Topic 5 weathering and sediments1

Topic 5: Weathering and Sediments

OutlineIntroduction

Weathering

Mechanical Weathering****

Chemical Weathering****

Factors That Control the Rate of Weathering

Erosion, Transport and Deposition

Depositional Environments

Lithification

Soil

IntroductionSedimentary rocks are one of three main rock types. Others rock types being igneous and metamorphic rocks.

Sediments are formed from the weathering of previously lithified igneous, metamorphic and sedimentary rocks. Sedimentary rocks are classified as one of two types:

(a) Detrital/Clastic Sedimentary Rocks

(b) Chemical/Non-Clastic Sedimentary Rocks

(a) Detrital/Clastic Sedimentary RocksConsist of detritus.Also referred to as “clastic” sedimentary rocks.

e.g. sandstone, mudstone, shale.

(b) Chemical/Non-Clastic Sedimentary RocksOriginate from substances that were taken into solution.

Includes biochemical sedimentary rocks. Inorganic or organic processes may remove these dissolved materials from solution.

e.g. limestones, evaporites (salt), coal.

Before I begin to discuss sedimentary rocks, I will examine how the

detrital or clastic sedimentary rock particles or sediments originate.

Clastic sediments are formed through a series of sedimentary

processes. Start with bedrock of any type:

- weathering

- erosion

- transport

- deposition

- lithification (compaction and cementation)

Sediments deposited in rivers (Assiniboine), lakes (Winnipeg) and

oceans (Hudson Bay). Ultimate resting place of the sediment eroded

from the land is in the world’s oceans.

WeatheringWeathering is defined as the “physical breakdown (disintegration) and chemical alteration (decomposition) of rocks and minerals at or near the surface”.

Can occur rapidly or slowly. e.g. landslide vs. waves on a calm lake.

Rocks do not weather at the same rate. Leads to differential weathering of a rock surface.

Differential weathering produces uneven rock surfaces.

e.g. Niagara Falls (Horseshoe Falls) has more resistant rocks at the top (limestone and dolomite) and less resistant rocks beneath (shale and limestone). Lower part erodes further back. Eventually causes the upper rocks to break off and fall into the river.

Interbedded limestone and lime-

mudstone, Spain.

Drumheller is in central Alberta, 135 km NE of Calgary.

more resistant

less resistant

Differential weathering. e.g. Hoodoos in Alberta. Cap rock of resistant sandstone is well cemented. Less resistant/cemented sandstone below.

Differential weathering. Spires and pillars (hoodoos) in Bryce Canyon NP, Utah.

Erosion of Deformed Sedimentary Rock

Differential weathering. Resistant versus nonresistant rock layers.

A few definitions:

parent material

Original rock material that is being weathered.

erosion

Removal of the weathered material from its place of origin. Transported by water, wind or ice.

soil

Regolith consisting of weathered material, water, air and organic matter that can support plants. Rock materials are usually the dominant parent material for soil.

 Two main kinds of weathering:

Mechanical

Chemical

Delicate

Arch.

Arches

National

Park, Utah.

Weathering

and

erosion.

Mechanical Weathering

Breaking of rock materials by physical forces. Into smaller pieces that

retain the chemical composition of the parent material. Mechanically

breaks down the rock.

Five main mechanical weathering processes:

(i) Frost Action: Ice Wedging

(ii) Pressure Release

(iii) Thermal Expansion & Contraction

(iv) Salt Crystal Growth

(v) Plant Root Wedging

First two processes (Frost Action, Pressure Release) are most

significant.

(i) Frost Action: Ice Wedging

Very effective type of mechanical weathering. Water in the rock will

periodically freeze and thaw. In areas where the temperature fluctuates

about the freezing point.

As freezing occurs, additional water tends to be attracted to the ice.

When water freezes to form ice, its volume increases by about 9 %.

Leads to high stresses in the rock. Causes mechanical disruption. Can

force apart large blocks, some weighing many tons.

