Plate Boundaries Explorer

The lines the disasters follow

Plot every earthquake of the last century on a world map and you do not get a scatter. You get lines: down the middle of the Atlantic, around the rim of the Pacific, in a belt running from the Mediterranean through Iran to the Himalayas. Roughly 90 per cent of the world's earthquakes and about 75 per cent of its active volcanoes sit on the Pacific rim alone, the belt people call the Ring of Fire. Hazards cluster because the outer shell of the Earth is not one piece. That shell, the lithosphere, is broken into seven major plates and a dozen or so smaller ones, and they move: slowly, constantly, and in different directions. For a century nobody could say why. Alfred Wegener noticed in 1912 that the coastlines of South America and Africa fit together and that matching fossils and rock sequences turn up on both sides, but he could not name a force strong enough to shove a continent, so geologists dismissed him. The mechanism only arrived in the 1960s, when surveys of the sea floor found strips of rock either side of the mid-ocean ridges carrying mirror-image magnetic signatures, proof that new crust was being made at the ridge and pushed outwards. The engine is heat. Radioactive decay deep inside the Earth keeps the mantle hot, and hot rock is less dense, so it rises, spreads sideways, cools and sinks again in slow convection currents. Geologists now think the bigger force is slab pull: once a cold, heavy plate edge starts sinking it drags the rest of the plate after it. Either way the result is the same. Plates meet, and at every kind of meeting something breaks. This module is about what happens at each kind.

Five tectonic terms, each with its number

These five carry the figures that turn a definition into an explanation. The density and thickness numbers in particular do a lot of work: they are the reason one plate goes under and the other does not.

Order the subduction chain

These six stages describe what happens where an oceanic plate meets a continental one. Put them in the order they occur.

• Two plates move towards each other, driven by convection and by the pull of the sinking slab
• The denser of the two is forced downwards where they meet
• The sea floor is dragged down with it, opening a long, deep notch
• Heat and water carried down with the slab cause melting above it
• The molten rock is less dense than what surrounds it, so it forces its way upwards
• It reaches the surface and builds a steep-sided cone over many eruptions

Which plate goes under, and why?

Where the Nazca plate meets the South American plate, it is always the Nazca plate that sinks. Which answer explains that correctly?

• The Nazca plate is oceanic, so it is thin and made of basalt with a density around 3.0 grams per cubic centimetre, while the South American plate is continental, thick and granitic at around 2.7. The denser plate is pulled down into the mantle and the lighter one rides over it, which is why continental crust survives for billions of years and ocean floor rarely lasts 200 million.
• The Nazca plate is moving faster, and the faster plate always sinks.
• The Nazca plate is smaller, and smaller plates are always subducted.
• The Nazca plate is moving east, and plates moving east always sink.

The three margins, with real places attached

AQA expects the margin type, the process, the landforms and the hazards, and expects a named example for each. Read the columns downwards as three complete answers. A fourth case is worth knowing: where two continental plates converge neither can sink, so the crust crumples upwards instead. The Indian plate is driving north into Eurasia at about 45 mm a year, which is what raised the Himalayas and still lifts them roughly 5 mm a year. There are no volcanoes there, but the earthquakes are severe.

Label the destructive margin

The cross-section shows an ocean on the left and land on the right, with the plates moving towards each other. Tap a label, then tap the pin it belongs to. Two of the labels do not belong on this diagram at all.

Match each place to what it tells you

What "explain" is actually asking for

A 6-mark question on this topic almost always uses the word explain, and it almost always means: give me the chain. Something like "Explain the formation of ocean trenches and fold mountains at a destructive plate margin. [6 marks]" The marks are not for naming the features. They are for the links between them. A useful test is whether every sentence in your answer connects to the next with a word like because, this causes or as a result. If your sentences could be shuffled without the answer getting worse, you have written a list and you will lose half the marks. Two further habits. First, put a named example in even when the question does not demand one, because a named plate and a real figure cost you five words and lift the answer immediately. Second, be careful with the word "pressure": examiners see a lot of answers claiming that pressure builds up and the plates explode. What actually happens at a conservative margin is that friction locks the two plates while the rest of each plate keeps moving, so strain accumulates in the rock until it is released suddenly as the plates jerk past one another. That is worth writing properly. Finally, distribution questions want "describe", not "explain". If you are asked to describe the distribution of volcanoes from a map, name the pattern and quote the map: long belts rather than a scatter, concentrated around the Pacific rim and along the mid-ocean ridges, with a few isolated ones such as Hawaii sitting in the middle of a plate over a hot spot.

