Lesson Note on the Process of Rock Formation For SS1

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TOPIC:  ROCK FORMATION:

CONTENT:

1. Meaning/process of rock formation
2. Types of rock

Sub topic 1: Meaning /process of rock formation:

Rock is defined as any minerals materials of the earth. Rock can also be defined as the aggregates of minerals made up of hard and soft materials like stone, sands etc. Rock constitutes the significant of earth crust. The earth crust consists of rocks which is the combination of different mineral elements such as silica which contain silicon and oxygen. Rocks are aggregates (mixtures) of minerals.

ROCK FORMATION (PROCESS OF ROCK FORMATION)

Rocks are formed as a result of volcanic eruptions from earth’s crust. During the volcanic eruption, the molten material or magma may remain within or escape to the surface. The magma or lava subsequently cools and solidifies to form glassy crystalline textured mass. In Nigeria rocks are found in Benue, Bauchi, Zaria, Plateau,

llorin, Ondo , Ado-Ekiti , Abakiliki and Ogoja in Cross River State , etc.

Some of the magma may cool before reaching the surface of the earth thus forming plutonic rocks (intrusive rocks) while some may reach the surface to form volcanic rocks (extrusive rocks)

Examples of plutonic (intrusive) rocks are granite, diorite, gabbro, etc.

Examples of volcanic (extrusive) rock are basalt, andesite, obsidian, pumice, rhyolite, scoria and tuff. Rapid cooling gives rise to fine grained rocks, while slow cooling gives rise to larger-grained rocks.

Rock contains primary and secondary minerals. The Primary Rock Minerals that exist in the natural form consist of quartz, biotite, olivine, magnesium, iron, and aluminium, sulphate and carbonate of iron, magnesium and calcium, clay minerals such as the silicate and hydrous oxide.

Secondary Rock Minerals are:

(a) Oxides of metal e.g. sodium oxide (Na₂O).

(b) Mortmorilonite.

Evaluation:

1. Define rock or what is rock? (b) explain the following with examples (i) plutonic rock (ii) extrusive rock
2. Enumerate the primary and secondary minerals found in rocks.

Sub-topic 2: TYPES OF ROCK.

Rocks are grouped according to their mode of formation, appearance and minerals composition. Based on this analysis, there are three types of rock. They are as follows:

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

Igneous rock:  This is formed as a result of high temperature and pressure beneath the earth’s crust which causes the liquid material (molten magma) below the earth surface to solidify or erupt in the form of a volcano. There are two types of igneous rock;

• Intrusive or plutonic
• Extrusive or volcanic

Examples of igneous rock rocks include: granite, diorite, gabbro, (they contain minerals like quart, feldspar, and mica) –plutonic,

While basalt and andesite are extrusive.

CHARACTERISTICS OF IGNEOUS ROCK:

1. They are coarsely crystalline in nature.
2. They are glassy in appearance.
3. They are usually hard and impervious.
4. They are resistant to erosion and other elements of climate
5. They do not contain fossils.
6. Their colours vary from pink, green, grey, or black.
7. They do not form layers i.e. they are non-stratified rocks

SEDIMENTARY ROCK:

They are formed when weathered particles are carried from their place of origin and deposited in another area as solid particles or when the pre-existing rocks are broken down by geological denudation agents. The materials carried are usually laid down in strata/sediments/ layers. They are usually cemented together by organic matter, cemented agents or pressure.

Examples of sedimentary rocks are limestone, sandstone, coal, dolomite, clay, shale, mudstone, peat, lignite, and gypsum.

Types of sedimentary rock

1. Mechanically formed sedimentary rocks
2. Organically sedimentary rocks
3. Chemically formed sedimentary rocks.

Mechanically formed sedimentary rocks are known as secondary rocks. They are formed as a result of the accumulation and cementation of weathered materials over a period of time.e.gs are dolomite, conglomerates, sandstone, limestone and quartzites.

Organically formed sedimentary rocks are formed as a result of deposition and hardening shell sand skeleton of aquatic organisms such as fish, oyster shells and snails e.gs are coal, peat, graphite and lignite. The remains of plants and animals found in sedimentary rocks are known as fossils.

Chemically formed sedimentary rocks are as a result of the chemical reaction of minerals in the rocks, precipitates are formed and deposited in layers over time .e.g sodium chloride, gypsum and nitrate.

Characteristics of sedimentary rocks :

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