Earth Structure, Rocks, and the Rock Cycle

Section 1Earth's layers

Earth is made of layers. The crust is the thin solid outer layer. The mantle is hot, mostly solid rock that can flow slowly. The outer core is liquid metal, and the inner core is solid metal under huge pressure.

Section 2Composition and conditions

The crust is mainly silicate rock. The mantle is denser silicate rock rich in minerals such as magnesium and iron. The core is mostly iron and nickel. As you go deeper, temperature and pressure increase, and the layers generally become denser.

Section 3Evidence for Earth's structure

Scientists use earthquake waves, volcanoes, rock samples, meteorites, and measurements of Earth's magnetic field to infer the inside of Earth. We cannot drill to the core, so the evidence is indirect. Seismic waves are especially important because they travel differently through solid and liquid layers.

Section 4Three rock types

Igneous rocks form when molten rock cools and solidifies, such as basalt and granite. Sedimentary rocks form from layers of sediment compacted and cemented, such as limestone and sandstone. Metamorphic rocks form when existing rocks are changed by heat and pressure, such as marble and slate.

Section 5The rock cycle

Rocks can change from one type to another over time. Weathering and erosion break rocks into sediments; compaction and cementation make sedimentary rock; heat and pressure make metamorphic rock; melting makes magma; cooling makes igneous rock. In the Caribbean, volcanic islands have igneous rocks, while limestone areas are common in Jamaica.