PET—518 Weathering and Erosion

• Objectives• 1. Difference between weathering and erosion.

2. Difference between chemical and mechanical

weathering.3. Types of mechanical weathering.4. Types of chemical weathering.5. Products resulting from the chemical

weathering of Igneous rocks.6. Factors that influence the type and rate of

rock weathering. • 7.How Mass Wasting occurs.

PET—518 The Rock Cycle

PET—518 The Rock Cycle

Siliciclastics: Composition• In sedimentary rocks the constituents are organized

into framework grains, matrix and cement. Framework grains and matrix are allogenic (transported to the site of deposition), whereas cements are authigenic (precipitated at the site of deposition).

The three most common framework grain types are: Quartz: both monocrystalline (single grains) and polycrystalline (e.g., chert) Feldspar Lithic Fragments (any pre-existing rock fragment)

• The three major types of rocks, igneous, sedimentary, and metamorphic rocks are interrelated by a series of natural processes

• Igneous rocks form from the cooling and crystallization of hot molten lava and magma. Igneous rocks undergo weathering and erosion to form sediments.

• Sediments are deposited and lithified by compaction and cementation to form sedimentary rocks.

• Sedimentary rock become buried by additional sedimentary deposition, and when they are deep within the Earth, they are subjected to heat and pressure which causes them to become metamorphic rocks.

• Further complications within the rock cycle include (1) weathering of sedimentary and metamorphic rocks (in addition to igneous rocks),

• (2) metamorphism of igneous rocks & repeated metamorphism of metamorphic rocks.

PET—518 Rock Cycle

PET—518 Definition of weathering:

• Before we discuss the other types of rocks (Sedimentary and Metamorphic) we need to have an understanding of the processes that cause the breakdown of rocks, either to form new minerals that are stable on the surface of the Earth, or to break the rocks down into smaller particles . 

• This process is called weathering, and is also the first step in a process that we call erosion

Weathering Process

• The weathering process occurs when rocks are exposed to the hydrosphere (water) and atmosphere (air).

• These weathering agents can change the physical and chemical characteristics of rocks.

• As rocks are broken down (weathered) they can be classified as different types of sediments, which are: boulders, cobbles, pebbles, sand, silt, clay, and colloids.

• The chart explains the sizes of each of these sediments

PET—518 Types of Weathering

• Geologists recognize three categories of weathering processes 

• A. Physical Weathering  - disintegration of rocks and minerals by a physical or mechanical process.

• B. Chemical Weathering - chemical alteration or decomposition of rocks and minerals.

• Although we separate these processes, as we will see, both work together to break down rocks and minerals to smaller fragments or to minerals more stable near the Earth's surface.

• C. Biological weathering• Organisms can assist in breaking down

rock into sediment or soil.

PET—518 A. Physical or mechanical weathering

• Physical weathering takes place by a variety of processes.  Among them are:  

• Development of Joints - Joints are regularly spaced fractures or cracks  in rocks that show no offset across the fracture (fractures that show an offset are called faults). – Joints form as a result of expansion due to cooling or relief of

pressure as overlying rocks are removed by erosion.

– Joints form free space in rock by which other agents of chemical or

physical weathering can enter.

PET—518 A. Physical or mechanical weathering

• Crystal Growth - As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals.  As these crystals grow they may exert an outward force that can expand or weaken rocks.

• Thermal expansion - – repeated daily heating and cooling of rock; – heat causes expansion; cooling causes contraction. – different minerals expand and contract at different rates

causing stresses along mineral boundaries.

PET—518 Biological Weathering

• Plant and Animal Activities - – Plant roots can

extend into fractures and grow, causing expansion of the fracture.  Growth of plants can break rock -  look at the sidewalks of  New Orleans for example.

– Animals burrowing or moving through cracks can break rock.

PET—518 A. Physical or mechanical weathering

Frost Wedging – Upon freezing, there is an 

increase in the volume of the water.  

As the water freezes it expands and exerts a force on its surroundings. 

Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles.

Frost wedging - water expands when it freezes

Frost action/ice wedging• Physical weathering occurs when rocks are broken in to

smaller pieces without changing the chemical composition of the rock. The sample will change in size but all its other characteristics will remain the same. There are a few types of physical weathering such as:

• Frost action/ice wedging is the breakup of rock caused by the freezing and thawing (contracting and expansion) of water. Water can seep into the cracks of a rock and as the climate cools the water freezes and expands breaking the rock apart..

PET—518 A. Physical or mechanical weathering

• Exfoliation or unloading - – rock breaks off into leaves

or sheets along joints which parallel the ground surface;

– caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial;

In this photo from Yosemite National Park, the exfoliation sheets are sub parallel to the valley walls

Abrasion

Physical grinding of rock fragments. 

Here, the photo shows some pits that have been eroded into the rock by sandblasting. 

Along with the physical weathering (the sandblasting), chemical weathering has taken place as the rock shows some signs of solution weathering as well.

Effect of Wind

• This photograph shows the powerful effect of wind generated abrasion is the Double Arch from Arches National Park. 

• The edges of the arches have weathered along joints, preexisting tectonically controlled vertical surfaces in the rock. 

• Then mechanical abrasion took over and carved out the arches. Double Arch from Arches

National Park. 

PET—518 Surface volume EffectSurface Area and Weathering

Chemical Weathering

• Chemical weathering occurs when a rock is broken down by chemical action resulting in a change in the composition of a rock.

• The main agents of chemical weathering are oxygen, rainwater, carbon dioxide, and acids produced by decaying plants and animals that leads to the formation of soil.

• There are a few types of chemical weathering such as:

Types of Weathering

Hydration

• Driving force of all weathering hydrates solutes attraction of water molecules for charged ions

• Hydrolysis is breaking a bond with the splitting of water 

•   KAlSi3O8 + H2O = H

• AlSi3O8    +    K+    +    OH-   Feldspar                      unstable feldspar

Carbonation

• Carbonation

• Reaction with carbonic acid (or CO2) 

• CO2   +   H2O     =     H2CO3     =  H+  + HCO3-

• carbonic acid = weak acid, but inexhaustible supply (roots)

• acidifies water to pH 5.6 or below

• CaCO3   +    H2CO3    =   Ca2+    +    2HCO3-

Acidification

• Reaction with a stronger acid• organic acids from plant decomposition,

bacteria, etc. 

• feldspar                                unstable feldspar  KAlSi3O8    +    RCOOH   =   HAlSi3O8    +    K+    +    RCOO

• Strong acids - acid rain; sulfuric and nitric acids:      (not enough to affect most soils)

Hydration Occurs when water interacts chemically with minerals.

For example, when hornblende and feldspar unite with water they eventually form into clay.

• 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. 

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

Oxidation

• occurs when oxygen interacts chemically with minerals.

• For example, when a nail rusts oxygen combines with the iron in the nail to form iron oxide

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 minerals are hematite, limonite, and goethite.

Hydrolysis• 2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 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 as kaolinite (stable at the earth's surface). 

• Feldspars typically weather to produce clay minerals.

Carbonation

• Carbonation occurs when carbon dioxide interacts chemically with minerals.

• When carbon dioxide is dissolved in water, it forms weak carbonic acid.

• Carbonic acid when it comes in contact with the surface of the earth dissolves large masses of limestone, creating caves and caverns.

• Other common terms associated with carbonation are sink holes, karst topography, stalactites and stalagmites

Chemical weathering processes

PET—518 Water Action

Water Cycle

Water Cycle

The Water Cycle

• Water on Earth is always changing. Its repeating changes make a cycle. As water goes through its cycle, it can be a solid (ice), a liquid (water), or a gas (water vapor). Ice can change to become water or water vapor. Water can change to become ice or water vapor. Water vapor can change to become ice or water.

