Granite

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Granite
I.G.Kenyon
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Major Granite Outcrops in Devon and Cornwall
All the separate outcrops link up at depth to
form a huge batholith of granite which comprises
many thousand cubic kilometres of rock
Granites dated at 270 300 million years
Bodmin Moor 287 288 Ma
Carn Brea 290 Ma
Lands End 275 280 Ma
St.Austell Moor 270 285 Ma
Dartmoor 280 - 300 Ma
Carnmanellis 290 295 Ma
Tregonning-Godolphin 280 Ma
At 6 km depth the granite is still at a
temperature of 200C to 225C
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The Process of Granite Emplacement
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Essential Minerals in Granite

• Quartz colourless, greyish or glassy, shows
  hexagonal cross section if crystals well formed.
  Minimum 10 to gt60
• Plagioclase Feldspar white/creamy colour,
  rectangular shaped crystals
• Orthoclase Feldspar flesh/pink colour,
  rectangular shaped crystals
• Biotite Mica black in colour, pearly lustre,
  shows perfect basal cleavage

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Accessory Minerals in Granite

• These may make up just 5 of the total rock
• Muscovite Mica silvery/straw colour pearly
  lustre
• Hornblende greenish-black, prismatic crystals
• Iron Pyrite brassy gold colour, metallic
  lustre
• Tourmaline greenish black, needle-like
  crystals

