The Earth’s surface is like a ball-shaped jigsaw puzzle. It is made up of about 15 giant pieces, called tectonic plates. These plates are always shifting about—but only very gradually. Some plates are moving apart, others are pushing together. In some places, the edge of one plate slides down below another. In others, two plates slide past each other, moving in opposite directions. All this pushing, sliding or bumping at the edges of the plates causes earthquakes.

Continental drift

Each plate carries a piece of continent or a piece of the ocean floor, or both. The plates float like giant rafts on top of a molten layer of the mantle. In this way, the continents gradually move to new positions around the globe. This is called continental drift. Over hundreds of millions of years, the map of the world has changed completely.

When two plates push together, the land slowly crumples up, forming mountains. The Himalayan mountains rise up where a plate that carries India is pushing into another plate carrying the rest of Asia. The Himalayas, which are already the highest in the world, grow taller by about the length of your thumb every year.

San Andreas Fault

Two plates slide past one another along what is called the San Andreas Fault in California. Sometimes the plates lock together for a time. The pressure becomes too much and the plates suddenly snap apart. When they do, an earthquake is the result.

Mid-Oceanic Ridge

Two tectonic plates are slowly moving apart from one another under the Atlantic Ocean. There is an enormous, jagged crack that runs down the middle of the ocean floor. Molten rock oozes up from the mantle to push the plates apart. As the rock cools and hardens it forms a long, narrow mountain range. This is called the Mid-Oceanic Ridge. The two plates are gradually pushing Europe and Africa further and further away from the Americas.