Plate
Tectonics
Cape Town and Buenos Aires are drifting apart at ~2.5 cm a year — about as fast as your fingernails grow. The same hot, slowly-flowing mantle that powers a volcano in Iceland is splitting Africa in two right now, along the East African Rift Valley. Every coastline, every mountain range, every fossil match between continents is a clue to a single great theory: plate tectonics.
In 1915, a German meteorologist called Alfred Wegener noticed something obvious that no one took seriously: the eastern coast of South America looks like it would fit, almost perfectly, into the western coast of Africa. He proposed that the continents had once been joined and had since drifted apart. He was laughed at for decades. How was he right — and what proved it?
Wegener was right about the what — Africa and South America really were joined, about 200 million years ago, in a supercontinent called Pangaea. What he couldn't explain was the how: what force could possibly drag entire continents thousands of kilometres? The breakthrough came in the 1960s, when oceanographers mapping the seafloor discovered the Mid-Atlantic Ridge — an underwater mountain chain running down the middle of the Atlantic, with brand-new rock being created at its centre and pushed outwards in both directions.
That rock carries a recorded signature of Earth's magnetic field, frozen in place when it cooled. Magnetic stripes on either side of the ridge match like a mirror. The seafloor itself is spreading, dragging the continents along for the ride. Convection currents in the hot, plastic asthenosphere (the layer you met in Topic 10) drive the whole machine. Wegener's "drifting continents" was upgraded to plate tectonics — and it became the unifying theory of every coastline, mountain, earthquake and volcano on Earth.
A Planet of Drifting Rafts
Earth's lithosphere is broken into about 15 plates — 7 major ones and a handful of smaller ones. They drift on the hot, plastic asthenosphere below — pulled and pushed by convection currents in the mantle.
The lithosphere (Earth's rigid outer layer — crust plus the top of the mantle) is not one continuous shell. It's cracked into giant slabs, called continental plates, which fit together like a planet-sized jigsaw puzzle. There are 7 major plates and several smaller ones. South Africa sits on the African Plate.
The plates ride on top of the asthenosphere — the hot, plastic-flowing layer of the mantle. Inside the mantle, convection currents are constantly at work: hot rock rises, cools, sinks, and rises again. That slow circulation drags the plates around at 1 to 20 cm per year. Where plates meet, three things can happen: they can collide, pull apart, or slide past each other. Every mountain range, ocean basin, earthquake and volcano on Earth can be traced to one of those three movements.
Three Kinds of Plate Boundary
Tap each boundary type to see what happens at that kind of edge, and where on Earth you can find one. The same convection engine inside the mantle drives all three — only the direction of motion changes.
Convergent
~ plates collide
Divergent
~ plates pull apart
Transform
~ plates slide past
Local connection — the African Plate is busy
South Africa sits on the African Plate, which is doing every one of the three things at the same time. To the west, the Mid-Atlantic Ridge (divergent) is pushing us slowly away from South America — about 2.5 cm a year. To the east, the African Rift Valley (divergent) runs through Ethiopia, Kenya and Tanzania — Africa is literally splitting in two. In a few tens of millions of years, the eastern Horn of Africa will be its own continent, separated by a brand-new ocean. The Pilanesberg (North-West Province) is a long-dead volcano from 1.3 billion years ago. The Prince Edward Islands and Marion Island, far south in the Southern Ocean, still have active volcanoes — the last eruption was on Marion in 2008. And Tristan da Cunha and Gough Island, where South Africa keeps a research station, sit on the same side of the Mid-Atlantic Ridge as us — Tristan about 400 km east of the ridge axis, and Gough further east again. They are not built by the ridge itself. They sit over a hotspot: a stationary plume of hot mantle rock that keeps punching volcanoes through the plate as the plate slides over it. Tristan last erupted in 1961 and the whole island had to be evacuated to England.
The Break-Up of Pangaea
200 million years ago, every continent was joined together into one giant landmass called Pangaea, surrounded by a single ocean called Panthalassa. Tap each milestone to see how it broke apart and made today's world.
The Evidence — Two Separate Lists
This is where exam answers most often go wrong, because CAPS asks for two different lists and they are easy to merge into one. Wegener's three were the observations that made him propose continental drift in the first place, back in 1915 — and none of them explained how. CAPS's six are the modern evidence for crustal movement, which came decades later and finally supplied the mechanism.
① Wegener's evidence for Continental Drift — three
What a meteorologist noticed by looking at a map and reading other people's fossil reports. Enough to propose the idea; not enough to convince anyone.
1 · The geographic fit
The western coastline of Africa and the eastern coastline of South America fit together like a jigsaw puzzle. Match the continental shelves (at 2 000 m depth) rather than the modern shorelines and the fit is near-perfect.
2 · Matching rock types and geology
Rock types, mountain belts and formation ages line up across the Atlantic. The Cape Fold Mountains continue as the Sierra de la Ventana in Argentina — same rock, same age, same structure.
3 · Matching fossils on matching margins
Fossils of the freshwater reptile Mesosaurus appear only in southern Africa and South America. Glossopteris (a fern) is found across India, Africa, Antarctica and Australia — only Gondwana makes sense of it.
