as you might expect

Mantle xenoliths in basalt

What’s this?

Pegmatite

What’s this? (look at the

massive crystals)

Forms when late-stage volatiles are injected into country rock when granite batholith is mostly crystallised

With many thanks to:

•The original author – someone from University of Massachusetts!

•Additional photos from Roger Weller, Conchise College, Arizona: http://skywalker.cochise.edu/wellerr/rocks/igrocksL.htm