Predict first: what force could move entire continents made of solid rock? The answer is convection currents in the semi-molten mantle — the same mechanism that moves water in a heated pan, scaled to 2,900 km depth and millions of years. Plates move roughly 2–5 cm per year: about the rate your fingernails grow.
What are the layers inside the Earth?
The Earth has a layered internal structure, inferred entirely from seismic wave data because no drilling has ever reached deeper than about 12 km:
| Layer | State | Composition | Thickness |
|---|---|---|---|
| Inner core | Solid | Iron and nickel | ~1,200 km radius |
| Outer core | Liquid | Iron and nickel | ~2,200 km thick |
| Mantle | Semi-solid (plastic) | Silicate rock | ~2,900 km thick |
| Crust | Solid | Silicate rock (continental/oceanic) | 5–70 km thick |
The crust and the solid upper portion of the mantle together form the lithosphere, which is broken into a series of large and small fragments called tectonic plates.
What drives plate movement?
Convection currents in the mantle are the driving force. The mantle, despite being solid over short timescales, flows slowly over millions of years when subjected to the enormous heat and pressure from the core:
- Rock near the core–mantle boundary is heated; it becomes less dense and rises.
- As it moves horizontally near the surface, it cools, becomes denser, and sinks.
- This circular flow drags the rigid tectonic plates above it, moving them at about 2–5 cm per year.
The heat source driving this convection is radioactive decay within the Earth (releasing heat energy continuously) combined with residual heat from the planet's formation.
What are the three types of plate boundary?
| Boundary type | Plates move | Features | Examples |
|---|---|---|---|
| Constructive (divergent) | Apart | Mid-ocean ridges; new oceanic crust formed; shallow earthquakes; volcanoes | Mid-Atlantic Ridge; East African Rift |
| Destructive (convergent) | Together | Subduction zones; ocean trenches; deep earthquakes; volcanoes; mountain building | Pacific Ring of Fire; Himalayas (continent–continent) |
| Conservative (transform) | Past each other (sideways) | No crust created or destroyed; no volcanoes; shallow but powerful earthquakes | San Andreas Fault, California |
At destructive boundaries where oceanic crust meets continental crust, the denser oceanic crust is forced down (subducted) beneath the continental crust, melting as it descends and producing magma that erupts as volcanoes. Where two continental plates collide (neither subducts easily because both are relatively light), the crust crumples and folds upward — creating mountain ranges such as the Himalayas (India colliding with Eurasia).
What evidence supports plate tectonics?
Alfred Wegener proposed continental drift in 1912, initially rejected by geologists who could not imagine what force could move continents. He was vindicated decades later when the mechanism (convection in the mantle) and the evidence of seafloor spreading were discovered.
Key evidence:
- Jigsaw fit: the coastlines of South America and Africa match so well they appear to have once been joined — particularly striking along the continental shelves rather than the coastlines.
- Matching fossils: identical fossils of land animals (e.g. the reptile Mesosaurus) are found on both sides of the Atlantic, in continents now 5,000 km apart. A land animal cannot swim such distances.
- Matching rock formations: ancient mountain ranges and rock types of the same age and composition are found on both sides of the Atlantic.
- Seafloor spreading: the ocean floor is youngest at mid-ocean ridges and oldest at the edges near subduction zones. The pattern of magnetic reversals in the rock record shows new crust forming symmetrically on both sides of the ridge.
- GPS measurements: modern GPS satellites can directly measure the movement of tectonic plates — e.g. North America is moving away from Europe at about 2.5 cm per year.
Why was Wegener's theory rejected in 1912?
Wegener presented compelling evidence for the shapes and fossils matching, but he could not propose a convincing mechanism — how could solid continents plough through solid oceanic rock? Without a mechanism, most geologists rejected the theory, even though the evidence was striking.
It was not until the 1960s, when oceanographic surveys mapped mid-ocean ridges and confirmed seafloor spreading (and the physical mechanism of mantle convection was better understood), that plate tectonics was accepted as the consensus theory. Wegener died in 1930, before his theory was vindicated.
This is an important lesson in the nature of science: evidence is necessary but sometimes not sufficient to overturn an established theory — a plausible mechanism is also needed.
Frequently asked questions
Why can't we drill to the centre of the Earth to directly study its structure?
The deepest borehole ever drilled — the Kola Superdeep Borehole in Russia — reached 12.2 km in 1989, after 20 years of drilling. The Earth's radius is 6,371 km. The rock temperature and pressure at depth make drilling deeper essentially impossible with current technology (the rock starts to flow plastically). Instead, all knowledge of the deep Earth comes from interpreting seismic waves produced by earthquakes, which travel through the Earth and are detected at surface seismometers.
Why are most earthquakes and volcanoes concentrated near plate boundaries?
Earthquakes occur when tectonic plates move suddenly, releasing stored elastic energy as seismic waves. The zones of greatest stress are at plate boundaries, where plates are colliding, separating, or sliding past each other. Volcanoes form where magma can reach the surface — at constructive boundaries (where plates separate and magma fills the gap) and above subduction zones (where the subducting plate melts and magma rises). The Pacific Ocean is surrounded by subduction zones and constructive boundaries — the Ring of Fire — accounting for about 90% of the world's earthquakes.
How do we know the outer core is liquid?
Seismology provides the key evidence. Two types of seismic wave exist: P-waves (compressional — travel through solids and liquids) and S-waves (shear — travel only through solids). After an earthquake, seismometers on the opposite side of the Earth detect P-waves but never S-waves that have passed through the centre of the Earth. This absence of S-waves through the core proves the outer core is liquid — S-waves cannot travel through it. The solid inner core is inferred because P-waves passing through it travel faster than expected for a liquid at those depths.
Could a major earthquake or volcanic eruption in Europe affect the UK?
The UK is far from active plate boundaries and experiences only minor, infrequent earthquakes — typically from ancient fault lines reactivating, with magnitudes rarely exceeding 5. The largest UK earthquake in recorded history was the Dogger Bank event of 1931 (magnitude 6.1), centred in the North Sea. Major volcanic events elsewhere can affect UK climate — the 1783 eruption of Laki in Iceland caused crop failures and unusually cold winters in Europe for two years. However, the UK is geologically stable and not at direct risk from plate-boundary volcanism.
For GCSE physics that predicts first and then builds to the evidence — explore Professor Newton at aitutors.me.