Granite
I.G.Kenyon
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
Lands End
275 – 280 Ma
Bodmin Moor
287 – 288 Ma
Granites dated at 270 – 300 million years
Carn Brea
290 Ma
St.Austell Moor
270 – 285 Ma
TregonningGodolphin
280 Ma
Carnmanellis
290 – 295 Ma
Dartmoor
280 - 300 Ma
At 6 km depth the granite is still at a
temperature of 200°C to 225°C
The Process of Granite Emplacement
Essential Minerals in Granite
• Quartz – colourless, greyish or glassy,
shows hexagonal cross section if crystals
well formed. Minimum 10% to >60%
• 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
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
Porphyritic Granite
Groundmass is
much finer grained
1-5mm in diameter
Black biotite mica
Large feldspar
phenocrysts up
to 4cm long
Well formed
euhedral crystals
4cm
Colourless or
glassy quartz
Orthoclase Feldspar phenocrysts show simple twinning
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
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
Kaolinised granite is often referred
to as Growan, a mixture of clay
minerals and quartz crystals
Brown/orange iron oxide staining
due to breakdown of biotite mica
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.
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
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
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)
The Products of Weathering
Tee peg for scale
Quartz remains as it not affected
by chemical weathering
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
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
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
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
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
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
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
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
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
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
Economic uses of Granite
Granite intrusions often have tin, copper zinc
and lead deposits associated with them
Processing plant
Conveyor belt
from mine
South Crofty Tin Mine closed in 1998.
It produced 1900 tonnes of tin per year.
Average grade of tin ore was 1.4%
Mine Head Gear
Economic use of Granite
The Wellington
Hotel, St.Just
Granite
blockwork
Economic uses of Granite
Granite Gatepost
Penweathers Lane
Near Truro
Granite sculpture outside
Sainsbury’s supermarket, Truro
3m
1m
Granite blockwork used for the base
Economic uses of Granite
Memorial Stone, Truro School Main building, Front Terrace
Economic uses of Granite
Truro School Chapel
Granite doorframes and
arch above made of granite
Economic uses of Granite
Granite wall, Tesco Camborne
Granite climbing wall
Truro School Sports Hall
Economic use of Granite
Tomb – St.Just Churchyard
Samuels Jewellers, Boscawen Street,Truo
Economic uses of Granite
Granite mini-roundabout, Tesco Camborne
Granite doorstep to butcher’s shop, St.Just
Granite at the Golf Club
Large granite boulders installed in front of the Pro
shop at Perranporth to stop potential ram raiders
Granite on the Golf Course
Details of the holes are
inscribed on large
granite blocks with
polished front surfaces
The 9th Tee
Perranporth Golf Club
Tee markers are a cylindrical core of granite
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
Economic uses of Granite
Granite planter
Village Square, St.Just
Granite speed-bumps
at Tesco Camborne
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
Granite Boulders – Carnon Downs Garden Centre
Individual boulders retail at £4-50!!
Granite boulders approximately 30cm x
30cm for decorative use in the garden
Economic uses of Granite
Granite chippings sold at garden centres 6mm diameter
£2-95 for 25kg at Carnon Downs Garden Centre
Economic uses of Granite
Granite fenceposts
St.Just Churchyard
Old granite
gravestones
laid flat
Economic uses of Granite
Granite chippings 6mm in
diameter used on a patio
Free draining, yet acts
as a weed suppressant
Free draining and
rather dry and
rough going for
snails and slugs!
Granite chippings 6mm
in diameter used as the
flooring in a greenhouse
Economic Uses of Granite
Penweathers Lane, Near Truro
1m
Granite millstone, now used
for decorative purpose
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
Economic use of Granite
Sainsbury’s, Truro
Granite kerbstones
Granite blocks from the former
Richard Lander School on this site
used to face the Sainsbury’s building
Granite wall
Economic uses of Granite
Front façade of Truro Cathedral
Truro Cathedral Spires
Economic uses of Granite
War memorial
Boscawen Street, Truro
St.Just Church
Clock Tower
Economic uses of Granite
Granite Cobbles in the road
Boscawen Street, Truro
Granite used as pavement
stone, outside Abbey National,
Boscawen Street, Truro
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
Granite used for building in the CBD of Truro
Lloyds/TSB Bank,
Boscawen Street,Truro
Granite
Blocks
Truro Library, Pydar Street
The whole façade is
made of granite blocks
The Cheesewring
Bodmin Moor
Cornwalls’ most
famous tor and
close to Brown
Willy, the highest
point in Cornwall
at 420m high

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