Plate Boundaries
Explain how plate boundaries cause earthquakes and volcanoes: plates pushing together, pulling apart, or sliding past each other create the forces that trigger these events, and mountains form where plates collide
What a learner can do afterwards
- Describe three types of plate boundary movement: convergent, divergent, and transform
- Explain that earthquakes occur when plates grind or collide at boundaries
- Explain that volcanoes form where plates pull apart or one slides under another, allowing magma to rise
1 · Read
You know Earth's outer shell has huge, slow-moving plates. Where two meet is a boundary. At a convergent boundary, plates push together. Two colliding continents can squeeze and thicken the crust, raising tall mountains. At a divergent boundary, plates pull apart. At a transform boundary, plates slide past each other sideways. Sideways grinding and sudden jolts point to a transform boundary.
Earthquakes can happen where plates grind or collide. Parts of their edges may stick while the plates keep moving. Stress builds as the moving plates push or pull the stuck rock. When the rock suddenly slips, stored energy spreads as earthquake waves.
Slow plate movement can lead to a sudden slip.
At some convergent boundaries, one plate sinks under another. This is subduction. As the sinking plate heats up, it releases water. The water helps some hot mantle rock above it melt into magma. Magma can rise and feed volcanoes. The usual cause is melting above the sinking plate, not the whole plate melting. Volcanoes can also form where plates pull apart, so they occur at both kinds of boundary.
At a divergent boundary, plates pull apart. Hot mantle rock rises and partly melts as pressure drops. The magma can cool into new crust, often under the ocean. At a transform boundary, sideways movement can cause earthquakes. The sliding itself does not usually produce magma.
The San Andreas Fault in California is a transform boundary. Some parts stick as the plates move past each other. Stress builds until the stuck rock slips, sending out earthquake waves. Other parts creep more steadily.
Push together can raise mountains or feed volcanoes. Pull apart can grow new crust. Slide past can cause earthquakes. Earthquakes occur at all three kinds of boundary.
Plate movement helps explain mountains, volcanoes, and earthquakes. Water from a sinking plate helps mantle rock above it melt.
2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.