weathering and sediment production

7/31/2019 Weathering and Sediment Production

http://slidepdf.com/reader/full/weathering-and-sediment-production 1/18

A TERM PAPER

ON

WEATHERING AND SEDIMENT PRODUCTION

BY

EZENWOBI, NKIRU .H.

M.SC. SEDIMENTARY/PETROLEUM GEOLOGY

DEPARTMENT OF GEOSCIENCES

UNIVERSITY OF LAGOS, AKOKA,

NIGERIA.

March, 2012.



TABLE OF CONTENT

CHAPTER ONE ……………………………….……………………………….INTRODUCTION

CHAPTER TWO………………………………………… MECHANICAL WEATHERING PROCESSES

CHAPTER THREE……………………………………… CHEMICAL WEATHERING PROCESSES

CHAPTER FOUR…………………………………………SEDIMENT PRODUCTION

CHAPTER FIVE…………………………………………..CONCLUSION



CHAPTER ONE

INTRODUCTION

MEANING OF WEATHERING

This refers to the general process by which rocks are broken down at the Earth’s

surface and as a result produces clays, soils, and dissolved substances in rivers.

Weathering is a twofold process, It is fragmentation (i.e. mechanical weathering) and,

decay (i.e. chemical weathering), operating together, each helping and reinforcing the

other.

Mechanical Weathering - physical disintegration of a rock into smaller fragments, eachwith the same properties as the original. Occurs mainly by temperature and pressurechanges.

Chemical Weathering - process by which the internal structure of a mineral is altered bythe addition or removal of elements. Change in phase (mineral type) and compositionare due to the action of chemical agents. Chemical weathering is dependent onavailable surface for reaction temperature and presence of chemically active fluids.Smaller particle sizes weather by chemical means more rapidly than large particles dueto an increase of surface area.

Erosion - the incorporation and transportation of weathering products by a mobile agentsuch as wind, water, ice.



All three processes may act independently, but will more often than not, occursimultaneously. Different circumstance will have one weathering process moreimportant than another. The processes may also act in concert with one another.

Since igneous rocks form at high temperatures, and under pressure conditions ranging

from one to several atmospheres. However, the conditions at the Earth's surface aresomewhat different than the conditions at which most rocks and minerals form.Therefore, the materials are no longer at equilibrium when they are exposed to surfaceconditions. Under these conditions, there is a tendency for all ordered systems to seek lower levels of energy or order. This is all done through weathering.

Differential weathering is referred to varying rates of weathering in an area where somerocks are more resistant to weathering than others. Resistant rocks weather slowly andmay protrude above softer rocks that weather rapidly.

Spheroidally weathered boulder is a rock that has been rounded by weathering from an

initial blocky shape. it is rounded because chemical weathering acts more rapidly orintensely on the corners and edges of a rock than on the smooth rock surfaces

WHY DOES ROCK WEATHER?

This is simply because Weathering is response of Earth’s materials to a changingenvironment. When a rock undergoes mechanical weathering, it is broken into smallerpieces and thereby increasing the amount of surface area available for chemicalweathering. Chemical weathering initially works along contact between mineral grains.Tightly bound crystals are loosened as weathering products form at contacts.



CHAPTER TWO

MECHANICAL WEATHERING PROCESSES

This is induced by processes such as- Frost Action, Pressure Release, Abrasion and

Biological Activities.

Frost Action

Types of Mechanical Weathering:

Frost Wedging - water expands

when it freezes. This photograph

shows the individual layers within

the sedimentary rock breaking apart

through repeated cycles of freeze-thaw. A similar process happens

when the rock is repeatedly wetted

and dried as salt crystals dissolve

from the rock then grow when it is

dried. Both processes can result in

the rocks being heaved - so what

was once a nice regular pattern of

bricks set in a pavement will

eventually become a chaotic jumbleof bricks oriented every which way.

Thermal Expansion and Contraction -

heating causes rock to expand,

cooling results in contraction;

different minerals expand and

contract at different rates. This

phenomen0n will look very similar to

frost wedging and salt crystal

growth, but will typically happen in

climates that undergo extreme

diurnal temperature changes.

Mechanical Exfoliation - rock breaks

apart in layers that are parallel to

the earth's surface; as rock is



Another photograph which shows

the powerful effect of wind

generated abrasion is the Double

Arch from Arches National Park. The

edges of the arches have weatheredalong joints, preexisting tectonically

controlled vertical surfaces in the

rock. Then mechanical abrasion

took over and carved out the arches.

Biological Activity

Plants roots in search of minerals and water grow into cracks and as the roots grow,

they wedge the rock part.

Burrowing in animals further break down the rock by moving the fresh material to thesurface where physical and chemical processes can attack it more effectively.

Also the impact of humans particularly noticeable where rock has been blasted insearch of minerals or for construction.



