1. ROCK REVISION Focus on hand specimens for GL2 exam Three rock types:
IGNEOUS
SEDIMENTARY
METAMORPHIC

2. Igneous rocks CRYSTALLINE Mass of interlocking crystals
Cooled from molten magma
Above ground (EXTRUSIVE) cool rapidly – so have small crystals (crystals do not have time to grow)
Underground (INTRUSIVE) are insulated so cool slowly – so have large crystals (crystals have a long time to form)

3. CRYSTAL SIZES COARSE crystals bigger than 2mm
Slow cooling, insulated, intrusive
MEDIUM crystals smaller than 2mm but bigger than 0.5mm
Some insulation, probably an intrusion near surface, or thick lava flow
FINE crystals smaller than 0.5mm
Surface cooling, fast, no insulation, extrusive

4. Granite
Credit: British Geological Survey
CRYSTALLINE

5. GRANITE 1cm This is a very coarse granite – a pegmatite
Credit: British Geological Survey
This is a very coarse granite
– a pegmatite

6. GRANITE – labels?
1cm

7. GRANITE Biotite mica, black, flaky mineral Mass of interlocking
crystals, large size
so slow cooling
- indicates an
intrusive igneous
rock
Quartz, grey glassy mineral
Feldspar, white mineral
1cm

8. RHYOLITE Mass of interlocking crystals, small size so rapid cooling
indicates an
extrusive igneous
rock
flow banding
Credit: British Geological Survey

9. BASALT – labels?
Credit: British Geological Survey

10. BASALT Individual crystals not visible to eye,
indicates rapid cooling of extrusive
igneous rock
n.b. Some basalts contain
holes called VESICLES.
These formed as the lava
Solidified. Vesicles mark
where gases escaped from
the cooling lava.
Credit: British Geological Survey
Overall dark colour to rock, shows
very little quartz is present, so a
BASIC composition

11. BASALT
Columnar jointing in basalt

12. DOLERITE Slightly larger crystals, some seen with
hand lens, shows cooling was fairly rapid
probably insulated by thickness of magma
Some lighter coloured
minerals, shows some
Quartz content of magma,
still of basic composition
Credit: British Geological Survey

13. GABBRO Coarse grain size – showing cooling was slow Pale coloured
mineral =
mainly feldspar
Green-black
mineral = augite
Credit: British Geological Survey

14. IGNEOUS SUMMARY Crystal size (use measurements/numbers)
link to cooling history
and hence
link to environment eg. Volcano or pluton
and
plate tectonic environment (constructive / destructive)
Mineralogy (named minerals)

15. Sedimentary rocks FRAGMENTAL TEXTURE - T.O.S.S.S …….
Orientation of fragments (lined up)
Size of fragments (large – strong current)
Shape of fragments (rounded – long transport)
Sorting of fragments (well sorted to poorly sorted – dumped material)

16. CONGLOMERATE – labels? Credit: British Geological Survey

17. CONGLOMERATE Large rounded fragment so LONG transport history
Fine grained
MATRIX
Credit: British Geological Survey
Variety of fragments
n.b.
Large fragments show - strong current
If poorly sorted - shows rapid dumping of material
If no orientation to fragments - shows rapid dumping of material/ no strong currents

18. SANDSTONE Credit: British Geological Survey

19. SANDSTONE Ripples – beach environment
Cross bedding – indicates current flow
Credit: British Geological Survey
Credit: British Geological Survey

20. SANDSTONE Slightly coarser layer, small increase in
strength of current
Red colour indicates
land
deposition, iron
rusted to
Iron Oxide
Cross bedding
shows successive
fronts of sand dune
Current flow direction 
Credit: British Geological Survey
Fine grains indicate wind transport
Grains are frosted under hand lens

21. LIMESTONE
Credit: British Geological Survey
Fossil shells

22. LIMESTONE How do you know it is limestone?
Test it with dilute hydrochloric acid
FIZZ = calcium carbonate
Fossil content
Broken up – strong current
Aligned – current flow/direction
Delicate preservation – lack of
current
-Type – environment (eg. Corals
Clear, warm, shallow
Marine, aerated sea
Credit: British Geological Survey

23. SEDIMENTARY SUMMARY FRAGMENTAL TEXTURE - T.O.S.S.S …….
Orientation of fragments
Size of fragments
Shape of fragments
Sorting of fragments

24. Metamorphic Rocks CHANGED rocks HEAT – causes new mineral growth
PRESSURE – lines up the minerals
PRODUCT depends on ORIGINAL rock
Key words:
CRYSTALLINE (shows rock has been heated severely);
NEW MINERAL GROWTH (heat);
ALIGNMENT (layers, shows pressure).

25. MARBLE Not many features to draw! Originally LIMESTONE
FOSSILS and structures DESTROYED (streaks of colour remain?)
FIZZES with dil HCl – still a carbonate – so there has been NO CHANGE TO MINERAL COMPOSITION
Credit: British Geological Survey

26. GNEISS Crystalline, BANDED, shows high heat
Credit: British Geological Survey
Credit: British Geological Survey
Crystalline, BANDED, shows high heat
to almost MELT the original rock and
then allow it to CRYSTALLISE as it
cools
Bands also show some PRESSURE
to separate out the minerals into
light and dark layers

27. “Shiny” SCHIST Layers of mica, give rock very smooth surface
Credit: British Geological Survey
Credit: British Geological Survey
Shiny, reflective surfaces
MICA – growth of new mineral
(heat) in parallel layers (pressure)
Garnet porphyroblast
(larger crystal)

28. A few extra metamorphics
That we will see in the Lake District!
Visit walking through the metamorphic aureole of the Skiddaw Granite
Slate
Chiastolite slate/Andalusite hornfels
Spotted rock

29. SLATE CLEAVES (splits) into layers Shows minerals have ALIGNED
Due to intense PRESSURE
Original rock: mudstone
Credit: British Geological Survey

30. ANDALUSITE HORNFELS Original rock probably shale
Minerals -long thin white
Growth of new mineral
so HEAT
RANDOM – not lined
up, so NO PRESSURE
Reddish colour, iron
staining
These pieces are found in the footpath walking up to the Skiddaw Granite,
ie. Walking through a metamorphic aureole around a granite pluton

31. “SPOTTED ROCK” Area of less spots Area of more spots RANDOM spots
Credit: British Geological Survey
Area of less
spots
Area of more
spots
RANDOM spots
(looks like measles!)
Growth of new mineral
HEAT, but lacks pressure
(not aligned)
Original rock probably a
Mudstone or shale
Credit: British Geological Survey
Please look at hand specimen – it is much darker than these!

32. Shale contains many minerals and produces a wide range of new minerals if heated during metamorphism
Limestone has restricted composition (calcium carbonate) so can’t form anything other than calcium carbonate when metamorphosed! – UNLESS new minerals are introduced (by percolating fluids).

33. METAMORPHISM SUMMARY CRYSTALLINE – if enough HEAT
NEW MINERAL – if HEATED
ALIGNED – if enough PRESSURE
Mix of both – alignment of new minerals – shows heat and pressure were both factors.

34. EXAM HINTS
Examiners will chose something that has recognisable features (ie. Something to identify – minerals, shape, alignment)
If asked to DRAW, only draw a small part of the rock, and get the features in!
Give all diagrams lots of LABELS and/or ANNOTATIONS
Don’t forget a SCALE
Don’t forget the basics: Igneous/Sedimentary/Metamorphic? – and why? (Crystalline, fragmental, crystalline/new minerals/banded)