Earth's hard shell is cracked into giant plates that creep around — and where they meet, the ground can heave, split, or shudder. Today you'll learn the three kinds of boundary, then you will predict what each one builds.
Before we start — you've felt or heard of the ground shaking in an earthquake — what do you think is moving, deep below your feet?
Earth's hard outer shell is broken into huge plates that float on the hot, slow-flowing mantle below. They creep about as fast as your nails grow. The cracks where two plates meet are called plate boundaries.
Plates can meet in three ways: convergent (push together → mountains — and where an ocean plate meets land, the heavier ocean plate dives under, called subduction, carving a deep trench and feeding volcanoes), divergent (pull apart → new crust), and transform (slide past → just shaking).
Plates don't slide smoothly — they get stuck, and stress builds for years. When they finally slip, the stored energy races out as seismic waves — an earthquake. The slip starts at the focus underground; the spot right above it on the surface is the epicentre.