GEO • SHS 1Term 2WAEC / WASSCE SyllabusVIP Lesson
Plate Tectonics, Vulcanicity & Earthquakes
Internal earth processes, plate boundaries (convergent, divergent, transform), intrusive and extrusive volcanic landforms, earthquakes, and seismic fault zones in Ghana.
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Real-World Context & Relevance in Ghana
Greater Accra experiences recurrent earth tremors due to the active Coastal Boundary Fault line running beneath Weija and McCarthy Hill.
Key Syllabus Learning Objectives
- 1Examine the mechanism of plate tectonics and distinguish between the three types of plate margins
- 2Differentiate clearly between intrusive volcanic features (batholiths, dykes, sills) and extrusive cones
- 3Identify earthquake epicenter versus focus and evaluate earthquake risk along the Weija fault zone in Ghana
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Comprehensive Syllabus Study Notes
• Plate Tectonics Theory:
- The lithosphere (crust and upper mantle) is broken into major and minor tectonic plates floating on the semi-molten asthenosphere.
- Plate Boundaries:
1. Divergent (Constructive): Plates move apart; magma rises to form mid-ocean ridges and rift valleys (e.g. East African Rift Valley).
2. Convergent (Destructive): Plates collide; denser oceanic plate subducts under continental plate forming ocean trenches, fold mountains (e.g. Himalayas), and volcanic arcs.
3. Transform (Conservative): Plates slide horizontally past one another, triggering severe earthquakes (e.g. San Andreas Fault).
• Vulcanicity:
- Intrusive Landforms (magma cools below earth's surface): Batholiths (huge plutonic granite mass), Sills (horizontal sheets concordant to bedding planes), Dykes (vertical/steep sheets discordant to rock strata), Laccoliths (dome-shaped intrusions).
- Extrusive Landforms (magma erupts onto surface as lava): Composite cones (stratovolcanoes like Mt. Cameroon), Shield volcanoes, Calderas, Geysers and Hot springs.
• Earthquakes & Seismic Activity:
- Origin: Sudden release of stress along geological faults. Focus (hypocentre) is the subterranean point of origin; Epicentre is the point directly above on the Earth's surface.
- Measurement: Richter Scale (magnitude of energy released) and Mercalli Scale (observed physical damage).
- Seismic Zones in Ghana: The Coastal Boundary Fault running through Weija, McCarthy Hill, Nyanyano, and the Akwapim-Togo ranges experiences frequent seismic tremors.
- Plate Tectonics Theory:
- The lithosphere (crust and upper mantle) is broken into major and minor tectonic plates floating on the semi-molten asthenosphere.
- Plate Boundaries:
- Vulcanicity:
- Intrusive Landforms (magma cools below earth's surface): Batholiths (huge plutonic granite mass), Sills (horizontal sheets concordant to bedding planes), Dykes (vertical/steep sheets discordant to rock strata), Laccoliths (dome-shaped intrusions).
- Extrusive Landforms (magma erupts onto surface as lava): Composite cones (stratovolcanoes like Mt. Cameroon), Shield volcanoes, Calderas, Geysers and Hot springs.
- Earthquakes & Seismic Activity:
- Origin: Sudden release of stress along geological faults. Focus (hypocentre) is the subterranean point of origin; Epicentre is the point directly above on the Earth's surface.
- Measurement: Richter Scale (magnitude of energy released) and Mercalli Scale (observed physical damage).
- Seismic Zones in Ghana: The Coastal Boundary Fault running through Weija, McCarthy Hill, Nyanyano, and the Akwapim-Togo ranges experiences frequent seismic tremors.
Practical Example:
Greater Accra experiences recurrent earth tremors due to the active Coastal Boundary Fault line running beneath Weija and McCarthy Hill.
Core Takeaway:
A sill is CONCORDANT (parallel to existing rock bedding planes), while a dyke is DISCORDANT (cuts vertically across existing rock layers).
WAEC / WASSCE Examiner Tips
- A sill is CONCORDANT (parallel to existing rock bedding planes), while a dyke is DISCORDANT (cuts vertically across existing rock layers).
Common Candidate Pitfalls & Errors to Avoid
- Using the S-wave speed where the P-wave speed belongs, which puts the epicentre hundreds of kilometres away.
- Naming the epicentre as the point where the rock first breaks, when that is the focus or hypocentre.
- Reporting a magnitude figure as though it were an intensity, and forgetting that intensity is what people and buildings suffer.
- Arguing that earthquakes occur only on plate boundaries, although the 1939 shaking in Ghana happened far from any margin.
- Saying S-waves prove the inner core is liquid, when it is the outer core that stops them.
Topic Mastery Checklist
- Examine the mechanism of plate tectonics and distinguish between the three types of plate margins
- Differentiate clearly between intrusive volcanic features (batholiths, dykes, sills) and extrusive cones
- Identify earthquake epicenter versus focus and evaluate earthquake risk along the Weija fault zone in Ghana
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