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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
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 ScreesLake 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 initiatedAnd 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 ReleaseRocks 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
HydrolysisThe 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
The End