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Various rocks
River rocks are smoothed by continuously bumping into each other over time

What Are Rocks?

A rock is a solid, naturally occuring mass of minerals. Rocks aren't usually uniform, meaning they can't be described easily by a single chemical formula—such as sodium chloride (NaCl) for salt. Instead, geologists classify rocks based on their different mineral components and the ways that they were formed.

How Are Rocks Formed?

There are three main ways for rocks to form, each with their own name:

  • Sedimentary rocks are formed by years and years of sediment (like dirt or sand) squeezing and compacting together. Generally, something like a stream or river will carry lots of small pieces of rocks and minerals to a larger body of water. These pieces will settle at the bottom and over a really long time (perhaps millions of years), they will form into solid rock. Some examples of sedimentary rocks are shale, limestone, and sandstone.
  • Igneous rocks (pronounced igg-nee-us) are formed when magma or lava cools and hardens. Below Earth's surface, magma hits temperatures of between 700-1,300°C (1,292-2,372°F), and cools after reaching Earth's surface or somewhere within the crust. Examples of igneous rocks include obsidian, basalt, and granite.
  • Metamorphic rocks (pronounced met-ah-more-fick) are formed by extreme heat and pressure, often below the Earth's crust. Metamorphosis means "change," and metamorphic rocks are often made from other types of rock. For example, shale, a sedimentary rock, can be changed, or metamorphosed, into a metamorphic rock such as slate or gneiss (pronounced nice). Other examples of metamorphic rocks include marble, anthracite, soapstone, and schist.

The Rock Cycle

The rock cycle is a model geologists use to describe the transformation of rocks over long periods of time. The cycle is driven by geological forces, such as pressure and erosion, and can take millions of years. Although the cycle doesn't always follow these steps exactly, here is a simplified idea of how rocks might undergo changes:

  1. Formation of Igneous Rocks: Rocks are formed through the cooling and solidification of molten magma or lava.
  2. Weathering and Erosion: Over time, these rocks can undergo weathering and erosion, breaking them down into smaller particles.
  3. Transportation and Deposition: The smaller particles are transported by wind, water, or ice and deposited, or brought to new locations.
  4. Formation of Sedimentary Rocks: Through processes like compaction, these deposited particles gradually form sedimentary rocks.
  5. Formation of Metamorphic Rocks: Under intense heat and pressure deep within the Earth's crust, both igneous and sedimentary rocks can undergo metamorphism. The rocks are transformed into different, metamorphic rocks.
  6. Repeated Cycle: This cycle repeats over millions of years, driven by geological forces, shaping the Earth's surface and leading to the formation of various rock types we see today.

Top Facts About Rock for Kids

  • 1. Fluorescent rocks can glow in the dark.

    When ultraviolet (UV) light hits these special rocks, they absorb the energy and re-emit it as visible light, creating a beautiful glow. This is known as fluorescence, and can also be seen in certain minerals, paints, inks, and dyes.

  • 2. There's a type of volcanic rock that can float on water.

    It might sound unbelievable, but a certain type of rock, called pumice, is so full of tiny air bubbles that pumice stones can actually float on water! Pumice is formed when lava from a volcano quickly and traps gas bubbles within it. This makes pumice rocks extremely light and therefore less dense than water, allowing them to float!

  • 3. There are types of rocks that can grow.

    Okay, well, while not in the same way as plants or animals, there are certain types of rocks that can slowly increase in size over time. One example is a rock type called a "concretion." Concretions are formed when minerals in groundwater slowly build up around a grain of sand or a seashell. Over hundreds or thousands of years, these mineral layers accumulate and harden, causing the concretion to grow larger.

  • 4. A lithophone is a kind of primitive instrument... made from rocks!

    These ancient xylophone-like instruments were likely some of the first ever created by humans, though they are still often made and used today. Some rocks, when struck with a stick or another rock, produce different musical notes or tones. This happens because certain rocks have unique crystal structures or composition that vibrates and resonates when struck. Now that's some classic rock.