Planet earth weathering and soils notes

Prof. C.ValentiPlanet Earth Weathering and Soils Notes 1

WEATHERING INTRODUCTION Weathering weakens rocks to prepare it for erosion, transportation and deposition.

Erosion is the removal of weathered rock from where they were weathered, transportation is the movement of material from one place to another (wind, water, ice, gravity) and deposition is the deposition of material elsewhere.

Denudation processes are all processes that break down and wear away large features.

Weathering, part of the rock cycle, is one process of a continuum of processes operating at the earth’s surface. Weathering is defined as the physical disintegration of rocks and minerals at the earth's surface by mechanical, chemical and biological processes. It is 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 .

Weathering is the first step to produce soils, which contain the nutrients to supply life on earth.

Two types of weathering: Physical and Chemical. Physical weathering is the physical disintegration of rock into smaller pieces without changing the chemical composition of the mineral matter. Chemical weathering is the alteration of mineral constituents in rock by chemical action or reaction, commonly producing new materials. 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.

CALCITE DEMONSTRATION. calcite on overhead, show reaction, show enhanced reaction when physically demolished (increasing the surface area). Physical disintegration creates more surface area for chemical processes to act upon the minerals. Smaller pieces have greater surface area:volume ratio and are therefore easily weathered chemically. More surface area for chemicals to attack.

Planet Earth Weathering and Soils Notes 1


ROLE OF WATER WATER IS A UNIVERSAL SOLVENT. Due to its polarity. Water is slightly negatively

charged near the oxygen atom and slightly positively charged near the hydrogen atoms. (Simply due to the angles made with hydrogen and oxygen atoms). An oxygen atom has more electrons on its side giving this end a slight negative charge.

WATER CAN SPLIT/HYDROLYSIS. Water can split into H+ and OH- ions. The number of H+ ions in a solution determines the pH of the solution where pH=1 is acidic with many H+ ions, 7 is neutral and pH=14 Basic with few H+ ions.

WATER EXPANDS AT ITS FREEZING POINT. Water is the only substance that expands when frozen (solid state). It takes up more volume as a solid than a liquid. Changing states of matter requires a change in volume. So as temperature drops, atoms become tightly packed, at 4 degrees C for water. This is the densest form of water (still a liquid). As temperature drops below 4 degrees, water begins to expand (to accommodate the crystalline structure of the water crystals) as it becomes a solid.



PHYSICAL WEATHERING (MECHANICAL)FROST WEDGING Repeated growth and melting of ice crystals in the pore spaces of soil and in rock

fractures. Water undergoes a 9% volumetric expansion that can generate great pressures. Rocks require cracks or joints so that water can seep in. Requires a closed system. If there is any space for water to escape, it will not break

the rock because no pressure will form. In a crack, the first to freeze is near the surface (within the rock) where the air temperature is cold. A thin cap of ice forms, trapping the remaining liquid water. When frozen, the water expands and cracks the rock. DEMONSTRATION two bottles frozen with water, one that has room left, and one filled to the max will be puffy.

Rocks must be strong enough to withhold a lot of pressure, but be weaker than the force of the frozen water pressure. Otherwise the rock will simply change shape (like water bottle)

Only occurs in areas with numerous freeze/thaw cycles occurring daily. (Potholes). Indications: Piles of talus cones and rocky bases along road cuts. Frost heaving lifts rock and soil vertically.

ABRASION Bumping and grinding of rock fragments and sediments while in transport by wind,

water, or glaciers. As particles are in transport, they chip and abrade against each other and the

bedrock becoming rounder and smoother. The longer in transport the more rounded the edges.

PRESSURE RELEASE/UNLOADING Large igneous intrusions form beneath the earth's surface at some confining

pressure. The conditions at which a mineral forms are the conditions at which a mineral is stable.

Under pressure, rocks may behave plastically. If the rock is greatly compressed when it forms 30-40 km depth, and is later exposed at the surface by weathering and eroding of sedimentary layers above, the decrease in confining pressure causes the rock to expand. Batholiths can be exposed by weathering and eroding of sedimentary layers above it).

The Expansion produces a tensile stress on the rock causing it to crack and break. Layers of rock slabs will slide off the batholith forming an onion peel layering called exfoliation layers.

Water can get into the joints and enhance weathering by frost wedging (pending temperate climate).



THERMAL CONTRACTION/EXPANSION When objects cool, they contract and when they heat up, they expand. Areas with

very high daily temperature ranges cause rocks to contract/expand on a daily basis, weakening the rock.

Occurs near the surface of the rock which experiences the most differential heating. (Insulation doesn't penetrate rocks at great depths).

BIOLOGICAL WEATHERING Plant and tree roots and animal burrows open spaces in rock or soil allowing water to

enter. Forces of root systems may be strong enough to break rock. Byproducts of plant and animal respiration adds acids to soils.



