Why does the Earth have mountains, earthquakes, and volcanoes? The answer is plate tectonics — the single big idea that ties geology together.

The Earth's rigid outer shell (the lithosphere from Lesson 1) is cracked into large pieces called plates. They ride on the slowly-flowing mantle and drift very slowly — about as fast as your fingernails grow. Where plates meet, things happen.

Where plates meet (kept simple)

• Pulling apart — new crust forms in the gap (think mid-ocean ridges).
• Pushing together — crust crumples into mountains, or one plate dives beneath another (where many volcanoes live).
• Sliding past — plates grind sideways, a classic setting for big earthquakes.

The words for breaking and bending

• Fault — a crack in rock where the two sides have slipped past each other. When a fault slips suddenly, the ground shakes — that's an earthquake.
• Fold — when rock layers are squeezed and bend, like a rug pushed from both ends, instead of breaking. Folds build many mountain ranges.

This is also where metamorphic rock is born: the intense heat and pressure at plate boundaries cook and squeeze rock into new forms.

A few more terms you'll hear everywhere

• Erosion — the wearing away and carrying off of rock and soil.
• Deposition — the dropping of that material somewhere new (erosion's partner).
• Bedrock — the solid rock lying beneath the loose soil.
• Outcrop — a spot where bedrock pokes up through the surface, a geologist's favourite thing to find.