② Evidence for crustal movements — six
The modern list, and the one CAPS enumerates a–f. These are what turned a laughed-at hypothesis into accepted theory.
a · Polar reversals
Earth's magnetic field flips every few hundred thousand years. When magma cools at a ridge, iron minerals lock in whichever direction the field pointed at the time. The seafloor therefore carries a barcode of mirrored magnetic stripes either side of every ridge.
b · Polar wandering
Plot the magnetic north recorded in rocks of different ages and you get a path that appears to wander across the globe. The pole did not move that far — the continents did. And the "wander paths" from two continents only line up when you slide those continents back together.
c · Sea floor spreading
Rocks nearest the ridge are youngest; they get steadily older toward the continents, with thicker sediment on top. The pattern is symmetrical on both sides. New crust is being made at the ridge and pushed outward.
d · Epicentres of earthquakes
Plot every earthquake epicentre on a world map and they trace the plate boundaries exactly — shallow ones along spreading centres, and deep ones sloping down beneath subduction zones. That match is itself evidence for seafloor spreading.
e · Rift zones
A rift zone is a region where the crust is actively being pulled apart and thinned, dropping a valley between parallel faults. You can watch it happening: the East African Rift is splitting a continent right now, and the Red Sea is a rift that has already flooded.
f · Volcanic hotspots
A stationary plume of hot mantle burns through the plate moving over it, leaving a chain of volcanoes that gets older in one direction. The chain is a tape recording of the plate's movement — Hawaii is the classic; Tristan da Cunha and the Walvis Ridge are ours.
Why magnetic stripes are the smoking gun
The matching paleomagnetic stripes on either side of a mid-ocean ridge are the single most powerful piece of evidence for plate tectonics. They prove three things at once: (1) the seafloor is spreading, because new rock is being added at the ridge and pushed outward; (2) the rate of spreading can be measured exactly, because each stripe corresponds to a known polar reversal; and (3) the same width of stripes on both sides means the spreading is symmetrical. Wegener couldn't have known any of this — the technology to measure it didn't exist in his lifetime. By the 1960s, marine geologists had read enough of the seafloor's barcode to convince every sceptic.
What a Spreading Centre Builds
A mid-ocean ridge is not just a seam where crust appears. It is the most geologically and biologically extraordinary place on the planet — and CAPS names two of its features as terms you must define: hydrothermal vents, and megaplumes and spreading centre.
A spreading centre is the line along a divergent boundary where new oceanic crust is actually created.
It is the ridge crest itself — the narrow zone where plates separate, magma rises to fill the gap, and fresh seafloor is welded on to both plates at once. Everything below happens because a spreading centre is hot, cracked and leaking.
Hydrothermal vents
Seawater seeps down through cracks in the young, fractured crust near a spreading centre. Close to the magma chamber it is heated to over 350 °C — it stays liquid only because the pressure at 2 500 m is crushing — and it dissolves metals and sulphides out of the rock. That superheated, mineral-loaded water then blasts back out of the seafloor as a hydrothermal vent. The instant it hits near-freezing deep water the minerals crystallise, turning the plume black and building a chimney: a black smoker.
The distinction to hold: a vent is a persistent outlet, running for years. A megaplume is a single massive event. Both occur at spreading centres, and both are CAPS definition (i)'s territory.
The investigation CAPS sets: vent ecosystems versus ecosystems on land
CAPS gives this block exactly one investigation — "Investigate the ecosystems related to hydrothermal vents. How are they similar to or different from ecosystems on land?" — and it is a genuinely deep question, because vents break the rule that everything else in this course depends on.
The difference that matters: every ecosystem you have studied so far is ultimately powered by sunlight, through photosynthesis. A vent community is 2 500 m down in total darkness. Its producers are bacteria that build sugar using the chemical energy in hydrogen sulphide instead of light — chemosynthesis. It is the only major ecosystem on Earth that does not run on the sun.
What is the same: the structure is identical. Producers at the base, primary consumers grazing them, predators above, decomposers recycling. Giant tube worms, vent crabs and clams occupy exactly the trophic roles that grass, antelope and lions do in the Karoo. Same architecture, different energy source — and that is the whole answer to CAPS's question.
Full comparison table and the write-up in the practical sheet.
Local connection — a vent field on our doorstep
The Southwest Indian Ridge, running south-east of South Africa between the African and Antarctic plates, is one of the slowest-spreading ridges on Earth — and it hosts active hydrothermal vent fields with animals found nowhere else. South African researchers aboard the SA Agulhas II work in this region, and the vent fauna of the Indian Ocean ridges is a live research question, not settled textbook material.
It also matters commercially and ethically: vent chimneys are rich in copper, zinc and gold, and deep-sea mining companies have applied for exploration licences on Indian Ocean ridges. Mining a vent field destroys an ecosystem that exists nowhere else and took centuries to grow. That is a decision your generation will have to make.
Key Vocabulary
Tap each card to flip it. CAPS names nine terms for this block outright — asthenosphere · mid-ocean ridge · subduction zone · rift zones · divergent, convergent and transform plate boundaries · hydrothermal vents · megaplumes and spreading centre. All nine are below, with the other eight this topic runs on.