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Porphyritic Granite
Black biotite mica
Large feldspar phenocrysts up to 4cm long
Groundmass is much finer grained 1-5mm in diameter
Well formed euhedral crystals
4cm
Colourless or glassy quartz
Orthoclase Feldspar phenocrysts show simple
twinning
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Shap Granite or Ademallite
Porphyritic texture, large phencrysts up to 3cm
surrounded by a finer groundmass 1-3mm in
diameter. Produced by a two-stage cooling process
Black biotite mica
3cm
Simple twinning shown by euhedral orthoclase
feldspar crystal
Orthoclase feldspar phenocrysts up to 3cm long
Colourless, grey or glassy quartz
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Kaolinised Granite
Feldspar phenocrysts decompose to kaolinite
(china clay) by hydrolysis
Grey/colourless and glassy quartz remains
unaltered
4cm
Kaolinisation is achieved by hydrolysis and
hydrothermal action
Brown/orange iron oxide staining due to breakdown
of biotite mica
Kaolinised granite is often referred to as
Growan, a mixture of clay minerals and quartz
crystals
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Tor Formation
A tor is an isolated outcrop of jointed granite
that rises above a generally flat,
plateau-like area. For example Haytor on
Dartmoor 454m above sea level The tors represent
areas of granite that are relatively more
massive than the surrounding granite by having
a low concentration of joints. These areas
undergo slow rates of denudation (surface
lowering) due to the fact water penetration
into the rock is limited. Consequently mechanical
and chemical weathering proceeds slowly. Areas
of high joint concentration undergo relatively
rapid disintegration and decomposition and the
land surfaces are lowered more rapidly
by weathering and erosion. Freeze thaw and
hydrolysis are the most important
processes in granite weathering.
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The Effects of Freeze-Thaw Weathering
Granite consists of cuboidal or rectangular
blocks due to 2 sets of joints intersecting at 90
Car keys for scale
Joints widened by freeze-thaw activity
Ice occupies 9 more volume than water and can
generate immense internal stress in confined
spaces
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Sculptured Granite Blocks
Micro-landforms in granite up to 70cm wide x 35cm
deep
No debris contained within the hollows
30cm
Rounded forms suggest hydrolysis followed by wind
action to remove any fine material
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The Effects Of Weathering
Surface of granite is crumbly here-a mixture of
clay and quartz crystals
Joints widened by freeze-thaw activity
Car keys for scale
Edges of the granite blocks have a rounded form
suggesting chemical weathering is important
(hydrolysis)
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The Products of Weathering
Quartz remains as it not affected by chemical
weathering
Tee peg for scale
This represents the first stage in the
formation of a new sedimentary rock (sandstone)
The products such as clay from chemical
weathering have been washed or blown way
Angular quartz grains ranging in size from 0.5 to
8mm
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Jointed Granite-Carn Brea
Rounded edges suggest chemical weathering by
hydrolysis
Granite is divided into a series of rectangular
or cuboidal blocks by the two sets of joints
intersecting at 90 degrees
As the overlying sediment (more than 5km)
thickness was eroded away, the granite expanded
and cracked parallel to its own surface to form
near horizontal joints
Vertical joints formed as the granite cooled,
contracted and solidified
Recently fallen block, triggered by freeze-thaw
action
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Granite Tors Carn Brea
Vertical joints formed by cooling and contraction
Horizontal joints produced by dilatation or
pressure release
Granite block or corestone is massive, having no
vertical or horizontal joints
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Logan Stones
This large granite boulder weighing many tonnes
can be rocked by hand
Eventually this block will be pushed away from
the tor by freeze-thaw activity
Only a small area of the block is in contact
with the granite underneath and acts as a pivot
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Nightjar Rock A Logan Stone
Head
Several persons standing on the head causes the
stone to rock, even though it weighs many tonnes
Wings
Beak
Very small surface area of block in contact with
the underlying granite, this acts as a pivot
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Clitterblocks around the Tors
Solifluction (Soilflow) is responsible for the
movement of the granite blocks further downslope
mainly between 10,000 and 2 million years ago
Large angular clitterblocks of granite up to
2.5m in diameter
Freeze-thaw activity is responsible for levering
the joint-bounded blocks away from the tors
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Solifluction
Occurs under Periglacial conditions Similar
climate to the Canadian Tundra and Siberia
today For most of the year the ground is frozen
solid During the brief spring/summer, the top
part of the soil begins to thaw out,
this is the active layer At depth the ground is
still frozen and impermeable The active layer is
saturated as drainage is impeded The saturated
mass of rock fragments and soil can flow
down slopes as gentle as just 2 degrees
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Granite is Impermeable
Typical vegetation of granite areas is heather
gorse and bracken
Granite protrudes through where soil worn away on
paths
Soil is very thin, just a few centimetres deep
Surface water collects in natural hollows after
rainfall as granite is impermeable
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Granite Country Rock Contact
St.Agnes Beacon in the distance, another
granite outcrop
Killas-metamorphosed mudstones and siltstones
Form lower, flatter ground and are used
for agriculture
Junction between granite and killas de-limited by
change in vegetation
Granite bracken and gorse
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Economic Uses of Granite
Granite is used mainly for its bulk physical
properties, rather than any minerals it
contains The exception is kaolinite (china clay)
which is formed by the chemical breakdown of
Orthoclase Feldspar It is igneous, comprised of
tightly interlocking crystals of quartz,
feldspar and mica It is relatively resistant to
weathering and erosion It is exposed at the
surface in Cornwall and
Devon and can be readily quarried The
interlocking joint pattern aids the quarrying
process
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Economic uses of Granite
Granite intrusions often have tin, copper zinc
and lead deposits associated with them
Mine Head Gear
Conveyor belt from mine
Processing plant
South Crofty Tin Mine closed in 1998.
It produced 1900 tonnes of tin per
year. Average grade of tin ore was 1.4
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Economic use of Granite
The Wellington Hotel, St.Just
Granite blockwork
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Economic uses of Granite
Granite sculpture outside Sainsburys
supermarket, Truro
Granite Gatepost Penweathers Lane Near Truro
3m
1m
Granite blockwork used for the base
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Economic uses of Granite
Memorial Stone, Truro School Main building, Front
Terrace
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Economic uses of Granite
Truro School Chapel Granite doorframes and arch
above made of granite
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Economic uses of Granite
Granite climbing wall Truro
School Sports Hall
Granite wall, Tesco Camborne
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Economic use of Granite
Tomb St.Just Churchyard
Samuels Jewellers, Boscawen Street,Truo
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Economic uses of Granite
Granite mini-roundabout, Tesco Camborne
Granite doorstep to butchers shop, St.Just
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Granite at the Golf Club
Large granite boulders installed in front of the
Pro shop at Perranporth to stop potential ram
raiders
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Granite on the Golf Course
The 9th Tee Perranporth Golf Club
Details of the holes are inscribed on large
granite blocks with polished front surfaces
Tee markers are a cylindrical core of granite
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Granite on the Golf Course
Perranporth Golf Club 7th Hole
7th Green
Large granite block used to indicate 150m from
the front edge of the green
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Economic uses of Granite
Granite planter Village Square, St.Just
Granite speed-bumps at Tesco Camborne
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Economic uses of Granite
Carnon Downs Garden Centre
Prices range from 30 to 450
Water Feature
Garden Seat
Birdbath
Garden ornaments made of imported Chinese granite
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Granite Boulders Carnon Downs Garden Centre
Individual boulders retail at 4-50!!
Granite boulders approximately 30cm x 30cm for
decorative use in the garden
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Economic uses of Granite
Granite chippings sold at garden centres 6mm
diameter 2-95 for 25kg at Carnon
Downs Garden Centre
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Economic uses of Granite
Granite fenceposts
St.Just Churchyard
Old granite gravestones laid flat
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Economic uses of Granite
Free draining and rather dry and rough going for
snails and slugs!
Granite chippings 6mm in diameter used on a patio
Granite chippings 6mm in diameter used as the
flooring in a greenhouse
Free draining, yet acts as a weed suppressant
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Economic Uses of Granite
Penweathers Lane, Near Truro
1m
Granite millstone, now used for decorative purpose
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Economic Uses Of Granite
Gravestone, St.Just Churchyard Surface is highly
polished
Granite chippings 12mm in diameter used for
gravel drive Also acts a good weed suppressant
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Economic use of Granite
Sainsburys, Truro
Granite blocks from the former Richard Lander
School on this site used to face the Sainsburys
building
Granite kerbstones
Granite wall
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Economic uses of Granite
Front façade of Truro Cathedral
Truro Cathedral Spires
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Economic uses of Granite
St.Just Church Clock Tower
War memorial Boscawen Street, Truro
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Economic uses of Granite
Granite Cobbles in the road Boscawen
Street, Truro
Granite used as pavement stone, outside Abbey
National, Boscawen Street, Truro
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Truro City Centre
Cathedral Lane Leats to drain away rain water
and wash away any debris
20cm
Kerbstones in Boscawen Street
Semi-circular channel cut in granite slab to
carry water
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Granite used for building in the CBD of Truro
Lloyds/TSB Bank, Boscawen Street,Truro
Granite Blocks
The whole façade is made of granite blocks
Truro Library, Pydar Street
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The Cheesewring Bodmin Moor Cornwalls most
famous tor and close to Brown Willy, the highest
point in Cornwall at 420m high