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Unit 6 Rocks.pdf

  1. UNIT 6: ROCKS
  2. LEARNING OBJECTIVES In this unit, the will should be able to: • Define the term rocks. • State the types of rocks. •State the characteristics of rocks. • Outline the importance of rocks.
  3. DEFINITION OF ROCKS A rock is a naturally occurring solid made up of two or more minerals. Rocks form the solid part of the earth’s crust. A mineral: is a natural inorganic substance possessing a definite chemical composition. Minerals are found below the earth’s surface. Rocks are studied under the scientific study called Petrology. Quartz - Mineral Feldspar Biotite
  4. TYPES OF ROCKS Rocks are classified based on three main characteristics: a) Chemical composition: mineral composition b) Physical properties such as permeability, texture, and the size of mineral particles c) Their mode of formation Therefore, there are three main types of rocks namely; 1. Igneous rocks, 2. Sedimentary rocks, and 3. Metamorphic rocks.
  5. IGNEOUS ROCKS Igneous rocks are formed when magma cools and solidifies inside or on the earth’s surface. Igneous rocks have four classes according to the chemical composition; acidic, intermediate, basic, and ultrabasic rocks. Type of rock Presence of silica (%) Presence of basic oxides (%) Acidic Less than 65 35 intermediate 55-65 35-45 Basic 45-55 45-65 Ultrabasic Less than 45 Greater than 55 Table showing chemical composition of igneous rocks
  6. CHARACTERISTICS OF IGNEOUS ROCKS ✓They are formed by the cooling of magma which solidifies into or onto the earth’s surface ✓ They contain crystals after cooling ✓ They do not contain strata (layers) ✓ They do not have fossils (plant and animal remains).
  7. TYPES IGNEOUS ROCKS There are two types of igneous rocks. They are intrusive and extrusive igneous rocks. a) Intrusive igneous rocks: these are formed when magma cools and solidifies inside the earth’s surface. They are also known as plutonic rocks. Examples: Diorite, dolerite, granite, gabbro, peridotite, diabase, porphyry, and lamprophyre. Notice: rocks that cool and solidify near the earth’s surface are known as hypabyssal rocks. They include, porphyrite, diabase, lamprophyre, and granophyre. 1/11/2023 7
  8. b) Extrusive igneous rocks: these are formed when magma cools and solidifies on the earth’s surface. They are also known as Volcanic rocks. There are two types of extrusive igneous rocks formed by lava flows and volcanic ejecta. ❖Extrusive igneous rocks formed by lava flows: these are formed lava flows a long distance on the earth’s surface before it cools and solidifies to form extrusive igneous rocks. Example are basalt and obsidian ❖Extrusive igneous rocks formed by volcanic ejecta: these are formed when solid ash and semi-liquid materials ejected out of the earth’s surface cool and solidify as they fall back onto the earth’s surface. Examples are pumice and tuff rocks. 1/11/2023 9
  9. SEDIMENTARY ROCKS Sedimentary rocks are formed by erosion, deposition, and compaction of weathered or eroded materials of former rock masses at the earth’s surface and within water bodies. They are formed through the process of sedimentation. 1/11/2023 10 Sedimentation: this is the process that causes mineral and other organic particles to settle and accumulate. Sedimentation of Rocks Sedimentary rocks
  10. Sedimentary rocks
  11. CHARACTERISTICS OF SEDIMENTARY ROCKS Characteristics of sedimentary rocks are the following: ❖ They contain both organic (plant and animal) and inorganic deposits ❖ They are laid down in strata (layers) ❖ They contain fossils (plant and animal remains) ❖ They are non crystalline because they do not form under heat. They are laid down in strata Skeleton of the bird-like specimen (Aurornis xui) found in Yizhou Fossil & Geology Park, China
  12. HOW SEDIMENTARY ROCKS FORM There are three ways through which sedimentary rocks are formed: 1) The pre-existing rocks are weathered and eroded and broken into small crystals to form sediments (weathering and erosion) 2) The weathered and eroded particles are transported and deposited in layers by water, wind, and moving ice. 3) Thereafter, the sediments are compacted to become hard rocks by the influence of pressure from the overlying sediments and other materials.
