Weathering and soil forming processUnit 2

Weathering• Weathering is the process that breaks  rocks down to smaller fragments and alters minerals formed at

higher temperature and pressure to those stable under conditions present near the Earth's surface. Types

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

• 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 the can expand or weaken rocks.

• Heat - Although daily heating and cooling of rocks  do not seem to have an effect, sudden exposure to high temperature, such as in a forest or grass fire may cause expansion and eventual breakage of rock.  Campfire example.

Plant and animal activities– Plant roots can extend into fractures and grow, causing expansion of the fracture.  Growth of plants

can break rock.–   Animals burrowing or moving through cracks can break rock.

• 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. Chemical weathering

• The conditions present near the Earth's surface that are different from those deep within the Earth are: • Lower Temperature (Near the surface T = 0-50oC)

  • Lower Pressure (Near the surface P = 1 - several hundred atmospheres)

  • Higher free water (there is lots of liquid water near the surface, compared with deep in the Earth)

Higher free oxygen (although O is the most abundant element in the crust, most of it is tied up bonded into silicate and oxide minerals, at the surface there is much more free oxygen, particularly in the atmosphere). Because of these different conditions, minerals in rocks react with their new environment to produce new minerals that are stable under conditions near the surface. Minerals that are stable under P, T, H2O, and O2 conditions near the surface are, in order of most stable to least stable

• Iron oxides, Aluminum oxides - such as hematite Fe2O3, and gibbsite Al(OH)3.• Quartz • Clay Minerals Muscovite

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• Alkali Feldspar• Biotite• Amphiboles• Pyroxenes • Ca-rich plagioclase• Olivine• These are igneous minerals that crystallize from a liquid.  Note the minerals that occur low on this list, are

the minerals that crystallize at high temperature from magma.  The higher the temperature of crystallization, the less stable are these minerals at the low temperature found near the Earth's surface

• The main agent responsible for chemical weathering reactions is water and weak acids formed in water• An acid is solution that has abundant free H+ ions.

  • The most common weak acid that occurs in surface waters is carbonic acid.

Carbonic acid is produced in rainwater by reaction of the water with carbon dioxide (CO2) gas in the atmosphere.

Types of Chemical Weathering Reactions Hydrolysis - H+ or OH- replaces an ion in the mineral. 

• Leaching - ions are removed by dissolution into water.  In the example above we say that the K+ ion was leached.

• Oxidation - Since free oxygen (O2) is more common near the Earth's surface, it may react with minerals to change the oxidation state of an ion. 

• This is more common in Fe (iron) bearing minerals, since Fe can have several oxidation states, Fe, Fe+2, Fe+3.  Deep in the Earth the most common oxidation state of Fe is Fe+2.

Oxidation

• Dehydration - removal of H2O or OH- ion from a mineral

Complete Dissolution - all of the mineral is completely dissolved by the water

Weathering of Common 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 stable at the Earth's surface and left in the rock after weathering. Factors that influence weathering Types of Rock and structure

• Different rocks are composed of different minerals, and each mineral has a different susceptibility to weathering. 

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• For example a sandstone consisting only of quartz is already composed of a mineral that is very stable and will not weather at all in comparison to limestone, composed entirely of calcite, which will eventually dissolve completely in a wet climate.

• Bedding planes, joints, and fractures, all provide pathways for the entry of water. • A rock with lots of these features will weather more rapidly than a massive rock containing no bedding

planes, joints, or fractures. • If there are large contrasts in the susceptibility to weathering within a large body of rock, the more

susceptible parts of the rock will weather faster than the more resistant portions of the rock.  • This will result in differential weathering.

