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Lessons / Restless Earth

Restless Earth · 5 min · 4 steps

Ridges, trenches and faults

Plays here

Goal. Name the three main kinds of plate boundary and what each one builds.

Steps

  1. Predict

    Where two plates move apart on the ocean floor, what do you expect to find?

    • A deep trench
    • A ridge of underwater mountains
    • A flat, quiet seabed
    Show the answer

    A ridge of underwater mountains. A ridge: hot rock rises, partly melts as the pressure drops, and makes new seafloor.

    This is the Mid-Atlantic Ridge. The plates move apart here by two to five centimetres a year, and molten rock fills the gap to make new seafloor. Iceland sits right on top of it. Tap the line for its card.

  2. Where an ocean plate meets another plate, the denser one sinks into the mantle: subduction. It makes the deepest trenches, the biggest earthquakes and chains of volcanoes, as here off Japan.

  3. Here the Pacific and North American plates slide past each other sideways along the San Andreas Fault: a transform boundary. No crust is made or destroyed, but the locked fault breaks in earthquakes.

  4. Where two continents collide, neither sinks easily, so the crust crumples upwards. India's collision with Asia built the Himalaya, and India is still pushing north by about 4 cm a year.

The common mistake

Myth

In an earthquake the ground opens into deep cracks that swallow things and close again.

Fact. An earthquake is a sudden slip of rock along a fault, mostly kilometres below the surface. Strong shaking can crack and slump the ground, but the Earth does not open into a chasm.

Check yourself

Question 1

At which kind of boundary is new seafloor made?

  • A mid-ocean ridge
  • A subduction trench
  • A transform fault
Show the answer

A mid-ocean ridge. At a mid-ocean ridge the plates move apart and rising molten rock fills the gap.

Question 2

Which kind of boundary produces the largest earthquakes?

  • Mid-ocean ridges
  • Subduction zones
  • Transform faults
Show the answer

Subduction zones. Every earthquake of magnitude 9 or more on record happened at a subduction zone, where a huge area of fault can slip at once.

Question 3

What happens where India meets Asia?

  • New seafloor is made
  • Two continents collide and mountains rise
  • The plates slide past each other sideways
Show the answer

Two continents collide and mountains rise. Continental crust is too light to sink far, so it crumples and thickens into the Himalaya and the Tibetan Plateau.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
EIa-Vc-1 · EIa-Vc-2 · Ia-Ⅳ-3
US Next Generation Science Standards
MS-ESS2-3 · HS-ESS1-5
England national curriculum
KS3 Geography: geological timescales and plate tectonics