The Rock Cycle
Grade 7 · Science · Study Guide
Discover how rocks are constantly transformed between igneous, sedimentary, and metamorphic types over millions of years.
Rocks are not permanent — they are constantly being made, broken down, and remade in a continuous process called the rock cycle. All rocks on Earth can be classified into three major types based on how they form: igneous, sedimentary, and metamorphic. Over deep time, any type of rock can be transformed into any other type.
Igneous rocks form when molten rock (magma underground or lava at the surface) cools and solidifies. Granite is an intrusive igneous rock — it cooled slowly underground, giving crystals time to grow large. Basalt is an extrusive igneous rock — it cooled quickly at the surface, forming small or invisible crystals. When rocks are exposed at Earth's surface, they are broken down by weathering (physical or chemical breakdown) and erosion (movement by water, wind, or ice). These tiny particles, called sediments, are carried away and eventually deposited in layers. Over time the layers are buried, compacted, and cemented together to form sedimentary rocks. Limestone and sandstone are common sedimentary rocks. Sedimentary rocks often contain fossils because organisms can be buried in the sediment.
When existing rocks are subjected to intense heat and pressure — without actually melting — their minerals recrystallize into new forms. These are metamorphic rocks. Shale (a sedimentary rock) can become slate. Limestone can become marble. Granite can become gneiss. The word metamorphic comes from the Greek word for 'change of form.'
The rock cycle has no beginning or end. A piece of granite might melt into magma, cool into basalt, weather into sediment, compact into sandstone, then be buried and squeezed into quartzite. All of these transformations take millions of years, driven by Earth's internal heat and surface forces like water and wind.
| Rock Type | How It Forms | Examples |
|---|---|---|
| Igneous | Magma or lava cools and solidifies | Granite, Basalt, Obsidian |
| Sedimentary | Sediment layers are compacted and cemented | Sandstone, Limestone, Shale |
| Metamorphic | Heat and pressure change existing rock | Marble, Slate, Quartzite |
Key terms
- Igneous rock
- Rock formed when magma or lava cools and solidifies.
- Sedimentary rock
- Rock formed from compressed and cemented layers of sediment (bits of rock, minerals, or organic material).
- Metamorphic rock
- Rock formed when existing rock is changed by heat, pressure, or chemical reactions without melting.
- Weathering
- The physical or chemical breakdown of rocks at or near Earth's surface.
- Erosion
- The process by which weathered material is picked up and transported by water, wind, ice, or gravity.
- Sediment
- Loose particles of rock, mineral, or organic material deposited by water, wind, or ice.
Flashcards
- Rock cycle
- The continuous process by which rocks are formed, broken down, and reformed over time.
- Magma
- Molten rock below Earth's surface.
- Lava
- Magma that has reached Earth's surface.
- Intrusive igneous rock
- Igneous rock that cooled slowly underground; has large crystals (e.g., granite).
- Extrusive igneous rock
- Igneous rock that cooled quickly at the surface; has small crystals (e.g., basalt).
- Compaction and cementation
- Processes that turn sediment layers into sedimentary rock under pressure.
Practice questions
What type of rock forms when lava cools quickly on Earth's surface?
- Sedimentary
- Metamorphic
- Extrusive igneous
- Intrusive igneous
Show answer
Answer: Extrusive igneous
Extrusive igneous rocks form at Earth's surface where lava cools quickly. Rapid cooling leaves little time for crystals to grow, resulting in fine-grained or glassy rock.
Fossils are most commonly found in metamorphic rocks.
Show answer
Answer: False
Fossils are found in sedimentary rocks, where organisms get buried in sediment layers before the rock forms. Heat and pressure of metamorphism would destroy fossil structures.
The physical or chemical breakdown of rocks at the surface is called ___.
Show answer
Answer: weathering
Weathering breaks rocks into smaller particles at Earth's surface. Physical weathering (frost, abrasion) breaks rock mechanically; chemical weathering (acid rain, oxidation) changes mineral composition.
Marble is a metamorphic rock that formed from which sedimentary rock?
- Sandstone
- Granite
- Limestone
- Shale
Show answer
Answer: Limestone
Limestone is a sedimentary rock made mainly of calcium carbonate. When buried and subjected to heat and pressure, it recrystallizes into marble.
Any type of rock can eventually be transformed into any other type through the rock cycle.
Show answer
Answer: True
The rock cycle has no fixed starting point. Given enough time and the right conditions (heat, pressure, erosion, melting), any rock type can be converted to another.
Fun facts
- The oldest rocks ever found on Earth are over 4 billion years old — almost as old as the planet itself.
- Marble, used in famous statues like Michelangelo's David, is metamorphic rock that started out as limestone on the ocean floor.
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