CHEMICAL WEATHERING Decomposition or chemical alteration. Alters minerals to form new minerals through chemical reactions. Creates new

substances through the addition of water or through chemical change. Water can combine with ions making minerals easy to dissolve and transport as

dissolved load. As more and more minerals are removed, the number of pore spaces increases

(increasing surface area) allowing the rate of weathering to increase.

Since many rocks and minerals are formed under conditions present deep within the Earth, when they arrive near the surface as a result of uplift and erosion, they encounter conditions very different from those under which they originally formed. Among 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 to several hundred atmospheres) Higher free water (there is a lot of liquid water near the surface, compared with

deep in the Earth) Higher free oxygen (although O2 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, Quartz* Clay Minerals Muscovite* Alkali Feldspar* Biotite* Amphiboles* Pyroxenes* Ca-rich plagioclase* Olivine*

Of the silicate minerals, most stable minerals are found at the bottom of Bowen’s reaction series and least stable minerals at the earth’s surface are found at the top of the series. Carbonate rocks and salts are soft rocks and are weak, easily eroded by chemical and physical processes.CHEMICAL WEATHERING PROCESSESThe main agent responsible for chemical weathering reactions is water and weak acids formed in water. 1. An acid is solution that has abundant free H+ ions.2. 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 ReactionsDISSOLUTION. Dissolving minerals into their constituent ions or molecules into solution. Water is the universal solvent. Water is a polar molecule where it has a positive end and a negative end that attracts and separates positive and negative ions. Minerals that dissolve easily in water have high ionic bonding.

Recall halite; Sodium donates electron to become positive, chlorine accepts electron to become negative. The exchange forms an ionic bond. Water being polar, will break up NaCl into Na+ and Cl-; H+ ends attract to Cl- and O- ends attract to Na+. THE POSITIVE H ENDS ATTRACT TO NEGATIVE IONS IN A MINERAL AND THE NEGATIVE O ENDS ATTRACT TO THE POSITIVE IONS IN A MINERAL. THE ATTRACTION FOR WATER TO THE IONS IS GREATER THAN THE ATTRACTION FOR THE IONS TO EACH OTHER. IONS ARE PULLED APART, ISOLATED FROM EACH OTHER (WATER COMPLETELY SURROUNDS EACH ION) AND THE SOLID IS NOW DISSOLVED.

Leaching - ions are removed by dissolution into water.

CARBONATION. Dissolution of carbonate minerals (limestones and dolomites). Requires the solution to be acidic. Water can become acidic by dissolving gasses. (0.03% carbon dioxide in the atmosphere. but in urbanized areas, with increased emissions, more CO2 is dissolved in rainwater.)

Dissolving carbon dioxide to form carbonic acid equation:H2O + CO2 -> H2CO3

In water, carbonic acid 'dissociates' (splits up) into hydrogen ions and bicarbonate ions. Dissociation of carbonic acid places more H+ ions in solution decreasing the pH and increasing the acidity. Acids greatly accelerates water's ability to dissolve carbonate rocks.

Carbonic Acid Dissociation Equation:H2CO3 -> H+ + HCO3

-Carbonation Reaction Equation:CaCO3 + H+ + HCO3

- -> Ca+2 + 2 (HCO3)-

By dissolving calcium carbonate, H+ ions are removed from the solution and the acidity decreases.

Areas with silicate rocks have pollution problems because silicate minerals do not buffer the solution and the water is not neutralized. (See Hydrolysis).

HYDROLYSIS. Reaction of a substance with water. Decomposes silicates. Lysis of water creates H+ + OH- ions. The H+ ions 'attack' and replace other + ions in a mineral. With a new H+ ion incorporated within a crystalline structure (smaller in size) Planet Earth Weathering and Soils Notes 6


the 'orderly atomic arrangement' is disrupted and the mineral becomes substantially weaker. Its like taking a big brick out of a building and sticking in a smaller one in its place; the building's structure is weakened.

So when a granite is attacked by weak acid (either hydrolysis or carbonic acid) the feldspar minerals are easily weathered to clay minerals and the quartz may remain unchanged.

Granite (continental crust), clay removed and quartz left behind, transportation of quartz to beaches, fine materials get washed out to sea and the larger grains are left on the beach.

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 process is most commonly associated with the incorporation of oxygen within the rock’s atomic structure.

Iron rich minerals are weathered by oxidation. This commonly occurs with rust. Iron loses electrons to oxygen and 'oxidizes'.

4Fe + 3O2 -> 2Fe2O3 (ferrous to ferric).

Oxidation Reactions require water and free oxygen (dissolved in the water).

CHEMICAL WEATHERING EFFECTIVELY BREAKS DOWN ROCK MINERAL BY MINERAL, GRAIN BY GRAIN; WEAKENING IT SO THAT IT IS EASILY ERODED AND TRANSPORTED AWAY. ALSO INCREASES SURFACE AREA FACILITATING MORE CHEMICAL REACTIONS.

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

BasaltFeldspars 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.



CONTROLS ON WEATHERING TIME. Chemical reactions may occur slowly, therefore it requires a lot of time to

weather chemically. Physical weathering may take years and years.

