The Rock Cycle

Free sample questions, a clear explanation, and 5 practice skills with an AI tutor that guides without giving the answer away.

Concept Review

The Rock Cycle: Earth's Ultimate Recycling Program

What if I told you that the granite countertop in your kitchen and the pencil graphite you write with were once the same type of rock? Welcome to the rock cycle — Earth's amazing way of recycling the same materials over millions of years.

Every rock on Earth belongs to one of three families, each formed by completely different processes. Think of them as Earth's cooking methods — some rocks get "baked," others get "pressed," and some get "layered."

The Three Rock Families

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Igneous

Born from fire and heat

🏔️

Sedimentary

Built layer by layer

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Metamorphic

Transformed by pressure

Igneous rocks form when molten rock (magma or lava) cools and hardens. If it cools slowly underground, you get coarse-grained rocks like granite. If it cools quickly on the surface, you get fine-grained rocks like obsidian — volcanic glass so sharp that ancient people used it for tools.

Sedimentary rocks form when tiny pieces of other rocks, shells, or organic matter get pressed together over time. Limestone, for example, often forms from compressed sea creatures' shells — meaning you might find fossils of ancient ocean life on top of mountains!

Metamorphic rocks are the transformers of the rock world. Heat and pressure deep underground change existing rocks into entirely new ones. Limestone becomes marble, and ordinary shale becomes slate — the same rock that once covered schoolhouse roofs.

🔍 Rock Detective Skills

You can identify rocks like a geologist by looking for clues:

Texture:Are the grains big or tiny? Smooth or rough?
Layers:Can you see distinct bands or stripes?
Crystals:Do you see shiny, geometric shapes?

Here's what makes this truly amazing: the rock cycle never stops. The Rocky Mountains contain rocks that were once ancient seafloors. Your local playground might sit on rocks that were once deep underground. By studying the rocks in your area, you're reading Earth's diary — discovering what your neighborhood looked like millions of years ago.

🔑 Key Takeaway

That granite countertop and pencil graphite? Granite can become metamorphic gneiss under pressure, then melt into magma, then cool into new igneous rock. Meanwhile, graphite forms when organic matter gets compressed into metamorphic rock. In Earth's recycling program, everything is connected, and nothing is ever truly "done."

Sample questions

1. Maya finds a rock with layers of different colored sand and tiny shell pieces pressed together. The rock feels gritty and she can see individual grains. How did this rock most likely form?

○ Heat and pressure deep underground changed an existing rock

○ Melted rock cooled and hardened

○ Wind broke down the rock into small pieces

✓ Layers of sand and shells were pressed together over time

Answer: Layers of sand and shells were pressed together over time — The layers of sand and shell pieces that were pressed together are clues that sediments were compressed and cemented to form a sedimentary rock.

2. Which process creates igneous rocks?

✓ Melted rock (magma or lava) cools and hardens

○ Sediments get pressed and cemented together

○ Heat and pressure change existing rocks

○ Weathering breaks rocks into pieces

Answer: Melted rock (magma or lava) cools and hardens — Igneous rocks form when melted rock material cools and solidifies, either underground as magma or above ground as lava.

3. True or False: Marble is a sedimentary rock because it forms from layers of sediment. Explain your reasoning.

○ True - marble has visible layers like other sedimentary rocks

✓ False - marble is metamorphic because it forms when limestone is changed by heat and pressure

○ True - marble contains compressed particles of calcium

○ False - marble is igneous because it forms from cooled magma

Answer: False - marble is metamorphic because it forms when limestone is changed by heat and pressure — Marble is metamorphic because it forms when the sedimentary rock limestone is transformed by intense heat and pressure deep in the Earth, not from layered sediments.

Skills in this topic

Classify rocks as igneous, sedimentary, or metamorphic based on formation
Describe the processes that form each type of rock
Trace how rocks change from one type to another in the rock cycle
Identify rock samples using texture, color, and formation clues
Investigate local rock formations to determine the geological history of your area

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