l>Weathering and Soils
EENS 1110

Physical Geology

bromheads.tv University Prof. Stephen A. Nelson
Weathering and Soils

Earth is covered by a thin “veneer” of sediment. The veneer caps igneous and also metamorphic “basement.” This sediment cover varies in thickness from 0 to 20 km. It is thinner (or missing) wbelow igneous and metamorphic rocks outcrop, and is thicker in sedimentary basins.

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In order to make this sediment and also sedimentary rock, several actions are required:

Weathering – Breaks pre-existing rock into little fragments or new minerals Transportation of the sediments to a sedimentary basin. Deplace of the sediment Burial and also Lithification to make sedimentary rock.

Each Step in the procedure of forming sediment and sedimentary rocks leaves ideas in the sediment. These ideas deserve to be taken to recognize the history of the sediment and also hence the history of the Planet.


Geologists acknowledge two categories of weathering procedures Physical Weathering - disintegration of rocks and minerals by a physical or mechanical procedure. Chemical Weathering - chemical modification or decomposition of rocks and also minerals. Although we sepaprice these procedures, as we will certainly view, both work-related together to break dvery own rocks and minerals to smaller sized pieces or to minerals even more steady close to the Earth"s surchallenge.Both kinds are an answer to the low press, low temperature, and water and also oxygen well-off nature of the earth’s surface.

Physical Weathering

The mechanical breakup or disintegration of rock does not adjust mineral makeup. It creates broken pieces or “detritus.” which are classified by size: Coarse-grained – Boulders, Cobbles, and also Pebbles. Medium-grained – Sand Fine-grained – Silt and also clay (mud). Physical weathering takes place by a range of procedures. Amongst them are:

Growth of Joints - Joints are on a regular basis spaced fractures or cracks in rocks that present no counter throughout the fracture (fractures that present an offset are called faults). Joints form as a result of growth because of cooling or relief of press as overlying rocks are removed by erosion. Igneous plulots crack in onion choose “exfoliation” layers. These layers break off as sheets that slide off of a pluton. With time, this process creates domed remnants. (See number B.4 in your text) Examples: Half-Dome (CA.) (check out number 22.12a in your text) and Stone Mountain (GA.). Joints form totally free room in rock by which other agents of chemical or physical weathering deserve to enter. Crystal Growth - As water percolates through fractures and pore spaces it might contain ions that precipitate to form crystals. As these crystals flourish they might exert an outside pressure that have the right to expand or weaken rocks. Thermal Expansion - Although day-to-day heating and also cooling of rocks perform not seem to have actually an impact, sudden expocertain to high temperature, such as in a woodland or grass fire might cause expansion and ultimate breakage of rock. Campfire instance. Root Wedging - Plant roots deserve to extfinish into fractures and also prosper, resulting in development of the fracture. Growth of plants can break rock - look at the sidewalks of New Orleans for instance. Animal Activity - Animals burrowing or moving via cracks can break rock. Frost Wedging - Upon freezing, tright here is an increase in the volume of the water (that"s why we usage antifreeze in auto engines or why the pipes break in New Orleans in the time of the rare freeze). As the water freezes it broadens and also exerts a pressure on its surroundings. Frost wedging is even more widespread at high altitudes wbelow tright here might be many type of freeze-thaw cycles.

Chemical Weathering

Due to the fact that many kind of rocks and minerals are created under problems current deep within the Earth, once they arrive close to the surchallenge as an outcome of uplift and erosion, they encounter conditions very various from those under which they initially formed. Amongst the conditions current near the Earth"s surface that are various from those deep within the Earth are: Lower Temperature (Near the surchallenge T = 0-50oC) Lower Prescertain (Near the surconfront P = 1 to a number of hundred atmospheres) Higher totally free water (there is a lot of liquid water near the surface, compared with deep in the Earth) Higher complimentary oxygen (although O2 is the the majority of abundant aspect in the crust, a lot of of it is tied up bonded into silicate and oxide minerals - at the surconfront there is a lot more totally free oxygen, particularly in the atmosphere). Because of these differing problems, minerals in rocks react with their new environment to produce brand-new minerals that are secure under problems near the surface. Minerals that are stable under P, T, H2O, and O2 conditions near the surconfront are, in order of many secure to least stable:

