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