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Weathering and the formation of Sedimentary Rocks


Weathering and the formation of Sedimentary Rocks WJEC GCSE Geology I.G.Kenyon Factors Controlling the rate and type of weathering Lithology (Rock Type) Rock ... – PowerPoint PPT presentation

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Title: Weathering and the formation of Sedimentary Rocks

Weathering and the formation of Sedimentary
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 gt700C and 5-15km depth All silicate
minerals except quartz are unstable at the
earths 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 earths 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
  • 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 0C
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
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
Rock is breaking up into thin concentric layers
parallel to its own surface
Masca exfoliation of basalt
Common in regions where there is a large diurnal
temperature range
Layers peeling away parallel to the rock surface
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
Thin sheets of rock peeling off like the layers
of an onion
Contact between phonolite and the olivine basalt
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
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
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
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
  • Biological

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
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 15mm in
Any clay minerals such as kaolinite have been
washed or blown away
The products of hydrolysis are clay minerals such
as kaolinite, illite, montmorillianite and
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
Feldspar and olivine weathered to a mixture of
clay minerals and iron oxides
Roadside cutting, Masca, Tenerife
Augite phenocrysts up to 8mm in diameter
relatively unweathered
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
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
The Effects of Carbonation
St. Marys 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.
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
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
Plants and soil help trap moisture against the
rock and they also contribute organic acids
Enlarged joints
Mosses and lichen are succeeded by grasses and
heather as the organic content of the skeletal
soil gradually increases
Factors Controlling the rate and type
of weathering
Lithology (Rock Type) Rock Structure Temperature R
ainfall Relief Influence of Man Time
The End