Slope On steep slopes weathering products may be quickly washed away by rains. On gentle slopes the weathering products accumulate.  On gentle slopes water may stay in contact with rock for longer periods of time, and thus result in higher

weathering rates. Climate

• High amounts of water and higher temperatures generally cause chemical reactions to run faster.  • Thus warm humid climates generally have more highly weathered rock, and rates of weathering are higher

than in cold dry climates. • Example:  limestones in a dry desert climate are very resistant to weathering, but limestones in a tropical

climate weather very rapidly. Animals- burrowing organisms like rodents, earthworms, & ants, bring material to the surface were it can be exposed to the agents of weathering. Time - since a rate is how fast something occurs in a given amount of time, time is a crucial factor in weathering. Depending on the factors above, rates of weathering can vary between rapid and extremely slow, thus the time it takes for weathering to occur and the volume of rock affected in a given time will depend on slope, climate, and animals.

Pedogenic Process• Soil formation is a complex of or sequence of events, including both complicated reactions and

comparatively simple rearrangements of matter, that intimately affects the soil in which it operates.• Numerous events may take place simultaneously or in sequence to mutually reinforce or contradict each

other.• For example:Calcification and podzolization operate concurrently in certain Boralfs.

Most easily weathered

Least easily weathered

OlivinePyroxentAmphibloeBiotite

FieldsparsMuscoviteQuartz

Gypsum, halite, etcCalcite, apatiteOlivine, pyroxeneBiotite, GlauconiteAlbite,anorthite,etc and volcanic glassQuartz,cristobalitie etc.MuscoviteMontmorilloniteMontmorilloniteKaolinite, halloysite and allophane Gibbsite, boehmiteHematite, goethiteAnatase, rutile,zircon

Weathering Index of Clay-Size Mineral Practicals

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• Geochemical weathering of minerals is a geologic process in the formation of initial material of soil and continues in the soil profile where it is considered to be a process of soil formation.

• Pedogenic processes include gain and losses of materials from a soil body in accordance with the degradational, aggradational, or intermediate geomorphic character of the site as well as translocations within a soil body.

• Soil formation (pedogenesis or genesis) can be illustrated using the example of ecological theatre and the evolutionary play proposed by Hutchinson (1965).

• Soil forming factors can be envisioned as setting the stage or conditions for soil formation � the ecological theatre.

• The play that is enacted within the theatre is the processes that interact over time to modify the original solum.

Soil formation (genesis) is brought about by a series of specific changes that can be grouped into four broad processes as noted below : additions, transformation, translocation, and removal.

Additions Additions can be materials that are transported into the location where a soil is forming.

For instance, dust with a high calcium carbonate content could be blown on to the developing soil adding calcium to the evolving profile.

When plants die or leaves fall on to the surface they decompose adding organic matter to the soil. TransformationTransformation of the materials added to the developing soil occur by chemical and biological processes acting on them.

For instance, leaves falling on the surface and plant roots dying beneath may decompose into a dark brown, nutrient-rich material called humus.

Humus is responsible for the dark brown to black color of many soils, especially near the top of the soil profile. Iron and aluminum can be oxidized under warm, moist climates. Soil material is constantly being transformed in one way or another. TranslocationTranslocation involves the movement of soil-forming materials through the developing soil profile.

Translocation occurs by water running through the soil transferring materials from upper to lower portions of the profile.

Burrowing animals like earth worms, ants, etc., move soil materials within the profile. Burrowing animals create passage ways through which air and water can travel promoting soil development.RemovalRemoval of soil forming materials means that they are completely removed from the soil profile. Easily dissolved elements like calcium carbonate can be removed from the soil profile under rainy climates. Soil horizons develop in response to the relative importance of each of the above processes.

All soils are impacted by the horizon development processes to one degree or another. These processes determine the characteristics or properties of soil in each of the horizons.

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• Soil development involves many physical, chemical and biological factors acting over time on materials, such as rock, gravel and sand.

• This process is influenced by different factors these are Climate, Parent materials, Organisms, Relief and time

External Factors of Soil Formation• It has been pointed out that the character and development of soil are not controlled by genes but rather

by external factors.• A study and classifications of these factors are great helps in understanding the soils.• A factor of soil formation is an agent, force, condition or relationship, or combintaion of these, which

influences, has influenced, or many influence a parent material of a soil.Soil Forming FactorsSoil formation is the function of (S = f(Clorpt)

Parent material: The primary material from which the soil is formed.

