You are viewing a free preview of this lesson.
Subscribe to unlock all 12 lessons in this course and every other course on LearningBro.
Understanding the internal structure of the Earth and the theory of plate tectonics is the essential foundation for the entire Tectonic Processes and Hazards topic in Edexcel A-Level Geography (Paper 1, Topic 1). This lesson addresses Enquiry Question 1 (EQ1): Why are some locations more at risk from tectonic hazards? by examining the physical processes that drive tectonic activity and the evidence that supports plate tectonic theory.
The Earth has a layered structure, differentiated by composition, density and physical state. Understanding these layers is critical because plate tectonics is driven by processes occurring within and between them.
The outermost layer of the Earth is the crust, which is thin and rigid compared to the layers beneath it.
| Property | Oceanic Crust | Continental Crust |
|---|---|---|
| Thickness | 5–10 km | 25–70 km (average ~35 km) |
| Density | 3.0–3.3 g/cm³ | 2.6–2.7 g/cm³ |
| Composition | Basaltic (silica + magnesium = sima) | Granitic (silica + alumina = sial) |
| Age | Generally < 200 million years | Up to 4 billion years |
| Behaviour at subduction zones | Subducts beneath continental crust due to greater density | Too buoyant to subduct; buckles and folds instead |
The difference in density between oceanic and continental crust is fundamental to understanding subduction — the process by which dense oceanic lithosphere sinks beneath lighter continental lithosphere at destructive plate boundaries.
The mantle extends from the base of the crust to a depth of approximately 2,900 km and accounts for about 84% of the Earth's volume. It is composed primarily of silicate minerals rich in iron and magnesium. The mantle is not uniform — it is divided into distinct zones:
Exam Tip: Distinguish carefully between the lithosphere (rigid; includes crust + upper mantle) and the asthenosphere (ductile; allows plate movement). Examiners frequently test whether candidates understand that tectonic plates are pieces of lithosphere, not just crust.
The core is divided into two parts:
graph TD
A["Earth's Structure"] --> B["Crust<br/>5–70 km<br/>Rigid, brittle"]
A --> C["Mantle<br/>~2,900 km thick<br/>Silicate minerals"]
A --> D["Core<br/>~3,470 km radius<br/>Iron & nickel"]
B --> B1["Oceanic: 5–10 km, basaltic, dense"]
B --> B2["Continental: 25–70 km, granitic, buoyant"]
C --> C1["Lithosphere: rigid upper layer"]
C --> C2["Asthenosphere: ductile, allows plate motion"]
C --> C3["Lower mantle: solid, slow convection"]
D --> D1["Outer core: liquid, generates magnetic field"]
D --> D2["Inner core: solid, extreme pressure"]
The theory of plate tectonics — that the Earth's lithosphere is divided into rigid plates that move relative to one another — is supported by multiple independent lines of evidence accumulated over more than a century.
Alfred Wegener proposed that the continents were once joined in a single supercontinent called Pangaea (meaning "all lands"), which began to break apart approximately 200 million years ago. His evidence included:
Jigsaw fit of continents: The coastlines of South America and Africa show a remarkable match, particularly when continental shelves are considered rather than present-day coastlines. Edward Bullard (1965) used computer modelling to demonstrate a near-perfect fit at the 500-fathom (approximately 900 m) depth contour.
Fossil evidence: Identical fossils of land-dwelling and freshwater organisms have been found on continents now separated by thousands of kilometres of ocean:
Geological evidence: Mountain belts and rock sequences match across continents. The Caledonian Mountains of Scotland and Scandinavia align with the Appalachian Mountains of North America. Precambrian cratons (ancient rock formations older than 2 billion years) in Brazil match those in West Africa.
Palaeoclimatic evidence: Glacial deposits (tillites) of Carboniferous-Permian age are found in tropical regions including India, equatorial Africa and South America. Coal deposits indicating tropical swamp conditions are found in present-day northern Europe. These can only be explained if these landmasses were at different latitudes in the past.
Exam Tip: Wegener's hypothesis was initially rejected not because his evidence was wrong, but because he could not explain the mechanism driving continental movement. He suggested centrifugal force and tidal drag, which physicists quickly showed were far too weak. This is a valuable example of how science progresses — good observations can precede an adequate explanatory mechanism.
