Weathering and the formation of Sedimentary Rocks

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Weathering and the formation of Sedimentary
Rocks
WJEC AS Geology
I.G.Kenyon
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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
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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
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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
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Products of Weathering
Rock fragments Unreactive quartz grains
Clay minerals (kaolinite, illite, smectite)
Ions in solution (Ca, K, Si, Fe,)
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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

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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.
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Mechanical Processes

• Freeze-Thaw
• Insolation - Exfoliation
• Insolation - Granular Disintegration
• Salt Crystal Growth
• Dilatation
• Biological
• Hydration

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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
off
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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
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Freeze-Thaw Activity results in the bedrock being
broken down into smaller angular fragments
Periglacial Head, Perranporth, Cornwall
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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
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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
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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
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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
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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
30cm
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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
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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
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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
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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
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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
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Biological Weathering Tree Roots Widen
Joints/Faults in Rocks
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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 and
fall apart Analogy weetabix soaked in milk
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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

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Chemical Processes

• Hydrolysis
• Carbonation
• Solution
• Oxidation
• Reduction
• Biological

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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
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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
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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
diameter
Any clay minerals such as kaolinite have been
washed or blown away
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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
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Chemical Weathering of Basalt by Hydrolysis and
Oxidation
Feldspar and olivine weathered to a mixture of
clay minerals and iron oxides
2cm
Roadside cutting, Masca, Tenerife
Augite phenocrysts up to 8mm in diameter
relatively unweathered
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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
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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
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The Effects of Carbonation
20cm
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.
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Solution
Few minerals are readily soluble in
water with a pH of 7.0 Halite and gypsum are the
most soluble minerals that occur at the earths
surface, but have limited occurrence The
important role of solution is in transporting
away the products of other chemical processes
such as hydrolysis
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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
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Oxidation
Iron reacts with oxygen to form iron oxide as
seen on these rusted barrels
Oxidation of pyrite from mine tailings
results in an orange brown ochre in many
streams
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Oxidation
Basalt, previously black in colour now reddish
brown due to oxidation of iron from
ferromagnesian minerals such as augite and olivine
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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
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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
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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)
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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
5cm
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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)
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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
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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
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Factors Controlling the rate and type
of weathering
Lithology (Rock Type) Rock Structure Temperature R
ainfall Relief Influence of Man Time
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The End