Erosion• Once a rock material has been weathered, it is ready

to be transported, or eroded.• Erosion refers to the transportation of rock, soil, and

mineral particles from one location to another. • Erosion is different from weathering since erosion has

the moving element. • The main driving force behind all agents of erosion is

gravity. • Without gravity the other major natural agents of

erosion such as: wind, running water, glaciers, waves, and rain would not occur.

Factors Affecting Transportation of Sediments

• Factors Affecting Transportation of Sediments

• Running water is the primary agent of erosion on Earth.• Most running water is found in streams and rivers. • There are many factors that affect the movement of

sediments in a stream.• Gradient (slope), discharge, and channel shape influence a

stream’s velocity and the erosion and deposition of sediments.

• Sediments carried by a stream are almost always rounded due to the grinding action of the water on the rocks, a process called abrasion.

• Streams are usually formed in V-shaped valleys; and deltas, flood plains, and meanders are results of what a stream can form. The watershed of a stream is the area drained by a stream and its tributaries (smaller feeder streams

Streams

• Streams carry materials in 4 distinct ways:

• Floatation, solution (dissolved particles),

• Suspension (within the water profile),

• and bed load (bouncing and dragging along the stream bed

Meandering• If river water runs into resistant sediments, the

movement of the meander can slow downstream. • As other meanders continue to migrate through softer

sediments upstream, they eventually intersect the slower-moving meander and cut off the channel between the two, forming an independent loop that will become a lake (see below).

Deposition

• Deposition is the final step in the erosional-depositional system. Rock particles that are picked up and transported by one of the eroding agents will ultimately be deposited somewhere else, and agents of erosion become agents of deposition. Final deposition of particles (sediments) usually occurs at the mouth of a stream. This is due to the faster flowing stream emptying into a slower larger body of water. When deposition occurs at the mouth of a stream or river a process called horizontal sorting takes place.

• The sediments that were once carried down the stream are arranged from largest to smallest.

Factors Affecting Deposition

• The major factors that affect the rate of deposition are particle size, shape, density, and the velocity of the transporting stream.

• Size: The smaller particles settle more slowly than the larger particles, due to the pull of gravity. The smaller particles tend to stay in suspension for longer periods of time. This form of deposition is called graded bedding or vertical sorting. The diagram below shows graded bedding.

Shape

• A round sediment compared to a flat (skipping stone) sediment of equal size will settle faster in a body of water. This is due to the resistance the flat particle will undergo as it settles through the water. The round particle will meet little resistance and settle at a must faster rate. The graphic below shows the relationship.

• Density: The density of particles also influences the rates at which sediments settle out of running water and wind. If particles are the same size but have different densities the higher density particle will settle faster.

• Velocity: The velocity of the transporting stream determines when sediments will be deposited. If the stream slows down during a drought period the carrying power will decrease and the particle sizes carried and deposited will also decrease. If a stream is flowing faster due to flood conditions then the carrying power of the stream will increase and the sizes of particles deposited will increase as well.

THE CHARACTERISTICS OF SEDIMENT

• Terrigenous sediment is derived from the weathering of pre-existing rocks. (Sometimes it is also called clastic or SILICICLASTIC OR detrital sediment). The grain size of sediment depends on the types of rocks in the source area from which the sediment was derived. The textures and mineralogy of the rocks in the source area control the grain size and composition of the resulting sediment.

DESCRIBING THE TEXTURE OF SANDS

• Texture refers to the size and shape of the grains in a sediment.

• Sediment can be separated into four main groups based on grain size. These four size groups are gravel, sand, silt, and clay. Some of these groups (gravel and sand) can be further subdivided.

• The sediment grain size scale is known as the Wentworth Scale

Grain Size Distribution• The range in grain size in a siliclastic rock is commonly known as

sorting. The sorting can be computed by from a histogram of the grain size distribution; it is most often estimated using a visual chart such as the one you see on the right. Sorting is one of the parameters used to determine Textural Maturity.

