Transcript Rocks Revision

ROCK REVISION
Focus on hand specimens for GL2 exam
 Three rock types:

IGNEOUS
 SEDIMENTARY
 METAMORPHIC

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)

CRYSTAL SIZES

COARSE crystals bigger than 2mm
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MEDIUM crystals smaller than 2mm but
bigger than 0.5mm
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Slow cooling, insulated, intrusive
Some insulation, probably an intrusion near
surface, or thick lava flow
FINE crystals smaller than 0.5mm

Surface cooling, fast, no insulation, extrusive
Granite
Credit: British Geological Survey
CRYSTALLINE
GRANITE
1cm
Credit: British Geological Survey
This is a very coarse granite
– a pegmatite
GRANITE – labels?
1cm
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
RHYOLITE
Mass of interlocking
crystals, small size
so rapid cooling
indicates an
extrusive igneous
rock
flow banding
Credit: British Geological Survey
BASALT – labels?
Credit: British Geological Survey
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
BASALT
Columnar jointing in basalt
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
GABBRO
Coarse grain
size – showing
cooling was slow
Pale coloured
mineral =
mainly feldspar
Green-black
mineral = augite
Credit: British Geological Survey
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)
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)

CONGLOMERATE – labels?
Credit: British Geological Survey
Credit: British Geological Survey
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
SANDSTONE
Credit: British Geological Survey
Credit: British Geological Survey
Credit: British Geological Survey
Credit: British Geological Survey
SANDSTONE
Ripples – beach environment
Cross bedding – indicates current flow
Credit: British Geological Survey
Credit: British Geological Survey
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
LIMESTONE
Credit: British Geological Survey
Fossil shells
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
SEDIMENTARY SUMMARY
FRAGMENTAL
 TEXTURE - T.O.S.S.S …….
 Orientation of fragments
 Size of fragments
 Shape of fragments
 Sorting of fragments

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).
MARBLE
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Credit: British Geological Survey
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
GNEISS
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
“Shiny” SCHIST
Credit: British Geological Survey
Layers of mica, give rock very
smooth surface
Credit: British Geological Survey
Shiny, reflective surfaces
MICA – growth of new mineral
(heat) in parallel layers (pressure)
Garnet porphyroblast
(larger crystal)
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
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SLATE
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Credit: British Geological Survey
CLEAVES (splits)
into layers
Shows minerals
have ALIGNED
Due to intense
PRESSURE
Original rock:
mudstone
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
“SPOTTED ROCK”
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!
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).

METAMORPHISM SUMMARY
CRYSTALLINE – if enough HEAT
 NEW MINERAL – if HEATED
 ALIGNED – if enough PRESSURE

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Mix of both – alignment of new minerals –
shows heat and pressure were both
factors.
EXAM HINTS
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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)