Weathering and the formation of Sedimentary Rocks WJEC

Weathering and the formation of Sedimentary Rocks WJEC AS 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 -15 km 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

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

Mechanical/Physical 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 Insolation - Exfoliation Insolation - Granular Disintegration Salt Crystal Growth Dilatation Biological Hydration

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 Scree in profile Wastwater Screes Lake District 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 Granite blocks weighing many tonnes are forced apart as water freezes and expands by 9% in volume as it turns to ice Car keys for scale 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 Caused by insolation weathering over thousands of years Rock is breaking up into thin concentric layers parallel to its own surface

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

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

Granular Disintegration Occurs in areas with large diurnal temperature ranges (within 24 hours) Affects coarse grained igneous rocks like granite Different coloured minerals in the rock heat up and expand at different rates Immense stresses set up at crystal boundaries Black biotite mica may cleave and weaken the whole rock structure Rock crumbles into constituent grains

Salt Crystal Growth Same effect as freeze-thaw but halite and gypsum crystallise instead of ice Common in coastal locations Sea spray penetrates rock structure Evaporation occurs and halite/gypsum crystallise Process repeated-crystals grow larger Eventually internal stresses fracture rock

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

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 rabbits create natural conduits water to reach the bedrock and for

Biological Weathering – Tree Roots Widen Joints/Faults in Rocks

Hydration Often classified as a chemical process Involves minerals taking up water into their atomic structures Only readily affects clay minerals produced by chemical processes such as hydrolysis Minerals absorb water, expand fall apart Analogy: weetabix soaked in milk

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

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

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

Carbonation Rainwater falling through the atmosphere picks up carbon dioxide to form a weak carbonic acid p. H 6. 0 Water infiltrating into the soil picks up more carbon dioxide from the soil air Weak carbonic acid p. H 5. 5 is capable of dissolving carbonate minerals Limestones, made of calcite (calcium carbonate) are most susceptible to this process

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

The Effects of Carbonation 20 cm 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.

Solution Few minerals are readily soluble in water with a p. H of 7. 0 Halite and gypsum are the most soluble minerals that occur at the earth’s surface, but have limited occurrence The important role of solution is in transporting away the products of other chemical processes such as hydrolysis

Oxidation The ability for minerals to incorporate oxygen atoms into their atomic structure Affects iron rich minerals most readily Biotite mica, hornblende, augite, olivine Results in red, brown, orange and yellow colouration of the soil The brown staining on granite is due to Fe ions being oxidised following hydrolysis of biotite mica

Oxidation of pyrite from mine tailings results in an orange brown ochre in many streams Iron reacts with oxygen to form iron oxide as seen on these rusted barrels

Oxidation Basalt, previously black in colour now reddish brown due to oxidation of iron from ferromagnesian minerals such as augite and olivine

Reduction The loss of oxygen atoms from the atomic structure of minerals Often aided by bacteria under anaerobic conditions Converts iron compounds From a ferric to a ferrous state Results in blue, grey, green colouration in soil profiles

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

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

Spheroidal Weathering Rectangular blocks outlined by joints undergo chemical weathering (a) The corners and edges weather more rapidly (b) Once spherical, its entire surface is weathered evenly with no further change in shape (c)

Masca – spheroidal weathering of basalt Car key for scale The third joint direction is parallel to the surface of the photograph Joints 2 Joints 1 The rock here has 3 sets of joints which intersect at right angles-dividing it up into cuboidal and rectangular blocks

Masca – spheroidal weathering of basalt More rapid chemical weathering occurs at the joint intersections Angular corners of the cuboidal blocks become rounded as chemical weathering proceeds Spheroidal mass of basalt Euro coin for scale

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

The End