Iron oxides, Aluminum oxides - such as hematite Fe2O3, & gibbsite Al(OH)3. Quartz* Clay Minerals Muscovite* Alkali Feldspar* Biotite* Amphiboles* Pyroxenes* Ca-rich plagioclase* Olivine* Note the minerals via a *. These are igneous minerals that crystallize from a liquid. Keep in mind the minerals that occur low on this list are the minerals that crystallize at high temperature from magma. The greater the temperature of crystallization, the much less stable are these minerals at the low temperature found near the Earth"s surchallenge (view Bowen"s reaction series in the igneous rocks chapter).

The main agent responsible for chemical weathering reactions is water and weak acids developed in water. An acid is solution that has plentiful complimentary H+ ions. The a lot of common weak acid that occurs in surface waters is carbonic acid. Carbonic acid is developed in rainwater by reactivity of the water with carbon dioxide (CO2) gas in the atmosphere.


H+ is a little ion and have the right to quickly enter crystal frameworks, releasing various other ions into the water.

Types of Chemical Weathering Reactions

Hydrolysis - H+ or OH- replaces an ion in the mineral. Example:


- ions are rerelocated by dissolution into water. In the example above we say that the K+ ion was leached. Oxidation - Since totally free oxygen (O2) is even more widespread near the Earth"s surchallenge, it might react via minerals to adjust the oxidation state of an ion. This is even more prevalent in Fe (iron) bearing minerals, considering that Fe deserve to have actually numerous oxidation states, Fe, Fe+2, Fe+3. Deep in the Earth the the majority of common oxidation state of Fe is Fe+2.


Dehydration - removal of H2O or OH- ion from a mineral.
Complete Dissolution
- every one of the mineral is totally liquified by the water.


Living Organisms - Organisms like plants, fungi, lichen, and bacteria have the right to secrete organic acids that can reason dissolution of minerals to extract nutrients. The function of microorganisms prefer bacteria has actually just current been uncovered.

Weathering of Typical Rocks
Rock Primary Minerals Residual Minerals* Leached Ions
Granite Feldspars Clay Minerals Na+, K+
Micas Clay Minerals K+
Quartz Quartz ---
Fe-Mg Minerals Clay Minerals + Hematite + Goethite Mg+2
Basalt Feldspars Clay Minerals Na+, Ca+2
Fe-Mg Minerals Clay Minerals Mg+2
Magnetite Hematite, Goethite ---
Limestone Calcite None Ca+2, CO3-2
*Residual Minerals = Minerals secure at the Earth"s surface and left in the rockafter weathering.

Interaction of Physical and Chemical Weathering Because chemical weathering occurs on the surface of minerals, the water and acids that manage chemical weathering call for access to the surface. Physical weathering breaks the rock to provide that surface. Fracturing the rocks, as occurs throughout jointing, rises the surface location that deserve to be exposed to weathering and also also gives pathmeans for water to enter the rock. (See number B.7 in your text). As chemical weathering proceeds, new softer minerals, favor oxides or clay minerals, will certainly develop areas of weakness in rock that will allow for better physical weathering. Dissolution of minerals will rerelocate product that holds the rock together, therefore making it weaker.

When rock weathers, it generally does so by working inward from a surface that is exposed to the weathering process. If joints and also fractures in rock beneath the surface develop a 3-dimensional network, the rock will be damaged into cube prefer pieces separated by the fractures. Water can pass through more quickly alengthy these fractures, and also each of the cube-prefer pieces will start to weather inward. The price of weathering will be greatest alengthy the corners of each cube, adhered to by the edges, and also lastly the deals with of the cubes. As a result the cube will weather right into a spherical shape, via unweathered rock in the facility and weathered rock towards the exterior. Such development of weathering is referred to as spheroid al weathering (See numbers B.8 in your text).

