Major Granite Outcrops in Devon and Cornwall All

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 Tregonning. Godolphin 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 -5 mm in diameter Black biotite mica Large feldspar phenocrysts up to 4 cm long Well formed euhedral crystals 4 cm Colourless or glassy quartz Orthoclase Feldspar phenocrysts show simple twinning

Shap Granite or Ademallite Porphyritic texture, large phencrysts up to 3 cm surrounded by a finer groundmass 1 -3 mm in diameter. Produced by a two-stage cooling process Black biotite mica 3 cm Simple twinning shown by euhedral orthoclase feldspar crystal Orthoclase feldspar phenocrysts up to 3 cm long Colourless, grey or glassy quartz

Kaolinised Granite Feldspar phenocrysts decompose to kaolinite (china clay) by hydrolysis Grey/colourless and glassy quartz remains unaltered 4 cm 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 454 m 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 70 cm wide x 35 cm deep No debris contained within the hollows 30 cm 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 8 mm

Jointed Granite-Carn Brea 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 5 km) thickness was eroded away, the granite expanded and cracked parallel to its own surface to form near horizontal joints Rounded edges suggest chemical weathering by hydrolysis 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 Beak Several persons standing on the head causes the stone to rock, even though it weighs many tonnes Wings 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. 5 m 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 are used for agriculture Junction between granite and killas de-limited by change in vegetation Granite – bracken and gorse

Economic Uses of 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 of quartz, feldspar and mica crystals 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 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 Gatepost Penweathers Lane Near Truro Granite sculpture outside Sainsbury’s supermarket, Truro 3 m 1 m 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 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 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 9 th Tee Perranporth Golf Club Tee markers are a cylindrical core of granite

Granite on the Golf Course Perranporth Golf Club 7 th Hole 7 th Green Large granite block used to indicate 150 m from the front edge of the green

Economic uses of 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 30 cm x 30 cm for decorative use in the garden

Economic uses of Granite chippings sold at garden centres 6 mm diameter £ 2 -95 for 25 kg at Carnon Downs Garden Centre

Economic uses of Granite fenceposts St. Just Churchyard Old granite gravestones laid flat

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

Economic Uses of Granite Penweathers Lane, Near Truro 1 m Granite millstone, now used for decorative purpose

Economic Uses Of Granite Gravestone, St. Just Churchyard Surface is highly polished Granite chippings 12 mm 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 Spires

Economic uses of Granite War memorial Boscawen Street, Truro St. Just Church Clock Tower

Economic uses of 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 20 cm 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 Cornwall’s most famous tor and close to Brown Willy, the highest point in Cornwall at 420 m high