Earth's structure consists of layers: the crust (thin outer layer), mantle (thick layer of semi-solid rock), outer core (liquid metal), and inner core (solid metal).

The theory of plate tectonics explains that Earth's crust is divided into large plates that float on the semi-liquid mantle. Where plates meet, we see earthquakes, volcanoes, and mountain formation. Convergent boundaries (plates colliding) create mountains. Divergent boundaries (plates separating) create rift valleys. Transform boundaries (plates sliding past each other) cause earthquakes.

The rock cycle describes how rocks continuously change form. Igneous rocks form when magma cools. Sedimentary rocks form when layers of sediment are compressed over time. Metamorphic rocks form when existing rocks are changed by heat and pressure. Any rock type can transform into any other over geological time.

Weathering breaks rocks down. Erosion moves the broken material. Deposition drops it in new locations. These processes shape Earth's surface over millions of years.