Harry Hess proposed that new oceanic crust forms at mid-ocean ridges and spreads laterally, eventually being recycled at deep ocean trenches. Key evidence supporting sea-floor spreading includes:
Magnetic striping: The Earth's magnetic field periodically reverses polarity. As magma solidifies at mid-ocean ridges, iron-rich minerals align with the prevailing magnetic field. This creates symmetrical bands of normal and reversed polarity on either side of the ridge — confirmed by Vine and Matthews (1963). This pattern can only be explained by new crust forming at the ridge and moving outward.
Age of ocean floor: Radiometric dating and deep-sea drilling (DSDP/ODP programmes) revealed that oceanic crust is youngest at mid-ocean ridges and progressively older with distance from the ridge. The oldest oceanic crust is approximately 200 million years old — far younger than the oldest continental crust (~4 billion years), because oceanic crust is continuously recycled at subduction zones.
Sediment thickness: Sediment cover increases with distance from mid-ocean ridges, consistent with older crust having had more time to accumulate pelagic sediment.
Heat flow: Heat flow measurements show highest values at mid-ocean ridges (where hot magma rises) and lower values on the abyssal plains, consistent with the cooling and thickening of lithosphere as it moves away from the ridge.
Global Positioning System (GPS) measurements now provide real-time confirmation of plate motion:
| Plate Boundary | Rate of Movement |
|---|---|
| Mid-Atlantic Ridge | ~2.5 cm/year (spreading) |
| East Pacific Rise | ~6–16 cm/year (fastest spreading ridge) |
| Pacific Plate (NW movement) | ~7–10 cm/year |
| India–Eurasia convergence | ~4–5 cm/year |
| San Andreas Fault (lateral) | ~4.6 cm/year |
Seismological evidence also reveals the geometry of subduction zones. Wadati-Benioff zones — inclined planes of earthquake foci — trace the descending slab of oceanic lithosphere into the mantle, providing direct evidence of subduction at destructive plate boundaries.
Understanding what drives plate motion is essential for explaining why tectonic activity occurs where it does. The current scientific consensus is that multiple forces act together, with slab pull being the dominant mechanism.
Radioactive decay of isotopes (primarily uranium-238, thorium-232 and potassium-40) within the mantle generates heat. Combined with residual heat from the Earth's formation, this creates temperature differences that drive convection currents — hot, less dense material rises, while cooler, denser material sinks. Arthur Holmes (1929) first proposed that mantle convection could drive continental drift.
However, modern understanding recognises that simple convection cells are an oversimplification. The mantle behaves as a complex system with:
Slab pull is now considered the most significant driving force. At subduction zones, dense oceanic lithosphere (made denser by cooling and phase changes in minerals at depth) sinks into the asthenosphere under gravity. The weight of the descending slab pulls the rest of the plate with it. Evidence for the dominance of slab pull includes:
At mid-ocean ridges, newly formed lithosphere sits at a higher elevation than the surrounding abyssal plains. Gravity causes the lithosphere to slide down the slope of the ridge, pushing the plate laterally. Ridge push is estimated to contribute roughly 5–10% of the total driving force — significantly less than slab pull.
graph LR
A["Ridge Push<br/>Elevated ridge → gravitational sliding<br/>~5-10% of driving force"] -->|"pushes plate laterally"| B["Plate Motion"]
C["Slab Pull<br/>Dense subducting slab sinks under gravity<br/>Dominant driving force"] -->|"pulls plate into trench"| B
D["Mantle Convection<br/>Heat-driven circulation<br/>Basal drag on plates"] -->|"drags or resists"| B
E["Basal Drag<br/>Friction at base of lithosphere<br/>May drive or resist motion"] -->|"variable effect"| B
The interaction between the base of the lithosphere and the flowing asthenosphere creates frictional forces. Depending on the relative motion of the plate and the underlying mantle flow, basal drag may either assist or resist plate motion. Its net contribution remains debated.
Exam Tip: When answering questions about plate motion, avoid stating that mantle convection is the sole driver. Examiners at A-Level expect you to recognise that slab pull is the dominant mechanism and that multiple forces interact. Stating only "convection currents drag plates along" is a GCSE-level answer.
