Weathering and the formation of Sedimentary Rocks WJEC GCSE Geology I.G.Kenyon

Why do rocks and minerals weather? Because they are out of equilibrium with the conditions under which they formed Minerals in granite originally formed at high temperatures and at considerable depth, typically >700°C and 5-15km depth All silicate minerals except quartz are unstable at the earth’s surface and are trying to re-adjust to the new conditions

Weathering – A Definition The breakdown in situ of rock materials at or near the earth’s surface, under the influence of low pressures, low temperatures and the presence of air and water

Weathering and Erosion Do not confuse weathering with erosion Erosion is the removal of weathered products by agents such as gravity, water, wind and ice Weathering is simply the chemical and physical breakdown of the bedrock in situ

Rock fragments Unreactive quartz grains Clay minerals (kaolinite, illite, smectite) Ions in solution (Ca, K, Si, Fe,) Products of Weathering

Leads to disintegration of the bedrock into smaller, angular, but chemically identical fragments Results in an increase in the surface area of rock exposed for chemical weathering to act upon Mechanical/Physical Weathering

As a rock is reduced into smaller and smaller particles, its surface area increases but its volume remains the same. Small particles have more surface area in proportion to their volume than do large particles.

Mechanical Processes Freeze-Thaw Exfoliation Pressure Release/Dilatation Biological

Freeze Thaw Activity Water penetrates joints, bedding planes, cleavages, faults and pore spaces Temperature falls below 0°C and water turns to ice Ice occupies 9% greater volume than water Immense internal stresses set up within rocks Process repeated many times, leading to angular fragments fracturing off

Freeze-Thaw activity often leads to the formation of Scree Slopes Wastwater Screes Lake District Scree in profile Scree shows crude grading finer at top, coarser at the base

Freeze-Thaw Activity results in the bedrock being broken down into smaller angular fragments Periglacial Head, Perranporth, Cornwall

The Effects of Freeze-Thaw Car keys for scale Granite blocks weighing many tonnes are forced apart as water freezes and expands by 9% in volume as it turns to ice Blocks are cuboidal or rectangular in shape due to the two sets of joints in the granite intersecting at 90 degrees Carn Brea Cornwall

Exfoliation/Onion Skin Weathering Common in areas with large diurnal temperature ranges (Over 24 hours) Outer layers of rock heat up and expand more rapidly than the layers at depth during the day At night outer layers cool and contract more rapidly than those at depth A series of concentric fractures are initiated And the rock peels off in layers like an onion

Masca – exfoliation or onion weathering of basalt Rock is breaking up into thin concentric layers parallel to its own surface Caused by insolation weathering over thousands of years

Car key for scale Basalt shows two sets of joints intersecting at right angles Masca – exfoliation of basalt Layers peeling away parallel to the rock surface Stress fractures produced by differential rates of expansion and contraction with depth Common in regions where there is a large diurnal temperature range

Olivine basalt dyke showing Exfoliation or Onion Weathering 30cm Thin sheets of rock peeling off like the layers of an onion Contact between phonolite and the olivine basalt dyke

Dilatation/Pressure Release Rocks at depth under great confining pressure Erosion removes overlying material Removal of mass causes rock to expand parallel to its own surface Rock fractures to form horizontal joints Process also occurs in quarries following blasting

Dilatation/Pressure Release The granite here has an absence of vertical joints and the tor is composed of large slabby blocks As overlying material has been eroded away the granite has expanded and cracked parallel to its own surface Dilatation joints

Biological Activity The action of tree roots widening joints and bedding planes Root growth in confined spaces can exert immense stresses within rocks and widen any natural lines of weakness Burrowing animals such as moles and rabbits create natural conduits for water to reach the bedrock

Biological Weathering – Tree Roots Widen Joints/Faults in Rocks

Leads to the decomposition of the bedrock Only quartz is unreactive and not affected Results in the formation of clay minerals from the breakdown of silicate minerals such as feldspars, mica, augite and olivine Ions are also released into solution Chemical Weathering

Chemical Processes Hydrolysis Carbonation Biological

Hydrolysis Silicate minerals react with water Clay minerals and ions in solution are produced Orthoclase feldspar decomposes to kaolinite (china clay) and releases ions of potassium and silicon into solution Biotite mica decomposes to chlorite and releases ions of iron into solution

Hydrolysis - Kaolinised Granite Unaltered grey, glassy quartz Iron oxide staining due to release of Fe ions from biotite mica Orthoclase feldspar altered to kaolinite by hydrolysis Biotite mica breaking down to form chlorite Granite is very crumbly and is described as Growan

Residual quartz grains following kaolinisation of granite on Carn Brea Any clay minerals such as kaolinite have been washed or blown away Loose, angular quartz grains mainly 1–5mm in diameter These grains represent the first stage in the formation of a new sedimentary rock, a sandstone Tee peg for scale

Hydrolysis The products of hydrolysis are clay minerals such as kaolinite, illite, montmorillianite and serecite. Clay deposits on the floor of Las Canadas Caldera, Tenerife. The clay has been derived from the breakdown of silicate minerals in igneous rocks such as feldspars, augite, olivine and micas

Chemical Weathering of Basalt by Hydrolysis and Oxidation Augite phenocrysts up to 8mm in diameter relatively unweathered Feldspar and olivine weathered to a mixture of clay minerals and iron oxides 2cm Roadside cutting, Masca, Tenerife

Carbonation Rainwater falling through the atmosphere picks up carbon dioxide to form a weak carbonic acid pH 6.0 Water infiltrating into the soil picks up more carbon dioxide from the soil air Weak carbonic acid pH 5.5 is capable of dissolving carbonate minerals Limestones, made of calcite (calcium carbonate) are most susceptible to this process

The Effects of Carbonation Large cave systems are often produced by carbonation as here in the Kango Caves, South Africa Stalactites represent calcite being re-precipitated from solution as Tufa

The Effects of Carbonation St. Mary’s Church forms part of the rear of Truro Cathedral, much of the original carvings in the limestone are badly affected by carbonation and most of the detail has been lost in places. 20cm

Biological-Chelation Rainfall percolating through humus becomes an organic acid.(eg fulvic acid) Organic acids or chelating agents attack clay minerals, releasing iron and aluminium into the soil Chelation is Greek meaning ‘to claw’ The chelating agents combine with the metallic ions (Fe, Al) to form organic-metal compounds called chelates. Chelates are soluble and are washed down the profile to accumulate at depth

Biological Weathering Car keys for scale Lichen and moss have colonised the surface of the granite, particularly in the joints (Lithosere) Moisture is trapped between the moss/lichen and the granite leading to more rapid weathering by hydrolysis A skeletal soil begins to develop in the joints etched by the moss/lichen

Biological Weathering Mosses and lichen are succeeded by grasses and heather as the organic content of the skeletal soil gradually increases Enlarged joints 5cm Plants and soil help trap moisture against the rock and they also contribute organic acids

Factors Controlling the rate and type of weathering Lithology (Rock Type) Rock Structure Temperature Rainfall Relief Influence of Man Time

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