# Rocks - Source: Kiddle (kids.kiddle.co), CC BY-SA 3.0, Kiddle encyclopedia, adapted from Wikipedia - Reading level: Grade 5 - Subjects: Rocks and Earthquakes, Nature, Science - Topics: rocks, rock, minerals, earth's, form, igneous - Length: 1256 words, about 9 min - Published: 2026-06-29 - Original: - Page: - Open in Sprouts: ![Marsh Butte and Geikie Peak, Grand Canyon](https://sprout.rootsofreason.org/media/kiddle-rocks-a88321b403.jpg) The Grand Canyon, a deep cut through layers of rocks. A **rock** is a natural, solid material made of [minerals](https://sprout.rootsofreason.org/r/kiddle/minerals). Rocks make up the Earth's outer layer, called the crust. They also form most of the Earth's inside, except for the hot, liquid outer core. Scientists who study rocks learn about their minerals, what they are made of, and how they formed. This helps us understand Earth's history and even rocks on other planets. Rocks are usually put into three main groups. These are igneous rocks, sedimentary rocks, and metamorphic rocks. Igneous rocks form when hot, melted rock (magma or lava) cools down. Sedimentary rocks form from bits of older rocks, minerals, or living things that get pressed together. Metamorphic rocks form when existing rocks change due to high heat and pressure. People have used rocks for a very long time. Early humans used rocks to make tools during the Stone Age. Later, rocks became important for building houses and roads. We also dig for rocks to get useful minerals and metals from them. Today, we even make new rock-like materials, like concrete. ## Studying Rocks ![Kummakivi balancing rock in Ruokolahti, Finland](https://sprout.rootsofreason.org/media/kiddle-rocks-6765d27719.jpg) A balancing rock called *Kummakivi* (meaning *"strange stone"*) in Finland. Studying rocks is a big part of geology, which is the study of Earth. Petrology is the study of what rocks are like and where they come from. Mineralogy focuses on the tiny mineral parts that make up rocks. Learning about rocks helps us understand Earth's past. It also helps us learn about human history and how we developed new tools and buildings. People have thought about rocks for a long time. But studying rocks as a real science began in the 1800s. In 1896, the discovery of radioactive decay allowed scientists to figure out the age of rocks. Later, in the 1900s, we learned about plate tectonics. This helped us understand how Earth's big plates move and shape the land. ## How Rocks Are Grouped Rocks are mostly made of tiny pieces of minerals. Minerals are solid materials with a clear, orderly structure. Some rocks also have "mineraloids," which are like minerals but don't have that orderly structure. An example is volcanic glass. The types of minerals in a rock depend on how the rock was formed. Most rocks contain silicate minerals. These minerals have silica in their structure. They make up about 95% of the Earth's crust. How much silica a rock has helps scientists name it and understand its features. ![DirkvdM rocks](https://sprout.rootsofreason.org/media/kiddle-rocks-d647b767a6.jpg) Rock outcrop along a mountain creek in Costa Rica. Scientists group rocks based on many things. These include the minerals they contain and their chemical makeup. They also look at how easily water can pass through them. The size and shape of the tiny bits that make up the rock are also important. These features show us how the rocks were formed. Over time, rocks can change from one type to another. This process is called the rock cycle. The rock cycle creates the three main types of rock: igneous, sedimentary, and metamorphic. These three main types are then divided into many smaller groups. There are no strict lines between similar rocks. They can slowly change from one type to another as their minerals increase or decrease. This means that the names we give rocks are just points in a continuous series of changes. ### Igneous Rocks ![GabbroRockCreek1](https://sprout.rootsofreason.org/media/kiddle-rocks-7a6eec6679.jpg) Sample of igneous gabbro rock. Igneous rocks get their name from the Latin word for "fire." They form when magma or lava cools down and becomes solid. Magma is melted rock found deep inside Earth. Lava is melted rock that flows out onto the Earth's surface or seabed. Rocks can melt due to high heat, less pressure, or changes in their makeup. Igneous rocks are split into two main kinds: - **Intrusive rocks** form when magma cools slowly inside the Earth's crust. This slow cooling allows large crystals