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WEATHERING

Turning Rock to Sediment and Solutions

Igneous rocks form at

high temperatures; at

the Earth’s surface they

are chemically unstable

and will begin to

disintegrate and

decompose in a process

known as weathering.

The products of

weathering are particles

known as sediment

(sand grains, gravel,

etc.) and rock

constituents dissolved

in water.

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Weathering of Rocks to Form Sediment

Earth is a very dynamic environment:

Volcanic & other mountain building processes elevate portions of the

Earth's surface, while opposing forces move material from high

elevation to lower elevation.

• Erosion -- The physical

removal of material by mobile

agents such as water, wind or

ice.

• Weathering – The physical

breakdown and chemical

disintegration of rocks at the

earth’s surface

Weathering

Two Types:

Mechanical Weathering - physical forces that break rock into smaller

and smaller pieces without changing the rock's mineral composition

(same minerals).

Chemical Weathering - involves a chemical transformation of rock

into one or more new compounds.

Weathering is the response of rocks to a changing environment:

Plutonic rocks form under high pressures and temperatures. At the

Earth's surface they are not as stable as the conditions under

which they formed. In response they gradually change: weathering.

Although we will look at them separately, mechanical and chemical

weathering work simultaneously and aid one another.

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Mechanical Weathering

Mechanical weathering increases the surface area available for

chemical weathering.

Physical processes lead to mechanical weathering:

1. Frost Wedging, Crystal Wedging, Thermal Expansion

2. Unloading/Mechanical Exfoliation

3. Biologic Activity

Erosional agents (water, ice) also result in "breaking rocks“/abrasion.

Jointed Rock, Sierra Nevada

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Mechanical Weathering (cont.)

1. Frost Wedging - caused by repeated cycles of freezing and thawing.

• H2O has the unique property of expanding (~9%) when it freezes.

• H2O works its way into cracks in rock, and upon freezing,

expands and enlarges these openings. After many freeze-thaw

cycles, the rock is broken into angular fragments.

•Frost wedging is most

prevalent in mountainous

regions where there is a daily

freeze-thaw cycle.

• It is sometimes frost wedging

that causes potholes in roads

during the winter.

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Wave Erosion with Salt

Wedging

Salt water gets into cracks,

evaporates, and the resulting growth

of salt crystals expands the cracks

and mechanically breaks the rock

apart. Erosion sweeps the rock

fragments away.

• Thermal Expansion seems to require

water to effectively break apart rock.

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Exfoliation and Domes

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Mechanical Weathering (cont.)

3. Biological Activity

•Mechanical weathering can be accomplished by organisms such as

trees and burrowing animals.

• Plant roots grow into fractures, and as they grow larger, they

wedge the rock apart.

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Mechanical and Chemical

Weathering by Plants

Chemical Weathering

Chemical Weathering is a chemical process that breaks down minerals.

1. Dissolution

2. Oxidation

3. Hydrolysis

Products of Chemical Weathering

Caves

Clay Minerals

Metal Ores – Aluminum

Rounded Boulders -Spheroidal Weathering

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Dissolution

Pure water acts as a solvent, however the presence of even a small

amount of acid in water dramatically increases the corrosive force of

water.

Carbonic acid is produced by atmospheric CO2 dissolving in rainwater

and surface waters.

Other sources of acid:

•organic acid from the decay of organic matter

•sulfuric acid from the weathering of sulfide minerals like

pyrite.

The mineral calcite is particularly susceptible to dissolution by acid.

CaCO3 + 2H+ Ca2+ + CO2 + H2O

In this process, calcite is dissolved. This is important for the formation

of caves in limestone.

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H2CO3 = Carbonic

Acid =

H2O + CO2 Water + Carbon Dioxide

(Gas)

Blanchard Springs Cave, Arkansas

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Hydrolysis

Hydrolysis is the major chemical process that produces clay minerals. Feldspar disintegrates through hydrolysis, the addition of water to its Lattice structure. 2KAlSi3O8 + 2H2CO3 + H2O AlSi2O5 + 4SiO2 +2K+ + 2HCO3

Feldspar acid water Clay silica K ion carbonate

When feldspars add water to their lattice structure, the expand. This expansion breaks apart the rock and releases quartz as sand or silt.

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Hydrolysis-The most

abundant product of the

chemical weathering of

rocks is the formation of

clay minerals - clays are

stable at the Earth’s surface

and are a dominant

component of soils.

Clay minerals are major

portion of many

sedimentary rocks such as

shale.

Boulders? Anza Borrego Desert,

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Granite Boulders and Outcrop

Weathering of Granite to Clay

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Spheroidal Weathering

Oxidation

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Chemical Weathering (cont.)

2. Oxidation — oxidation is a chemical process that is responsible

for rust forming from iron.

4Fe + 3O2 2Fe2O3

The oxidation of iron is greatly accelerated in water.

Igneous ferromagnesian minerals such as olivine, pyroxene and

amphibole oxidize in the presence of water at surface conditions.

Another important oxidation reaction occurs when sulfide minerals

such as pyrite (FeS2 ) oxidize to form iron oxide minerals.

Grand Canyon of Kawai - Oxidation

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Types Description of process and results

Dissolution

Hydrolysis

Oxidation

Types Description of process and results

Dissolution Water molecules gang up on ions on outside of

mineral lattice (surface) and break the mineral

bonds, releasing the ions into solution. Water

carries ions away.

Hydrolysis

Oxidation

Types Description of process and results

Dissolution Water molecules gang up on ions on outside of

mineral lattice (surface) and break the mineral

bonds, releasing the ions into solution. Water

carries ions away.

Hydrolysis Water molecules enter mineral formula, replacing

other components and changing mineral to a new

one: a clay mineral. Example:

2KAlSi3O8 + 2H+ + 9H2O =

Al2Si2O5(OH)4+ 4H4SiO4 + 2K+

K-feldspar + water =

Kaolinite clay + silicic acid and potassium ions

Oxidation

Types Description of process and results

Dissolution Water molecules gang up on ions on outside of mineral lattice (surface) and break the mineral bonds, releasing the ions into solution. Water carries ions away.

Hydrolysis Water molecules enter mineral formula, replacing other components and changing mineral to a new one: a clay mineral. Example:

2KAlSi3O8 + 2H+ + 9H2O =

Al2Si2O5(OH)4+ 4H4SiO4 + 2K+

K-feldspar + water =

Kaolinite clay + silicic acid and potassium ions

Oxidation Oxygen bonds with Fe ions on outside of mineral lattice (surface) removing Fe from mineral and producing Hematite (rust).

Weathering Activity

Na, Ca, K, Mg, other cat ions, AND excess silica (silica not in quartz or not associated with Al and O) – Dissolves and is removed by water.

Al, Si, O – Hydrolyzes: combines with water to form clays (only if all three available in mineral).

Fe – Oxides: combines with O to form insoluble iron oxides, giving red to yellow soil cover.

Quartz – Stays in place; doesn’t break down chemically.

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Unlike other silicate minerals, quartz is particularly resistant to chemical weathering - that is why it tends to accumulate on beaches and sand dunes since everything else has weathered away.

What’s Left?

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Group Activity:

List all the characteristics of a rock outcrop in the mountains that would make it weather fastest – include information about the type of rock as well as the environment and local

activity.

Differential Weathering – same

cemetery

Granite

Marble

(1870)

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Differential Weathering Soft rock weathers rapidly and easily

Harder rock protrudes and

forms ridges

Differential Weathering

Harder sandstone forms ridges,

softer shale forms valleys