To classify a rock, read the evidence first: crystals, layers or bands. Each points to a formation process, and the process gives the family.
This lesson is part of rocks and rock-cycle reasoning. When you can classify, move to tracing two possible pathways through the rock cycle.
What evidence points to which family?
| Evidence in the description | Process | Family |
|---|---|---|
| Interlocking crystals, large | Slow cooling of magma underground | Igneous, intrusive |
| Interlocking crystals, tiny or glassy | Fast cooling of lava at the surface | Igneous, extrusive |
| Layers, fossils, rounded grains | Deposition, compaction and cementation | Sedimentary |
| Bands, aligned minerals, very hard | Heat and pressure change an older rock | Metamorphic |
Use the table to match clues to families, not to memorise by rote.
Worked example: three invented specimens
Specimen A. Large, visible crystals of different minerals fit together like a jigsaw. No layers.
The evidence is crystal size, so the rock cooled slowly from magma. It is intrusive igneous.
Specimen B. Thin, flat layers of fine sand, with an impression of a shell.
Layers and a fossil show that sediment was laid down and buried, then compacted and cemented. It is sedimentary.
Specimen C. Hard, compact rock with wavy bands where dark and light minerals line up. The description says it was buried near a large mass of magma.
Heat and pressure changed an earlier rock and aligned its minerals. It is metamorphic.
The mistake that costs marks
The common slip is to write “it is sedimentary because it is a rock found on land”. That is not evidence, and it would fit any rock.
A stronger sentence quotes the clue and the process: “The rock has layers and a shell impression, which shows deposition and compaction of sediment, so it is sedimentary.” The answer names the clue, the process and the family in one sentence.
Check yourself
A rock has tiny crystals you cannot see without a hand lens and small holes where gas escaped. It was found beside an old volcano. Classify it and explain.
Answer
It is extrusive igneous rock. The tiny crystals show fast cooling, because lava at the surface loses heat quickly and crystals have no time to grow. The gas holes and the volcano location support a surface eruption.
What to study next
Go on to tracing two possible pathways through the rock cycle. Use the claim, evidence and explanation revision desk to rehearse the clue, process, family sentence.
If you want a teacher to give you fresh specimen descriptions, see online one-to-one Geography tuition.