Rock formation and Types: Igneous, Sedimentary and metamorphic Rocks

CONTENT

• Meaning of Rock
• Types of rock

ROCK

A rock is any mineral material of the earth. It may be a combination of different mineral elements such as silica (silica contain silicon and oxygen).

TYPES OF ROCKS

Rocks can be classified into three major groups based on how they are formed and their appearances. These include

1.     Igneous rock

2.     Sedimentary rock

3.     Metamorphic rock

MODE OF FORMATION OF IGNEOUS ROCK

They are formed as a result of cooling and solidification of molten magma erupted from the earth crust. Magma occurs from high temperature and pressure underneath the earth and forces itself towards the earth surface through the cracks. As the magma approaches the earth surface, it cools as a result of lower temperature of the earth surface and solidify to form igneous rocks.

TYPES OF IGNEOUS ROCK

1. PLUTONIC [OR INTRUSIVE] IGNEOUS ROCK:These are rocks formed when molten magma solidifies slowly before it gets to the surface of the earth to form large crystals. Prolonged erosion makes the rock to be exposed later to the surface. Examples include granite, gabbro, and diorite.
2. VOLCANIC [EXTRUSIVE] IGNEOUS ROCK: These are formed when the molten magma cools and solidifies rapidly when it gets to the surface to form crystals. Examples include basalt, pumice.

CHARACTERISTICS OF IGNEOUS ROCKS

1. They have glassy appearance
2. They contain crystals i.e they are crystalline in nature.
3. They do not occur in layers.
4. They do not contain fossils.
5. They are hard and impervious
6. They are resistant to erosion

EVALUATION

1. Describe the mode of formation of igneous rock
2. Outline four characteristics of igneous rocks

MODE OF FORMATION OF SEDIMENTARY ROCKS

They are rocks formed from deposits of organic materials and existing weathering rocks which accumulate and consolidate over time. They are cemented together after a long time to form sedimentary rocks. The sediments are deposited by natural agents such as water, wind and ice and settle down in layers one on top of another.

There are three types of sedimentary rocks based on the process of formation.

1. Mechanically formed sedimentary rocks:These are rocks formed from sediments of other rocks that are accumulated and cemented over a long period of time. Examples include sandstones, shale, clay, breccia, and conglomerate.
2. Organically formed sedimentary rocks: These are rocks formed from the remains of living organisms. When the rocks are formed from the remains of animals like coral or shellfish, they are called calcareous rocks e.g. lime stone and chalk. When the rocks are formed from vegetable matter such as swamps and forest are called carbonaceous rocks e.g. coal, peak, and lignite.
3. Chemically formed sedimentary rocks: These are rocks formed from precipitated chemically from rock solution. Examples include gypsum, potash, and dolomite.

CHARACTERISTICS OF SEDIMENTARY ROCKS

1. They occur in layers or strata.
2. They are not in crystal form i.e they are not crystalline in nature.
3. They contain fossils of plants and animals.
4. They are not resistant to erosion
5. They may be coarse, fine soft or hard.

EVALUATION

1. Describe the mode of formation of sedimentary rocks
2. State four characteristics of sedimentary rocks

MODE OF FORMATION OF METAMORPHIC ROCKS

These rocks result from igneous or sedimentary rocks due to combined effect of pressure and heat. The composition and texture of unstable parent rock minerals are changed under pressure and heat. Examples are marble, quartzite, gneiss, schist, graphite and slate.

• Slate is formed from clay (sedimentary)
• Marble is formed from limestone (sedimentary)
• Quartzite is formed from sandstone (sedimentary)
• Gneiss is formed from granite (igneous)
• Schist is formed from shale (sedimentary)
• Graphite is formed from coal (sedimentary)

CHARACTERISTIC OF METAMORPHIC ROCKS

1. Some may occur in layer or strata
   1. The rock may be hard or soft
   1. They are not in form of crystals
   1. They exist in different colours and texture
   1. They may contain fossils

EVALUATION

1. Describe the mode of formation of metamorphic rocks
2. Give four examples of metamorphic rocks and their parent materials

GENERAL EVALUATION

1. What is a rock?
   1. State three types of rock
   1. Describe the mode of formation of the two types of igneous rock
   1. State four characteristics of sedimentary rocks.
   1. Classify sedimentary rocks based on based on mode of formation and give two examples in each case

BOOK REVIEW

Essential Agricultural science by O. A. Iwena chapter 10, page 61 – 64

READING ASSIGNMENT

Answer question 6 Essential Agricultural science by O. A. Iwena chapter 10, page 64

WEEKEND ASSIGNMENT

1. Which of the following is not a sedimentary rock?  A. mica B. sandstone C. limestone

D. dolomite

• Granite is ——— rock A. igneous B. sedimentary C. metamorphic D. weathered
• Schist is formed from A. shale B. sandstone C. quartzite D. coal
• A metamorphic limestone is A. marble B. gneiss C. schist D. shale
• The type of rock formed from molten magma is known as A. sedimentary B. schist C. igneous D. sandstone 

THEORY

1. Write short note on the mode of formation and characteristics of A. granite B. limestone
2. State three characteristics for each of the three rock types.