First part of Chapter 6.

Weathering

Weathering is the name of the process in which the original rocks disintegrate and decompose. It is somewhat similar to the equilibrium processes we have learned in metamorphic rocks; Some minerals such as igneous and metamorphic minerals are not stable under surface condition (STP), and they will essentially alter to adjust to the new environment.

Weathering involves physical, chemical and biological processes, but the two major types are:

Physical (mechanical) and chemical weathering.

Mechanical weathering:

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

1. 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 frost wedging to happen we need:

          http://www.calstatela.edu/faculty/acolvil/weathering/frost_wedging.jp

          1. enough moisture

          2. falling, rising temperature

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

Sheeting

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)

   (www.utexas.edu/.../ 10_weathering/2.htm)

Thermal expansion

Daily temperature cycle, in hot deserts

expansion - contraction

 http://geography.sierra.cc.ca.us/booth/Physical/chp15_gradation/disinteg_granite.jpg

Organic Activity

root action

animal burrow

CHEMICAL WEATHERING

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

Therefore, chemical weathering can not be very important in deserts.

Major processes of chemical weathering are:

1. DISSOLUTION

It is easy to imagine dissolution.

(salt or sugar in water)

The 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 distilled water.    this fact is abruptly changing by the presence of different weak acids:

          carbonic acid:H2O+CO2 

          HCl: think of acid test

          H2SO4 

Ions from such a dissolution makes the water HARD

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

2. HYDROLYSIS

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 mostly clay.

The original framework silicate structure will become sheet silicate.  It is true for most silicate minerals. Some silica is also released from the silicate mineral, which in the water forms weak a acid, and may then be carried away from the site of reaction.

Not every hydrolysis will produce kaolinite. Other clay minerals might form depending on the original silicate mineral.

3. OXIDATION

oxidation in an other word is called rusting.

4Fe+3O₂=Fe₂O₃ 

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

GEOMETRY OF ROCK INTEGRATION

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

Stability of minerals is the OPPOSITE of BOWEN REACTION SERIES

          Texture is also very important (porosity, permeability)

       2.Climate  

       3.Topography (Amount of rocks exposed)

examples:

GRANITE

          Feldspar - clay min Ca goes first,

               Na later

          pyroxene-amphib. mica - clay + Limonite

          Quartz remain unaltered (light redish soil)

Basalt

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

Sandstone

          Quite resistant

Limestone

          Humid clim

                   dissolves away easily, cave systems

          aride clim.

                   resistant, forms cliffs

Shale

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

DIFFERENTIAL WEATHERING

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

SOIL

Regolith and SOIL

Regolith is coming from the Greek word REGO:meaning blanket

It is a layer of soft disintegrated 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.

          UPPERMOST LAYER OF REGOLITH IS CALLED SOIL.

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

 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

SOIL PROFILE

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.

O HORIZON

       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:

       1.Climate

       2.Parent rock material

       3.Plants and animals

       4.topography

1.Climate:

       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%

4.Topography

       It effects the rate of erosion and drainage

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

SOIL TYPES IN THE US

PEDalfER

       pedon=soil (greek)

       Al-alumina

       Fe-iron

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)

PEDOCAL

Accumulation of calcium carbonate

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

CALICHE

Pure caCO3 layer arid-semi-aride climate

LATERIT

Hot wet climate of the tropics

Great amount of silica is dissolved---

Bauxite

Problems in soil management

SOIL EROSION

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

Soil erosion can happen by:

 Wind

 water

       Eroded soil is moved to lakes rivers,

       shrink the reservoir capacity

       Pollution carried by soil into reservoirs drinking water becomes toxic