Tectonic plates interact at their boundaries in three fundamental ways. The type of boundary determines the nature of tectonic activity, landforms and hazards.
| Boundary Type | Plate Movement | Key Processes | Associated Landforms | Seismicity |
|---|---|---|---|---|
| Divergent (constructive) | Plates move apart | Sea-floor spreading, rifting | Mid-ocean ridges, rift valleys | Shallow, low-moderate magnitude |
| Convergent (destructive) | Plates move together | Subduction, mountain building | Trenches, fold mountains, island arcs | Shallow to deep, high magnitude |
| Conservative (transform) | Plates slide laterally | Friction, stress accumulation | Fault lines, offset features | Shallow, can be very high magnitude |
| Collision | Continental plates converge | Compression, folding, thrusting | Fold mountains, plateaux | Shallow, moderate-high magnitude |
These boundary types are explored in detail in Lessons 2 and 3.
Not all tectonic activity occurs at plate boundaries. Hotspots are areas of anomalously high volcanic activity located away from plate margins, caused by mantle plumes — narrow columns of exceptionally hot material rising from deep within the mantle (possibly from the core-mantle boundary at approximately 2,900 km depth).
The Hawaiian Islands provide the classic example of a hotspot volcanic chain. The Pacific Plate moves north-westward over a stationary mantle plume at approximately 7–10 cm/year. Each island in the chain formed over the hotspot and was then carried away by plate motion:
| Island | Age (million years) | Distance from current hotspot |
|---|---|---|
| Big Island (Hawaii) | < 0.7 Ma (active) | Over the hotspot |
| Maui | ~1.3 Ma | ~170 km NW |
| Molokai | ~1.8 Ma | ~250 km NW |
| Oahu | ~3.0 Ma | ~350 km NW |
| Kauai | ~5.1 Ma | ~520 km NW |
| Midway Atoll | ~28 Ma | ~2,400 km NW |
The progressive increase in age along the chain provides independent confirmation of both the direction and rate of Pacific Plate motion. A sharp bend in the Hawaiian-Emperor seamount chain at approximately 47 million years ago indicates a change in the Pacific Plate's direction of motion.
Yellowstone National Park sits above a continental hotspot. The North American Plate has moved south-westward over the hotspot, leaving a trail of progressively older calderas across Idaho. The current Yellowstone caldera last erupted catastrophically ~640,000 years ago. The geothermal features (geysers, hot springs, fumaroles) and the elevated topography of the Yellowstone Plateau are all manifestations of the underlying mantle plume.
While the vast majority of earthquakes occur at plate boundaries, significant seismic events can also occur within plates. Examples include:
Intra-plate earthquakes are often more damaging than their magnitudes suggest because they occur in regions with little seismic preparedness, lower-frequency shaking that affects buildings differently, and populations unaccustomed to earthquakes.
Exam Tip: Hotspots and intra-plate earthquakes are important for Edexcel EQ1 because they challenge the simplistic view that all tectonic hazards occur at plate boundaries. Being able to explain anomalies strengthens your answers and demonstrates higher-order understanding.
The Wilson Cycle (named after J. Tuzo Wilson, 1966) describes the cyclical opening and closing of ocean basins over geological time — a process taking approximately 300–500 million years. The cycle consists of several stages:
The Wilson Cycle integrates the concepts of rifting, sea-floor spreading, subduction and collision into a coherent framework. It also explains why mountain belts contain marine fossils — they represent the uplifted remains of former ocean floors.
| Key Concept | Detail |
|---|---|
| Earth's layered structure | Crust, mantle (asthenosphere/lithosphere distinction), outer core, inner core |
| Wegener's continental drift | Jigsaw fit, fossils, geology, palaeoclimate — but lacked a mechanism |
| Sea-floor spreading | Magnetic striping, age of ocean floor, sediment thickness, heat flow |
| Modern evidence | GPS confirms real-time plate motion; Wadati-Benioff zones confirm subduction |
| Driving mechanisms | Slab pull (dominant), ridge push, mantle convection, basal drag |
| Boundary types | Divergent, convergent, conservative, collision |
| Hotspots | Mantle plumes; Hawaii chain confirms plate direction and speed |
| Intra-plate earthquakes | Ancient fault reactivation; challenge boundary-only model |
| Wilson Cycle | Cyclical opening/closing of ocean basins over 300–500 Ma |
Exam Tip: For Edexcel 20-mark essays on plate tectonic theory, structure your answer around the progression of evidence: Wegener (observational) → Hess/Vine-Matthews (mechanism) → GPS/seismology (confirmation). This chronological approach demonstrates the evolution of scientific understanding, which examiners value highly.
This lesson covers the foundational knowledge for Edexcel A-Level Geography Paper 1, Topic 1: Tectonic Processes and Hazards (EQ1). The next lesson examines divergent and convergent plate boundaries in greater detail.