Soil forms from different parent materials; one such parent material is bedrock. Sandstone, limestone, and shale are types of sedimentary rocks that contain quartz sand, lime, and clay, respectively.

Quartz is very resistant to weathering; therefore, sand grains are larger in diameter compared to silt and clay particles.

Basalt weathers slowly and is high in iron, which imparts a red color to the soil.

Other types of parent material that mineral soils form from are called Recent Cover Deposits and include alluvium, colluvium, glacial deposits, lacustrine (lake) deposits , loess deposits , marine deposits, and volcanic ash deposits.

Climate

Parent materials

O

rgan

ism

s

Relief

Time

Soil

Transported Parent Materials

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• If the glacial till is derived primarily from limestone and shale,montmorillonite may be the dominant clay mineral, and the pH and base saturation are high. If parent materials deriveds from sandstone and granites, the pH and base saturation tend to be low,and reflected to the acid soil.

• Strongly weathered sediments cover the underlying igneous and metamorphic rock characterized by the inclusion of one or several layers of pediment gravel, and stone lines. They range in thickness from less than one meter to several meters.

• Many of these sediments have high clay contents , possibly high iron content, has aggregated in a very stable fine granular structure

• Silicate clays present are usually of the 1:1 kaolinite type and gibbsite contents may be relatively high.• Limestone and dolomites are greater than 50% carbonates by defination, and the balance of the rock is

composed of silt and clay and \or quartz and \or iron and other contaminats. These soil relatively high pH and base satyration.

• Siliceous Crystalline Rocks include the more acidic quartzone igneous and metamorphic rocks.Volcanic Ash

• Generally thick soil profiles, friable in upper part, and ordinarily with distinct stratification.• Presence of intensely dark humic compounds in the topsoils.• Prominent yellowish brown subsoil colour.• Very low bulk density• High water-holding capacity• Weak structural aggregates, with porous peds. High CEC, lack of stickness or plasticity.

Climate• Climate has two major components for soil formation.

• The first is the temperature. As the mean annual soil temperature increases, the weathering of the rocks and minerals in the soil will be faster.

• For every 10°C rise in temperature, the rate of biochemical reactions doubles.

• Tropical soils will also weather faster because the chemical reactions will be taking place faster and the reactions will occur throughout the year.

• If all other environmental conditions are similar, the cooler the climate the greater the quantity of organic matter in the soil.

Rainfall and soil properties• pH of the soil increase with increasing rainfall• The depth to carbonates in the soil increase as rainfall increase.• The nitrogen content of the soil increases with increasing rainfall• The content of clay in the soil solum increases as rainfall increases.

Temperature and soil properties• With increasing temperature, soil colour tend to become less gray and more reddish. • Bases are most completely leached in warm areas.• Nitrogen and organic matter contents decrease as the temperature increase• Clay contents increase with increase in temperature.• Along with temperature is the climate factor of precipitation or rainfall.

• In general, areas with more rainfall will have greater weathering and greater leaching.

• Areas that are cool and wet will have more leaching compared to areas that are hot and wet because more of the rainfall in a hot, wet climate evaporates back into the atmosphere before leaching can occur.

• Wind redistributes sand and other particles especially in arid regions.

• The amount, intensity, timing, and kind of precipitation influence soil formation.

• Seasonal and daily changes in temperature affect moisture effectiveness, biological activity, rates of chemical reactions, and kinds of vegetation.

Biological factors• Plants, animals, micro-organisms, and humans affect soil formation.

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• Animals and micro-organisms mix soils and form burrows and pores. Plant roots open channels in the soils.

• Grass roots are "fibrous" near the soil surface and easily decompose, adding organic matter. Taproots open pathways through dense layers.

