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Lessons / Restless Earth

Restless Earth · 5 min · 5 steps

Volcanoes and the Ring of Fire

Plays here

Goal. Explain why most volcanoes sit on plate boundaries and how hot spots make the exceptions.

Steps

  1. Predict

    Where do you expect most of the world's volcanoes?

    • Spread evenly over the land
    • Along plate boundaries, especially around the Pacific
    • Mostly near the poles
    Show the answer

    Along plate boundaries, especially around the Pacific. Along plate boundaries, and above all in the ring around the Pacific Ocean.

    Each triangle is a volcano from NOAA's catalogue, coloured by its last recorded eruption. Most line up along the plate boundaries, especially in the Ring of Fire around the Pacific.

  2. Where an ocean plate sinks, it carries water down into the mantle. The water lowers the melting point of the hot rock above, which melts and rises: a chain of volcanoes grows where the sinking plate is about 100 km below the surface, as in Japan and the Philippines.

  3. At mid-ocean ridges, rock melts as the plates pull apart and the pressure on it drops. Iceland rises above the sea because a hot spot sits right under the ridge there.

  4. Hawaii is far from any boundary. It sits over a hot spot, where unusually hot rock rises from deep in the mantle. As the Pacific Plate slides over it, older islands are carried away to the north-west.

  5. Taiwan has volcanoes too: the Tatun Volcanic Group just north of Taipei, including Qixing Mountain. Its last known eruption was small, more than 1,000 years ago, but hot springs and gas vents show heat below. Scientists monitor it and count it as active.

The common mistake

Myth

A volcano is a mountain full of molten rock all the way down to the Earth's core.

Fact. Magma forms only in certain places, tens of kilometres down, where plates meet, pull apart or pass over hot spots. It gathers in chambers a few kilometres beneath a volcano. The mantle itself is almost entirely solid.

Check yourself

Question 1

Why do volcanoes form above a sinking plate?

  • The sinking plate is made of lava
  • Water carried down lowers the melting point of the rock above
  • Friction between the plates melts them
Show the answer

Water carried down lowers the melting point of the rock above. Water released from the sinking plate makes the hot mantle rock above it melt at a lower temperature.

Question 2

Why is there a chain of islands at Hawaii?

  • The hot spot moves around the Pacific
  • The plate slides over a hot spot that stays roughly in place
  • Earthquakes break one island into many
Show the answer

The plate slides over a hot spot that stays roughly in place. Each island forms over the hot spot and is then carried away by the moving plate, so the islands get older towards the north-west.

Question 3

What is the Ring of Fire?

  • A ring of volcanoes and earthquakes around the Pacific Ocean
  • A circle of volcanoes around the North Pole
  • One giant volcano in the Pacific
Show the answer

A ring of volcanoes and earthquakes around the Pacific Ocean. Subduction zones surround most of the Pacific, so volcanoes and earthquakes form a ring around it.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
Ia-Ⅳ-3 · Ia-Ⅳ-4
US Next Generation Science Standards
MS-ESS2-2 · MS-ESS3-2
England national curriculum
KS3 Geography: geological timescales and plate tectonics