Weathering

Weathering is the physical breakdown and chemical alteration of rocks and minerals.

•Weathering happens on or near the surface.

Weathering is caused by

•Atmospheric gases

•Water

•Acids

•Living organisms

There are two types of weathering, mechanical and chemical

Why is Weathering important?

It produces the raw products for both

Sedimentary rocks and soils

It also products some important mineral resources like aluminum ores

What is erosion?

Erosion is the removal of weathered material from the weathering source area.

Mechanical Weathering

This is the physical breaking of bedrock into smaller pieces.

Types of Mechanical weathering include

•Frost wedging

•Salt crystal growth

•Pressure release

•Thermal expansion & contraction

•Activities of organisms

Frost wedging:

This occurs in regions where temperatures fluctuate above and below freezing

The material produced by frost wedging accumulates as talus at the base of mountains forming a talus slope.

Pressure release:

This common in intrusive rocks that were formed deep under ground. E.g. granite batholiths.

When this rock is exposed to the surface by uplift and erosion the rock expands and sheet joints form parallel to the rock surface. This is also called exfoliation.

Exfoliation domes form as the slabs of rock slip or slide off the bedrock. E.g. Yosemite

Thermal expansion and contraction:

A common feature in desert environments where temperatures can vary by as much as 30°C (86 °F)

Rock is a poor conductor of heat so the outside expands more than the interior.

But this is not an important agent of mechanical weathering.

Chemical Weathering

This is the decomposition of the rocks by chemical alteration of the bedrock.

Water (H2O)

Acids (H2SO4, HNO3 , H2CO3)

atmospheric gases (carbon dioxide and oxygen)

and organism are important in chemical weathering.

Water:

Some minerals will dissolve in water like the mineral halite (NaCl)

H2O + NaCl Na+ + Cl-

This minerals are transported as dissolved salts in water.

Living organisms:

Many minerals will not dissolve in water, but they will in acid.

Lichen is a fungi and algae living together.

Lichen covered rocks weather faster than rocks not covered by lichen. Why is this?

During the process of photosynthesis, lichen removes CO2 from the atmosphere.

This gas then reacts with water to form an acid.

The chemical reaction:

Water + carbon dioxide Carbonic acid

H2O CO2 H2CO3

Carbonic acid hydrogen ion + bicarbonate ion

H2CO3 H+ HCO3-

Rocks that dissolve in acid easily are

Limestone and marble.

CaCO3 + HCl CO2 + CaCl2 + H2O

Another important chemical reaction is oxidation

Iron minerals in the rock react with oxygen to form iron oxides

This often appears as red stains in the rock

Fe 2+ + O2 Fe2O3

Plant action: 1.Tiny root hairs seek out small cracks and pits in rock. 2.Once the root hairs find a place they grow and expand. 3.The expansion causes great pressure and cracks the rock.

Factors controlling Chemical Weathering:

•Surface area of rock

•Climate

•Rock type

Surface area:

The greater the surface area the more area of the rock that is exposed to the elements and so it weathers faster.

Rocks with many joints and fractures will weather faster.

Climate and the speed of weathering

The wetter and warmer the climate the faster chemical weathering will occur.

So where in the world we you find the fastest weathering?

Tropical regions with high rainfall and warm temperatures.

Weathering here can extend to depths of several tens of meters.

In cold areas and deserts it will only go down a few centimeters!

Soil:

What is soil? Soil is a mixture of:Air,water and its solutes,mineral particles from parent bedrock,decaying organic matter,And organism responsible for its decay.

Soil is a living skin!

Soil Horizons

Soil Horizons:O composed mainly of organic material

a few centimeter thicknot fully decomposed

A Top soil

organic matter

biological activity, bacteria, plant roots, fungi, worms etc.

minerals can be leached by water into lower layers

B sub-soil

fewer organisms

soluble minerals build up here

C partially altered bedrock with little organic matter

The most important factor effecting soil formation is Climate

Different climates produce different soils

Soils and climate:

Tropical climate have laterite soils.

These are deep, but not not very fertile as the minerals have been leached out. Nutrients are in the O horizon.

The soil is often a deep red color from weathering.

If soil is rich in aluminum, then leached aluminum hydroxide may accumulate in the B horizon. as bauxite (aluminum ore).

Humid areas form pedalfer soils.

usually rich in organic matter

abundant water has leached most soluble minerals from A horizon

Pedocal soils are found in arid and semi-arid areas

Western and southwest regions of the USA

Contain less organic matter than pedalfers

Soil Degradation:

Soil is one of the most important factors in human civilization.

Its mismanagement and destruction has been the collapse of many civilizations in the past.

E.g. Sumeria in Iran/Iraq

Easter Island

Teotihuacan

Maya

The decrease in soil productivity or loss of soil to erosion is called

soil degradation