Layers in a jar: how to show children where rocks come from
W4C Science · Earth Science · 4 min read · Evidence: established science; activity is classroom practice
Shake sand, gravel and soil in a jar of water and wait ten minutes. The layers that form are the start of the story of sedimentary rock, and children can see it happen.
Geologists sort rocks into three families by how they formed. Igneous rocks cool from melted rock; granite cools slowly underground and has big crystals, while rock that cools fast at the surface is fine-grained or glassy. Sedimentary rocks, such as sandstone, form from pieces of older rock that settle in layers and are pressed together. Metamorphic rocks form when heat and pressure change an existing rock. Over very long times, any of the three can turn into another. This is the rock cycle.
Most of this happens far too slowly to watch. The first step of sedimentary rock, settling in layers, is the exception.
The investigation
Fill a clear jar one third with a mix of gravel, sand and soil.
Add water to near the top, close the lid tightly and shake for 30 seconds.
Before putting it down, ask children to predict which will settle first.
Draw the jar at 1 minute, 5 minutes and 10 minutes.
Fair-test question: does shaking longer change the layers? Use a second jar with the same mix.
Big, heavy grains sink first and fine grains last, so the jar builds its own stripes. Show a photo of a striped cliff and ask children what it has in common with their jar.
A common confusion
Children often use "big" and "heavy" as if they meant the same thing. Ask: is a big pebble always heavier than a small one? Is a big piece of polystyrene? This is a good chance to separate size from weight.
Try this tomorrow. Collect 20 small, washed stones. Ask children to sort them into groups using their own rule, then explain the rule to a partner. Scientists classify rocks the same way: by properties they can observe and test.
Related resource: W4C-120 Young Scientists Lab includes this session plus rock sorting, an erosion tray and crystal growing. Coming soon to EduStore; subscribe to the newsletter to hear when it opens.
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