The three main types of rocks include Igneous, sedimentary and metamorphic rocks.

What is a Rock?

A rock is any substance that forms part of the earth crust. The solid substance may be as hard as granite or may even be as soft as mud. The earth crust consists of various types rocks and some of these rocks are fine grained while others are large grained. Rocks consist of different minerals.Rock type is defined as a particular kind of rock containing particular known characteristics and properties different or similar to others. These rock types are collected, classified, and analysed to identify the mineral compositions and their formation processes; however, rocks are much easier to defined than minerals. Even rocks within one rock type may have a highly variable composition. All rocks are generally classified into three groups according to their mode of origin, mode of formation and physical appearance.

The outer cover of the earths surface is known as the earths crust. The earths crust is basically formed of the rocks composed of different minerals. These earth forming minerals function in different ways. For example, mineral oil and coal are used as fuel. Moreover, the economic use of gold, copper, zinc, aluminium, nickel, iron etc. is also very much important. Different land forming processes like volcanism, depositional and erosional activities are highly influenced by the structure of the rocks and minerals. It is therefore very important to learn about these minerals and rock types since they are the elements responsible for forming the earths crust. Rocks are made up mostly of crystals of different kinds ofminerals, or broken pieces of crystals, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants.

The most common rocks examples and their Properties are:

• Limestone- quartz, feldspar, micas
• Sand stone- quartz cacite, feldspar
• Shale- quartz, micas feldspar
• Basalt calcite, dolomite
• Granite- feldspar, augite

Types of Rocks

There are three types of rocks and they are classified according to their mode of origin, mode of formation and physical appearance. These are namely:

1. Igneous rock
2. Sedimentary rock
3. Metamorphic rock

Igneous and metamorphic rocks are formed by the processes accomplished in the interior of the earth. On the other hand, the processes acting on the surface of the earth form the sedimentary rocks. Though the modes of formation of these rocks are different, yet they are interrelated.

Igneous Rocks

Picture of igneous rock

These are rocks formed from the cooling and solidification of molten magma from the earth beneath. Igneous rocks can be formed under two environmental conditions which are intrusive and extrusive.

Igneous rocks can be classified based on many criteria such as texture, crystal or grain size, color mineralogy, chemical composition, mode of occurrences and genesis. Since it is easy to give a rock a name in the field, the first order of classification should not be based on its chemical composition since chemical composition requires chemical analysis. Genetic classification should be avoided if it is possible to do, but this is not always the case. For instance, the main subdivision of rocks into sedimentary, igneous and metamorphic is genetic and cannot be avoided. However, igneous rocks are classified based on their content of essential minerals, for example, those that make up the bulk of the rock, which is known as the mode of the rock. Minor amount of accessory minerals are not considered. Igneous rocks are divided into two main groups on the basis of their grain size such as

1. Intrusive rocks– Intrusive rocks are sub-divided into plutonic and hypabyssal. Extrusive rocks are again sub-divided into explosive and quiet type. In intrusive type of rock, the magma solidifies in the deep interior of the earth. Granite, gabbro, syenite, diorite etc. are the examples of intrusive rocks. In hypabyssal type, the magma solidifies in the shallow interior as Porphyry. Magma when thrown by violent explosions into the air after it has been solidified gives rise to the explosive igneous rocks. Tuff and breccia are the common examples of this type of rock. In the quite type of igneous rock, the molten magma flows out quietly in all directions and solidifies. Rhyolite, basalt, andesite etc. are best examples of this type of rock. Batholiths, Laccoliths, Dikes, and sills are the common examples of igneous rock formed into the interior of the earth.
2. Extrusive Rocks– extrusive igneous rocks are rocks that have been poured on the earths surface where they are referred to as lavas. Intrusive igneous rock is also called volcanic rocks. These rocks do not contain fossils. An example of extrusive igneous rocks is Basaltic rock and they can be located in the North Eastern part of Nigeria. In the deep interior of the earth due to cooling, magma may be crystallized to form intrusive igneous rock. If therefore magma moves its way to the surface of the earth through the narrow vents, then it is termed as lava. This lava after cooling becomes extrusive rock.

