الأسبوع 29 · المحور الرابع، الوحدة الرابعة، المفهوم 4.3 «البراكين» (ب) ومشروع الوحدة «القوى التي تشكل سطح الأرض» · 3 دروس
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The rock cycle
Discovery Education Science P4: Term 2, Unit 4, pp. 179–195
GoalName the three kinds of rock, say how each forms, and describe the rock cycle as a loop with no start or end.
New words
igneous rock · rock formed when melted rock (magma or lava) cools and hardenssedimentary rock · rock formed when layers of sediment are pressed and stuck togethermetamorphic rock · rock changed by great heat and pressure, without melting completely
Explanation
Rocks are not fixed for ever. They change from one kind to another in the rock cycle. When magma cools slowly underground it forms igneous rock with big grains, such as granite. The red granite of Aswan is an example. When lava cools quickly on the surface, it forms igneous rock with fine grains, such as basalt. Weathering and erosion break rocks into sediment, and deposition lays the sediment in layers. Over a very long time the layers are pressed and cemented together into sedimentary rock such as sandstone and limestone. Heat and pressure can change any rock into metamorphic rock: limestone can become marble. If rock gets hot enough, it melts into magma, and when this cools, igneous rock forms again. A cycle has no start and no end, and the steps can happen in many orders.
Worked examples
Which kind? Omar sorts five rocks. Granite and basalt cooled from melted rock, so they are igneous. Sandstone and limestone are made of pressed sediment, so they are sedimentary. Marble was changed by heat and pressure, so it is metamorphic.
granite→sand→sandstone→changed by heat and pressure
A path through the cycle. A mountain of granite weathers into sand. Rivers carry the sand to the sea. Over a long time the sand is pressed into sandstone. If it is buried deep, heat and pressure can change it again.
Model
Stands for
pressed sand layers
sedimentary rock
squashed play dough
metamorphic rock
Model it safely. Press layers of damp sand and a little soil in a small cup with a flat spoon: this makes layers like sedimentary rock. Squash layered play dough with the palm of your hand: it bends and folds a little like metamorphic rock. Neither model uses heat.
Class activity
Rock cycle models (no heat, a grown-up near). Press three thin layers of different sand and soil into a cup with a flat spoon, then tip the block out onto a plate and draw its layers. Next squash a ball of two-colour play dough flat, fold it and draw the swirls. Write which kind of rock each stands for. Then draw the rock cycle as a circle with five arrows and label the steps.
Practice
1
Which kind of rock forms when magma or lava cools?
Igneous rock.
2
Marble is made from limestone. What changes the limestone?
Great heat and pressure.
3
A cliff has 5 layers, each 2 cm thick in a model. How tall is the cliff?
5 × 2 = 10 cm.
4
Why do we say the rock cycle has no start or end?
Any kind of rock can change into another, and so on, again and again.
