Difficulty: Introductory | Prerequisites: None
The rock cycle is the slow, continuous process by which Earth recycles its own material. Rocks do not stay one type forever: given enough heat, pressure, weathering or melting, any rock can become any other type. This sits at the foundation of geology and Earth science, and it connects to plate tectonics, erosion, soil formation and the carbon cycle. If you understand the rock cycle, you have a map for nearly every landscape process the course will cover.
Earth has three rock families: igneous (cooled from molten material), sedimentary (compressed fragments) and metamorphic (transformed by heat and pressure). They convert into one another through weathering, erosion, melting, cooling, compaction and metamorphism, forming a continuous loop with no fixed starting point.
Igneous rock
Rock formed when magma or lava cools and hardens (solidifies). Think of it as: rock that was once liquid.
Sedimentary rock
Rock formed when sediments (fragments of other rocks, minerals, or organic material) are compressed and cemented together over time. In simple terms, this means layers of debris get squeezed into solid stone.
Metamorphic rock
Rock that has been changed in form by intense heat and/or pressure without fully melting. Think of it as: an existing rock that got cooked and squeezed into something new.
Magma
Molten (liquid) rock beneath Earth's surface. When it reaches the surface, it is called lava. In simple terms, this is rock so hot it has turned to liquid.
Sediments
Small fragments of rock, mineral grains, or organic particles produced by weathering and erosion. Think of it as: the crumbs left over when rock breaks down.
Weathering
The breakdown of rock at or near Earth's surface by physical, chemical or biological processes. The rock does not move; it breaks apart in place.
Erosion
The transport of weathered material (sediments) by wind, water, ice or gravity. Often paired with weathering because one produces the fragments and the other carries them away.
Compaction and cementation
The two-step process that turns loose sediments into sedimentary rock. Compaction squeezes out water and air; cementation glues the grains together with dissolved minerals.
Melting
The process by which solid rock is heated until it becomes magma. This typically happens deep underground or at tectonic plate boundaries.
Cooling and hardening
The process by which magma or lava loses heat and solidifies into igneous rock. Slow cooling underground produces coarse crystals; rapid cooling at the surface produces fine crystals or glass.
Heat and pressure (metamorphism)
The conditions that transform existing rock into metamorphic rock. The rock does not melt; its mineral structure rearranges. This usually happens at depth, near tectonic boundaries or next to magma intrusions.
Igneous rock forms when magma cools and hardens.
Intrusive (plutonic) igneous rock cools slowly underground, producing large crystals (e.g. granite).
Extrusive (volcanic) igneous rock cools quickly at the surface, producing small crystals or glassy texture (e.g. basalt, obsidian).
Sedimentary rock forms when sediments are compacted and cemented together.
Clastic sedimentary rock is made from fragments of other rocks (e.g. sandstone, shale).
Chemical sedimentary rock forms from mineral precipitation out of water (e.g. limestone, rock salt).
Organic sedimentary rock forms from accumulated biological material (e.g. coal).
Metamorphic rock forms when existing rock is changed by heat and/or pressure.
Foliated metamorphic rock shows layered or banded texture from directional pressure (e.g. slate, schist, gneiss).
Non-foliated metamorphic rock has no banding because the pressure was uniform (e.g. marble, quartzite).
Weathering and erosion break down any rock type into sediments and transport those sediments elsewhere.
This is the entry point to the sedimentary pathway.
All three rock types (igneous, sedimentary, metamorphic) can be weathered.
Compaction and cementation turn loose sediments into sedimentary rock.
Burial under more layers increases pressure, squeezing out water.
Dissolved minerals act as natural glue between grains.
Heat and pressure (without melting) transform any rock into metamorphic rock.
This typically occurs at convergent plate boundaries or near magma intrusions.
The deeper the burial, the greater the transformation.
Melting converts any solid rock into magma.
This happens at subduction zones, hotspots and mid-ocean ridges.
Sedimentary and metamorphic rocks can melt just as igneous rocks can.
Cooling and hardening turn magma into igneous rock.
The rate of cooling determines crystal size.