Frost wedging is most effective at -5° to -15° C. Therefore very common

in colder climates.

Frost action/ice wedging. Freeze-thaw cycles. Talus slopes.

Frost action/ice wedging is responsible for most of the rock debris observed on mountain slopes. e.g. Rocky Mountains, Canada. This debris

forms talus cones. One of the most effective types of mechanical weathering.

talusWeathered material that accumulates at the bases of slopes.

Physical Weathering

(ii) Pressure ReleaseRock masses that were originally buried deep beneath the Earth’s surface are subjected to very high confining pressures due to the weight of the overlying rock. As erosion wears down the upper surface, both the weight of the overlying rock and the confining pressure are reduced. A rock responds to the decrease in weight and pressure by expanding upwards. Examples are found in many granitic batholiths. e.g. Yosemite National Park, CA. Batholiths have exfoliation domes (large rounded rock domes). exfoliation“Peeling-off” of successive shells or layers. Like the ‘skins’ of an onion, around a solid rock core. e.g. granitic batholith.  Joints are also produced during the pressure release. Sheet joints and vertical joints. These are near-surface phenomena. jointA fracture along which no movement has occurred. Or where movement has been perpendicular to the fracture surface.

Small exfoliation dome in the Sierra Nevada near Donner Pass, CA.

Large exfoliation dome, Stone Mountain, Georgia.

Slabs of granitic rock bounded by sheet joints,

Sierra Nevada, CA.

Sheet-joints formed by expansion in the Mt. Airy Granite in North Carolina

(iii) Thermal Expansion & Contraction

Daily heating of rock in bright sunshine followed by cooling each night

causes the mechanical breakdown of that rock. Common rock forming

minerals expand by different amounts when heated.

Also individual minerals expand differently. Most expansion is on the

surface. Little expansion in the interior of the mineral.

Surface temperatures over 80°C have been measured on some desert

rocks. Daily temperature fluctuations of up to 40°C can occur in a

desert.

Not a very significant mechanical weathering process.

(iv) Salt Crystal Growth

Groundwater moving slowly through fractured rocks contains ions.

These ions may precipitate out of solution to form salts. Salt crystals

growing within rock cavities or along grain boundaries. Can exert

enormous pressure. Results in disaggregation or rupturing of rocks.

Process often observed in deserts. High rates of evaporation. High

temperatures.

Also observed in Antarctic granite formations. Salt grows between

mineral grains in the granite. Rock slowly disintegrates.

Resulting debris transported by winds. Leaves an unusual landscape.

Granite rocks that look like Swiss cheese or a honeycomb texture.

Only observed in a few specific environments. Not widespread.

Honeycomb weathering example, Pebble Beach, CA: caused by disaggregation of granular rocks.

(v) Plant Root Wedging

Seeds germinate in cracks in the rocks.

Plants extend their roots further into the

crack. Roots can wedge apart adjoining

blocks of rock. Further widen the cracks.

Analagous to sidewalk breaks and cracks.

Mostly associated with the positions of

trees.

Not volumetrically significant.

Could work in parallel with ice wedging.

Lichen (fungi and

algae), from an island

in the Irish Sea.

Lichens derive their

nutrients from the rock

and contribute to

chemical weathering.

Chemical WeatheringProcess whereby rock materials are decomposed by chemical alteration of the parent material. Chemical breakdown of rocks.

Minerals in igneous and metamorphic rocks that formed at high temperatures and pressures are chemically unstable when exposed to the lower temperatures and pressures at the Earth’s surface. Such minerals break down and their components form new, more stable minerals.

Ferromagnesian silicates are the most likely to react. e.g. olivine and pyroxene. e.g. host rock for diamonds (kimberlite) weathers easily. NWT diamonds located at the base of a lake/swamp.