• Micro-organisms affect chemical exchanges between roots and soil.

• Humans can mix the soil so extensively that the soil material is again considered parent material. • The native vegetation depends on climate, topography, and biological factors. • Leaves from plants fall to the surface and decompose on the soil. • Organisms decompose these leaves and mix them with the upper part of the soil. • Trees and shrubs have large roots that may grow to considerable depths.

There is a direct relationship between the kind of native vegetation and the climate. The different vegetation that developed on a soil will impart special characteristics to the soil.

For forested areas like the Big Woods or coniferous forest the soil develops a thin surface horizon, a leached, light colored zone below the surface, and an accumulation zone that is often brown or red in color

The true "tall grass prairie Soils developing under a prairie vegetation will have a thick, black surface horizon, a result of the thick prairie roots contributing large amounts of organic matter to the surface soil, making it black.

In summary, climate affects the amount of leaching that takes place in the soil and the speed with which soil horizons develop. Vegetation affects the thickness and color of the surface horizons. Soil properties and organic matter

• Much of the organic material of the organic horizons has acquired a granular or fine blocky structure.• Irregularities in the lower boundaries of soil horizons may be determined by biotic activity. • The termite is a primitive insect that works in a humid atmosphere at all times and is particularly active in

intertropical regions.• Channels are excavated in the soil to depths of usually between 1 and 30 cm by grasshoppers, wild bees

and spider.

Topography: Slope and aspect affect the moisture and temperature of soil. Topography refers to the shape or contour of the land surface. Steep soils may be eroded and lose their topsoil as they form. Thus, they may be thinner than the more nearly level soils that receive deposits from areas upslope. Deeper, darker colored soils may be expected on the bottom land.

Another complicating factor is the depth of the water table in relation to the slope position.

Well Drained (WD) and do not have any characteristics that indicate poor drainage. These features are called redoximorphic features and are reflected in the soil as splotches of red or gray color and are called mottles.

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Well drained soils have mottles greater than 4 feet. For the Somewhat Poorly Drained (SPD) soils the mottles will occur from 2 to 3 feet. For the Poorly Drained  (PD) soils the entire soil below the dark surface will be gray in color. Indicating that most of the iron has been reduced due to the low O2 levels. Poorly drained soils occur in depressions and low areas. Slope and soil properties

• depth of the solum• Thickness and organic matter content• relative wetness of the profile• Color of the profile• Degree of horizon differentiation• Soil reaction• Soluble salt content• kind degree of pan development

Well drained Poorly drain soil

Aspect of the slope or the direction the slope faces.

This is important where the slopes are steep enough to allow the southern aspect to become hotter from the direct rays of the sun.

As this occurs more water is evaporated and less leaching occurs, thus less soil development takes place on southern exposures.

In summary, topography affects soil development by the location of the soil on the slope profile, the direction the steep slope faces, and the depth to the water table.Time The length of time required for a soil to form depends on the intensity of the other active soil forming factors of climate and organisms, and how topography and parent material modify their affect. All of the above factors assert themselves over time, often hundreds or thousands of years. Soil profiles continually change from weakly developed to well developed over time. A given period of time may produce change in one soil and the same time period will have little affect on another soil, due to the other 4 soil forming factors.Thus young soils have minimal soil development and few horizons while old soils have well developed horizons.

Soil forming factors that hasten the rate of soil development are:

Permeable, unconsolidated, parent material,

In general soils at the summit and backslope develop horizons the fastest. The steeper the slope the less development the soil Soils on the shoulder develop horizons slower because :rainfall will run off this slope position faster,there will be more soil erosion ,and there will be less leaching. At the foot-slope position the soils will collect sediment from upslope that can bury the horizons and slow down their development

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warm, humid, climateforest vegetationsummit or backslope landscape position that is well drained.

Conditions that are prone to retard soil development are:

Impermeable, hard, consolidated, parent materialcold, or dry, climateprairie vegetationsteeply sloping back slopes or shoulders.

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