Minerals and the Three Rock Families
A rock is an assemblage of minerals, and which minerals are present plus how they are arranged says how the rock formed. Grain size in an igneous rock records cooling rate, sorting in a sedimentary one records transport, and banding in a metamorphic one records directed pressure.
What a learner can do afterwards
- Identify a hand specimen as igneous, sedimentary or metamorphic from texture rather than colour
- Explain what crystal size in an igneous rock tells you about where it cooled
- Relate the sorting and rounding of sediment grains to how far they travelled
1 · Read
A rock is a pack of minerals grown or glued together. Which minerals sit inside, and how they are arranged, tells you how the rock formed. That is why you judge a rock by its texture and never by its colour.
Picture four mystery rocks. Granite holds interlocking crystals that grew as melt cooled. Sandstone and conglomerate hold separate grains and pebbles pressed and glued together. Phyllite shines because its minerals were pressed flat into lined up sheets.
Each family hides a measuring tape. In an igneous rock, big crystals mean the melt cooled slowly deep down, and tiny ones mean it cooled fast at the surface. In a sedimentary rock, rounded well sorted grains travelled far, while angular mixed grains stayed near home. In a metamorphic rock, bands record pressure that pushed harder from one side.
Hold any hand specimen to the light and run this check. Interlocking crystals mean igneous. Loose looking grains or pebbles mean sedimentary. Shine plus bands plus splitting into flat sheets mean metamorphic.
Minerals plus their arrangement name the family, and grain size, sorting and banding tell where and how each rock formed.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.