Principal agents of chemical weathering are water solutions that behave as weak acids. Chemical weathering is therefore most pronounced in regions where both temperature and precipitation are high. i.e. tropical regions. Activity of organisms also plays an important role.

TEST******DIAGR

AM

er in which silicate

minerals react

during weathering

is the reverse of

the Bowen’s

reaction series.

3 main chemical

weathering

processes:

(i) Hydrolysis

(ii) Leaching or

Solution

(iii) Oxidation

(i) Hydrolysis

As rainwater falls through the atmosphere it dissolves small quantities of

carbon dioxide (CO2), producing carbonic acid.

Carbonic acid (H2CO3) is slightly acidic.

i.e. a weak acid.

Carbonic acid breaks down in soil to form hydrogen ions (H+) and

bicarbonate ions (HCO3-). Hydrogen cations are so small that they can

enter a crystal and replace other ions.

CO2 + H2O H2CO3 H+ + HCO3-

Hydrates minerals. e.g. feldspars to clay minerals.

A chemical reaction where the H+ cation or OH- anion of water replace the

ions of a mineral. One of the chief processes involved in the chemical

breakdown of common rocks.

Hydrolysis of potassium feldspar produces a clay mineral called kaolinite.

i.e. changes mineralogy.

4KAlSi3O8 + 4H+ + 2H2O 4K+ + Al4Si4O10(OH)8 + 8SiO2

Potassium Kaolinite

Feldspar Clay

Above is a balanced chemical equation (elements and charge).

Spheroidal weathering.

Mostly a chemical

weathering process.

Spherical granite boulders.

(ii) Leaching or Solution

Another common process of chemical weathering. Continued removal

by water solutions of soluble matter from bedrock.

Soluble substances leached from rocks during weathering are present in

all surface water and groundwater. Sometimes their concentrations are

high enough to give an unpleasant taste.

e.g. solution of halite (NaCl) or table salt.

e.g. calcite (CaCO3) is insoluble in pure water.

But is very soluble in the presence of a weak acid.

i.e. carbonic acid H2CO3.

Headstones in a cemetery, Deerfield, Massachusetts. Both are made of sandstone. One on the right is older. Leached/weathered for a longer time interval. Ca-rich cement in the sandstone?

CaCO3 + H2CO3 Ca2+ + 2(HCO3)-

Calcite + Carbonic Acid Calcium Ion + Bicarbonate Ion

Karst Topography

Leaches carbonate rocks. Weak acid dissolves minerals into ions. Dissolution. Dissolution of calcite in limestone rocks produces large caves and caverns. e.g. Carlsbad Caverns, New Mexico.

(iii) Oxidation

Refers to reactions with oxygen to form oxide minerals. Important chemical weathering process in the alteration of ferromagnesian silicates. Such as olivine, pyroxene, amphibole and biotite.

Fe in these minerals combines with oxygen to form the mineral goethite (4FeO*OH) which subsequently dehydrates to form the mineral hematite

(Fe2O3).

Change minerals in the presence of oxygen.

e.g. Fe-oxide minerals are generated.

oxidation of Fe to form goethite:

4FeO + 2H2O + O2 4FeO*OH

dehydration of goethite to form hematite:

4FeO*OH 2Fe2O3 + 2H2O

Oxidation

of pyrite

(FeS2) in

mine

tailings.

Factors That Control the Rate of Weathering

At least 7 main factors control the rate of weathering:

(i) Particle Size

(ii) Climate

(iii) Parent Material or Rock Type

(iv) Rock Structure and Texture

(v) Local Topography

(vi) Burrowing Animals

(vii) Time

(i) Particle Size

Relationship of surface area to volume. As a rock is reduced into smaller

and smaller particle sizes, the volume remains the same but the surface

area increases dramatically. Smaller particles have more surface area in

proportion to their volume. More area for weak acids (chemical) or frost

action (mechanical) to have an effect.

(ii) Climate

Moisture and

heat promote

chemical

reactions.

