Help for A-level Geology students in identifying the main igneous rocks - interactive so you can test yourself as you go...

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

2. Characterizing 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

3. 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???

4. Characterizing Rocks
The three major characterizing features of
rocks are:
• Colour
• Composition (Mineralogy/Chemistry)
• Texture
Note: Even the most sophisticated
geological classification schemes are
based on these features

5. Characterising Rocks
Classification by Colour
Colour Index (used mainly for igneous rocks)
• Leucocratic – light colour
• Mesocratic – intermediate colour
• Melanocratic – dark colour
and/or
• Felsic – rich in light coloured minerals
• Mafic – rich in dark coloured minerals

6. Igneous Rocks
Textures of Igneous Rocks
• Aphanitic – fine-grained. Individual grains
can’t be seen with naked eye
• Phaneritic - grains easily seen with the naked
eye
• Porphyritic – larger grains in finer grains
•
Inclusions
> Xenoliths
> Xenocrysts

10. Vesicular &
Glassy
Textures

11. Xenoliths

12. 1
9
5
2
4
8
3
7
6
10
11
12
13
14
15

13. What % Si02??
66
52
45
1
3
2
4

14. Colour?
Leucocratic (light)?
i.e. silicic v mafic

Melanocratic (dark)?
Mesocratic (medium)?
Grain size /
Texture?
Phaneritic (coarse)

Aphanitic (fine)?
Medium?
Black
Biotite
shiny?
mica
K feldspar
Pink?
Mineralogy?
Granite
Quartz
Grey vitreous?

15. A quicker way?
– pink crystals, coarse grained – granite
(then check for other minerals to be sure)

16. Granodiorite
…plenty of quartz… but no / not much K feldspar…

17. Colour?
i.e. silicic v mafic
Leucocratic (light)?
Melanocratic (dark)?
Mesocratic (medium)?

Grain size /
Texture?
Phaneritic (coarse)

Aphanitic (fine)?
Medium?
Gabbro
Not
Mafic easy!! – are these green or grey?
Dark?
(pyroxene or I think pale green!! – so…
Plagioclase
white?
amphibole)
Olivine
feldspar
…but if grey 
Mineralogy?
Quartz

18. Colour?
Leucocratic (light)?
i.e. silicic v mafic

Melanocratic (dark)?
Mesocratic (medium)?
Grain size /
Texture?
Phaneritic (coarse)
Aphanitic (fine)?

Medium?
So what is it?
Rhyolite

19. Flow banding texture – distinctive 
rhyolite

20. Colour?
Grain size /
Texture?
Leucocratic (light)?
Melanocratic (dark)?

Mesocratic (medium)?
Phaneritic (coarse)
Aphanitic (fine)?
Larger crystals called??
phenocrysts
…white, so…  ??
Plagioclase feldspar
NB – story?... Think
Bowen’s Reaction
series…
These aren’t crystals
(round – bubble-shaped!)
amygdales
(infilled vesicles)
Medium?
So what is it?
Basalt


21. What are the holes??
Vesicles
Colour?
mesocratic
so…  ??
andesite
BUT…
What are the GREEN
phenocrysts?
Olivine!!
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…
So can’t be andesite
lighter coloured than usual??
So what is it?
Basalt
(vesicular
basalt)

22. Grain size /
Texture?
Colour?
Leucocratic (light)?
Melanocratic (dark)?

Mesocratic (medium)?
Phaneritic (coarse)
Aphanitic (fine)?
Medium?
So what is it?
Dolerite


23. Looks pretty light-coloured –
and obviously course-grained
So might think ‘granite’… BUT…
What minerals would you expect?
K feldspar and quartz for a start….
So what is it?
Diorite
Whereas what’s here???
Plagioclase feldspar and mafics…

24. So what is it?
Andesite
Colour?
Leucocratic (light)?
Melanocratic (dark)?
Mesocratic (medium)?

Grain size /
Texture?
Phaneritic (coarse)
Aphanitic (fine)?
Medium?


25. So what is it?
Andesite
Colour?
Leucocratic (light)?
Melanocratic (dark)?
Mesocratic (medium)?

Grain size /
Texture?
Phaneritic (coarse)
Aphanitic (fine)?
Medium?


26. So what is it?
Andesite
Colour?
Leucocratic (light)?
Melanocratic (dark)?
Mesocratic (medium)?

Grain size /
Texture?
Phaneritic (coarse)
Aphanitic (fine)?
Medium?


27. Yep, they’re all
andesite!!

28. Glassy texture, black coloured… so…
How formed?
Super-cooled – so no
time for crystals to
form – usually when
erupted into water
obsidian

29. Volcanic glass is chemically unstable – out of
equilibrium with surface temperatures – so begins to
crystallise (de-vitrify) – snowflake obsidian

30. If you see this rock, just note the large green crystals  ??
olivine
…so it’s peridotite (ultramafic) – not dark black as you might

31. What’s this?
Mantle xenoliths
in basalt

32. What’s this?
(look at the
massive crystals)
Pegmatite
Forms when late-stage volatiles are injected into country
rock when granite batholith is mostly crystallised

33. 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