All rocks fall into one of 3 main groups:


  • These are the oldest rocks and were originally formed from cooling magma (liquid rock) when the earth’s crust was cooling down many millions of years ago. Magmas solidify at temperatures between 600⁰C ‐ 1200°C;
    • They are made up of tightly interlocking crystals of various minerals. Slow cooling gives large crystals and large grained rocks. Rapid cooling gives small crystals and fine grained rocks and;
    • A good example of an igneous rock is granite which is a large grained rock made up of 3 minerals.


GRANITE                              Mica


Granite is common in Africa. There are many more types of igneous rocks such as, basalt, diorite, rhyolite, gabbro and peridotite.


  • These are divided, into 2 types:
    • Clastic rocks made up from pieces of other rocks,
    • Non‐clastic rocks formed from vegetable matter or by chemical reaction
    • Sedimentary rocks are formed by the destruction and weathering of igneous rocks. Weathering can be by the action of frost, temperature changes, wind and water or by the dissolving effect of rainwater which is slightly acidic.

Example of the formation of sedimentary rocks formed by the action of water e.g. a river. As the river flows downhill it carries particles of rock and soil with it. As the river bed levels out the water flows more slowly and the particles fall to the bottom of the river bed with the largest particles (Gravel) falling out first. These particles gradually become compressed to form rocks.

  • Gravel particles form conglomerates;
    • Sand particles form sandstones and;
    • Silt and Clay particles form shale and mudstones.

Figure 1: Deposition of soil particles to form sedimentary rocks

Source: spot.pcc

Table 1: Formation of soil particles to sedimentary rocks

 ConglomeratesSandstoneShale and mudstoneLimestone and chalk

Where the river slows down enough for marine life to become established (e.g. fish, snails, etc.), some of these animals die and fall to the bottom of the river and the calcium in their bones forms limestone. This whole process can take many thousands of years.

Conglomerates, Sandstones, Shale and Mudstones are Clastic Sedimentary rocks. Limestone and Chalk are Non‐Clastic Sedimentary rocks.

Coal is an example of a sedimentary rock formed from vegetable matter.


  • These are formed from other rocks, igneous or sedimentary by great pressure and heat
  • Examples of Metamorphic rocks are: schisits, gneiss, slate and marble.


The following is a list of some of the minerals which are found in rocks.

Table 2: Minerals which are found in rocks

QuartzSilica Oxide. Commonest of all minerals. Occurs in many sedimentary and Metamorphic rocks. Colourless and water clear
    FeldsparsOrthoclase. Potash Feldspar. Essential constituent of igneous rocks. Occurs in metamorphic rocks. White/ Pink crystals, duller than quartz.
Plagioclase Soda lime feldspar. Albite‐Sodium. Anorthite ‐ Lime. Essential constituent of some igneous rocks. Also occurs in Metamorphic rocks. Usually white.
  MicasMuscovite White/Potassium Mica
Biotite Black Mica. Constituents of acid igneous rocks, many metamorphic rocks and sediments.
  AmphibolesHornblende. Occurs in many igneous and metamorphic rocks. Black or Greenish black crystals.
Anthophyllite Long fibrous crystals Asbestos Heat resistant, high electrical resistance, no chemical action or decay.
Pyroxenesa) Augite. Black, and similar to hornblende. Constituent of more basic igneous and many metamorphic rocks.
  OlivinesOlivine. Green, crystalline. Basic and ultra‐basic rocks.
Serpentine. Varied colour
Talc. Secondary mineral, formed by hydration of igneous rocks or dolomitic limestone’s
    Iron MineralsHematite:      Iron ore. Reddish
Limonite:      (Hydrated) Iron ore. Brown
Magnetite: Black Iron ore. Lodestone
Iron Pyrites: Sulphur ore.
Ironstone: Seams in sandstone.
  CopperCopper Pyrites. Ore of copper, often associated with Iron Pyrites
Chalcocite.         Copper Glance. Green
Malachite.           Ore of Copper. Green
Leada) Galena. Lead ore, nearly always contains some silver.
      CarbonatesDolomite. Magnesium Limestone. Usually honey colour.
Magnesite. Ore of magnesium
Calcite. Crystalline, and usually colourless. Occurs in igneous, sedimentary and metamorphic rocks and in veins. Softer than Quartz.
Stalactites and Stalagmites. Formed from the evaporation of lime charged water.
Phosphatesa) Apatite. Calcium Phosphate. Found in igneous rocks, and small amounts in metamorphic rocks. Crystalline, Green.