Picture showing the stages of intrusive and extrusive igneous rocks formation

Examples of Igneous Rocks

1. Andesite
2. Basalt
3. Dacite lava
4. Diorite
5. Granite
6. Gabbro
7. Obsidian
8. Pegmatite
9. Peridotite
10. Pumice
11. Rhyolite
12. Scoria
13. Welded Tuff

Sedimentary Rocks

Picture of sedimentary rock

These are rocks that are formed by the deposition of sediments either by running water, ice, glacier or wind; hence the name sedimentary rocks. In this type of rock, the sediments are deposited in layers or in strata. Sedimentary rocks cover an area of 5 per cent of the total area of the Earths crust. However, about 75per cent of the total exposed part of the continental landmass is formed of sedimentary rocks.

All sedimentary rocks are non-crystalline and they seldom contain fossil. All rocks are due to weathering processes, these rocks are transported by water wind or ice, small fragments of rocks are easier to transport than large fragments and these fragments can be deposited on the surface to form sediments.

Types of sedimentary rocks

• Clastic sedimentary rocks
• Chemically sedimentary rock
• Biochemical sedimentary rocks
• Organic sedimentary rocks

1. Clastic sedimentary rocks- Clastic sedimentary rocks consist of consolidated fragments. As rock and mineral fragment are being transported from their original source, they become smaller and change shape. The sediments that are accumulated near the source will be different from those deposited afar off after transportation. A clastic sedimentary rock with large rounded fragments in a finer grain matrix is called conglomerate for instance in breccia, the nature of large particles in a conglomerates show the type of rock that was eroded.
2. Chemically sedimentary rock– these are rocks which are form as a result of chemical composition, chemical sedimentary rocks is made up of minerals deposited directly from hydrous solutions. There are three types of chemical sediments namely evaporites, travertine, dolomite or chert.
3. Biochemical sedimentary rocks- In biochemically formed sedimentary rock, living organisms play an important role; many organisms have shells of calcite or it polymorph aragonites (CaCO3) while others have shells of silica (SiO₂). When these organisms die, their shells are then accumulated on the surface to form biochemical sedimentary rocks, the soft part of the organism rots away or turn into oil. Some of the notable rocks form from the remains of animals includes chalk and corals.
4. Organic sedimentary rocks- organic sedimentary rocks consist of carbon-rich plants remains. Coal is an example of organically formed sedimentary rocks, it is formed from the remains of plants that grew in the forests or swamps environment, the plants remains are buried in the soil after being exposed to harsh temperature and pressure, they are converted to a black, combustible rock known as coal which consists of >50% of carbon. However, plankton float in the water, but the soft part could mix with mud on the sea floor to form shale, this organic material is gradually converted in oil, changes the shale to black color. These types of shale are called oil shale.

Some common examples of sedimentary Rocks

• Conglomerate
• Breccia
• Sand stone
• Siltstone
• Shale
• Oil shale
• Coal
• Fossiliferous limestone
• Chert
• Limestone
• Selenite
• Iron Ore
• Halite or rock salt
• Dolomite

Sources of sediments and their characteristics

Different types of mechanical and chemical weathering processes are acting on the earths crust. Due to the effect of these processes, the outer crust of the earth is gradually eroding out. These eroded materials are known as sediments. The sediments, which are formed by detrital created by the mechanical weathering, is known as clastic sedimentary rocks. Clay, sandstone and pebbles are the examples of clastic sedimentary rocks. Sedimentary rocks which originate from the sediment accumulated by the chemical weathering is known as organic sedimentary rock. In this process, the materials which remain in soluble condition in the water gradually settle down at the bottom by different inorganic processes and ultimately become consolidated. For example, if evaporation occurs in any confined seawater, then the salts remain as residue. In this case, evaporation is an inorganic process and the salt that remains as residue is considered sediments. In some cases, the dead bodies of different aquatic animals are accumulated as organic sediments and ultimately these are consolidated and transformed into organic sedimentary rocks. Coal, limestone are the proper examples of such rock. Besides, dolomite, chert and gypsum are the examples of chemical sedimentary rocks.