The single supercontinent that contained all of Earth's modern land masses, joined together about 250 million years ago. Surrounded by one ocean called Panthalassa. Began breaking up about 200 mya.
The southern supercontinent that split off from Pangaea. It contained today's Africa, South America, Antarctica, Australia and India. Its break-up opened the modern Atlantic and Indian Oceans.
The large-scale motion of continental plates across the surface of Earth — driven by convection in the mantle. The unifying theory that explains ocean basins, mountain ranges, earthquakes, volcanoes and fossil distributions.
Circular flow of hot, less-dense material rising and cool, denser material sinking. In Earth's mantle, these currents drag the plates above. The same mechanism heats your kitchen with hot air rising from a radiator.
The process at a convergent boundary where one plate is forced under another and pushed back down into the mantle. Creates ocean trenches, volcanic arcs and powerful earthquakes — the Pacific Ring of Fire is one giant subduction system.
The process at divergent boundaries in the ocean where new oceanic crust forms as magma rises at mid-ocean ridges, cools, and is pushed outward. The Atlantic Ocean is widening by ~2.5 cm per year because of this.
The underwater mountain range at a divergent plate boundary where new oceanic crust is created by upwelling magma. The Mid-Atlantic Ridge is the most famous example — visible above sea level at Iceland.
The study of ancient magnetic fields recorded in rocks. When magma cools, iron-bearing minerals lock in the direction of Earth's magnetic field at that moment. Reading these "magnetic fossils" gave scientists the smoking-gun evidence for seafloor spreading.
CAPS definition (a). The hot, plastic-flowing layer of the upper mantle beneath the lithosphere, roughly 80–200 km down. Solid, but soft enough to flow over geological time — which is what lets the rigid plates above it move at all.
CAPS definition (c). The region at a convergent boundary where the denser oceanic plate dives beneath the other and is destroyed back into the mantle. Marked by an ocean trench, deep earthquakes and a chain of volcanoes.
CAPS definition (d). A region where the crust is being actively pulled apart and thinned, dropping a valley between parallel faults. On land: the East African Rift. Flooded: the Red Sea. At sea: the rift valley along every mid-ocean ridge crest.
CAPS definition (e). Where two plates move apart and magma rises to fill the gap, creating new crust. Produces mid-ocean ridges, rift valleys and hydrothermal vents. Example: the Mid-Atlantic Ridge.
CAPS definition (f). Where two plates move together. Ocean meets continent → subduction, a trench and volcanoes. Continent meets continent → neither will sink, so the crust buckles upward into mountains (the Himalayas). Crust is destroyed here.
CAPS definition (g) — it writes it "transformative". Where two plates slide past each other sideways. No crust is created and none destroyed — but the plates snag and then jerk free, so these boundaries produce severe shallow earthquakes. Example: the San Andreas Fault.
CAPS definition (h). An opening at a spreading centre where seawater that has seeped into the crust, been heated past 350 °C by magma and loaded with dissolved minerals, blasts back out. Minerals crystallise on contact with cold deep water, blackening the plume and building a black smoker chimney.
CAPS definition (i). A spreading centre is the line along a divergent boundary where new oceanic crust is created. A megaplume is a single enormous pulse of superheated mineral-rich water released there in one event — usually a seafloor eruption — forming a lens of warm water tens of km across. Vent = continuous. Megaplume = one event.
The apparent movement of the magnetic pole traced by plotting the magnetism locked into rocks of different ages. The pole barely moved — the continents did. Two continents' wander paths only line up when you slide them back together, which is why this is evidence for crustal movement. Do not confuse with polar reversals, where the field flips direction.
A stationary plume of hot mantle that burns through the plate sliding over it, leaving a chain of volcanoes that ages in one direction — a tape recording of the plate's motion. Hawaii is the classic; Tristan da Cunha and the Walvis Ridge are ours. Not the same as a ridge volcano.
Building sugar using the chemical energy in hydrogen sulphide instead of light. It is what bacteria at hydrothermal vents do, and it makes vent communities the only major ecosystem on Earth not ultimately powered by the sun. You meet it properly in Topic 21.
In the Lab — the CAPS Investigations
CAPS sets two activities for this block, and neither needs anything you do not already own. One is three blocks of wood; the other is a research and comparison task built on the vent ecosystems in Section 5.
🔬 The two investigations
- Wooden blocks to model the three boundary types — then sketch the movements. CAPS names the apparatus outright. Two blocks pushed together, pulled apart and slid past each other, with a sketch and a real-world example for each. The sketching is the assessed part. 🟢 two blocks of wood
- Investigate hydrothermal vent ecosystems — how are they similar to, and different from, ecosystems on land? A structured comparison ending in the point that matters: same trophic architecture, completely different energy source. 🟢 desk
The practical also adds a plate-boundary identification drill — given a described location, name the boundary type and predict what you would find there. That is the form this topic takes in an exam.
📄 Practical sheet: Open the Topic 11 practical →
Quick Check
Question 1
Well done!
Next: Geological Time — how scientists date the rocks plate tectonics builds.