The rock cycle has no beginning or end. Any rock type can become any other rock type, and the path does not have to follow a set sequence. A sedimentary rock can become metamorphic (via heat and pressure), or it can weather back into sediments, or it can melt into magma. Every arrow in the diagram represents a real geological pathway.
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Every arrow is a real geological pathway. The cycle has no fixed start: any rock type can become any other.
Granite (igneous) and marble (metamorphic) are chosen for buildings and countertops precisely because of the processes that formed them: slow cooling or intense pressure produces dense, durable stone. Sandstone (sedimentary) erodes more easily, which is why old sandstone buildings weather visibly over decades. Understanding the rock cycle also matters for the petroleum industry, since oil and gas accumulate in porous sedimentary rock layers.
Students often think the rock cycle follows a fixed sequence (igneous → sedimentary → metamorphic → back to igneous). It does not. Any rock type can transform into any other, and steps can be skipped entirely.
Students often confuse weathering with erosion. Weathering is the breakdown of rock in place. Erosion is the movement of that broken material to a new location. They are related but distinct.
Students sometimes assume magma only produces igneous rock. In fact, magma is a stage that any rock passes through when melted, regardless of what it was before.
Students sometimes think metamorphic rock has melted. It has not. Metamorphism happens while the rock remains solid; if it melts, it becomes magma, and the product when it cools is igneous rock, not metamorphic.
⚠️ Be able to trace a path between any two rock types on the diagram. A common exam question gives you a starting rock and an ending rock and asks which processes are involved.
⚠️ Know the difference between weathering and erosion. This distinction appears frequently in multiple-choice questions.
⚠️ Understand that the rock cycle has no fixed start. If a question asks "where does the rock cycle begin," the answer is that it does not have a single starting point.
⚠️ Be able to name all five stages/materials in the cycle (sediments, sedimentary rock, magma, igneous rock, metamorphic rock) and all five processes (weathering/erosion, compaction/cementation, melting, cooling/hardening, heat/pressure).
True or false: Sedimentary rock can only form from the weathering of igneous rock.
Fill in the blank: The process that turns sediments into sedimentary rock is called ________ and ________.
True or false: Metamorphic rock has been melted and re-solidified.
Fill in the blank: When magma cools and hardens, the result is ________ rock.
True or false: The rock cycle must follow the sequence igneous → sedimentary → metamorphic.
Answers: 1. False (any rock type weathers into sediments). 2. Compaction and cementation. 3. False (it changed form under heat/pressure but stayed solid). 4. Igneous. 5. False (any rock can become any other type).
Q: A piece of granite is subjected to intense heat and pressure deep underground but does not melt. What type of rock does it become, and what is this process called?
A: It becomes metamorphic rock. The process is called metamorphism (heat and pressure without melting). The resulting rock might be gneiss.
Q: Describe the complete path by which a metamorphic rock could eventually become a new sedimentary rock.
A: The metamorphic rock is broken down by weathering, the fragments are transported by erosion, those sediments are deposited in layers, and over time compaction and cementation turn them into sedimentary rock.
Q: Why is it incorrect to say the rock cycle has a starting point?
A: Because any rock type can transform into any other through multiple pathways. There is no single entry point; the cycle is continuous and can be entered at any stage.
Q: What determines whether magma becomes intrusive or extrusive igneous rock?
A: The location of cooling. Magma that cools slowly beneath the surface becomes intrusive (plutonic) igneous rock with large crystals. Magma that reaches the surface as lava and cools quickly becomes extrusive (volcanic) igneous rock with small crystals or glassy texture.
Q: A sedimentary rock is carried deep underground at a subduction zone. Describe two different things that could happen to it next in the rock cycle.
A: (1) It could be subjected to heat and pressure and become metamorphic rock. (2) It could be heated enough to melt entirely, becoming magma, which could later cool into igneous rock.
The rock cycle connects directly to plate tectonics: subduction zones drive melting and metamorphism, while mid-ocean ridges produce new igneous rock. It also underpins the study of soil science, since soils form from the weathering of rock. In biology courses, the rock cycle matters because sedimentary rock is where most fossils are preserved, linking geological processes to the fossil record and evolution.
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