G2: Plate Tectonics Theory
The theory of plate tectonics, types of plate boundaries, and the evidence supporting continental drift and seafloor spreading.
Plate Tectonics Theory
The theory of plate tectonics, types of plate boundaries, and the evidence supporting continental drift and seafloor spreading.
Key Fact: Plate tectonics states the lithosphere is divided into large plates that move on the asthenosphere, driven by convection currents in the mantle
Key Fact: Divergent boundaries: plates move apart (e.g. Mid-Atlantic Ridge), creating new oceanic crust
Key Fact: Convergent boundaries: plates collide; oceanic-continental produces subduction zones and volcanoes; continental-continental produces fold mountains
Key Fact: Transform boundaries: plates slide past each other (e.g. San Andreas Fault), causing earthquakes
Key Fact: Wegener proposed continental drift in 1912, citing fossil, rock and coastline evidence, but the mechanism was unknown
Key Fact: Seafloor spreading was confirmed by paleomagnetic striping in oceanic crust and age dating of ocean floor rocks
📋 Key Vocabulary and Concepts
For Plate Tectonics Theory, you must know:
- Mid-ocean ridge: A submarine mountain range where new oceanic crust is formed by magma rising at a divergent plate boundary
- Subduction: The process by which denser oceanic crust sinks beneath lighter continental crust at a convergent boundary
- Paleomagnetism: The study of the record of Earth's magnetic field preserved in rocks; used as evidence for seafloor spreading
- Convection current: A circular current in the mantle caused by heat from the core; hot material rises, cool material sinks, driving plate movement
- Isostasy: The equilibrium between the Earth's crust and mantle; thicker, less dense crust floats higher
❓ Practice Questions
Q: What are the three types of plate boundary?
Q: What evidence did Wegener use to support continental drift?
Q: How does paleomagnetism support seafloor spreading?
Q: What happens at an oceanic-continental convergent boundary?
Q: Why was Wegener's theory initially rejected?
✅ Answers
- Divergent (constructive, plates move apart), convergent (destructive, plates collide), and transform (conservative, plates slide past each other).
- 1) Jigsaw fit of continents (e.g. South America and Africa). 2) Matching fossils on separated continents (e.g. Mesosaurus). 3) Matching rock types and mountain chains across oceans. 4) Evidence of past glaciation in now-tropical regions.
- As new oceanic crust forms at mid-ocean ridges, iron-rich minerals align with Earth's magnetic field. The field reverses periodically, creating symmetrical magnetic stripes on either side of the ridge, proving new crust forms at ridges and spreads outward.
- The denser oceanic plate subducts beneath the lighter continental plate. This produces a deep ocean trench, volcanic arc on the overriding plate, and earthquakes along the Benioff zone.
- He could not explain the mechanism driving continental drift. The forces he proposed were too weak. The theory was only accepted after seafloor spreading was discovered in the 1960s.
🎯 Exam Tips
- Three boundary types: divergent, convergent, transform
- Wegener's evidence is commonly tested, know at least 3 lines of evidence
- Paleomagnetic striping is key evidence for seafloor spreading, symmetrical about the ridge
- Subduction: denser oceanic ALWAYS sinks beneath lighter continental
📝 Exam Technique
GCSE Geology Exam Tips — Plate Tectonics Theory:
1. For Plate Tectonics Theory questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Plate Tectonics Theory to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Plate Tectonics Theory questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Plate Tectonics Theory to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ Continental drift and plate tectonics are the same theory ✓ Continental drift (Wegener, 1912) proposed continents moved through ocean crust; plate tectonics (1960s) explains that the lithosphere is divided into plates that move on the asthenosphere
✗ Mantle convection currents drag plates from above ✓ Convection currents push plates from below and also create ridge push and slab pull; slab pull is the dominant driving force
✗ Only oceanic crust can subduct ✓ While oceanic crust typically subducts due to higher density, continental crust can also be forced down, though it is more buoyant and resists subduction
✍️ Model Answer
Full-Mark Response
Describe the three types of plate boundary and explain the evidence for plate tectonics. [6 marks]
At divergent (constructive) boundaries, plates move apart. Magma rises to fill the gap, creating new oceanic crust at mid-ocean ridges (e.g. Mid-Atlantic Ridge). At convergent (destructive) boundaries, plates collide. Where oceanic meets continental crust, the denser oceanic plate subducts, forming a trench, volcanic arc and earthquakes (e.g. Andes). Where two continental plates collide, fold mountains form (e.g. Himalayas). At transform (conservative) boundaries, plates slide past each other without creating or destroying crust, causing earthquakes (e.g. San Andreas Fault). Evidence includes: 1) Continental jigsaw fit, South America and Africa match. 2) Matching fossils (Mesosaurus, Glossopteris) on now-separated continents. 3) Matching rock types and mountain chains across oceans. 4) Paleomagnetic striping, symmetrical magnetic reversal patterns either side of mid-ocean ridges prove seafloor spreading. 5) Ocean floor dating, rocks are youngest at ridges and get older with distance. 6) Earthquake and volcano distributions align with plate boundaries.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of plate tectonics theory, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of plate tectonics theory to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to plate tectonics theory, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
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