Weathering and Soil

Summary of Important Concepts

• Soil is a layer of weathered rock, minerals, and organic matter at the earth’s surface that supports plant life.

• The main factors that determine the characteristics of a soil are climate (mainly temperature and rainfall), topography, type of parent rock material, organic activity, and the amount of time that the soil has been forming.

• Soil forms from regolith - broken up rock material at the earth’s surface. Soil forms as regolith undergoes weathering - the physical and chemical breakdown of rock. - Physical weathering is breakdown of rock by physical forces. Example: rock wedged apart by freezing water or by plant roots.- Chemical weathering is breakdown of rock by chemical reactions. Examples: dissolution, oxidation, and hydrolysis reactions.

What is SOIL?

It has been said that soil represents “a few inches between humanity and starvation”. This phrase puts the importance of soil in perspective. Soil is a layer of weathered rock, minerals, and organic matter at the earth’s surface that supports plant life. Without soil, human life would not be possible.

This figure illustrates the composition of a typical soil. Soil is composed of mineral matter from weathered rock; water, gases, and organic matter (the remains of plant and animal material and bacteria).

Soil’s main important uses for humanity are summarized here.

Weathering and Soil Formation

Soil forms from regolith - the term used for broken up rock material at the earth’s surface.

Rock (regolith) exposed at the earth’s surface undergoes weathering - the physical and chemical breakdown of rock at the earth’s surface. Two main types of weathering occur: physical weathering and chemical weathering.

Physical weathering is breakdown of rock by physical forces.

Chemical weathering is breakdown of rock by chemical reactions that occur when rock and mineral matter interact with water and air.

Physical Weathering and Chemical Weathering work hand in hand: Physical disintegration creates more surface area for chemical processes to act upon minerals. Smaller

pieces have a greater surface area: volume so they are easily ‘attacked’ by chemical weathering.

Water is a Polar Molecule: Although

it is neutral, the charges ‘build up’

on either side of the molecule. It is more positive on one side

(H) and more negative on the

other (O). Hydrogen bonding

between water molecules gives it surface tension.

Role of Water.

1. Universal Solvent: Right shows a water molecule. Note the slightly positive charge on one end and slightly negative

charge on the other. Although it is electrically neutral, the ‘slight’

difference in charge on either side makes the molecule easily dissolve weakly (ionic) bound

minerals.

2. Hydrolysis: Water can split to form 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.

3. Expands When Frozen; densest at 4oC.

Physical Weathering: Physically disintegrating but

not changing chemically.

Frost Wedging, very effective at breaking up rocks. Only in climates with frequent freeze/thaw cycles. Water seeps into cracks in rock. Temperature drops resulting in the ice at the surface to freeze

(thus forming a CAP). The remaining water freezes and expands exerting a great force against the rock causing it to crack. This

process occurs in temperate climates that has significant precipitation (to supply the water to freeze and thaw).

Thermal Contraction and Expansion

• Thermal contraction and expansion is also related to temperate climates, especially those with high daily temperatures and extremely low night time temperatures (like the desert)—that is a WIDE daily range in temperature.

• During the day time, hot high temperatures cause atoms to spread further apart (expand) and at night, cold drops in temperatures cause atoms to contract.

• Repeated on a daily basis, thermal contraction and expansion due to wide daily ranges in temperatures effectively render rocks weak and they break.

Abrasion: The longer sediments or rocks or

pebbles are in transit, the more bumping and abrading

it undergoes. Right: Well rounded

sediments indicate LONG TRANSPORTATION TIME.

Left: The shorter the transportation time, the more angular the clasts (sediments/rocks). Not enough time for abrasion.

The shorter the transportation time, the more angular the clasts (sediments/rocks). Not enough time for abrasion.

Pressure Release/Unloading: Intrusive rocks form within the earth’s crust under severe confining pressures (and so are

stable under pressure). Once the pressure (layers of rock above are removed) is relieved, the rock expands and cracks.

Biological Weathering: plant/tree roots can be very powerful. Root systems can

squeeze through cracks, and as the plant grows, enlarge the

cracks.

Biological weathering by lichens growing on rocks! Metabolic byproducts of the lichen can chemically weather the minerals.

Chemical Weathering: breakdown of rock by

chemical reactions that occur when rock and

mineral matter interact with water and air.

The main types of chemical weathering are:

Dissolution by Water: Dissolving minerals into their constituent ions or molecules into solution. .

Carbonation: Dissolution of carbonate minerals (limestones and marble). Requires the solution to be acidic.

Hydrolysis: Decomposes silicates; this reaction produces clay minerals - the most important minerals in soil.

Oxidation: Free oxygen atoms dissolved in water combine with iron in the rock, producing iron oxide minerals.

Important: nearly all chemical weathering proceeds faster under wetter conditions.

Dissolution by Water

Dissolving minerals into their constituent ions or molecules into solution. Water being a polar molecule can gang up on charged ions (i.e. sodium and chloride). Because of the polar nature of the water molecules, they are able to ‘pull’ Na from Cl—once this is done, water molecules surround the ions preventing them from forming ionic bonds. Straight dissolution by water dissolves ionic bonds easily.

Dissolution can also take place a little quicker if the water is acidic (has many H+ ions dissolved in it)—see carbonation and hydrolysis.

Dissolution by Acid (Carbonation using carbonic acid example)

Carbonation: Dissolution of carbonate minerals specifically (calcite in limestones and marble). Requires the solution to be

acidic.