Classification of sedimentary rocks

Primarily, the sedimentary rock is of three types viz. Clastic, chemical and organic sedimentary rocks. The names of sedimentary rocks with their elements are shown in Table beow

Sources: elements and the associated names of the sedimentary rocks

Characteristics of sedimentary rocks

• Bedding, ripple marks, clay, cracks etc. are found in the sedimentary rocks
• The presence of fossils is one of the major characteristics of the sedimentary rock.

Metamorphic rocks

Metamorphic rock

Metamorphic rocks are rocks, which through temperature, compression and chemical actions have been changed either in form or in composition, are known as metamorphic rocks. Metamorphic processes usually takes place in the solid state, the process does not cause the rock to melt, but instead changes them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Pressure or temperature can even change previous metamorphosed rocks into new types. Metamorphic rocks are often squished, smeared out and folded. Despite these uncomfortable conditions, metamorphic rocks do not get hot enough to melt; else, they would change to igneous rocks.

The agents of metamorphism: Heat, compression and chemically active fluids are the main agents that operate singly or collectively to transform rocks. These metamorphic agents increase the internal density of the rocks and the dimension of the crystals and create clear foliation. Due to the application of temperature and pressure, the minerals of the rock become compressed like the leaves of trees and are arranged in parallel layers. These characteristics of layered structure of rocks are called foliation.

Examples of the important metamorphic rocks:

Foliated metamorphic rocks- foliated metamorphic rocks have a layered or banded appearance that is produced by exposure to heat and directed pressure. These are formed when the pressure squeezes the flat or elongate minerals within arockso they become aligned. Theserocksdevelop a platy or sheet-like structure that reflects the direction that pressure was applied. Examples of foliated rocks are slate, Hornblende etc.

Non-foliated metamorphic rocks– non-foliated metamorphic rocks are formed when metamorphism takes place under constant pressure, when this happens, an intrusion of granite magma will heat its so-called country rock envelope. These rocks do not have a layered or banded appearance. Examples of non-foliated rocks are marble, quartzite etc.

Picture showing the formation of foliation

Examples of Metamorphic Rocks

1. Gneiss
2. Slate
3. Phyllite
4. Schist
5. Hornfels
6. Amphibolite
7. Marble
8. Quartzite
9. Novaculite
10. Lapis Lazuli
11. Soapstone

Uses of rocks

1. Rock masses on the mountains serves as tourist centers for humans. For example, the Zuma rock in Abuja, Ayers Rocks (Uluru) in Australia Horseshoe Bend, Arizona in USA and Aphrodite Rocks, Cyprus etc.
2. Rocks form the main source mineral deposits such as Gold, Diamond, Copper and Petroleum for commercial purposes.
3. Rocks provide the essential raw materials needed for both light and heavy industries for example, limestone derived from sedimentary rock is used for the manufacture of cement, whereas marble, derived from metamorphic rock is used for manufacturing glasses.
4. Rocks could be used for domestic purposes such as grinding stone for grinding nuts, seeds and herbs.
5. Some beautiful rocks like marble can be polished as ornaments for decorating our houses offices, churched, mosques and tombstones.
6. Rocks such as coal and limestone serve as sources of power or fuel for our domestic and industrial uses.
7. From rocks, we can extract minerals such as kyanite and muscovite which can be used in ceramics or for heat shielding.
8. Rocks sometimes contain rare gemstones that we can extract, cut, and polish so we can exchange for money.
9. Rocks provide us with mineral salt (common salt) which serves as a source of food nutrient and seasoning.
10. Rocks are of economic importance because they aid construction work. For example, rocks like granite and sandstone are used for roads, bridge and building construction.
11. Rock like Pumice which is an example of igneous rock can be used for Bath Scrub
12. Rocks are sometimes used to reveal to us about past climate, past environments, past continents and tectonic events that once took place, and about the formation of our planet and solar system.
13. Igneous rocks such as granite is very high in silica, potassium, sodium, and quartz, it is commonly used for architectural purposes, ornamental stone and monuments.