Rock (geology)

Rocks are solid parts of the Earth made from tiny pieces called minerals or mineral-like substances. They are found everywhere and make up the Earth’s outer layer, called the crust. Scientists study rocks to learn about Earth’s history and how our planet formed.

The Grand Canyon, an incision through layers of sedimentary rocks.

Rocks come in three main types: igneous, sedimentary, and metamorphic. Igneous rocks form when hot, melted rock called magma cools down. Sedimentary rocks are made from bits of older rocks that have been broken apart, moved, and then pressed together. Metamorphic rocks change when they are pushed and heated so much that they turn into new kinds of rock without melting.

People have used rocks for thousands of years. Long ago, during a time called the Stone Age, people made tools from rocks. Today, rocks are used to build houses and roads. We also dig up rocks to get useful materials like metals from them. Even modern inventions like concrete are made to act like natural rocks.

Study

Further information: Geology, Petrology, and Mineralogy

Geology is the study of Earth and its parts, including rocks. Petrology looks at rocks and where they come from. Mineralogy studies the tiny parts called minerals that make up rocks. Learning about rocks helps us understand Earth's past, how humans lived long ago, and how we build and create new things.

In the 1800s, people started studying rocks as a science. They learned new ideas about how rocks form. Later, in the 1900s, scientists found ways to tell the age of rocks and learned about how Earth's plates move.

Classification

Igneous rock

Main article: Igneous rock

Rock outcrop along a mountain creek near Orosí, Costa Rica.

Igneous rocks form when melted rock called magma or lava cools and hardens. This can happen deep inside the Earth or on the surface. Igneous rocks are divided into two groups: those that cool slowly underground, like granite, and those that cool quickly on the surface, like basalt. Igneous rocks make up most of the Earth's crust.

Sedimentary rock

Main article: Sedimentary rock

Sample of igneous gabbro

Sedimentary rocks form from pieces of older rocks, minerals, or tiny parts of living things that settle and stick together. These pieces can be moved by water, wind, or ice from erosion. Sedimentary rocks often have fossils and usually form in layers.

Metamorphic rock

Main article: Metamorphic rock

Metamorphic rocks form when any rock is changed by heat and pressure deep inside the Earth. This process can create new minerals and change how the rock looks. Metamorphic rocks can have layers or not have layers. Examples include schist, gneiss, and marble.

Extraterrestrial rocks

Main article: Planetary geology

Rocks are not just found on Earth; they are also part of many objects in space. Planets like Mars, Venus, and Mercury, as well as many moons, asteroids, and meteoroids, are made of rock. When pieces of these space rocks, called meteorites, fall to Earth, they give us clues about what other rocks in space are like. Scientists have also collected rocks from the Moon and Mars through special space missions, such as the Hayabusa mission, to learn more about them.

Human use

Ceremonial cairn of rocks, an ovoo, from Mongolia

Humans have used rocks for a very long time — at least 2.5 million years! Rocks help us build things, make tools, and create new materials. For example, people mine rocks to get metals. These metals help us make many things we use every day.

Some rocks are very strong, like quartzites. Others are soft and easy to work with, like limestone. People have used soft rocks to build buildings and bridges for thousands of years. Mining is how we get useful materials like metals, coal, and even diamonds from the earth. Mining can help us, but it can also affect the environment. Many places have rules to help protect nature.