The soil fractions are produced from rocks by:‐

  • Physical weathering;
    • Chemical weathering and;
    • Biological weathering.

The type of soil produced will depend on the parent material of the rock. Physical weathering: This is a mechanical process.


Agents of Physical Weathering are:


Is the expansion and contraction of a rock due to heating during the day and cooling at night. Where a rock is partly buried, the exposed part will be heated by the sun and the part underground will remain cool, causing stresses inside the rock. Rocks crack, crumble and the outside layers peel off ‐ this process is called exfoliation.

      Inert: chemically inactive.


This is the most destructive of all forces on rocks. All rocks contain some water between the grains and in cracks. When this water freezes it exerts pressure inside the rook causing it to break up.


The abrasive or sandpapering action of sand blown by wind acts on exposed rock surfaces and wears them away.


Running streams and rivers wear rocks away and carry the material downstream.

The products of physical weathering are stones, sand and silt. These fractions are produced simply by the mechanical grinding down of rock fragments into very small pieces. These fractions have not undergone any chemical changes and have no chemical properties, e.g. they are chemically inert. This explains why sandy soils have poor water and nutrient holding properties and are basically infertile.


This is a chemical process and parts of the rock undergo chemical changes. It is mainly caused by water which is usually very slightly acidic and has a dissolving effect on the rock.

The most important rock forming minerals are silicates and over very long periods these will slowly dissolve in water. They are also altered by the acid in the water.

Table 3: Examples of soil formation through weathering

  Potash Feldspar + Water                             Kaolin (Clay) + Silicic Acid + Potash Soda Lime Feldspar + Water                      Clay + Silicic Acid + Calcium + Sodium  
Consider the breakdown of granite rocks. Granite is made up of Quartz, Mica and Feldspar and breaks down as follows:
QUARTZPhysical WeatheringSAND & SILT

This process changes rock from a hard crystalline substance to a soft dull mass and eventually into soil containing sand, silt and clay. A rock containing a lot of Quartz will produce a very sandy soil, while one with a lot of Mica and Feldspar will produce a clay soil.

It is important to remember that because of the way it is formed, e.g. by ‘Chemical Weathering’, all the Chemical and water holding properties of a soil are provided by the clay fraction. The sand and silt fractions have no chemical properties.


This is carried out by soil micro‐organisms, e.g. bacteria and fungi, which work to break down soils into smaller particles. Earthworms mix the soil by burrowing and plant roots penetrate cracks in  rocks causing exfoliation.

See Diagram on Development of Soils on Page 8.


  • Quartz ‐The commonest rock forming mineral in central Africa
  • Mica ‐ This is black mica, the other form of Mica is white.
  • Feldspar ‐ This is Potash Feldspar which is pink. Soda Feldspar is white.
  • Granite ‐This is the commonest rock in Central Africa. It is made Up of Quartz,
  • Mica and Feldspar ‐ If you examine this rock carefully, you can see the black pieces of mica, the white of Soda Feldspar and the clear crystals of quartz. It is an Igneous rock.
  • Sandstone ‐ This is a Sedimentary Rock. It has been built up in Layers and these can be seen clearly on the sample.
  • Schist ‐ This is a Metamorphic Rock and been formed by Heat and pressure. Again note the characteristic layering.
  • Copper ‐The green colour is copper in a chemical form. This is a rock‐forming mineral. Soils weathered from this type of rock will contain copper.
  • Chrome ‐ This is a rock‐forming mineral and also an important mineral exported from Zimbabwe. This is Chrome Ore.
  • Iron Pyrites ‐ This is one form of Iron in rocks, and is known as Fool’s Gold because it is sometimes mistaken for gold. It is a rock‐forming mineral.