Hazardous Earth: The Restless Earth — Plate Tectonics

What you'll learn

This guide covers the fundamental plate tectonic theory that explains earthquakes, volcanoes and mountain formation. You'll understand Earth's internal structure, the evidence supporting continental drift and seafloor spreading, and the specific processes occurring at different plate boundaries. This topic forms the foundation for understanding tectonic hazards examined in IGCSE Geography.

Key terms and definitions

Plate tectonics — the theory that Earth's lithosphere is divided into large plates that move slowly over the asthenosphere, causing earthquakes, volcanoes and mountain building at plate boundaries.

Lithosphere — the rigid outer layer of Earth comprising the crust and upper mantle, broken into tectonic plates approximately 80-100 km thick.

Asthenosphere — the semi-molten, plastic layer of the upper mantle (100-700 km deep) on which tectonic plates move due to convection currents.

Subduction — the process where a denser oceanic plate sinks beneath another plate (oceanic or continental) at a convergent boundary, forming deep ocean trenches and volcanic arcs.

Convection currents — circular movements of heated mantle rock rising from the core, spreading sideways, cooling, and sinking back down, which drive plate movement.

Continental drift — the theory proposed by Alfred Wegener (1912) that continents were once joined as Pangaea and have since moved apart over millions of years.

Seafloor spreading — the process at mid-ocean ridges where magma rises to create new oceanic crust, pushing plates apart and causing ocean basins to widen.

Constructive (divergent) boundary — a plate boundary where two plates move apart, allowing magma to rise and create new crust, typically forming mid-ocean ridges or rift valleys.

Core concepts

Earth's structure and composition

Earth consists of distinct layers with different properties:

The core:

• Inner core: solid iron and nickel (5,000-6,000°C)
• Outer core: liquid iron and nickel (4,000-5,000°C)
• Generates Earth's magnetic field through liquid metal movement

The mantle:

• Comprises approximately 84% of Earth's volume
• Temperature range: 1,000-3,700°C
• Upper mantle includes the rigid top (part of lithosphere) and plastic asthenosphere below
• Lower mantle is solid but can flow very slowly over geological time

The crust:

• Oceanic crust: 5-10 km thick, denser (3.0 g/cm³), composed mainly of basalt, younger (less than 200 million years)
• Continental crust: 30-70 km thick, less dense (2.7 g/cm³), composed mainly of granite, older (up to 4 billion years)
• This density difference explains why oceanic crust subducts beneath continental crust

Evidence for plate tectonic theory

Continental drift evidence (Wegener, 1912):

• Jigsaw fit: South America and Africa coastlines match, especially at continental shelf edge
• Fossil evidence: Identical Mesosaurus (freshwater reptile) fossils found in Brazil and South Africa; Glossopteris (fern) fossils across South America, Africa, India, Antarctica and Australia
• Geological evidence: Mountain ranges align across continents (Appalachians in North America match Scottish Highlands); rock types and ages match across continents now separated by oceans
• Glacial deposits: Evidence of ancient glaciation in now-tropical regions (India, Africa, South America) suggests continents were once positioned differently

Seafloor spreading evidence (Hess, 1960s):

• Magnetic striping: Oceanic crust shows symmetrical patterns of normal and reversed magnetic polarity either side of mid-ocean ridges, recording Earth's magnetic field reversals
• Age of rocks: Oceanic crust becomes progressively older with distance from mid-ocean ridges; youngest rocks always at ridge centres
• Ocean sediment thickness: Sediment is thinnest near ridges and thickens away from them
• Heat flow measurements: Higher heat flow recorded at mid-ocean ridges where new crust forms

Mechanisms of plate movement

Three main processes drive tectonic plate motion:

Convection currents:

• Radioactive decay in the core generates intense heat
• Hot mantle material becomes less dense and rises toward the lithosphere
• At the base of lithosphere, currents spread horizontally, dragging plates
• Cooled material sinks back toward the core, creating circular movements
• Timescale: approximately 2-10 cm per year

Slab pull:

• The most powerful driving force
• Dense oceanic lithosphere sinks into the mantle at subduction zones
• Weight of descending slab pulls the rest of the plate behind it
• Particularly effective at older, colder, denser oceanic crust

Ridge push:

• Elevated mid-ocean ridges create a slope
• Newly formed lithosphere slides downslope away from the ridge
• Gravity assists this movement
• Weaker force than slab pull

Types of plate boundaries and associated features

Constructive (divergent) boundaries:

Plates move apart; new crust forms.

