EVSC10001 Lecture Notes - Lecture 23: Saprolite, Thermal Shock, Regolith
10 Jun 2018
School
Department
Course
Professor
Weathering: breaking down of rocks + minerals through contact with Earth's
atmosphere, water and biological organisms
Produces clay minerals, loose rock debris (regolith) and/or sediment
-
Physical
Biological
Chemical
Insolation weathering
Root wedging
Hydrolysis
Fire and thermal shock
Lichen thalli wetting and drying
Dissolution of limestone
Salt weathering
biochemical
Iron oxidation
crystallisation
biodissolution
Hydration
hydration
chelation
Thermal expansions
Involves no moving agent of transport
-
Produces
Clay minerals
○
Regolith (layer of broken and partly decomposed rock that covers
bedrock )
○
Source material for clastic sediment
○
Saprolite: not reworked (autochthonous) chemical weathering of
bedrock (Quartz, Kaolinite) - maintains structural features of parent
rock
○
-
PHYSICAL WEATHERING: mechanical breakage + disintegration
Chemical composition unchanged
-
Sediment
-
Temp changes -> expansion of chemicals in rocks -> splitting and cracking of
rocks
Insolation weathering / expansion
Radiation/heat energy working on rocks
§
Heating + cooling -> cracks
§
Most effective with moisture involved
§
○
-
Angular basalt clasts - lithological control is probs critical to shape
Cracks depend on MINERALS
○
-
Efficiency driven by presence of moisture + role of fire
Water shrinks + expands
○
-
Frost cracking: temp w/ moisture go below 0
Results in blockfields
○
-
Salt weathering
Multiple splits forming through expanding clays, and in salt-affected
rocks
○
Minerals/salt can dissolve in water -> evaporation -> precipitation ->
exert pressure on rocks
○
Relies on chemical transformation
Pressure on pore walls
§
Repeated crystallization + dissolution
§
Hydration
§
Expansion on heating
§
○
Results: spalling and flaking
○
-
Weathering by ocean -> walls from small circular CAVES like a sponge
Alveoli: small (<1m) cavernous weathering features - honeycomb
features
○
Tafoni: large (>1m) cavernous weathering features - flaking backwall
when active + overhanging lip - cave-like in granular rocks
○
-
Eg. Yosemite national park
Exfoliation + sheeting
○
Granite mountains
○
Large slabs fall off -> slide down
○
Batholith: igneous intrusion deep in Earth's crust
Granite intrudes into lithosphere -> stuck -> cools slowly -> forms
body of rock -> over time surface erodes -> batholith exposed ->
pressure release -> expansion of rock body -> SHEETING +
EXFOLIATION
§
○
-
BIOTIC WEATHERING
Organisms as agents of weathering / breaking rocks
Roots
Squeeze -> stretch/grow -> expand (physical process induced by
plants)
§
○
Funghi
○
Lichens
○
Bacteria
○
-
Chemical: produce acids that react with rock surface
-
CHEMICAL WEATHERING: decomposition by reaction with water
Chemical or mineralogical composition changes
-
Cohesion -Mineral assemblage from quartz, feldspar, etc. -
-
Formation of fissures - cracks - incipient weathering
-
Decay -Grain bonds loosen
-
HYDRATION!!
Adsorption of water molecules in crystal lattice or on crystal surfaces of
minerals
Form film - water sticks to surface
§
○
1)
Increase in volume - no change in chemical composition
-
Water can interact + exchange w/ ions in rocks
-
(granite) Feldspar + water -> Kaolinite + Silicic Acid + Potassium
HYDROLYSIS: Exchange of ions in crystal lattice with water
Chemical composition change -> forms clay minerals
§
2)
-
Feldspar reaction is very slow - in nature it is very fast
Acid as solvent - H+ ions strong tendency to bond
○
CO2 + H2O -> H2CO3 - ACID RAIN
○
Feldspar + water + carbonic acid -> kaolinite + silicic acid + potassium +
bicarbonate
○
-
OXIDATION (and reduction)3)
Iron released from minerals, reacts with O2 (takes up e- from oxygen)
Changes to yellow/red colour
○
-
DISSOLUTION (carbonate)4)
Transition of mineral into AQUEOUS solution (needs water) - but carbonate
needs carbonic acid
-
Products of chemical weathering
Clay minerals
Recombination of silica, alumina and metal cations released
during weathering
§
1)
-
Formed by diff environmental conditions
§
Primary minerals -> transformed to 2:1 layered clays -> 1:1
layered clays -> layered clays to hydrous oxides of iron and
aluminium
§
Quartz gravel - dissolution
-
Basalt - oxidation
-
Hardening of near surface layer by migration of pore fluids ->
evaporate _. Leave behind soluble minerals
-
Detachment of individual grains - produces rough surface
Solution of cement
§
Volumetric expansion
§
Stress release
§
Water adsorption
§
Spalling: peeling off of platy fragments
§
Flaking: removal of surface layer sections - coarse grained debris
Forms REGOLITH -> clastic sediment □
§
Granular disintegration: loosening of surface -> fall out
§
-
WEATHERING FACTORS
Rock strength1.
