How do mountains grow?

In a picture of mountains, you might trace the tall peaks with your finger. They look enormous and still, so how do mountains grow?

Earth's rocky outside has huge pieces called plates, like pieces of a giant cracked eggshell. These plates fit beside each other and carry the land and ocean floor.

Deep inside Earth, heat helps rock move slowly, even though most of that rock stays solid. This slow movement helps move the plates above, too.

Some plates move toward each other until the land along their edges meets. Neither piece of land can slide freely forward anymore, so the plates squeeze the rock between them.

Imagine pushing the two ends of a rug toward its middle. Can you guess what happens to the middle when the ends keep pushing?

The rug bunches upward, and squeezed land can bunch upward too. Rock bends and breaks under the enormous push, piling into a thicker stack that rises into mountains.

As the plates keep pressing, the mountains can keep rising. That growth takes millions of years, far longer than a person lives, so you cannot watch a peak stretch taller.

Volcanoes offer another way for mountains to grow. Deep below a volcano, melted rock pushes upward through cracks. When it spills out onto the ground, we call it lava.

Outside, the lava loses heat and cools into hard rock. It leaves a new rocky layer on the ground, like another blanket spread over a bed.

When a volcano sends out more lava, more layers can pile on top. Time after time, those layers can build a mountain taller and wider.

Plates, lava. Mountains grow when moving plates squeeze land upward, or when a volcano adds layers of rock.

As you settle down to rest, Earth's mountains have all the time they need to grow.