Discover the Rock Cycle: How Rocks Form, Change, and Transform

Igneous Rock

Igneous rock forms when magma or lava cools and hardens. When magma cools slowly deep underground, it forms intrusive igneous rocks like granite, which have large, visible crystals because the slow cooling gives minerals time to grow. When lava cools quickly at the surface, it forms extrusive igneous rocks like basalt or glassy obsidian, which have very small crystals or no crystals at all.

Sedimentary Rock

Sedimentary rock forms from layers of sediment tiny pieces of rock, sand, mud, and organic material that are deposited, then compacted and cemented together over time. This process, called lithification, happens in two steps: compaction (the weight of overlying layers squeezes sediment together) and cementation (dissolved minerals fill spaces between particles and harden like glue). Sedimentary rocks are the only type that commonly contain fossils, because organisms can be buried in soft sediment and preserved as the layers harden.

Metamorphic Rock

Metamorphic rock forms when existing rocks are buried deep underground and subjected to intense heat and pressure without melting completely. For example, marble forms when limestone (a sedimentary rock) is transformed by heat and pressure. If any rock type melts completely, it becomes magma, which can then cool to form new igneous rock. You can explore more about these rock types in Rock Types: Igneous, Sedimentary, and Metamorphic.

Weathering breaks rocks into smaller pieces through physical forces (like freezing water) and chemical reactions (like acid rain). Erosion then moves those weathered particles called sediment from one place to another by water, wind, or ice. Deposition occurs when the agents of erosion slow down and drop the sediment in a new location, where it builds up in layers. These surface processes connect to what you studied in Surface Features: Mountains, Valleys, Oceans and Matter Cycles: Water, Carbon, Nitrogen Cycles.

A mineral is a naturally occurring, solid, inorganic substance with a definite chemical composition and crystal structure. Rocks are mixtures of one or more minerals. You can identify minerals using several physical properties, which you can study further in Mineral Properties: Physical and Chemical Properties.

Key mineral identification properties include hardness (measured on the Mohs hardness scale from 1 to 10, where talc is softest and diamond is hardest), streak (the color of powder left on a white ceramic plate), and luster (how the mineral's surface reflects light metallic, glassy, or dull). You can also connect mineral properties to Physical Properties: Mass, Volume, Density and Chemical Properties: Reactivity, pH, Combustibility.

Rock Cycle: The continuous process by which rocks change from one type to another over time through melting, cooling, weathering, erosion, and pressure. You can think of it as Earth's never-ending recycling system.

Magma: Molten rock material that exists beneath Earth's surface inside the mantle or crust. When magma erupts onto the surface, it is then called lava.

Lava: Molten rock that has reached Earth's surface through a volcanic eruption. Lava and magma are the same material the only difference is their location.

Igneous Rock: Rock that forms when magma or lava cools and solidifies. Examples include granite (slow cooling, large crystals) and basalt (fast cooling, small crystals).

Sedimentary Rock: Rock that forms when layers of sediment are compacted and cemented together over time. Limestone and sandstone are common examples, and these rocks often contain fossils.

Metamorphic Rock: Rock that forms when existing rocks are changed by intense heat and pressure deep within Earth's crust, without melting completely. Marble and quartzite are examples.

Sediment: Tiny pieces of rock, sand, mud, shells, and organic material produced by weathering and carried by erosion. Sediment eventually settles and can form sedimentary rock.

Weathering: The process that slowly breaks rocks apart into smaller pieces through physical forces (like ice expanding in cracks) and chemical reactions (like acid rain dissolving minerals).

Erosion: The movement of weathered rock particles (sediment) from one place to another by water, wind, or ice. Erosion transports sediment away from its original location.

Deposition: The process where transported sediment settles and is dropped in a new location when the water or wind carrying it slows down. Deposited layers eventually build up into sedimentary rock.

Compaction: The squeezing together of sediment layers by the weight of material piling up on top, pushing particles closer together as part of forming sedimentary rock.

Cementation: The process where minerals dissolved in water fill the spaces between sediment particles and harden, gluing them together to form solid sedimentary rock.

Mineral: A naturally occurring, solid, inorganic substance with a definite chemical composition and crystal structure. Minerals are the building blocks of rocks.

Hardness: A mineral property that measures how resistant its surface is to being scratched. You measure hardness using the Mohs hardness scale, which runs from 1 (softest, like talc) to 10 (hardest, like diamond).

Mohs Hardness Scale: A scale that ranks minerals by their resistance to scratching, from 1 (softest) to 10 (hardest). You can test hardness using everyday objects like a fingernail (~2.5), copper coin (~3), or steel file (~6.5).

Streak: The color of the powder a mineral leaves when rubbed across a white ceramic plate. Streak is often different from the mineral's surface color and is a reliable identification tool.

Luster: The way a mineral's surface reflects light, described as metallic, glassy (vitreous), or dull. Luster is one of the key properties you use to identify an unknown mineral.

Fossil: The preserved remains or traces of a living organism found in rock. Fossils are most commonly found in sedimentary rock because the formation conditions preserve organic material.

Intrusive Igneous Rock: Igneous rock that forms when magma cools slowly deep underground, producing large crystals. Granite is a well-known example.

Extrusive Igneous Rock: Igneous rock that forms when lava cools quickly at Earth's surface, producing small crystals or a glassy texture. Basalt and obsidian are examples.

You can deepen your understanding of the rock cycle by connecting it to related concepts. Think about how Types of Changes: Physical vs. Chemical Changes apply to weathering and metamorphism, and how Reactions: Signs of Chemical Reactions explain chemical weathering. You can also use Scientific Models: Creating and Testing Predictive Models to build and test your own model of the rock cycle.

Try identifying rock samples by their texture and crystal size coarse-grained rocks like granite cooled slowly, while fine-grained rocks like basalt cooled quickly. You can also practice the streak and hardness tests to identify common minerals using the Mohs scale.

Before exploring the rock cycle, you should be comfortable with Phase Changes: Temperature Effects on State, since melting and solidification are central to igneous rock formation. Your knowledge of Scientific Models: Creating and Using Models will also help you interpret rock cycle diagrams. Understanding Physical Properties: Mass, Volume, Density and Chemical Properties: Reactivity, pH, Combustibility prepares you to analyze mineral and rock characteristics.

Mastering the rock cycle prepares you for several exciting topics ahead. You will use this knowledge when you study Plate Tectonics: Continental Drift Theory and Earth's Structure: Internal Layers, where you will see how moving plates drive rock transformation. You will also apply it in Geological Events: Earthquakes and Volcanoes and Introduction to Mineral Resources: Formation and Extraction.

The rock cycle also connects to life science Evidence of Change: Fossil Record and Similarities relies on your understanding of how sedimentary rocks preserve fossils. Additionally, the phase change concepts you see in the rock cycle link directly to Phase Changes: Energy in Transitions and States of Matter: Kinetic Molecular Theory.

Among peer topics, Rock Types: Igneous, Sedimentary, Metamorphic and Mineral Properties: Physical and Chemical Properties work hand-in-hand with the rock cycle to give you a complete picture of Earth's materials.