GCSE · Geography · AQA · Spec 8035

Destructive plate margins

Some of the most violent eruptions and strongest earthquakes on Earth happen where one plate sinks under another.

Where an ocean meets a continent

Follow the sinking plate

Two plates are moving towards each other: an oceanic plate and a continental plate. Step across from the ocean to the land and see what the sinking plate builds at each stop.

Zone 1 of 4

Ocean floor

Out here the oceanic plate is heading for the continent. It is the denser of the two plates, so when they meet it is the one forced down into the mantle and destroyed. That sinking has a name: subduction.

  • Denser plate
  • Moving towards the continent

One plate, three jobs: it bends down to make the trench, sticks and slips to make earthquakes, and melts deep in the mantle to feed the volcanoes further inland.

Check your thinking

Why aren't the volcanoes at the trench?

In the section above, the volcanoes sat some distance back from the trench — even though the trench is where the plates actually meet.

Which is closest to what you think is going on?
How sure are you?

Now take the continent away

When two oceanic plates meet

Swap the continent for a second oceanic plate. The denser of the two still sinks — but with open sea above it, look at what grows this time.

Zone 1 of 4

The denser oceanic plate

Both plates here are oceanic, so which one goes down? The denser one. It subducts beneath the other plate, exactly as before.

Same sinking plate, different landscape: against a continent it builds mountains and volcanoes; against another ocean it builds a volcanic island arc.

Why these volcanoes explode

Stratovolcano (destructive margin)vsShield volcano (constructive margin)

Read the magma row first — every other row follows from it.

Focus

Magma

Stratovolcano (destructive margin)

High in silica — made when heat and magma from the mantle melt parts of the continental crust

Shield volcano (constructive margin)

Runny basalt magma

The insight

Start any volcano explanation with the magma. Silica is what makes destructive-margin magma thick.

Gas

Stratovolcano (destructive margin)

Viscous (sticky) magma traps the gases

Shield volcano (constructive margin)

Gases are not trapped

Eruption

Stratovolcano (destructive margin)

Explosive

Shield volcano (constructive margin)

Effusive — the lava flows out

Shape

Stratovolcano (destructive margin)

Steep-sided cone built of layers of lava and ash

Shield volcano (constructive margin)

Gentle slopes

Danger

Stratovolcano (destructive margin)

More hazards

Shield volcano (constructive margin)

Fewer hazards

Hazards at destructive margins

Name that hazard

Pick a description, then choose the hazard it describes.

Still to sort

Ash cloud (0)

Fine ash thrown up by the explosion

Lava flow (0)

Sticky lava moving downhill

Gas emissions (0)

What you can't see

Lahar (0)

Ash and debris plus water

Where the line is: A lahar and a pyroclastic flow both carry ash and debris. The lahar is the one with water mixed in — a mudflow that can arrive long after the eruption.

Pyroclastic flow (0)

Scorching gas, ash and debris

Where the line is: No water here: a pyroclastic flow is a cloud of gas, ash and debris at up to 800 °C, moving at 700 km/h.

Landslide (0)

A slope that gives way

Tsunami (0)

Waves set off under the sea

7 of 7 still to sort.

Explosive eruptions and big earthquakes bring a whole range of dangers — some of them long after the event, or far out at sea. Each description below is one of them.

The Pacific Ring of Fire

How much happens in one ring?

Guess first: what percentage of the world's earthquakes happen in the Pacific Ring of Fire?

Your estimate

50%

0%100%

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One plate sinks — and that makes the trench, the volcanoes and the earthquakes.

What you need to know

  • At a destructive (convergent) margin two plates move towards each other and the denser oceanic plate subducts — it sinks into the mantle and is destroyed.
  • A destructive margin can be oceanic–continental (trench, fold mountains, volcanoes) or oceanic–oceanic (trench and a volcanic island arc).
  • Volcanoes form some distance from the trench, because the sinking plate only melts once it is deep in the mantle.
  • Earthquakes happen when the plates stick because of friction, pressure builds, and they suddenly slip; they are often high magnitude with a deep focus.
  • High-silica, sticky, gas-trapping magma makes eruptions explosive and builds steep-sided stratovolcanoes, with hazards from ash clouds to lahars — plus tsunamis and landslides.

