G2: Plate Tectonics

🌍 The Structure of the Earth

The Earth is made up of several layers, each with different properties:

📋 Plate Tectonic Theory

Historical Development

• 1912 — Alfred Wegener proposed the theory of continental drift, suggesting that continents were once joined as a supercontinent called Pangaea and had slowly moved apart
• Wegener's evidence included: matching fossil patterns across continents (e.g. Mesosaurus in South America and Africa), matching rock formations, matching coastlines (like the jigsaw fit of South America and Africa), and evidence of past climates (e.g. glacial deposits in now-tropical areas)
• Wegener's theory was rejected because he could not explain how the continents moved
• 1960s — Seafloor spreading was discovered, providing the mechanism: new oceanic crust is formed at mid-ocean ridges and destroyed at subduction zones
• This led to the modern theory of plate tectonics, which explained the mechanism Wegener could not

🔀 Convection Currents

Additional forces that drive plate movement:

• Slab pull — at destructive margins, the dense oceanic plate sinks into the mantle under gravity, pulling the rest of the plate with it (this is considered the main driving force)
• Ridge push — at constructive margins, newly formed oceanic crust is elevated, causing it to slide downhill under gravity away from the ridge

📐 Types of Plate Margins

1. Constructive (Divergent) Margins

• Plates move apart driven by convection currents
• Magma rises through the gap and cools to form new oceanic crust
• Creates mid-ocean ridges (underwater mountain ranges)
• Volcanoes form along the ridge — mostly gentle, effusive eruptions
• Shallow-focus earthquakes occur as plates fracture and move
• Example: The Mid-Atlantic Ridge, where the North American Plate and Eurasian Plate are moving apart at about 2.5 cm per year. Iceland sits on this ridge and is growing wider each year.

2. Destructive (Convergent) Margins

There are two types of destructive margin:

3. Conservative (Transform) Margins

• Plates move sideways past each other in opposite directions (or the same direction at different speeds)
• No crust is created or destroyed
• No volcanoes form because there is no subduction and no magma rising
• Powerful earthquakes occur when built-up friction is released
• Example: The San Andreas Fault in California, where the Pacific Plate moves north-west past the North American Plate. The 1906 San Francisco earthquake (magnitude 7.9) occurred along this fault.

🗺️ Global Distribution of Tectonic Hazards

Earthquakes and volcanoes are not randomly distributed — they are concentrated along plate boundaries:

• Constructive margins: Mid-Atlantic Ridge (divergent boundary creating Iceland's volcanoes and earthquakes), East African Rift Valley
• Destructive margins: Western coast of South America (Nazca subducting under South American Plate — Andes volcanoes), western Pacific (subduction zones creating island arcs like Japan and the Philippines)
• Conservative margins: San Andreas Fault (California), North Anatolian Fault (Turkey)

📊 Major Tectonic Plates

❓ Practice Questions

✅ Answers

1. The Earth has four main layers: the crust (5–70 km thick, solid, oceanic or continental), the mantle (2,900 km thick, solid but can flow slowly, contains convection currents), the outer core (2,200 km thick, liquid iron and nickel), and the inner core (1,220 km radius, solid due to extreme pressure, 5,500°C). The crust and upper mantle form the rigid lithosphere, which sits on the semi-molten asthenosphere.
2. Heat from the Earth's core causes the semi-molten rock in the mantle to rise. As it rises towards the crust, it cools, becomes denser and slowly sinks back down. This continuous cycle of rising and sinking creates convection currents. These currents drag the overlying tectonic plates, causing them to move across the Earth's surface. Additional forces include slab pull (gravity pulling dense subducting plates down) and ridge push (gravity pushing new elevated crust away from mid-ocean ridges).
3. At a constructive margin, two plates move apart (diverge). As the plates separate, magma rises from the mantle through the gap to fill the space. The magma cools and solidifies to form new oceanic crust — a process called seafloor spreading. Shallow-focus earthquakes occur as the plates fracture, and gentle volcanic eruptions happen along the mid-ocean ridge. Named example: the Mid-Atlantic Ridge where the North American and Eurasian plates move apart at approximately 2.5 cm per year. Iceland sits on this ridge and experiences frequent volcanic activity (e.g. Eyjafjallajökull 2010).
4. At destructive margins, two plates move towards each other. The denser oceanic plate is subducted beneath the less dense continental plate into the mantle, where it melts. The molten rock (magma) rises through the overriding plate to form volcanic eruptions at the surface. The friction and pressure of subduction and the melting process both cause earthquakes of varying depths (shallow to deep focus). At conservative margins, two plates slide past each other horizontally — no crust is destroyed (no subduction) and no magma is created, so there is no volcanic activity. However, the plates grind against each other, building up friction. When this friction is suddenly released, it causes powerful earthquakes (shallow to intermediate focus). Example: San Andreas Fault where the Pacific Plate slides past the North American Plate.
5. Wegener's evidence included: (1) Fossil evidence — the Mesosaurus fossil found in both South America and Africa, which could not have swum the Atlantic; (2) Geological evidence — matching rock sequences across the Atlantic, e.g. the Appalachian Mountains matching Scottish/Norwegian mountains; (3) Jigsaw fit — the coastlines of South America and Africa fit together; (4) Paleoclimatic evidence — glacial deposits found in now-tropical regions like India and Africa, suggesting these areas were once near the South Pole. His theory was initially rejected because he could not explain the mechanism of how continents moved.
6. The Pacific Ring of Fire experiences many tectonic hazards because it surrounds the Pacific Plate, which is bordered by multiple destructive and conservative margins. The Pacific Plate is being subducted beneath several surrounding plates (Eurasian, North American, Philippine, Australian), creating numerous subduction zones that generate both explosive volcanoes and powerful earthquakes. The conservative margin with the North American Plate (San Andreas Fault) also produces earthquakes. This concentration of plate boundaries means that approximately 75% of the world's active volcanoes and 90% of earthquakes occur within this zone.

🎯 Exam Tips

• Always use named examples of specific plate boundaries and countries — examiners reward specific knowledge
• When describing a plate margin, mention the direction of plate movement, what happens to the crust, and the hazards/landforms produced
• Remember that at conservative margins there are earthquakes but NO volcanoes — this is a common exam question
• Use the terms oceanic and continental correctly — oceanic crust is denser and always subducts
• Draw annotated diagrams in the exam if asked to explain plate margins — they can earn full marks
• Know the difference between continental drift (Wegener's incomplete theory) and plate tectonics (the modern theory with a mechanism)

📝 Exam Technique

⚠️ Common Errors

✍️ Model Answer

📊 AO Deep Dive

📝 Exam Questions by Topic

• AQA Past Papers - Topic Questions -- AQA
• Edexcel Past Papers - Topic Questions -- Edexcel
• OCR Past Papers - Topic Questions -- OCR
• Eduqas Past Papers - Topic Questions -- Eduqas
• CCEA Past Papers - Topic Questions -- CCEA

🎬 Video Resources

• BBC Bitesize
• Geography Online
• Pick Up A Geography Grade