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The Three Rock Families

Rock Families6 min read

Key takeaways

Igneous rocks crystallize from molten magma or lava.
Sedimentary rocks form from compacted sediment and often hold fossils and layers.
Metamorphic rocks are older rocks reshaped by heat and pressure.

One rock, many lives

Every rock on Earth belongs to one of three families, defined by how it formed. Over millions of years a rock can move between them — the 'rock cycle' — as it melts, erodes, or is squeezed deep underground. Knowing the family is the fastest way to organize an unknown specimen.

Igneous — from melt

Igneous rocks solidify from molten rock. Cool slowly deep underground and large crystals grow (granite, with visible grains). Cool fast at the surface as lava and crystals stay tiny or glassy (basalt, obsidian). Look for interlocking crystals and, often, no layering or fossils.

Sedimentary — from settling

Sedimentary rocks form when sand, mud, shells, or dissolved minerals accumulate and harden. They frequently show visible layers (bedding), rounded grains, and — crucially — fossils. Sandstone, shale, and limestone are classic examples. Many feel softer or more crumbly than igneous rock.

Metamorphic — from change

Metamorphic rocks are pre-existing rocks transformed in the solid state by intense heat and pressure, without fully melting. Shale becomes slate then schist; limestone becomes marble; granite becomes gneiss. Tell-tale signs are foliation (mineral grains aligned into bands or wavy stripes) and a harder, denser feel.

A quick sorting test

Ask three questions in order: Does it have flat layers or fossils? Likely sedimentary. Does it have wavy bands or a foliated, shiny sheen? Likely metamorphic. Is it a mass of interlocking crystals with no layering? Likely igneous. It's not foolproof, but it sorts most hand samples fast.

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