Fundamentals of Soil Science (1+1)

Rocks

Rocks
The rocks are generally composed of two or more minerals. Petrology ( Greek, petra means rock, logos means science) deals with science of rocks. It consists of
i) Petrography which deals with description of rocks
ii) Petrogenesis which is the study of the origin of rocks. Geologists have classified rocks into three major groups: Igneseous, Sedimentary and Metamorphic.

Igneous Rocks (Latin ignis, means fire):

• The whole surface of the earth passed through a molten stage and the first solid mineral was derived from this molten material known as magma.
• Igneous rocks are formed by cooling and crystallization of molten material-magma-on or beneath the surface of the earth.
• Igneous rocks formed from molten magma that cooling within the earth’s crust are called intrusive rocks. The volcanic rocks that cool on the surface of earth are termed extrusive. They are source of parent material for other rocks and ultimately for soils.
• The examples are Granite, Syenite, Diorite, Gabbro, Dolerite and Basalt.

Sedimentary Rocks

• Sediment is the material that settles on the bottom of something else, usually a liquid. Thus sedimentary rocks are formed from sediments, derived from the breaking down of pre-existing rocks. These are composed of materials that have been worn down by wind, water, or ice and deposited somewhere. Stratification is the most common feature of these rocks and as such these are also termed as stratified rocks.
• Sedimentary rocks are classified into three subgroups: Clastic sedimentary, Chemical particulates, and organic sedimentary. Clastic sedimentary are simply rock fragments e.g. sandstone and shale. The second subgroup is sedimentary rocks that are formed from chemical compounds settling out of water solutions e.g. Limestone and Dolomite. The last subgroup, organic sedimentary rock, is formed of living or once-living substances settling out of water. Coal is a well-known example of an organic sedimentary rock.

Metamorphic Rocks

• The word metamorphic means “change in form” and thus metamorphic rocks are those which have undergone some chemical or physical change from its original form.
• The change due to water is called Hydrometamorphism, due to heat is Thermometamorphism and due to pressure is called Dynamometamorphism.
• The examples are Gneiss- Formed from granite; Schist- Formed from basalt or shale; Quartzite Formed from sandstone; Slate- Formed from shale; Marble- Formed from lime stone

Relative abundance of rocks:

• Composition of earth’s crust as a whole

Igneous rocks – 95%, Sedimentary rocks – 5% (Shales 4%, Sandstone 0.75% and limestone 0. 25%)

• Composition of the upper five kilometers of the earth’s crust:
• Sedimentary rocks: Shales – 52%, Sandstones – 15%, Limestone & Dolomite – 7% (Total 74%)
• Igneous rocks: Granite – 15%, Basalt – 3% (Total 18%)
• Others – 8%

Note: Although sedimentary rocks forms only five per cent of the total earth crust, yet they are important as they occur to the extent of 74 per cent (almost 3/4th) at or near the surface of the earth (upper 5 km). As we go deeper, we find predominance of igneous rocks.

Minerals:

• The minerals may be defined as naturally occurring, homogeneous element or inorganic compound having definite chemical composition.
• More than 90 percent of all the minerals in the earth’s crust are made up of compounds containing Silicon and Oxygen.
• There are over 2500 minerals on earth. Only 30 minerals are commonly found on the earth’s surface and in surface rocks.
• Most minerals are made from nonliving substances. Many have crystal shapes. These are three-dimensional and hence easy to recognize.

Minerals are classified into primary and secondary.
1. Primary minerals: The original component of an igneous rock is known as primary mineral. Those primary minerals which are recognized as the characteristic component of that rock are known as essential mineral, while those which occur only in small quantities and whose presence and absence is not so important are called accessory minerals.
2. Secondary minerals: A mineral resulting from the decomposition of a primary mineral or from the reprecipitation of the products of decomposition of the primary minerals are called secondary mineral.

Relative abundance of important rock forming minerals:

Minerals (arranged in the order of their crystallization)	Important constituents	Percent distribution
Primary minerals
Ferro magnesium minerals
Ortho-ino silicates		16.8
Olivine	Fe, Mg
Pyroxenes	Ca, Na, Fe, Mg
Amphiboles	Ca, Na, Fe, Mg, Al, OH
Phyllo Silicates		3.6
Biotite	K, Fe, Mg, Al, OH
Muscovite	K, Al, OH
Non-Ferro Magnesium minerals
Tecto Silicates
Feldspars		61.0
Anorthite	Ca, Al
Albite	Na, Al
Orthoclase	K, Al
Quartz
Secondary minerals
Clay minerals	Na, K, Ca	11.6
Others	Mg, Fe, Al, OH	6.0