Oceanic example — Mid-Atlantic Ridge:

• Magma rises through the gap, solidifying to create new oceanic crust
• Forms underwater mountain ranges with central rift valley
• Shallow-focus earthquakes (tension/normal faulting)
• Gentle volcanic eruptions (basaltic lava, low viscosity)
• Iceland sits on the Mid-Atlantic Ridge, experiencing both volcanic and earthquake activity

Continental example — East African Rift Valley:

• Continental crust stretches and fractures
• Forms linear rift valleys with parallel faults
• Volcanic activity (Mount Kilimanjaro, Mount Kenya)
• Eventually may form a new ocean basin

Destructive (convergent) boundaries:

Plates move together; crust is destroyed or deformed.

Oceanic-continental (subduction):

• Denser oceanic plate subducts beneath less dense continental plate
• Forms deep ocean trench (Peru-Chile Trench reaches 8,000 m depth)
• Fold mountain range on continental plate (Andes Mountains)
• Violent explosive volcanic eruptions (andesitic/rhyolitic magma, high viscosity)
• Deep and shallow earthquakes along Benioff zone
• Example: Nazca Plate subducting beneath South American Plate

Oceanic-oceanic (subduction):

• Older, denser oceanic plate subducts beneath younger plate
• Forms deep ocean trench
• Volcanic island arc develops on overriding plate
• Violent volcanic eruptions and earthquakes
• Example: Pacific Plate subducting beneath Philippine Plate, forming Mariana Trench and volcanic islands

Continental-continental (collision):

• Neither plate subducts (similar densities)
• Crust crumples and thickens to form fold mountain ranges
• No volcanic activity (no subduction, no magma generation)
• Shallow-focus earthquakes
• Example: Indo-Australian Plate colliding with Eurasian Plate, forming Himalayas (still rising 5 mm/year)

Conservative (transform) boundaries:

Plates slide past each other horizontally; crust is neither created nor destroyed.

• No volcanic activity
• Frequent shallow-focus earthquakes
• Pressure builds as plates lock; sudden release causes earthquakes
• Example: San Andreas Fault, California (Pacific Plate moving northwest relative to North American Plate at 5 cm/year)
• Example: Alpine Fault, New Zealand

Distribution of earthquakes and volcanoes

Tectonic hazards occur in predictable patterns:

Ring of Fire:

• Encircles Pacific Ocean
• Contains 75% of world's active volcanoes
• Accounts for 90% of world's earthquakes
• Formed by subduction zones around Pacific Plate margins

Distribution patterns:

• Earthquakes: occur at all plate boundary types; also intraplate locations due to ancient fault lines or human activity
• Volcanoes: primarily at constructive and destructive boundaries; absent at conservative boundaries and continental-continental collision zones
• Mid-ocean ridges: continuous underwater volcanic and earthquake activity
• Hotspots: volcanic activity away from plate boundaries (Hawaiian Islands, Yellowstone) caused by mantle plumes

Worked examples

Example 1: Explain question (6 marks)

Question: Explain how convection currents in the mantle cause plates to move. (6 marks)

Mark scheme approach: Award 1 mark per valid point, up to 6 marks. Look for:

• Description of heat source
• Rising of heated material
• Lateral movement/spreading
• Cooling and sinking
• Circular/cyclical pattern
• Effect on plates/dragging mechanism

Model answer:

Radioactive decay in Earth's core generates intense heat (1), causing mantle material directly above to heat up and become less dense (1). This hotter, less dense material rises toward the base of the lithosphere (1). When rising currents reach the lithosphere, they spread horizontally/sideways (1), dragging tectonic plates along with them (1). As the mantle material moves away from the heat source, it cools, becomes denser and sinks back toward the core (1), completing the convection cell and maintaining the circular movement (1).

Example 2: Comparison question (4 marks)

Question: Compare the characteristics of earthquakes at constructive and destructive plate boundaries. (4 marks)

Model answer:

At constructive boundaries, earthquakes are shallow-focus only, typically less than 70 km deep (1), whereas at destructive boundaries earthquakes can be shallow, intermediate or deep-focus, occurring along the Benioff zone down to 700 km (1). Constructive boundary earthquakes are generally lower magnitude as tension causes normal faulting in brittle crust (1), whilst destructive boundary earthquakes can reach much higher magnitudes due to the immense pressure as one plate subducts beneath another (1).