The big picture

At a destructive margin two plates move towards each other, and the denser oceanic plate is forced down into the mantle and destroyed — that sinking is called subduction. Where it starts to sink, a deep ocean trench forms; where it sticks and suddenly slips, powerful earthquakes happen; and where it melts deep in the mantle, magma rises to feed volcanoes some distance from the trench. Against a continental plate that gives fold mountains and explosive stratovolcanoes; against another oceanic plate it gives a volcanic island arc. The sticky, gas-trapping magma and the violent earthquakes make these margins home to a whole range of dangerous hazards.

Key points

1Denser oceanic plate sinks: subduction.
2Trench where it starts to sink; volcanoes where it has melted.
3Stick, build pressure, slip: a high-magnitude earthquake.
4Focus = where the earthquake happens; epicentre = the point on the surface directly above.
5Sticky high-silica magma traps gas, so eruptions are explosive.
6Ash clouds, lava flows, gases, lahars, pyroclastic flows, tsunamis and landslides.
7Ring of Fire: about 90% of earthquakes and 75% of volcanoes.

Worked example

Problem

Explain why volcanoes at an oceanic–continental destructive margin are steep-sided stratovolcanoes.

⚠ Watch out

Drawing the volcano right on top of the trench. The trench is only where the plate starts to sink; it melts deep in the mantle, so the volcanoes sit some distance away.

🧠

Memory hook

Follow the sinking plate: it BENDS (trench), STICKS (earthquakes), then MELTS (volcanoes, set back from the trench).

✓

Check yourself

Without looking back, sketch a margin where two oceanic plates meet. Label the sinking plate, the trench, where the plate melts and the island arc — then say which plate sinks and why.

Flashcards

(16)
What happens at a destructive (convergent) plate margin?
Two plates move towards each other, and one sinks below the other and is destroyed.
What is subduction?
The process where a denser oceanic plate sinks beneath a less dense plate into the mantle.
Name the two kinds of destructive margin.
Oceanic plate under continental plate; and oceanic plate under oceanic plate (the denser one subducts).
Where does an ocean trench form at a destructive margin?
At the subduction zone, where the denser plate begins to sink — e.g. the Mariana Trench in the Western Pacific.
How do fold mountains form at an oceanic–continental margin?
The oceanic plate slides under the continental plate and pushes it up, making a chain of fold mountains near the ocean edge (e.g. the Andes).
Where does the magma for destructive-margin volcanoes come from?
The subducting plate heats up and melts as it descends deeper into the mantle; the magma rises through cracks in the crust.
Why aren't the volcanoes directly above the trench?
The plate only melts deep in the mantle, so magma reaches the surface some distance away from the margin.
What is a volcanic island arc?
Where an oceanic plate subducts under another oceanic plate, underwater volcanoes build up over time into islands — e.g. the Windward Islands in the Caribbean.
What causes an earthquake at a destructive margin?
The plates get stuck by friction, pressure builds in the subduction zone, then they suddenly slip and release seismic energy.
Focus vs epicentre?
Focus: the point where the earthquake occurs (can be deep in the subduction zone). Epicentre: the point on the surface directly above the focus.
What does magnitude measure, and why is it often high here?
The energy an earthquake releases. It is often high at destructive margins because of the intense pressure and friction in the subduction zone.
Explosive vs effusive eruption?
Explosive: gas bursts out violently (stratovolcanoes at destructive margins). Effusive: lava simply flows out and gases are not trapped (shield volcanoes).
How does an underwater earthquake cause a tsunami?
The seafloor suddenly shifts upwards and pushes the ocean water up; the waves build up to 20–30 m in shallower water near the coast.
Lahar
A mudflow of volcanic ash and debris mixed with water — it can happen weeks, months or even years after an eruption.
Pyroclastic flow
A deadly cloud of gas, ash and debris heated to up to 800 °C, travelling at 700 km/h.
The Pacific Ring of Fire — what share of earthquakes and volcanoes?
About 90% of earthquakes and 75% of volcanoes; its plate margins are destructive.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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