CLIMATE. Influences both chemical and physical weathering. Most chemical weathering requires water so moist climates favor chemical

weathering. Chemical reactions may be accelerated by warm temperatures, therefore

warm/moist climates favor chemical weathering. Physical weathering may require water (frost wedging) but will require large daily

ranges in temperature (to freeze/thaw). Thermal climates favor physical weathering.

Weathering TropicalWarm and Moist

TemperateWarm/Cold and Moist

SevereCold and Dry

Chemical GREAT MODERATE LOWPhysical LOW GREAT MODERATE

ROCK TYPE AND STRUCTURE. Different rocks are composed of different minerals, and each mineral has a

different susceptibility to weathering. For example a sandstone consisting only of quartz is already composed of a mineral that is very stable on the Earth's surface, 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.



Products of weathering, the formation of soils. What is soil? Soil is layers of weathered, unconsolidated material on top of bedrock (below soil,

may be parent rock). Others define it as layers of unconsolidated material that can support plant life. Soils are mixtures of rock debris, organic matter, liquids, gases which develop at the

earth’s surface and is formed in situ by weathering of parent material. The organic matter in soil is derived from the decay of dead plants and animals. The inorganic matter (rock debris) consist of the nutrients (mineral matter, dissolved ions) released during weathering.

Controls on soil formation. Soils are variable from place to place. Different soils develop from the influence of six factors – climate, vegetation cover, soil organisms, regolith composition, topography, and time.

Time. It takes thousands and thousands of years to develop soil meters thick. Unfortunately soils can be eroded away much faster than they form. As a general rule, the more time, the thicker the layers of soil.

Climate. Influences how quickly soil will develop and what type of weathering will occur. (1) influence what will live in the soil, (2) controls the rate of chemical and physical reactions, (3) controls the rate of decomposition of organic matter.

Parent material. The type of parent rock (mineralogy) will chemical composition of the soil.

Relief. Difference in topography (high and low elevations). Soils are thicker on flat slopes (accumulate and no runoff erosion) than on steep slopes.

Organisms and Vegetation Cover. Organisms aid in the development of soils, live in soils, grow in soils, and decay (place organics in soils). Organisms keep nutrients recycled for plant growth. Partially decayed organic

matter provides nutrients for further plant growth, increasing soil fertility, and organic breakdown.

Animals living in soils help to mix it, called bioturbation. Respiration results in humic acids, and acids contribute to chemical weathering. Soils need to be warm and moist to house critters. Tropical areas have thick soil horizons.



Soil system. The soil system is an open system with inputs and outputs and transfers making it very complex and diversified over area.

Inputs (1) weathered bedrock, supplies the inorganic components to the base of the soil

column (depends on geology of the area) (2) surface deposits from wind, water, and ice (all transported materials) and (3) organic debris, added to the surface from the decay/growth of vegetation, plant

litter, and root material (depends on the type of vegetation.

Outputs. (4) Surface erosion. remove soil by wind, water or ice (more prevalent in agricultural

areas or deforested areas). (5) Leaching, dissolved materials are transported down by water percolating through

the soil. (6) Lateral outflow through groundwater flow.

Translocation - movement of material up and down in a soil column.

Transformations - chemical changes, or reactions that may mix up the soil.

Constituents of soil. Soils can include some or all of the following; mineral matter (inorganics) primarily feldspars, silica, and iron compounds, organic components (leaves, branches, humus/partially decaying matter), liquids (predominantly water) but the water can have dissolved ions in it (solute rich gasses (oxygen, carbon dioxide/from bacterial decay and respiration, nitrogen)

similar to the atmosphere. All of these constituents vary with depth in a given profile. In general, inorganics increase with depth, organics are most prevalent near the surface, water remains nearly constant, and air decreases with depth as soil is compressed and the water table approaches.



General soil profile. Because soils build downward, and chemical and physical changes vary as you move downward, soils develop different horizons or layers. Horizons are near horizontal layers of soil that differ in chemical and or physical properties.

Soils vary with variations in climate (temp, vegetation, precipitation, parent material) which results in the thicknesses of the layers to be different, but in general layers are consistent in terms of chemical and physical properties. Therefore a general profile is recognized. O, A, E, B, C, R.

O, is the organic layer, can have undecomposed material (twigs, leaves, leaf litter) or can be partially decomposed beyond recognition but are black and mucky in color.

The remaining horizons are mineral layers.

A, is the uppermost layer of mineral matter that is rich in organic material (downwashed humus, roots, organic matter), is dark in color due to decay, and has a lot of bio activity.

E, if present, consists mainly of coarse material, usually sand and silt. Aluminum and iron are removed by leeching to the B layer through infiltration of water, leaving behind the coarser material (sand and silt) which is not easily weathered.

B, receives material (clays, iron and aluminum oxides) from the E giving it a brownish or reddish color.

C, layer is the layer just above the bedrock, it consists of chunks of parent material (bedrock) called regolith and soil. Bedrock is weakly weathered.

R, is the parent rock that has not been weathered yet.


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