Earth Science: Rocks

Quick Facts

  • Rocks are solid materials that make up the outer layer of Earth.
  • Minerals are the building blocks of rocks.
  • Geologists are scientists who study rocks.
  • Relative age describes the age of something compared with the age of another thing


Rocks are classified into three main groups: igneous, sedimentary, and metamorphic.

Igneous rocks, sometimes referred to as “Fire-Made”, are rocks made from hot, molten rock material that has cooled and hardened. Molten rock that is below Earth’s surface is called magma. Magma that reaches the surface of Earth is called lava. Molten rock that cools slowly will make large crystals. Molten rock that cools quickly will make small crystals.

Sedimentary rocks are formed from bits or layers of small rocks. Deposited rock particles, called sediments, settle in layers. The weight of the layers packs the particles together into rock. Some sedimentary rocks are made of once-living things. Limestone is made of the shells and skeletons of once-living sea animals.

Sediments are laid down in time order. Those at the bottom are laid down first, while those at the top are laid down last. Time order tells us about age. Relative age describes the age of something compared with the age of another thing, using words like older and younger.  ( activity/video )

A metamorphic rock is a rock that has been changed by heat and pressure. For example, marble is a metamorphic rock that once was limestone. The heat that causes the change may come from nearby magma, and the pressure comes from the weight of other rocks stacked on top.

Rocks are always changing. The rock cycle is a process by which rocks are changed from one type to another. Magma or lava cools and forms igneous rocks. Rocks on Earth’s surface break into small pieces. In time, the small pieces turn into sedimentary rocks. Temperature and pressure may also change rocks. Rocks below Earth’s surface may melt and form magma, and the cycle repeats.