Weathering is the name of the process in which the original rocks disintegrate and decompose. It is a little bit similar to the equilibrium processes we learned in metamorphic rocks, because certain minerals such as igneous and metamorphic are not stable under surface condition, and they will essentially alter to adjust to the new environment.

Weathering involves physical, chemical and biological processes, but the two major type is

Physical (mechanical) and chemical

Mechanical weathering:

It is strictly a physical process, involving no change of chemical composition

Ice or frost wedging:

Water penetrates easily into the openings of the rocks from rain or melting snow, than when freezes expands (stress is about 110 kg/cm2= to drop a 98 kg iron ball from a height of 3m).

Over a period of time the rock will be literally hammered apart.

For these we need:

          1. enough moisture

          2. falling, rising temperature

          3. rocks with cracks or weakening (problem on highways)


Rocks formed under confining pressure, when eroded up to the surface got released from the pressure and tend to expand. This internal pressure will break the rocks into fragments. Expansion joints will form parallel to the surface, and the process is called sheeting.

(Highway...... rocks taken away --- sheeting can cause problems rock slumping)


Thermal expansion

Daily temperature cycle, in hot deserts

expansion - contraction

Organic Activity

root action

animal burrow


During chemical weathering rocks are decomposed, the internal structures of minerals are destroyed, and new minerals are created.

Water is the primary source of chemical weathering

          -as part of the weathering or

          -as carrying material for dissolved ions

          on the other hand it removes the weathered particles, to expose the next fresh rock   layer

The degree and the rate of chemical weathering is essentially influenced by the amount of precipitation.

          chem w. is not that imp. in deserts.

Major processes are


You can imagine dissolution easily

(salt or sugar in water)

Most water-soluble mineral is Halite. This is caused by the mineral structure, and the fact that the water has polar molecules.

Lots of mineral are insoluble in dest. water, this fact is abruptly changing by the presence of different weak acids:

          carbonic acid:H2O+CO2

          HCl: think of acid test


Ions from such a dissolution makes the water HARD

Ions of hard water reacts with soap and makes insoluble compound (not desired)


The chemical union of water and a mineral is called HYDROLYSIS.

This is not an adsorption process as the pores of a sponge sucks up water, but a specific chemical change where a new mineral is forming.

In hydrolysis the ions of the mineral react with H+ and OH- ions of the water to produce new minerals.

ex. Feldspar

          NaAlSi3O8 (Na plagioclase)

          to weather this we need: CO2


                                                 and of course the feldspar

CO2 will react with the water to form carbonic acid


          H2O+ CO2 = H2CO3

4NaAlSi3O8+4H2CO3+ 18H2O = 4Na+ + 4HCO3- +        

            (plag)            (carb.acid)                   (diss. comp.)

          8H4SiO4 + Al4Si4O10(OH)8

                   (clay min.)

As you can see the clay mineral has no Na+ in it, on the other hand it contains (OH) in its structure.

The original framework silicatestucturewill become sheet silicate.  Some silica is also released from the feldspar, which in the water forms weak acid, and may then be carried away from the site of reaction.



oxidation in an other word is called rusting.


Oxidation involves every process where cations loose electrons from their outer electron shell to Oxygen.

For oxidation most of the time the Cation was released from a silicate structure by Hydrolysis

When in coal mining or volcanic area pyrite is exposed it will go through oxidation

2FeS2 + 4H2O= 2FeO(OH) + 2H2SO4 + H2

Big problem with soils around these mines.  Acidifying groundwater.

The activity of chemical weathering rises with rising temperature.

it is most intensive in warm hot tropical weathering


The Mechanical breakdown and the shape of the weathered rocks are usually inherited from the pattern of joints, bedding, and cleavage of the original rocks,

First usually the edges and faces became weathered

              Spheroidal weathering


During this PROCESS successive shells will separate down from the rock (spelling off)


Weathering of different rocks

Important factors:

          1.Mineral composition

          Check in headstones in old cemetery!

                   Quartz is very stable

                   olivine, pyroxene very unstable

IMPortant! Stability of minerals is the OPPOSITE of BOWEN REACTION    SERIES

          Texture is also very important (porosity, permeability)


       3.Topography (Amount of rocks exposed)




          Feldspar - clay min Ca goes first,

                         Na later

          pyroxene-amphib. mica - clay + Limonite

          Quartz remain unaltered (light redish soil)



          Soon goes into clay and iron oxides (red-brown soil)



          Quite resistant


          Humid clim

                   dissolves away easily, cave systems

          aride clim.

                   resistant, forms cliffs


weathers away quickly because it is fine grained and soft. However it is not changing, probably just transported away.


Different rock masses weather very differently.

More resistant zones will make cliffs, and stand out as ridges. The weaker zones tend to become walleyes or depressions


Regolith and SOIL

Regolith is coming from the Greek word REGO

meaning blanket!!!!!

It is a layer of soft desintegrated rock material formed in place and covers the fresh rock underlies beneath it.

The thickness of the regolith is variable from some cm to hundreds of meters, dep. on the composition of rocks, climate, + the length of time the weathering takes place.


       Composed of rock fragments, minerals and decomposed organic      matter (HUMUS)(50%), and pore space filled by air and water


Humus is very important, because it helps to retain the water in the soil zone. On the other hand it provides the nutrients for plants.(5%)

       Soil water: is not pure water, but contains lots of             very important ions, nutrients (25%)

       Soil air: supply the oxygene plants need (25%)

Soils formed in place are termed:

       Residual soils

Soils  developed on unconsolidated deposits are termed:

       Transported soils



Transition of the upper surface of the soil down to the bedrock.

Soil Profile contains a constant sequence of layers, or horizons, distinguished by composition, color and texture.


       thin surface of leaf mold (forest)


A horizon.

       topsoil layer, light leached layer (zone of leaching) (ions and fine   grained parts are washed out downward) ELUVIATION


B horizon.

       Subsoil, contains Fine clays washed down from the topsoil. This is the zone of accumulation and commonly reddish. ILLUVIATION


A and B together is called solum or true soil

C horizon:

Zone of partly disintegrated and decomposed bedrock, It is gradually grades into the fresh unaltered bedrock.

The soils which has no well developed zones are immature

The type of zones, and the thickness of soil is essentially depend on the following factors:


       2.Parent rock material

       3.Plants and animals



       Desert, arctic region:

                   Physical weathering major,

                   Organic matter is minimal (no humus)

                   SOIL: thin, mostly rock fragments

       Equatorial regions

              Chemical weathering is dominant

              Thick soil develops rapidly (60m).Rarely it                     even can be 150m.

2.Parent rock material:(ex.)

       Quartz sandstone: thin infertile soil

3.Plants and animals:

       organic decay----humus (mostly plants)

       Swamp almost 100%

       Desert 1%


       It effects the rate of erosion and drainage

       Slope:On steep slope no too much soil, few water, not         well dev.soil.



       pedon=soil (greek)




Lots of Fe, and Al-clays, mostly occurring underneath of forests in mid-latitude areas, with more than 65 cm annual rainfall (acidic conditions (from organic decomposition)



Accumulation of calcium carbonate

Dry climate, grasslands smaller amount of clay, bec. dry climate



Pure caCO3 layer arid-semi-aride climate



Hot wet climate of the tropics

Great amount of silica is dissolved---



Problems in soil management


       More dangerous lately, bec. of forest destroying       (plants are protective)

Soil removement can happen by:



       Eroded soil is moved to lakes rivers,

       shrink the reservoir capacity

       Pollution carried by soil into reservoirs drinking water         becomes toxic