The rock cycle

First establish your pupils' understanding of the terms 'rock' and 'mineral' - remember that rocks are made of minerals. Each rock is usually composed of several minerals mixed up as small grains or crystals. In sedimentary rocks the grains are usually rounded and glued (cemented) together loosely, making them porous and crumbly. In igneous and metamorphic rock the grains are interlocking, making the rocks harder and impervious to water. Minerals have been used by humans ever since the discovery that a piece of flint made an excellent tool.

To set the rock cycle in context try asking the following question:

■ What is the link between granite, sand, clay, mudstone and slate?

Begin your explanation by introducing volcanoes (use a DVD clip or download one from a website). Let the pupils examine a piece of granite and ask them to describe it. They should note the large light-coloured crystals of quartz, the darker feldspar. Granite is an igneous rock. [Note: explore the origins of the word 'igneous' - ignite; to do with fire (see Chapter 4, p. 34).] The crystals in granite are relatively large which indicates slow formation and slow cooling, suggesting that granite cooled from magma that never reached the Earth's surface as a volcano. There is opportunity for practical work on the formation of crystals here.

Inform the class that quartz is silicon oxide, the same as sand, and feldspars are complex aluminium silicates, the same as clay. So once granite has been broken down (weathered) by a series of physical and biological means the resulting grains will be carried away (eroded), usually by water, and deposited on beaches or river beds as sand (quartz) and clay (feldspar). If they shake any soil sample with water in a jar and leave it to stand they will see those two minerals - first the sand settles to the bottom, followed by the clay. These sediments become compacted by geological processes to form sedimentary rocks such as sandstone (silicon based) and mudstone (a derivative of the finer feldspar particles; clay, mud and silt). [Note: explore the etymology of 'sedimentary', similar to sedentary; to settle.] This is also an ideal time to explore and examine fossils, their formation and means of distortion, fragmentation and preservation.

As time goes by these sedimentary rocks become covered with more sediments, compacting them further. The heat and pressure sometimes causes changes in their chemical and physical make-up and the rocks begin to 'cook'. Shale and slate are examples of a rock transformed (metamorphosed) in this way from clay/mudstone.

Other metamorphic rocks, such as marble (from chalk and limestone) and quartz-ite (from sandstone), have been subject to even higher temperatures caused by being within close proximity of the mantle either through subduction of the plates or by being along a fault line. Discussion of the term 'metamorphic' will bring in cross-discipline uses such as the life cycles of insects [meta = from Latin and Greek, meaning to transform; morph = Greek, meaning form].

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