Transcript File - leavingcertgeography

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 5-15km 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
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
30cm
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
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
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
and rabbits create natural conduits
for water to reach the bedrock
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 and 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 1–5mm 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
2cm
Augite phenocrysts up to 8mm in
diameter relatively unweathered
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
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
20cm
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 pH 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
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
5cm
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