Example 3: Using evidence (8 marks)

Question: 'Wegener's theory of continental drift was proven by the evidence he presented in 1912.' To what extent do you agree with this statement? (8 marks)

Mark scheme approach: Level 1 (1-4 marks): Basic statements about evidence. Level 2 (5-6 marks): Clear explanation of supporting evidence. Level 3 (7-8 marks): Detailed evaluation of evidence strength; considers limitations and later supporting evidence.

Model answer:

Wegener presented several lines of evidence that supported continental drift. The jigsaw fit of continents, particularly South America and Africa, suggested they were once joined, especially when matching the continental shelf edges rather than just coastlines. Fossil evidence was particularly compelling – identical Mesosaurus fossils in Brazil and South Africa could not have crossed the Atlantic Ocean as it was a freshwater reptile. Geological evidence showed that rock types, ages and mountain ranges aligned across now-separated continents, such as the Appalachians matching Scottish Highlands.

However, Wegener's evidence alone did not "prove" continental drift. The scientific community rejected his theory primarily because he could not explain the mechanism of how continents moved. He suggested they ploughed through oceanic crust, which was physically impossible. The theory was only widely accepted in the 1960s when seafloor spreading was discovered, providing the missing mechanism. Magnetic striping patterns either side of mid-ocean ridges, progressively older oceanic crust away from ridges, and understanding of mantle convection currents provided the proof Wegener's original evidence lacked. Therefore, whilst Wegener's evidence strongly suggested continental drift, it required later discoveries to prove the theory conclusively.

Common mistakes and how to avoid them

• Confusing lithosphere and asthenosphere: Remember the lithosphere is rigid (crust + upper mantle) whilst the asthenosphere is semi-molten and allows plate movement. Don't say "plates float on magma" – they move on the plastic asthenosphere.

• Mixing up plate boundary types with hazards: Conservative boundaries produce earthquakes but never volcanoes. Continental-continental collision zones produce earthquakes and mountains but no volcanoes (no subduction means no magma generation).

• Vague explanations of subduction: Always specify which plate subducts and why (oceanic is denser than continental; older oceanic is denser than younger oceanic). Don't just say "one plate goes under another."

• Incorrect convection current diagrams: Show rising currents beneath mid-ocean ridges and sinking currents at subduction zones. Arrows should form complete cells. Heat source is the core, not "magma."

• Confusing evidence types: Keep Wegener's original evidence (fossils, jigsaw fit, glacial deposits, geology) separate from later seafloor spreading evidence (magnetic striping, age of rocks, sediment thickness). Questions often ask you to distinguish these.

• Imprecise locations: Use specific named examples – "the Nazca Plate subducting beneath the South American Plate forming the Peru-Chile Trench and Andes Mountains" scores better than "a destructive boundary somewhere."

Exam technique for "Hazardous Earth: The Restless Earth — Plate Tectonics"

• Command word precision: "Describe" requires characteristics/features (what it looks like). "Explain" requires reasons/processes (why/how it happens). "Compare" requires both similarities and differences. For 6-8 mark questions, develop each point with detail and linkage rather than listing many superficial points.

• Annotated diagrams: For questions about plate boundaries or Earth's structure, clear labelled diagrams with annotations explaining processes can earn full marks. Ensure labels identify features and annotations explain processes or give data (e.g., "oceanic crust 5-10 km thick, density 3.0 g/cm³").

• Using case studies effectively: Whilst this topic focuses on processes rather than case studies, naming specific examples (San Andreas Fault, Mid-Atlantic Ridge, Himalayas) demonstrates precise knowledge and can elevate answers from Level 1 to Level 2/3.

• Mark allocation guides time: Spend approximately 1 minute per mark. A 6-mark "explain" question needs roughly 6 minutes and should contain 6 developed points. Don't write a full-page answer for 2 marks or one sentence for 8 marks.

Quick revision summary

Earth's lithosphere comprises plates moving over the semi-molten asthenosphere, driven by convection currents, slab pull and ridge push. Wegener's continental drift theory (1912) was supported by fossil, geological and jigsaw fit evidence, later proven by seafloor spreading discoveries showing magnetic striping and age patterns. Constructive boundaries create new crust; destructive boundaries involve subduction or collision; conservative boundaries involve lateral sliding. Each boundary type produces characteristic earthquakes, volcanoes and landforms. Understanding plate tectonic theory explains the global distribution of hazards concentrated along plate margins, particularly the Pacific Ring of Fire.