Rock (geology)

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

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.

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

Classification

See also: Formation of rocks

Rocks are made up of tiny pieces of minerals, which are solid materials formed from atoms that stick together in a special pattern. Some rocks also have mineral-like substances without this pattern, such as volcanic glass. The types of minerals in a rock depend on how it was formed.

Most rocks contain silicate minerals, which include silica and make up about 95% of the Earth's crust. Rocks are grouped based on their minerals, chemical makeup, how their particles are arranged, and their size. Over time, rocks can change from one type to another through a process called the rock cycle. This creates three main types of rocks: igneous, sedimentary, and metamorphic.

Igneous rock

Main article: Igneous rock

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

Sedimentary rocks form from particles of older rocks, minerals, or organisms that settle and stick together. These particles can come from weather erosion carried by water, wind, or ice. Sedimentary rocks often contain fossils and usually form in layers.

Metamorphic rock

Main article: Metamorphic rock

Metamorphic rocks form when any type of rock is changed by heat and pressure deep within the Earth. This process can create new minerals and change the rock's appearance. Metamorphic rocks can be foliated, meaning they have a layered texture, or non-foliated, without 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. These space rocks are often heavier than the rocks we find on our planet. 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

Humans have used rock for a very long time — at least 2.5 million years! Rocks have helped us build, make tools, and even create new materials. For example, people have mined rocks to get metals, which helped us make many of the things we use today.

Some rocks are very strong, like quartzites, while 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. While mining can help us get these important resources, it can also affect the environment, so many places have rules to help protect nature.