Major Granite Outcrops in Devon and Cornwall

Dartmoor 280 - 300 Ma

Bodmin Moor 287 – 288 Ma

St.Austell Moor 270 – 285 Ma

Carnmanellis 290 – 295 Ma

Lands End 275 – 280 Ma

Carn Brea 290 Ma

All the separate outcrops link up at

depth to form a huge batholith of

granite which comprises many thousand cubic

kilometres of rock

At 6 km depth the granite is still at a temperature of 200°C to 225°C

Granites dated at 270 – 300 million years

Tregonning-Godolphin

280 Ma

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 GraniteLarge feldspar

phenocrysts up to 4cm long

Colourless or glassy quartz

Black biotite mica

Orthoclase Feldspar phenocrysts show simple twinning

Groundmass is much finer grained 1-5mm in diameter

4cm

Well formed euhedral crystals

Shap Granite or Ademallite

Orthoclase feldspar phenocrysts up to 3cm long Colourless, grey or glassy quartz

Black biotite mica

Simple twinning shown by euhedral orthoclase

feldspar crystal

Porphyritic texture, large phencrysts up to 3cm surrounded by a finer groundmass 1-3mm in

diameter. Produced by a two-stage cooling process

3cm

Kaolinised Granite

4cm

Brown/orange iron oxide staining due to breakdown of biotite mica

Feldspar phenocrysts decompose to kaolinite

(china clay) by hydrolysis

Grey/colourless and glassy quartz remains unaltered

Kaolinised granite is often referred to as Growan, a mixture of clay

minerals and quartz crystals

Kaolinisation is achieved by hydrolysis and hydrothermal action

Tor FormationA 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

Car keys for scale

Joints widened by freeze-thaw activity

Granite consists of cuboidal or rectangular blocks due to 2 sets of joints intersecting at 90°

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

30cm

Rounded forms suggest hydrolysis followed by wind action to remove any fine material

No debris contained within the hollows

The Effects Of Weathering

Car keys for scale

Joints widened by freeze-thaw activity

Surface of granite is crumbly here-a mixture of clay and quartz crystals

Edges of the granite blocks have a rounded form suggesting chemical weathering is important (hydrolysis)

The Products of Weathering

Tee peg for scale

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

Quartz remains as it not affected by chemical weathering

This represents the first stage in the formation of a new

sedimentary rock (sandstone)

Jointed Granite-Carn Brea

Vertical joints formed as the granite cooled,

contracted and solidified

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

Recently fallen block, triggered by freeze-thaw action

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

Granite Tors – Carn BreaHorizontal joints produced by dilatation or pressure release

Vertical joints formed by cooling

and contraction

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

Only a small area of the block is in contact with the granite

underneath and acts as a pivot

Eventually this block will be pushed away from the tor by freeze-thaw activity

Nightjar Rock – A Logan Stone

Very small surface area of block in contact with the underlying granite, this acts as a pivot

Head

BeakWings

Several persons standing on the head causes the stone to rock, even

though it weighs many tonnes

Clitterblocks around the Tors

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 (Soilflow) is responsible for the movement of the granite blocks

further downslope mainly between 10,000 and 2 million years ago

SolifluctionOccurs 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

Soil is very thin, just a few centimetres deep

Granite protrudes through where soil worn away on paths

Surface water collects in natural hollows after rainfall as granite is impermeable

Granite – Country Rock Contact

Granite – bracken and gorse

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

St.Agnes Beacon in the distance, another granite outcrop

Economic Uses of GraniteGranite 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

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

Conveyor belt from mine

Processing plant

Economic use of Granite

The Wellington Hotel, St.Just Granite

blockwork

Economic uses of GraniteGranite sculpture outside

Sainsbury’s supermarket, Truro

3m

Granite Gatepost Penweathers Lane

Near Truro

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

Tee markers are a cylindrical core of granite

Details of the holes are inscribed on large

granite blocks with polished front surfaces

The 9th Tee Perranporth Golf Club

Granite on the Golf Course

Large granite block used to indicate 150m from the front edge of the green

Perranporth Golf Club 7th Hole

7th Green

Economic uses of Granite

Granite planter Village Square, St.Just Granite speed-bumps

at Tesco Camborne

Economic uses of Granite

Garden ornaments made of imported Chinese granite

Carnon Downs Garden Centre

Garden Seat

Water Feature

Prices range from £30 to £450

Birdbath

Granite Boulders – Carnon Downs Garden Centre

Granite boulders approximately 30cm x 30cm for decorative use in the garden

Individual boulders retail at £4-50!!

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

St.Just ChurchyardGranite fenceposts

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

Granite chippings 6mm in diameter used as the flooring in a greenhouse

Free draining and rather dry and

rough going for snails and slugs!

Economic Uses of Granite

Granite millstone, now used for decorative purpose

1m

Penweathers Lane, Near Truro

Economic Uses Of Granite

Granite chippings 12mm in diameter used for

gravel drive Also acts a good weed suppressant

Gravestone, St.Just Churchyard Surface is highly polished

Economic use of Granite

Sainsbury’s, Truro

Granite wall

Granite blocks from the former Richard Lander School on this site

used to face the Sainsbury’s building

Granite kerbstones

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

Kerbstones in Boscawen Street

Cathedral Lane – Leats to drain away rain water and

wash away any debris

Semi-circular channel cut in granite slab to carry water

20cm

Granite used for building in the CBD of Truro

Lloyds/TSB Bank, Boscawen Street,Truro

Truro Library, Pydar Street

Granite Blocks

The whole façade is made of granite blocks

The Cheesewring

Bodmin Moor

Cornwall’s most famous tor and close to Brown

Willy, the highest point in Cornwall

at 420m high