ROCKS AND MINERALS
LEARNING OUTCOMES
By the end of this chapter, you be able to;
- Explain how igneous, sedimentary and metamorphic rocks are formed, that they are made of minerals and that some minerals are useful.
- Identify the physical properties of rocks and distinguish between the different types of rocks
- Explain the process of weathering and how weathered rock particles from the basis of soil
INTRODUCTION
The earth we live in is made up of rocks.
- We use the rocks for different purposes such as construction of houses and roads
- Rocks are naturally occurring solid aggregates of minerals, mineraloids, or organic material.
- They form the Earth's outer solid layer, the lithosphere, and are classified into three main types based on their formation processes
- Rocks are composed of various components that determine their characteristics and classification.
Components of Rocks
Minerals:
These are naturally occurring, inorganic solids with a specific chemical composition and crystalline structure. Common minerals found in rocks include quartz, feldspar, mica, calcite, and olivine.
Mineraloids: These are naturally occurring substances that do not have a crystalline structure, distinguishing them from true minerals. Examples include opal and volcanic glass (obsidian).
Organic
Material: In some sedimentary rocks, organic materials such as plant debris, shells, and other biological remnants can be significant components. Coal, for example, is a sedimentary rock composed primarily of plant material.
Matrix: This refers to the finer-grained material in which larger grains, crystals, or clasts are embedded. In sedimentary rocks, the matrix can consist of finer particles like clay or silt.
Cement: In sedimentary rocks, cement is the mineral matter that binds the grains together. Common cementing agents include silica, calcite, and iron oxides.
Clasts or Grains: These are the fragments of pre-existing rocks or minerals that are incorporated into a sedimentary rock. Clasts can range in size from clay particles to large boulders.
TYPES OF ROCKS
Rocks are classified into three main types based on their formation processes: igneous, sedimentary, and metamorphic.
a) IGNEOUS ROCKS
Igneous rocks form from the cooling and solidification of molten rock material called magma (beneath the Earth's surface) or lava (at the surface).
These are of 2 types;
Intrusive (Plutonic) Igneous Rocks
These form from magma that cools slowly beneath the Earth's surface, allowing large crystals to develop.
These rocks cool deep underground, resulting in large, visible mineral crystals.
Examples of intrusive (Plutonic) igneous rocks
Granite: A coarse-grained, light-colored rock rich in quartz and feldspar. Commonly used in building materials and kitchen countertops.
Gabbro: A coarse-grained, dark-colored rock primarily made of calcium-rich plagioclase and pyroxene. It makes up much of the deep oceanic crust.
Diorite: A coarse-grained, speckled rock with a mix of light and dark minerals, mostly feldspar and hornblende.
Pegmatite: An intrusive rock with exceptionally large crystals (often several centimeters to meters in length). Often mined for rare minerals and gemstones.
Peridotite: A dense, coarse-grained rock primarily composed of the mineral olivine. It makes up most of the Earth's upper mantle.
Extrusive (Volcanic) Igneous Rocks
These rocks cool quickly on the Earth's surface or in the air, resulting in tiny crystals or a glassy texture.
Examples of Extrusive (Volcanic) Igneous Rocks
Basalt: A fine-grained, dark-colored rock that makes up the majority of the ocean floor and volcanic islands (like Hawaii).
Obsidian: A smooth, dark, natural volcanic glass that forms when lava cools so rapidly that no crystals can grow.
Pumice: A light-colored, highly porous rock filled with gas bubbles. It is so lightweight that it can float on water.
Rhyolite: A light-colored, fine-grained rock with a high silica content, often pink or gray in color.
Scoria: A dark-colored, heavy volcanic rock riddled with significant cavities (vesicles) created by gas bubbles trapped in cooling lava.
Characteristics of igneous rocks
Texture:
Can be coarse-grained (phaneritic), fine-grained (aphanitic), glassy, or porphyritic (large crystals in a fine matrix).
Composition: Typically composed of silicate minerals such as quartz, feldspar, mica, pyroxene, and olivine.
Color: Varies widely; can be light (felsic), intermediate, or dark (mafic) based on mineral content.
Formation: From the cooling and solidification of magma or lava.
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