CHAPTER THREE

Types of Chemical Weathering:

Dissolution

H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-

water + carbon dioxide + calcite dissolve into calcium ion and

bicarbonate ion

Dissolution is very common in areas that have a great deal of

limestone. Acidic waters (from pollution or natural) dissolve

limestone allowing for additional water to gain entrance. This Can

cause sinkholes and karst features as well as dissolution of statutes and grave stones.

Oxidation (rust)

4Fe+2 +3O2 --> 2Fe2O3

ferrous iron + oxygen combine to form ferric iron oxide (hematite)

Will happen to all iron-bearing silicates to varying degrees. Common reaction mineralsare hematite, limonite, and goethite.

Hydrolysis

2KAlSi3O

8+ 3H

20 --> Al

2Si

2O

5(OH)

4+ 4SiO

2+ 2K(OH)

potassium feldspar in acidic water hydrolyses to kaolinite + quartz + potassium

hydroxide

Silicate minerals (unstable at the earth's surface) weather to form clay minerals such askaolinite (stable at the earth's surface). Feldspars typically weather to produce clayminerals.

Factors that affect the rate of chemical weathering

Particle Size –Smaller the particle size the greater the surface area and hence

the more rapid the weathering

Mineral make up – Calcite readily dissolves in mild acidic solution and silicateminerals that form first from magma are least resistant to chemicalweathering.



Climatic Factors –Chemical weathering is most intense where there isabundant liquid water and slow or even absent where water is scarce or isfrozen into ice.

Weathering Products of Rock-Forming Minerals

Quartz - slow process and largely ineffective. Quartz remains quartz. Grains are

rounded.

Feldspar - weathers to clay with the cations Na+, Ca+, and K+ going into solution.Clays that can form include kaolinite (pure aluminum silicate), illite and montmorillonite.

Muscovite – weathers to clay mineral with k + and SiO2 going into solution

Ferromagnesian minerals - weather to clay plus highly insoluble iron oxides, essentiallyvarieties of limonite (rust)

Factors which dictate clay formation are

(a) Climate;

(b) Time;

(c) Parent material.



Weathering of Feldspar

H2O + CO2 ------->> H2CO3

Acid then dissociates and the following happens:

2KAlSi3O8 (feldspar)+ 2H+ + H2O ------->> Al2Si2O5(OH)4 (clay)+ 2K + + 4SiO2

When feldspar with a framework silicate structure is attacked by the H+ of carbonic

acid, it forms clay mineral which is a hydrous aluminium silicate with a sheet-silicatestructure. Partly because of the complexity of the reaction, the chemical weathering of feldspar proceed at a much slower rate than the solution weathering by calcite



CHAPTER FOUR

SEDIMENT PRODUCTION

Definition of Sediment

This refers to loose, solid particles that can originate by the following processes-

Weathering and erosion of preexisting rocks.

Chemical precipitation from solution, usually in water and Secretion by organisms.

Role of weathering in sediment production

Weathering plays a very important role in sediment production. Both weathered andunweathered rock and sediment can be eroded and weathering does not stop after

erosion has taken place. Sand being transported by a river can be actively weatheringas can mud on a lake bottom.

4.1 The Rock Cycle

Weathered rocks are eroded, transported and deposited as sediment. When thesesediment accumulate, they lithify to form sedimentary rock, which in turn undergoesdeep burial with increase temperature and pressure, recrystallizes to form metamorphicrock. As the depth of burial becomes even greater, temperature becomes high enoughto partially melt the metamorphic rock and magma is created. The magma thensolidifies to form igneous rock.



Weathering products

Mineral stability

Sediments are the by-product of weathering. Sediments are particles of minerals,

some of them altered from the original rock, some simply reduced in sized, and some

new minerals by reaction. The stability of a mineral is its resistance to alteration or

destruction by weathering, abrasion during transport and post depositional solution. Atthe time of their formation, minerals are in equilibrium with their environment, andthese are stable, but as they brought into a new environment different from those inwhich they form. They are prone to go into solution or to be decomposed.

The stability of minerals can be predicted using the Bowen's reaction series,

however, in the case of the weathering series this is known as the GoldichDissolution Series:

The mineral at the top crystallize normally at high temperatures, while those at thebottom crystallize at lower temperatures. This therefore means that the first minerals to



crystallize from magma show the fastest rate of decay when attacked by weathering,whereas the last minerals to crystallize from magma show the slowest rate of decay.Hence Olivine and Calcic-plagioclase are less stable under the low temperature-lowpressure environment of weathering, decompose most readily. On the other handQuartz which is stable under the much lower temperatures and pressures that prevailed

as the magma finally solidified, is more stable in the environment of weathering.

Soil

Weathering rates depend on the composition of the rock, temperature range and

rainfall amount. Weathering produces soils.

Soil refers to the surficial material that forms due to weathering or layers of weathered,

unconsolidated material that contains organic matter and is capable of supporting plantgrowth.

Soil may or may not remain in place, and any soil may be a combination of residual

and transported material.

Residual soil: Remains in place; has not been transported (gruss).