Weathering

more intense

in a warm,

moist climate.

Compared to

a dry, cold

climate.

Tropics vs.

polar regions.

Rate of chemical weathering.

(iii) Parent Material or Rock Type

Quartz is resistant to chemical breakdown. Therefore granite is very

resistant to weathering. Quartz minerals make up the bulk of granite (>

50 %).

Minerals high up on the Bowen’s discontinuous reaction series are the

first to weather.

(iv) Rock Structure and Texture

Presence of joints or fractures will speed up the weathering process.

Susceptible to frost action.

Order in which silicate minerals react during mechanical and chemical weathering is the reverse of the Bowen’s reaction series.

Weathering has been concentrated along vertical fractures separating these panels of sandstone at Arches National Park, Utah.

Fluid seeps along fractures. Increases the rate of weathering.

(v) Local Topography

Higher slopes lead to an accelerated rate of mechanical weathering.

Solid products of weathering are washed away quickly. Exposes fresh

bedrock. Rocky Mountains vs. Hudson Bay Lowlands.

(vi) Burrowing Animals

Bring partly decayed rock up to the Earth’s surface. Move disaggregated

rock. Enormous volumes over geological time.

(vii) Time

More time of exposure at the Earth’s surface equals a greater exposure

to weathering processes.

Erosion, Transport and Deposition

“bridge” between solid rock (parent material) and subsequent sedimentary rock

Erosion– original rock broken down into sediments

Transport and Deposition– sediments are moved by air, water, ice and gravity

Lithification– sediments eventually harden to form a sedimentary rock

Erosion

• bedrock is weathered to become sediment

– chemical and mechanical weathering

sediment

– weathered material derived from pre-existing rocks

– sediment is then removed from its place of weathering

• process is called erosion

Transport

Bridge between solid rock (parent material) and subsequent sedimentary rock. Sediments are moved by air, water, ice and gravity (transport). Clastic sediment is transported in many different ways. Main agents of transport are:

- ice- wind- water- gravity

 Additional chapters go into more detail on transporting agents and environments. Subsequent topics in this course.

Two types of transport mechanics in water and air: Flowing water or wind carries the sediment in suspension (suspended in water or air; suspended load transport) or by traction (bouncing along the surface; bed load transport). Usually a flow of a certain strength is required to move the particles.Transport occurs from high areas towards low areas (generally). Sediments can be transported a considerable distance. From North Dakota all the way to the Gulf of Mexico. e.g. Mississippi River system. Vast majority of coarse grained transport occurs during flood stages of river systems. Highlights the impact and role that low frequency, high intensity events have on the geologic rock record. From Brandon to the Atlantic Ocean. Via Assiniboine River, Red River, Lake Winnipeg, Nelson River and Hudson Bay.

Mineral grains and rock particles are abraded during transport. Due to

collisions with each other and the edges of the transporting medium.

e.g. river channel.

Abrasion reduces the size of detrital particles. Also tends to round off

the corners or edges of the rock or mineral. Rounded grains indicate

that the material has undergone a considerable amount of transport.

More mature. Immature sediments are usually very angular.

– poorly sorted • a wide range in grain sizes

– from pebble to sand…

– well sorted• a narrow range in grain sizes

– almost all same size

Sediments can also be sorted during transport.Sorting refers to the size distribution of the sediments.

A wide range in grain sizes = poorly sorted.A narrow range in grain sizes = well sorted.

Sorting results from processes that selectively transport and deposit particles by size.

Deposition

Bridge between solid rock (parent material) and subsequent sedimentary rock. Sediments are laid down in their final resting place (deposition). Agents of deposition are the same as those of transport. Deposition occurs when the flow is no longer strong enough to carry sediment. Therefore deposition occurs because of a drop in energy. Flowing water or wind slows down. Can no longer carry the sediment as suspended load or bed load. Sediment settles onto the bed (i.e. is deposited). Sediments are laid down in horizontal layers called beds.