Differential weathering - Diff rock types react differently
-
Mafic materials more susceptible at higher temps
Olivine
-
-
Silicic low temps - last minerals to crystallize
Quartz, feldspar
-
-
Structure + grain size 2.
Large rocks = small surface -> slow weathering
-
Small rocks = large surface -> fast
-
Weather attacks cubes (edge, corner, and side)
-
Joints provide paths for water + surface area for weathering
-
Climate3.
Precipitation, temperature, and vegetation (produce organic acids)
-
Weathering type and depth change
Tropics have rocks altered VERY DEEP
-
-
Strong physical weathering with low rainfall, and low temps
-
Strong chemical when high rainfall and high temps
-
Saprolite: softy, thoroughly decomposed and porous rock - often rich in clay
formed by chemical weathering of rocks
Especially common in humid and tropical climates
-
Weathering
Wednesday, 6 June 2018
10:09 pm
Weathering: breaking down of rocks + minerals through contact with Earth's
atmosphere, water and biological organisms
Produces clay minerals, loose rock debris (regolith) and/or sediment
-
Physical Biological Chemical
Insolation weathering Root wedging Hydrolysis
Fire and thermal shock Lichen thalli wetting and drying Dissolution of limestone
Salt weathering biochemical Iron oxidation
crystallisation biodissolution Hydration
hydration chelation
Thermal expansions
Involves no moving agent of transport
-
Produces
Clay minerals
○
Regolith (layer of broken and partly decomposed rock that covers
bedrock )
○
Source material for clastic sediment
○
Saprolite: not reworked (autochthonous) chemical weathering of
bedrock (Quartz, Kaolinite) - maintains structural features of parent
rock
○
-
PHYSICAL WEATHERING: mechanical breakage + disintegration
Chemical composition unchanged
-
Sediment
-
Temp changes -> expansion of chemicals in rocks -> splitting and cracking of
rocks
Insolation weathering / expansion
Radiation/heat energy working on rocks
§
Heating + cooling -> cracks
§
Most effective with moisture involved
§
○
-
Angular basalt clasts - lithological control is probs critical to shape
Cracks depend on MINERALS
○
-
Efficiency driven by presence of moisture + role of fire
Water shrinks + expands
○
-
Frost cracking: temp w/ moisture go below 0
Results in blockfields
○
-
Salt weathering
Multiple splits forming through expanding clays, and in salt-affected
rocks
○
Minerals/salt can dissolve in water -> evaporation -> precipitation ->
exert pressure on rocks
○
Relies on chemical transformation
Pressure on pore walls
§
Repeated crystallization + dissolution
§
Hydration
§
Expansion on heating
§
○
Results: spalling and flaking
○
-
Weathering by ocean -> walls from small circular CAVES like a sponge
Alveoli: small (<1m) cavernous weathering features - honeycomb
features
○
Tafoni: large (>1m) cavernous weathering features - flaking backwall
when active + overhanging lip - cave-like in granular rocks
○
-
Eg. Yosemite national park
Exfoliation + sheeting
○
Granite mountains
○
Large slabs fall off -> slide down
○
Batholith: igneous intrusion deep in Earth's crust
Granite intrudes into lithosphere -> stuck -> cools slowly -> forms
body of rock -> over time surface erodes -> batholith exposed ->
pressure release -> expansion of rock body -> SHEETING +
EXFOLIATION
§
○
-
BIOTIC WEATHERING
Organisms as agents of weathering / breaking rocks
Roots
Squeeze -> stretch/grow -> expand (physical process induced by
plants)
§
○
Funghi
○
Lichens
○
Bacteria
○
-
Chemical: produce acids that react with rock surface
-
CHEMICAL WEATHERING: decomposition by reaction with water
Chemical or mineralogical composition changes
-
Cohesion -Mineral assemblage from quartz, feldspar, etc. -
-
Formation of fissures - cracks - incipient weathering
-
Decay -Grain bonds loosen
-
HYDRATION!!