to grow. Granite is a common example of this type. - **Extrusive rocks** form when magma reaches the surface as lava or volcanic ash. This material cools quickly. Examples include pumice and basalt. As magma moves towards the Earth's surface, it often becomes richer in silica. This happens because minerals with less silica crystallize out as the magma cools. Also, the magma can absorb some of the surrounding crustal rock, which is usually high in silica. So, the amount of silica is a key way to classify igneous rocks. Igneous rocks make up about 65% of the Earth's crust by volume. Most of the oceanic crust is basalt, which is an igneous rock. The continental crust is mostly made of granite and similar rocks. ### Sedimentary Rocks !["Liesegang banding" in quartzose sandstone (Upper Paleozoic; quarry near Crossville, Tennessee, USA) 2 (40280403530)](https://sprout.rootsofreason.org/media/kiddle-rocks-eaca5ccbfe.jpg) Sedimentary sandstone with iron oxide bands. Sedimentary rocks form on the Earth's surface. They are made from bits of older rocks, minerals, or living things that gather and get stuck together. They can also form when minerals separate from water. This process causes small pieces of rock or organic material to settle down. Then, these pieces get pressed and cemented together under moderate heat and pressure. Before they settle, sediments are created when older rocks break down due to weathering. Then, water, wind, ice, or glaciers carry these bits to a new place. Sedimentary rocks make up about 7.9% of the Earth's crust. Most of these are shales, with limestone and sandstone making up the rest. Sedimentary rocks often contain [fossils](https://sprout.rootsofreason.org/r/kiddle/fossil-facts-for-kids). They usually form in flat layers because of gravity. The tiny pieces in sedimentary rocks are also grouped by their size. The smallest are clay, then silt, sand, and gravel. Some systems also include larger pieces like cobbles and boulders. ### Metamorphic Rocks ![Skagit-gneiss-Cascades](https://sprout.rootsofreason.org/media/kiddle-rocks-28a2d624dd.jpg) Metamorphic banded gneiss rock. Metamorphic rocks form when any type of rock (sedimentary, igneous, or even another metamorphic rock) is put under different heat and pressure conditions. This process is called metamorphism, which means "to change in form." It causes big changes in the rock's physical features and chemistry. The original rock changes into new minerals or new forms of the same minerals. The heat and pressure needed for this are always higher than what's found on Earth's surface. For example, this happens when huge continental plates crash into each other. Metamorphic rocks make up about 27.4% of the Earth's crust. Metamorphic rocks are grouped by how they form. - **Contact metamorphism** happens when hot magma heats the rocks around it. Heat is the main factor here. - **Pressure metamorphism** happens when sediments are buried very deep. Pressure is the main factor, and heat plays a smaller part. This can create rocks like jade. - **Regional metamorphism** happens when both heat and pressure are important. This often occurs in areas where mountains are forming. Metamorphic rocks are also divided into two groups based on their structure. - **Foliated** rocks have a layered or banded texture. Schists are foliated rocks mostly made of flat minerals like mica. A gneiss has clear bands of different colors. Other foliated rocks include slates and phyllites. - **Non-foliated** rocks do not have layers. Common examples are marble, soapstone, and quartzite. Quartzite is a changed form of sandstone. ## Rocks from Space ![TallOvoo](https://sprout.rootsofreason.org/media/kiddle-rocks-d18a0da4c9.jpg) A ceremonial cairn of rocks, called an ovoo, in Mongolia. Most of what we know about rocks comes from Earth. But rocks also make up many objects in space. Planets like Mars, Venus, and Mercury are made of rock. So are many moons, asteroids, and meteoroids. Meteorites that fall to Earth give us clues about rocks from space. They are often heavier than Earth rocks. Space missions, like *Hayabusa*, have also brought asteroid rocks to Earth. Scientists have also studied rocks from the Moon and Mars. ## Picture credits - Yogendra Joshi, CC BY 2.0 () - Kotivalo, CC BY-SA 4.0 () - DirkvdM, CC BY-SA 3.0 () - Wilson44691 ( Mark A. Wilson at the Wayback Machine , Department of Geology, The College of Wooster)., Public domain () - James St. John, CC BY 2.0 () - Public domain () - Qfl247 ( talk ), CC BY-SA 3.0 ()