Factors that Influence Weathering

Rock Type & Structure Different rocks are created of various minerals, and also each mineral has a various susceptibility to weathering. For example a granite consisting mostly of quartz is already written of a mineral that is incredibly steady on the Earth"s surchallenge, and will not weather a lot in compariboy to limestone, composed completely of calpoint out, which will ultimately disfix entirely in a wet climate. Bedding planes, joints, and fractures, all provide pathways for the entry of water. A rock via lots of these attributes will weather even more swiftly than a substantial rock containing no bedding planes, joints, or fractures.

If tright here are big contrasts in the susceptibility to weathering within a big body of rock, the even more prone parts of the rock will weather much faster than the even more resistant portions of the rock. This will certainly lead to differential weathering.
Slope - On steep slopes weathering commodities might be easily wamelted ameans by rains. On gentle slopes the weathering assets accumulate. On gentle slopes water might continue to be in contact through rock for longer durations of time, and also for this reason lead to higher weathering prices. Climate- High amounts of water and also better temperatures generally cause chemical reactions to run quicker. Therefore warmth humid climates mostly have actually even more highly weathered rock, and rates of weathering are better than in cold dry climates. Example: limestones in a dry desert climate are exceptionally resistant to weathering, however limestones in a tropical climate weather incredibly promptly. Animals- burrowing organisms choose rodents, earthworms, & ants, carry material to the surface were it can be exposed to the agents of weathering.


“Soil is composed of rock and sediment that has actually been modified by physical and also chemical interactivity via organic material and also rainwater, over time, to produce a substprice that deserve to support the development of plants.” Soils are a vital herbal reresource. They represent the interface between the lithosphere and also the biospbelow - as soils administer nutrients for plants. Soils consist of weathered rock plus organic material that comes from decaying plants and also pets. The exact same components that regulate weathering manage soil development through the exception, that soils additionally requires the input of organic material as some develop of Carbon.

When a soil develops on rock, a soil profile establishes as presented below. These different layers are not the same as beds developed by sedimentation, instead each of the horizons forms and grows in area by weathering and the addition of organic material from decaying plants and also plant roots.

Although you will certainly not be expected to know every one of the soil terminology debated on page 196, the complying with terms are essential.

Caliche - Calcium Carbonate (Calcite) that develops in arid soils in the K-horizon by chemical precipitation of calmention. The Ca and Carbonate ions are dissolved from the upper soil horizons and precipitated at the K-horizon. In arid climates the amount of water passing through the soil horizons is not enough to entirely dissolve this caliche, and as result the thickness of the layer may rise through time. Laterites - In humid tropical climates intense weathering including leaching occurs, leaving behind a soil affluent in Fe and also Al oxides, and giving the soil a deep red shade. This extremely leached soil is referred to as a laterite.

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Soil Erosion In many climates it takes between 80 and 400 years to create about one centimeter of topsoil (an organic and nutrient wealthy soil suitable for agriculture). Hence soil that is eroded by poor farming methods is basically lost and also cannot be reput in a reasonable amount of time. This might come to be a critical aspect in controlling civilization populace.

Questions on this material that might be asked on an exam

What is the difference in between physical weathering and also chemical weathering? What forms of points reason physical weathering? Define the following: (a) acid, (b) leaching, (c) hydrolysis, (d) dissolution, (e) oxidation, (f) differential weathering, (g) joints, (h) caliche, (i) laterite How carry out physical weathering and also chemical weathering communicate through each other? Contrast the weathering process that would certainly take location in warm humid tropical climates via those that would take area in hot, dry, desert climates. What is a soil and also just how is a soil formed? Why is it crucial for human beings to proccasion the erosion of soils?