Adsorption of water molecules in crystal lattice or on crystal surfaces of
minerals
Form film - water sticks to surface
§
○
1)
Increase in volume - no change in chemical composition
-
Water can interact + exchange w/ ions in rocks
-
(granite) Feldspar + water -> Kaolinite + Silicic Acid + Potassium
HYDROLYSIS: Exchange of ions in crystal lattice with water
Chemical composition change -> forms clay minerals
§
2)
-
Feldspar reaction is very slow - in nature it is very fast
Acid as solvent - H+ ions strong tendency to bond
○
CO2 + H2O -> H2CO3 - ACID RAIN
○
Feldspar + water + carbonic acid -> kaolinite + silicic acid + potassium +
bicarbonate
○
-
OXIDATION (and reduction)3)
Iron released from minerals, reacts with O2 (takes up e- from oxygen)
Changes to yellow/red colour
○
-
DISSOLUTION (carbonate)4)
Transition of mineral into AQUEOUS solution (needs water) - but carbonate
needs carbonic acid
-
Products of chemical weathering
Clay minerals
Recombination of silica, alumina and metal cations released
during weathering
§
1)
-
Formed by diff environmental conditions
§
Primary minerals -> transformed to 2:1 layered clays -> 1:1
layered clays -> layered clays to hydrous oxides of iron and
aluminium
§
Quartz gravel - dissolution
-
Basalt - oxidation
-
Hardening of near surface layer by migration of pore fluids ->
evaporate _. Leave behind soluble minerals
-
Detachment of individual grains - produces rough surface
Solution of cement
§
Volumetric expansion
§
Stress release
§
Water adsorption
§
Spalling: peeling off of platy fragments
§
Flaking: removal of surface layer sections - coarse grained debris
Forms REGOLITH -> clastic sediment □
§
Granular disintegration: loosening of surface -> fall out
§
-
WEATHERING FACTORS
Rock strength1.
Differential weathering - Diff rock types react differently
-
Mafic materials more susceptible at higher temps
Olivine
-
-
Silicic low temps - last minerals to crystallize
Quartz, feldspar
-
-
Structure + grain size 2.
Large rocks = small surface -> slow weathering
-
Small rocks = large surface -> fast
-
Weather attacks cubes (edge, corner, and side)
-
Joints provide paths for water + surface area for weathering
-
Climate3.
Precipitation, temperature, and vegetation (produce organic acids)
-
Weathering type and depth change
Tropics have rocks altered VERY DEEP
-
-
Strong physical weathering with low rainfall, and low temps
-
Strong chemical when high rainfall and high temps
-
Saprolite: softy, thoroughly decomposed and porous rock - often rich in clay
formed by chemical weathering of rocks
Especially common in humid and tropical climates
-
Weathering
Wednesday, 6 June 2018 10:09 pm
Document Summary
Weathering: breaking down of rocks + minerals through contact with earth"s atmosphere, water and biological organisms. Produces clay minerals, loose rock debris (regolith) and/or sediment. Fire and thermal shock lichen thalli wetting and drying dissolution of limestone. Regolith (layer of broken and partly decomposed rock that covers bedrock ) Saprolite: not reworked (autochthonous) chemical weathering of bedrock (quartz, kaolinite) - maintains structural features of parent rock. Temp changes -> expansion of chemicals in rocks -> splitting and cracking of rocks. Angular basalt clasts - lithological control is probs critical to shape. Efficiency driven by presence of moisture + role of fire. Frost cracking: temp w/ moisture go below 0. Multiple splits forming through expanding clays, and in salt-affected rocks. Minerals/salt can dissolve in water -> evaporation -> precipitation -> exert pressure on rocks. Weathering by ocean -> walls from small circular caves like a sponge. Alveoli: small (<1m) cavernous weathering features - honeycomb features.
