The Three Rock Types
All rocks fall into THREE main types based on how they FORMED. IGNEOUS rocks form from cooling MAGMA or LAVA (molten rock). SEDIMENTARY rocks form when sediment (sand, mud, shells, plant matter) is compressed and cemented. METAMORPHIC rocks form when existing rocks are TRANSFORMED by heat, pressure, or chemical activity — usually deep underground.
Examples and clues. IGNEOUS: granite (slow-cooled, big crystals), basalt (fast-cooled, small crystals), pumice (volcanic, full of holes). SEDIMENTARY: sandstone (compressed sand), limestone (compressed shells), shale (compressed mud), coal (compressed plants). Often have LAYERS. METAMORPHIC: marble (from limestone), slate (from shale), schist, gneiss. Often have BANDS or sheen. Each rock type tells a story about its formation.
You see a rock with clear LAYERS, fossils visible. Most likely:
The rock cycle. Rocks change types over millions of years. Igneous can erode into sediment → sedimentary rock. Sedimentary heated and squeezed becomes metamorphic. Metamorphic melted becomes magma → igneous. There's no "final" rock type — Earth is constantly recycling its crust.
Identify Three
Find or look up images of: granite, sandstone, marble. Note what they look like. The same minerals can appear in different rock types depending on the formation conditions.
Three rock types. One eternal cycle. Knowing them lets you read any rock landscape — and Earth's story written in stone.
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