  13. TYPES OF SEDIMENTARY ROCKS a) Mechanically (Physically) formed sedimentary rocks: these rocks are formed when sediments are transported and deposited either on land or sea by the agents of erosion, then after they are consolidated and hardened to form hard rocks by the pressure from overlying layers. Examples: i. Sandstone and gritstone: composed of rock particles mainly sand ii. Conglomerate, breccia and boulder clay; composed of rock particles larger than 2mm in diameter, iii. clay stone: composed of very small rock particles. 1/11/2023 14
  14. b) Chemically formed sedimentary rocks: these are formed when salt and other soluble minerals are precipitated, evaporate and accumulate in layers by the influence of rain, temperature, and pressure. Classification of chemically formed sedimentary rocks 1/11/2023 16 Classification Examples of rocks Composition Carbonates Travertine, trona, dolomite Calcium carbonate compounds Sulphates Gypsum Calcium sylphate Chlorides Rock salt Sodium chloride Silicates Flint Silica Iron stones Limonite, hematite Iron oxides
  15. c) Organically formed sedimentary rocks: these rocks are formed when remains of plants and animals accumulate in layers over time. Then after they are compacted by the pressure from overlying materials hence to form hard rocks. Classification of organically formed sedimentary rocks 1/11/2023 Classification Examples of rocks Composition Calcareous Limestone, chalk, coral reef Skeletons of tiny marine creatures Ferruginous Iron stone Hydrated iron oxide Siliceous Diatomite Remains of diatoms rich in silica Carbonaceous Coral (lignite coal, bituminous coal, brown coal, anthracite coal) Remains of plants
  16. METAMORPHIC ROCKS These rocks are formed when pre-existing rocks are subjected to chemical or physical changes under the influence of heat and pressure. Types of metamorphic rocks 1/11/2023 Original rock Metamorphic rock Classification Type of rock Igneous Granite Gneiss Augite Hornblende Sedimentary Shale Schist Clay Slate Sandstone Quartzite Limestone Marble Mudstone Slate Coral Graphite
  17. CHARACTERISTICS OF METAMORPHIC ROCKS Characteristics of metamorphic rocks are the following: ❖ They are formed through a metamorphism process. ❖They are more resistant to erosion ❖ They are formed by chemical or physical changes from the existing rocks ❖ They are crystalline and often have a squashed (foliated or banded) texture.
  18. FORMATION OF METAMORPHIC ROCKS Metamorphic rocks are formed in two ways: a) Contact metamorphism: this is when igneous rocks come into contact with other rocks and change their chemical and physical structure to form metamorphic rocks. a) Dislocation (region metamorphism): this is when a large region is subjected to different temperature conditions by the advancing igneous rock. Instead dislocation occurs along a line of weakness or fault follows a volcanic pipe.
  19. CHARACTERISTICS OF ROCKS Rocks are characterized by the following features: ✓Rock color: the color of rocks gives a hint about its mineral composition. For example; gold is yellow and copper is reddish brown ✓Rock texture and grain size: this determines the size of the individual grain that form the rocks. ✓Rock structure: this is the arrangement of the minerals that form the rocks. It determines the shape and the mode of formation of the rocks. ✓Rock composition: this is the composition of minerals that form the rock. It determines the origin of the rocks ✓Rock hardness: this is the level of rock resistance to being broken or scratched. It determines how rocks may have been formed. ✓Rock density: this refers to the weight of rocks. It determines how rocks are heavy. ✓Rock luster: this refers to the rock brightness. It determines how rocks reflect the light. ✓Rock cleavage: this refers to how rocks break. It determines the way by which rocks break ✓Rock taste and odor: this refers to rock porosity. It determines how rocks allow water to pass or not pass through them ✓Rock solubility: this determines the level at which rocks dissolve in water. 1/11/2023 23
  20. IMPORTANCE OF ROCKS ❖Some rocks provide raw materials for industries leading to industrialization ❖Some rocks are extracted to provide building and constructional materials for example sandstone, limestone etc. ❖Some rocks contain valuable minerals such as gold, diamond, copper that are exported to earn foreign exchange ❖Some rocks are extracted and used in art in the making of carvings that are sold to earn income ❖Some rocks such as coal are source of energy for industrial and domestic uses ❖Some rocks such as rock salt are used as a healthy substitute of common salt ❖Some rocks when weathered provide fertile soils good for crop growing ❖Some rocks such as phosphates, calcium are used as fertilizers ❖Some rock formation like granite tors and coral reefs are tourist attraction sites ❖Some rocks store underground water which can be extracted and used for domestic and industrial and agriculture Therefore, rocks are used in promoting different economic sectors that boost the country’s development. However they have some disadvantages. 1/11/2023 25
  21. DISADVANTAGES OF ROCKS Some rocks have the following disadvantages on human activities: ❖Some rocks are obstacles and may hinder cultivation activities ❖They may also hinder road construction activities ❖Some rocks are very porous provide infertile soils that may reduce cultivation yield ❖Some rocks may hinder mining activities because they require much expenses for their extraction ❖Some rock formation results in rock mountainous may hinder transport and communication