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Lessons · Teaching · rocks, minerals and the rock cycle

Three kinds of rock, and one is made of the other two

Igneous rock forms when molten rock cools, sedimentary rock forms when fragments settle and are compressed, and metamorphic rock forms when existing rock is changed by heat and pressure without melting. The rock cycle turns any one into any other, given enough time.

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In beta. This lesson was written for Hone and has not yet been checked by a state certification officer. Practice material, not professional advice. What that means.

What it is for

The cycle is the idea, not the three names. A pupil who knows only the names thinks rocks are three kinds of thing; one who knows the cycle understands that the mountain and the beach are the same material at different points in a loop.

How to think about it

Identify by how it formed, not by how it looks. Crystals throughout suggest cooling from molten. Visible layers or fragments suggest settling. Banding and a hard, altered look suggest heat and pressure short of melting. Then place it on the cycle and say what it could become next.

Worked example

Igneous: cooled from molten rock, and cooling slowly gives larger crystals
Granite cooled slowly underground; basalt cooled fast at the surface.
Sedimentary: fragments settled in layers and were compressed, sometimes with fossils in them
Fossils are found here and almost never in the other two.
Metamorphic: changed by heat and pressure while staying solid
Limestone becomes marble; shale becomes slate.
Melting turns anything igneous again; weathering turns anything into sediment
Which is what makes it a cycle rather than a list.

Your turn

Name the rock type in which fossils are found.

Fossils are found in  rock

The trap

Believing metamorphic rock has melted. If it melts it is igneous when it cools. Metamorphic means changed while still solid, and that is the whole definition.

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