Rock Types
Elementary School
Definition
The three main categories of rock classified by how they form: igneous rock from cooled molten material, sedimentary rock from compacted sediments, and metamorphic rock changed by heat and pressure. Each type gives clues about Earth's history.
Worked examples
Granite forms when magma cools slowly underground, creating large interlocking crystals.
This is igneous rock — molten material cooled to form the rock you see.
Sandstone forms when sand grains are deposited in layers, then compacted and cemented together over time.
This is sedimentary rock — loose sediments were pressed into solid rock.
Marble forms when limestone is subjected to intense heat and pressure deep in Earth's crust.
This is metamorphic rock — an existing rock was changed by heat and pressure.
Common mistakes
- All rocks with crystals are igneous → Metamorphic rocks can also have crystals formed during transformation Marble and gneiss are metamorphic but show crystals created by heat and pressure, not cooling magma.
- Sedimentary rocks are always made of sand → Sedimentary rocks form from any compacted sediment: mud, shells, pebbles, or sand Shale forms from mud, limestone from shells, and conglomerate from pebbles — all sedimentary.
- Metamorphic rocks melt during formation → Metamorphic rocks change in the solid state without melting If the rock melts, it becomes magma and will form igneous rock when it cools, not metamorphic.
Where you'll use it next
Understanding rock types helps you study the rock cycle, plate tectonics, fossil formation, and Earth's geologic history in later Earth science units and geology courses.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026