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Geology · 200 Level
This is the complete lecture note for GEY 203 Introduction to Petrology, prepared for Geology students at 200 Level. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Course Code: GEY 203 Course title: Introduction to Petrology Units: 2 Units List of Series Series 1: Foundations of Igneous Petrology Series 2: Systematic Study of Metamorphic Rocks Series 3: Textures and Minerals in Metamorphism Series 4: Introduction to Sedimentary Petrology Series 5: Classification of Igneous, Metamorphic, and Sedimentary Rocks 1.1 Origin and Formation of Igneous Rocks - 1.1.1 Magma generation and sources - 1.1.2 Processes of crystallisation and solidification - 1.1.3 Geological settings of igneous activity 1.2 Occurrence and Geologic Setting of Igneous Rocks - 1.2.1 Extrusive and intrusive environments - 1.2.2 Distribution of igneous bodies in the crust - 1.2.3 Plate tectonic context of igneous rocks 1.3 Systematic Description of Igneous Rocks - 1.3.1 Textural features of igneous rocks - 1.3.2 Mineral composition of igneous rocks - 1.3.3 Field and laboratory identification methods 1.4 Classification of Igneous Rocks - 1.4.1 Basis of classification: texture and mineralogy - 1.4.2 Major categories: felsic, intermediate, mafic, ultramafic - 1.4.3 Common classification schemes (e.g., TAS diagram, modal classification)
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Introduction Igneous rocks form the very foundation of the Earth’s crust, shaping landscapes and providing vital clues about the planet’s internal processes. From the fiery eruptions of volcanoes to the slow cooling of magma deep underground, these rocks record the dynamic history of Earth’s geological activity. As learners of petrology, gaining a clear understanding of how igneous rocks originate, occur, and are classified will prepare you to explore the broader study of rocks and their role in Earth systems. The aim of this Series is to equip you with the knowledge and skills to explain the origin and formation of igneous rocks, describe their occurrence and geologic settings, and apply systematic methods for their description and classification. We shall commence this Series by examining the origin and formation of igneous rocks, focusing on magma generation, crystallization, and geological settings. Next, we will explore their occurrence and distribution within extrusive and intrusive environments, considering the role of plate tectonics. Following that, we will move on to the systematic description of igneous rocks, paying attention to their textures, mineral composition, and identification methods. Finally, we will wrap up by studying the classification of igneous rocks, highlighting the basis of classification and the major categories used in petrology. Series Learning Outcomes Upon completing this Series, you should be able to: - SLO 1.1 define the processes involved in the origin and formation of igneous rocks, - SLO 1.2 explain the occurrence and geologic settings of igneous rocks in both extrusive and intrusive environments, - SLO 1.3 describe the textural features and mineral composition of igneous rocks using systematic methods, and - SLO 1.4 classify igneous rocks accurately based on texture, mineralogy, and established classification schemes. 1.1 Origin and Formation of Igneous Rocks 1.1.1 Magma generation and sources The study of igneous rocks begins with magma, which is molten rock material found beneath the Earth’s surface. Magma is generated when parts of the mantle or crust melt due to changes in temperature, pressure, or the addition of volatiles such as water. The mantle is the thick layer of solid rock beneath the crust, and it is the primary source of most magmas.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Magma generation occurs in specific tectonic settings, including mid-ocean ridges, subduction zones, and hotspots. Fill in the blank: Magma is produced when parts of the _____ or crust melt due to changes in physical conditions. Figure 1.1: Magma generation zones in the Earth’s interior 1.1.2 Processes of crystallization and solidification Once magma is generated, it begins to cool and undergoes crystallization, which is the process by which minerals form from the cooling melt. The rate of cooling determines the texture of the igneous rock. Slow cooling deep underground produces coarse-grained rocks such as granite, while rapid cooling at the surface produces fine-grained rocks such as basalt. Solidification refers to the complete transformation of magma into solid rock. Fill in the blank: Slow cooling of magma produces -grained igneous rocks, while rapid cooling produces fine-grained rocks.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.2: Crystallization and cooling of magma 1.1.3 Geological settings of igneous activity Igneous activity occurs in diverse geological settings. At mid-ocean ridges, magma rises to form new oceanic crust. In subduction zones, melting occurs as oceanic plates descend into the mantle, producing volcanic arcs. At hotspots, plumes of hot mantle material rise to the surface, creating isolated volcanic centers such as Hawaii. These settings explain the distribution and diversity of igneous rocks across the Earth. Fill in the blank: Volcanic arcs are typically formed in ______ zones where oceanic plates descend into the mantle.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Plate 1.1: Volcanic arc formed at a subduction zone Pilot questions 1.1 1. Which part of the Earth is the primary source of most magmas? 2. What is crystallization in the context of igneous rocks? 3. How does cooling rate affect the texture of igneous rocks? 4. Name two geological settings where magma generation occurs. 5. What type of volcanic feature is associated with hotspots? 1.2 Occurrence and Geologic Setting of Igneous Rocks 1.2.1 Extrusive and intrusive environments Igneous rocks occur in two main environments: extrusive and intrusive. Extrusive rocks are formed when magma erupts onto the Earth’s surface as lava and cools rapidly, producing fine-grained textures. Examples include basalt and rhyolite. Intrusive rocks, on the other hand, form when magma cools slowly beneath the surface, producing coarse-grained textures such as granite and gabbro. These environments determine not only the texture but also the overall appearance and distribution of igneous rocks.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Fill in the blank: Igneous rocks that cool slowly beneath the Earth’s surface are called ______ rocks. Figure 1.3: Comparison of extrusive and intrusive igneous settings 1.2.2 Distribution of igneous bodies in the crust Igneous bodies are distributed throughout the Earth’s crust in various forms. Plutons are large intrusive bodies that crystallize deep underground, while dykes and sills are smaller intrusions that cut across or lie parallel to existing rock layers. At the surface, volcanic flows and pyroclastic deposits represent extrusive igneous activity. The distribution of these bodies provides important information about past geological processes and helps geologists reconstruct the history of crustal development. Fill in the blank: A tabular igneous intrusion that cuts across existing rock layers is called a_______.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.4: Distribution of igneous bodies in the crust 1.2.3 Plate tectonic context of igneous rocks The occurrence of igneous rocks is closely linked to plate tectonics, which refers to the movement and interaction of large plates that make up the Earth’s lithosphere. At divergent boundaries, magma rises to form new crust at mid-ocean ridges. At convergent boundaries, subduction leads to melting and the formation of volcanic arcs. At hotspots, mantle plumes generate isolated volcanic centers. Understanding these tectonic contexts helps explain why igneous rocks are found in specific regions and why their composition varies. Fill in the blank: New oceanic crust is formed at _____ boundaries where plates move apart.
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