Where we can get acid:H2O + CO2 -> H2CO3 -> H+ + HCO3

-

Water + Carbon Dioxide -> Carbonic Acid -> Hydrogen Ion + Bicarbonate Ion

The acid splits to H+ ions and Bicarbonate ions. The more H+ ions in solution, the more acidic the solution and the lower the

pH.

Carbonation Reaction Equation:CaCO3 + H+ + HCO3

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

Calcite + Hydrogen Ions + Bicarbonate Ion -> Calcium Ion + 2 Bicarbonate Ions.

After dissolving rocks with calcite, the H+ ions are no longer dissolved in solution rendering the solution no longer ACIDIC.

Chemical Weathering of Limestone: easy to

do so long as you have some ACID in the rain. Marble (limestone is

the parent) headstones don’t last

as long as granite headstones (or

buildings for that matter).

This limestone has been gradually dissolved by water from breaking waves, as shown by the pitted, honeycomb texture.

Karst Topography: Weathering of limestone (calcite) forms KARST (see little hills below). We’ll learn more about this during GROUNDWATER lecture but for now, groundwater essentially dissolves limestone. When rocks above these

underground tunnels and caves are eroded away, the limestone is exposed. More weathering of the limestone at the surface will occur, and eventually these little hills will

dissolve away.

Hydrolysis

Decomposes silicates; this reaction produces clay minerals - the most important minerals in soil.

4KAlSi3O8 + 4H+ + 2H2O -> 4K+ + Al4Si4O10(OH)8 + 8SiO2

Orthoclase + Hydrogen Ion + Water -> Potassium Ion Kaolinite (clay min.) + Quartz

Feldspar is a KAl silicate. If we take a weak acid and dissociate it into H+ + HCO3- in water, the H+ ions will act upon the feldspar. Note potassium feldspar’s chemical formula and then look at the chemical formula after hydrolysis. Where did the K+ ions go?

Basically hydrolysis weakens the mineral—H+ is the smallest ion (atomic number of 1). Hydrogen ions basically kick out the K+ ions (much larger in size) and shove themselves into the crystalline lattice—tremendously weakening the mineral.

Imagine a cinder block wall. Wherever there would be a K ion (lets just say every tenth cinder block for example), remove every tenth cinder block and replace it with a lego. You can see how the integrity and strength of the cinder block wall is significantly weakened by sticking smaller ions in place of larger ones!

The importance of water in chemical weathering is dramatically illustrated here. The granite obelisk at the left was carved about 3500 years ago in Egypt. In the dry climate of Egypt the rock surface remains fresh even after all those years.

A “twin” obelisk carved at the same time was moved to New York City about 100 years ago. After surviving unscathed for 3400 years in arid Egypt, the wetter climate of New York quickly broke down the rock surface by chemical weathering!

Stone Mountain, Georgia: made of granite and granite is made of different minerals which have different silicate structure

and thus different stabilities. Certain minerals are more susceptible to chemical weathering. Preferential weathering

forms little divots in Stone Mountain.

Recall the igneous pluton on the left. The silicate minerals are more stable at the earth’s surface than surrounding

(and once overlying sedimentary rocks)

OxidationThis would be the reaction between water, iron and oxygen. You know like the rust you’d find on my car! By bonding oxygen to iron atoms, the new mineral (hematite) is structurally weaker than straight metallic iron.

Oxidation of iron in sediments produces red color sediments and is an indication that 1. iron is present, 2. oxygen is present and 3. water is present.

All this red clay in my backyard—well it was produced by a stream! No there is no stream in my backyard today, but the principle of uniformitarianism suggests that red colored sediments are produced by streams today, and streams must have produced the red sediments in my backyard some time ago.

Weathering of Common Rocks CHEMICAL WEATHERING EFFECTIVELY BREAKS DOWN ROCK

MINERAL BY MINERAL, GRAIN BY GRAIN; WEAKENING IT SO THAT IT IS EASILY ERODED AND TRANSPORTED AWAY.

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

Controls on Weathering• Time: The longer the minerals are exposed to weathering agents,

the more weathering will occur.• Climate (Influences both chemical and physical weathering).

– Abundant precipitation allows for fresh, aggressive water to rapidly dissolve minerals without becoming saturated. Warm temperatures increase chemical reaction rates. Warm, tropical climates have accelerated chemical weathering.

– Climates with wide ranges in high and low temperatures (daily) will result in daily freeze thaw cycles (frost wedging) or aid in expansion and contraction (hot/cold temperatures).

– Chemical weathering is pronounced in warmer, tropical climates whereas physical weathering dominates temperate (not too hot, not too cold—seasonal) climates.

• Rock Chemical Composition (Silicate minerals higher on Bowen’s reaction series are weaker). The higher the percentage of stronger minerals (more covalent bonding), the longer weathering will take.

Soil Profiles

Digging down into a soil, you would notice that the soil zone has a layered appearance. This layering is called a soil profile. Each layer of a soil profile is called a horizon.

Soil profiles vary between different types of soils, but one can often recognize the “O”, “A”, “B”, and “C” horizons in many soils.

O horizon: a thin layer of partially decomposed organic matter called humus.

A horizon: called the zone of leaching; in this area mineral matter is dissolved by water percolating down.

B horizon: called the zone of accumulation; in this area particles and dissolved materials from the A horizon are deposited.

C horizon: a zone of transition from soil to rock, consisting of weathered parent bedrock.