ROUNDNESS

• Roundness is a measure of the sharpness or roundness of the corners of a sedimentary particle. Roundness is determined by comparing the sand grains with a visual comparison chart.

• As sediment is transported, it undergoes abrasion by coming into contact with the stream bottom, sea-floor, or other grains of sediment. The abrasion tends to "round-off" the sharp edges or corners. Rounding is also related to the size of the grains. Boulders tend to round much more quickly than sand grains because they strike each other with much greater force.

• Grain shape comprises attributes which refer to the external morphology of particles. These include surface texture, roundness and form. Grain shape (Bustin, 1995) is determined by:

• internal structure, (mineral cleavage); • characteristics of source rock such as jointing

and bedding; • lithology; • hardness • fracture • transport

Roundness

• Roundness is defined as the average radius of curvature of corners (ri in figure) to that of the largest inscribing circle (R in figure).

• As you can see, that type of measurement is very tricky.

• Gravel forms through physical weathering of rock. A piece of gravel is usually a "rock fragment" composed of more than one mineral. Sometimes a piece of gravel is a single mineral, most commonly quartz. This is because quartz is sometimes present as veins, which may be several inches wide (or more), thus producing gravel-sized clasts.

• Sand forms through the breakdown and disintegration of rocks which have sand-sized (1/16 - 2mm) grains, such as granite and gneiss.

• In humid climates, quartz sand grains are released from granite after the feldspar grains alter to clay by chemical weathering (hydrolysis). In more arid areas, granite breaks down by physical weathering (such as frost wedging), releasing both feldspar and quartz grains.

• Silt originates from the chipping of coarser grains during sediment transport, or from the disintegration of fine-grained crystalline rocks (such as slates, phyllites, and schists).

• Clay originates primarily through chemical weathering of feldspars and other alumino-silicate minerals (those which contain aluminum and silicon). The term "clay" refers to a particular size of sediment particle, which could be a quartz grain or a clay mineral flake, or some other very small mineral fragment. The term "clay" is also used to refer to a group of minerals. There are a number of clay minerals, including kaolinite (the white clay mined in central Georgia and used for shiny coatings on paper, and additives to rubber), illite (which contains potassium), and montmorillonite or smectite (a group of clays which can take in large amounts of water, and as a result these clays are commonly referred to as "swelling clays").

SORTING

• Sorting refers to the range in grain sizes in a sediment or sedimentary rock. Sediment (or rock) which is well sorted will have most of the grains roughly the same size. A poorly sorted sediment or rock has a wide range of grain sizes. Sorting can be estimated using a visual comparison chart.

SPHERICITY

• Grains of sediment are three dimensional. Sphericity refers to "equal dimensions". Is the sediment particle elongated (one dimension longer than the other two), flattened or sheet-like (one dimension much smaller than the other two dimensions), or is it spherical (its three dimensions roughly the same length)? Sphericity can be described as high or low. According to this definition, a ball would have highly sphericity, but so would a cube (high sphericity, but low roundness). In contrast, a submarine sandwich would have low sphericity, but high roundness. A shoebox would have both low sphericity and low roundness. Sand grains may have high or low sphericity. Some minerals may produce elongated or flattened grains, depending primarily on original crystal shape and cleavage.

• Be careful not to confuse rounding with sphericity. A well-rounded grain may or may not resemble a sphere. And a spherical grain may or may not be well rounded.

TEXTURAL MATURITY

• Textural maturity is a concept which proposes that as sediments experience the input of mechanical energy (the abrasive and sorting action of waves and currents), they pass through a series of four stages. – Stage 1- Immature - Sediment contains mud (clay and/or silt) – Stage 2 - Submature - Poorly sorted sediment with no mud – Stage 3- Mature - Well sorted sediment with no mud – Stage 4 - Supermature - Well sorted and rounded sediment with no

mud • Three steps are involved:

– Winnowing or washing out of fines - makes an immature sediment become submature

– Sorting of grain sizes - makes a submature sediment become mature – Rounding - makes a mature sediment become supermature