Horizontal BedsBook Cliffs, Utah

Depending on what was happening to the

sediment as deposited, the layers may show

sedimentary structures. e.g. ripple marks,

graded bedding, cross bedding, mud cracks.

Sedimentary structures can give some

indication as to the depositional environment

and strength of the transporting energy. i.e.

high- or low-energy environments.

Mud Cracks

Ripples

Cross Bedding

Ripples and Graded Bedding

Depositional EnvironmentsDepositional environment is defined as “any geographic area in which sediment is deposited”.

e.g. next time you are on a beach at the lake, take a look in the shallow water for ripples or ridges in the sand. These can be preserved in the rock record. These sedimentary structures indicate a relatively low energy environment in shallow water.

Sediments are mainly deposited in relatively low lying areas. Most sediment is ultimately deposited in the ocean. Fairly close to the coastline. Continental margins.

There are 3 general depositional settings:

- continental

- transitional

- marine

Continental

– glacial

– lake

– river/stream

– desert

– alluvial fan

Transitional

– beach

– barrier island

– delta

– lagoon

– estuary

– tidal flat

– swamp (e.g.

mangrove)

Marine

– beach

– barrier island

– delta

– shallow marine

or shoreface

– continental shelf

– continental

slope

– submarine

canyon

– submarine fan

– deep marine or

abyssal plain

LithificationBridge between solid rock (parent material) and subsequent sedimentary

rock. Sediments eventually harden to form a sedimentary rock

(lithification). Process by which sediment is transformed into a

sedimentary rock is called lithification. From the verb lithify, meaning

turning to stone. Relatively slow process.

Sediments are first compacted. Pore spaces (air or water-filled pockets

or voids) occur between the sediments. These are gradually reduced by

the weight of overlying sediments. Pressure increases over time.

Volume of the sediments decreases because of the reduction in pore

space. Sand grains fit more tightly together after compaction. Muds can

lose as much as 40 % of their volume. Mostly water is squeezed out

during compaction of muds.

Cementation is also required for the lithification of sandstones. Most

common cementing materials are calcium carbonate (CaCO3) and silica

(i.e. quartz, SiO2).

Circulating and

percolating ground

waters occur in the

pore spaces.

Substances

dissolved in these

waters can

precipitate and

cement grains

together. Process

called cementation.

Compaction and cementation work together on a sand in order to generate a lithified sandstone.

Compaction alone can lead to the lithification of mud to form mudstone or shale.

Soil• What is Soil and How Does it Form?

• Soil is a mixture of weathered rock material, water, air, and organic matter.

– Sand, silt, and clay

• weathered rock fragments

– Humus

• carbon rich decayed

organic material

– Residual soils

• develop on parent rock

– Transported soils

• Eroded and transported to

another location

Soil Profile

– O horizon: organic matter.

– A horizon: top soil, zone of leaching, intense biological activity.

– B horizon:

subsoil,

zone of

accumulation.

– C horizon:

partially

altered parent

rock, little

organic

matter.

Soil Profile

A

A

Caliche – crust of soluble calcium salts (Ca-carbonate, gypsum, Mg-carbonate).

• Factors Controlling Soil Formation

– Climate,

– topography (e.g. relief),

– vegetation cover,

– soil organisms,

– composition of parent rock

– time

– Three major soil types are recognized:

• pedalfers (humid climates)

• pedocals (arid climates)

• laterites (tropical climates): reddish, rich in Fe, Al.

Laterite from Madagascar.

Laterite soil is depleted within

2-5 years.

• Soil degradation is a decrease in soil productivity or loss of soil. Three types are recognized:

– Erosion.

– Chemical degradation: soils overused, insufficient use of fertilizers, pollution and salinization.

– Physical deterioration: when soil is compacted by weight of heavy machinery and livestock.

A large gully

(erosion) has

removed the

soil, Rio

Reventado,

Costa Rica.