The rock cycle
How igneous, sedimentary and metamorphic rocks form, why some rocks have big crystals and others small ones, and how weathering, erosion and deposition recycle rock. With a fair-test sugar-cube 'river'.
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A rock is a solid collection of minerals. Granite is mostly the minerals quartz, feldspar and mica, as crystals locked together. Sandstone can contain the same minerals, but as grains cemented together.[1]
Igneous rocks form when magma cools. Lava that cools fast at the surface makes rock with small crystals, such as basalt. Magma that cools slowly underground, maybe over thousands or millions of years, makes rock with large crystals, such as granite.[1][4]
Rocks are broken down by weathering. Water freezes in cracks and the ice pushes them apart; rain is slightly acidic and slowly changes minerals; roots and burrowing animals widen cracks.[3]
Erosion moves the broken pieces, by gravity, wind, rain, rivers, oceans and glaciers. When the pieces are laid down, usually in seas and lakes, this is called deposition.[2][1]
Layer after layer piles up. The weight squeezes the layers (compaction) and minerals glue the grains together (cementation): a sedimentary rock. Each layer is younger than the layer beneath it. Limestone can form from shells, and coal from dead plants.[4][1]
Metamorphic rocks form when rocks are buried deep and changed by very high temperatures and pressures, without melting. Limestone becomes marble, and shale can become slate.[1][4]
Together this is the rock cycle: natural processes, including the movement of the Earth's plates, slowly change each kind of rock into another, recycling rock over millions of years.[5][2]
Try it: Sugar-cube river: shaking and wearing
You need
- 12 sugar cubes per group
- 3 identical plastic jars with lids
- A stopwatch or a counter
- A sieve or tea strainer
- A sheet of dark paper
- A magnifying glass
Safety
Use plastic jars, not glass. Hold the lids on firmly. Do not eat the sugar. Wipe up spilt sugar so nobody slips.
What to do
- Put 4 sugar cubes into each jar and close the lids.
- Plan a fair test: what will you change, and what will you keep the same? (Change the number of shakes; keep the same cubes, jars and shaking strength.)
- Predict: what will the cubes look like after 50, 100 and 200 shakes?
- Shake jar 1 50 times, jar 2 100 times and jar 3 200 times.
- Tip each jar through the sieve over the dark paper. Look at the cubes and at the powder that falls through.
- Count the sharp corners left on the cubes in each jar, and compare the amount of powder.
Play: Which kind of rock?
Sort each rock or clue: igneous, sedimentary or metamorphic?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16