Complete the plate margin facts

Oceanic crust is around 5 to 10 km thick with a density of roughly _____ grams per cubic centimetre, while continental crust is far thicker and lighter, so where the two converge it is always the _____ plate that is forced downwards. That sinking opens a deep ocean _____ and melts rock above the descending slab, feeding steep composite volcanoes. Where plates pull apart instead, as along the Mid-Atlantic Ridge at about _____ cm a year, new crust is created and the eruptions are far gentler. And where they simply slide past one another, as in California, there are no volcanoes at all but the _____ can be sudden and severe.

Spot the three accurate tectonic claims

Select the THREE statements that are true.

• Not every volcano sits on a plate margin: Hawaii lies in the middle of the Pacific plate, above a fixed plume of rising heat, and the plate has dragged a chain of islands across it over millions of years.
• Conservative margins produce earthquakes but never volcanoes, because crust is neither created nor destroyed there and so no new magma is generated.
• Wegener proposed continental drift in 1912 using the fit of the coastlines and matching fossils, but it was rejected until sea-floor surveys in the 1960s found mirror-image magnetic stripes either side of the mid-ocean ridges.
• Where two continental plates meet, the denser one subducts and forms a trench.
• Earthquakes are scattered fairly evenly across the Earth's surface.
• Destructive margins produce the gentlest eruptions because the magma is very runny.

A 6-mark process answer, marked

Explain the formation of ocean trenches and fold mountains at a destructive plate margin. [6 marks] At a destructive margin two plates move towards one another, driven by convection currents in the mantle and by the weight of the already-sinking slab pulling the rest of the plate after it. Where an oceanic plate such as the Nazca meets a continental plate such as the South American, the oceanic plate is denser, at roughly 3.0 grams per cubic centimetre against about 2.7, and much thinner, so it is the one forced downwards. As it bends and descends it drags the sea floor down with it, and because the descent is steep and continuous this opens a long, narrow trough in the ocean floor: the Peru-Chile Trench, formed this way, reaches 8,065 m deep. At the same time the leading edge of the continental plate is being squeezed against the oceanic plate and cannot go anywhere, so the layers of sediment lying on it are crumpled and folded upwards, and this buckling is added to by magma rising from the melting slab beneath. As a result the Andes have been pushed up along the same margin that produced the trench, which is why the deepest water in the region lies only a few hundred kilometres from some of the highest peaks. What earned the marks: every sentence is tied to the next by "because", "as it", "at the same time" and "as a result", so the answer is a chain rather than a list; a named plate pair and two real figures are supplied without being asked for; and the trench and the mountains are explained as two outcomes of one process rather than as two separate facts, which is exactly what the question is testing.

Find the two sentences that actually explain

Below are five sentences from a student's answer on destructive margins. Three of them only describe. Tap the TWO that genuinely explain, by linking a cause to an effect.

• The oceanic plate is denser than the continental plate, so it is the one forced down into the mantle, and as it descends it drags the sea floor with it to open the trench.
• The Peru-Chile Trench is 8,065 m deep.
• Water carried down on the descending slab lowers the melting point of the rock above it, which is why magma forms here and rises to build the volcanoes.
• The Andes are a range of fold mountains in South America.
• The Nazca plate and the South American plate meet off the west coast of the continent.

Build the conservative margin sentence

Assemble a sentence that describes what happens in California without falling into the "pressure explodes" trap.

Read four margins from the evidence

Each stage gives you what a geologist can measure at an unfamiliar boundary. Work out what kind of margin it is.

• A ridge runs along the sea floor. The rock at its crest is brand new, and it gets steadily older in both directions away from the ridge. Earthquakes are frequent but weak and very shallow, and the lava is runny.
• A trough 8 km deep runs parallel to a coast. A chain of steep, ash-built volcanoes stands inland from it, and earthquake foci get deeper the further inland you measure them.
• A fault line crosses the land for hundreds of kilometres. Fences and roads built across it have been offset sideways over the decades. There are strong shallow earthquakes and not a single volcano.
• The highest mountains on Earth are still rising a few millimetres a year. The rocks at the summits are marine sediments that once lay on a sea bed. There are severe earthquakes but no volcanoes.

Write the process chain

Explain why earthquakes and volcanoes occur at some plate margins but earthquakes occur alone at others. [6 marks]

• Open by naming the margin types you are going to contrast, and name a real example of each
• For the margin with both hazards, give the chain: what moves, what sinks or separates, what melts, and what reaches the surface
• For the margin with earthquakes alone, explain why no magma is produced there, using the fact that no crust is created or destroyed
• Link every sentence to the next with "because", "this causes" or "as a result", so the answer reads as a chain rather than a list
• Describe the earthquake mechanism accurately: friction locks the plates, strain accumulates in the rock, and it is released suddenly