Transported soil: Transported by wind or water and deposited.

A complete soil profile will have the following components:

O horizon: Organic debris and leaf litter on the surface.

A horizon: Topsoil - leaching, water movement down, Organic and Mineral material

transported downward.

B horizon: Subsoil - accumulation of dissolved material and fine clays, hardpan.

C horizon: Partially altered parent rock material.

Bedrock: Unweathered parent rock material.

These horizons are not present in all soil profiles. In areas of rapid erosion, B & C

may be present or C only. In some areas no soil profile will develop at all.



Factors in Soil Formation

1. Climate: The greater the rainfall amount, the more rapid the rate of erosion

and leaching. Laterites form in humid climates where only Al2O3 (Bauxite) and

Fe(OH)3 remain.

2. Topography: The steeper the surface slope, the more likely any eroded

material is to be transported out of the system.

3. Parent Material: Granites are more resistant to weathering than gabbros.

Sandstones are more resistant to weathering than limestones in humid

climates, but limestones are more resistant than sandstones in arid climates.

4. Plant and Animal activity: Plant and animal activity produces humic acids that

are powerful erosion agents. Plants can physically erode as well as chemically

erode. Plants stabilize soil profiles; Animals (including man) tend to

destabilize the soil profile, increasing erosion.

5. Time: Reaction rates are slow, the longer a rock unit has been exposed, themore likely it is to be weathered.

These factors can be remembered by the acronym ClORPT - Climate, Organic activity

(plants and animals), Relief (topography), Parent material, and Time.

Clay minerals and quartz, the two minerals usually remaining after completeweathering of rock play important roles in soil development and plant growth. Quartzcrystals form sand grains that help keep soil loose and aerated, allowing good waterdrainage. Partially weathered crystals of feldspar and other minerals can also form sandsized grains.

Clay minerals help to hold water loosely enough that most of it is available for uptakeby plants.

Plants nutrients such as Ca++ and K +, supplied by the weathering of feldspar are alsoheld loosely on the surface of clay minerals. A plant root is able to release H+ fromorganic acids and exchange it for the Ca++ and K + that the plant needs for healthygrowth.

Laterites

These are usually red and are composed almost entirely of Iron and Aluminium Oxides,the least soluble products of rock weathering in tropical climates. If the soil is rich inHematite, it can be mined for iron ore. Laterites exposed to the sun are apt to bake intoa permanent bricklike layer that makes digging nearly impossible. This hard layer canbe quarried and makes a durable building material



Diamonds

These are widely scattered in diamond pipes when they form .They are oftenconcentrated by weathering. At the earth’s surface the ultramafic rock in the pipe isweathered and eroded away. The diamonds being resistant to weathering are left

behind concentrated in rich deposits on top of the pipes. Rivers may redistribute andreconcentrate the diamonds.

Sea Water

The solution of calcite supplies substantial amounts of Ca++ and HCO3- to underground

water. The weathering of calcium feldspar into clay minerals can also apply Ca++, HCO3-

and SiO2 to water. If these are not precipitated as solids, they remain in solution andmay eventually find their way into a stream and then into the ocean. Enormousquantities of dissolved material are carried by rivers into the sea. This is the mainreason sea water is salty.

Under ordinary chemical condition, the dissolved Ca++ and HCO3- can combine to form

solid CaCO3.The calcite and silica are the most common materials precipitated ascement, which binds loose particles of sand, silt and clay into sedimentary rock.



REFERENCES

Busch, Daniel. A., 1974, Stratigraphic Traps in Sandstones -Exploration Techniques, Memoir 21:

Tulsa, AAPG

Folk, R.L., 1974, Petrology of Sedimentary Rocks, Austin, Texas, Hemphill

Publishing Co.

Friedman, Gerald M., and John E. Sanders, 1978, Principles of Sedimentology,

New York, John Wiley & Sons

Greensmith, J.T., 1978, Petrology of the Sedimentary Rocks, London, George

Allen & Unwin Ltd.

Milner,H.B .1992. An Introduction to Sedimentary Petrography, Murby , London, 125pp.(6)

Pettijohn, F.J., 1975, Sedimentary Rocks, New York, Harper & Row, Publishers, Inc.

Scholle, Peter A., 1978, Carbonate Rock Constituents, Textures, Cements, and

Porosities, Memoir 27: Tulsa, AAPG

Scholle, Peter A., 1978, Deposition, Diagenesis, and HydrocarbonPotential of

Deeper-WaterLimestones, Tulsa, AAPG Department of Educational Activities

Scholle, Peter A., 1979, Constituents, Textures, Cements, and Porosities of

Sandstones and Associated Rocks, Memoir 28: Tulsa, AAPG

Shirley, Martha Lou, and James A. Ragsdale, eds., 1966, Deltas in Their

Geologic Framework, Houston Geological Society Research and Study Group:

Houston, H.G.S.



Refeferences

weathering and sediment production

Documents