Geology Rocks – rock around our homes
Rocks are made of minerals, which are solid chemical elements or compounds that occur naturally. They may be hard, such as granite, limestone and coal, or soft, such as sand and clay, but are always made up of minerals.
It may seem strange, but we need rocks for all our meals. They are used to make the crockery and cutlery we use at meal tables. More surprisingly, they – or the minerals they contain – are present in many of the things we eat and drink.
Setting the table
During meals, we use lots of bowls, plates, cups and saucers. This crockery is all made from clay.
We use special clays called china clay and ball clay to make crockery. The clays are formed when one of the minerals in another type of rock, granite, crumbles and rots. So, we find the clays in areas where there are granites, such as Devon and Cornwall. Crockery is made by forming these clays into the shapes we want and heating them in a kiln until they are baked hard.
If you look at a piece of granite through a magnifier, you can see that it is made up of crystals of different minerals. Only one of these – the white or pinkish one – breaks down to form the minerals in china clay and ball clay (in case you are curious to know, it is called feldspar). The clear crystals are quartz; the flat shiny silver or black specks are mica.
Things for cooking and eating
We need pans for cooking and kettles to heat water and these are usually made from steel or aluminium. So are the knives, forks and spoons we use. We may also wrap food in aluminium foil to protect it or to bake it in the oven.
‘Ores’ are the minerals or rocks that provide the metals we need, like steel and aluminium. Steel is made by melting a mixture or iron ore, limestone and coke (made from coal) in a large furnace. To improve steel – for example, to make it stainless – we add other metals such as nickel or chromium. All the materials we need to make steel come from rocks.
The metals are obtained from ores found in different countries. There are limestones in many parts of Britain – most were formed from the remains of shelly creatures that lived in ancient seas that no longer exist. Coal found in Britain was formed from plants that grew in swamps about 300 million years ago – these plants became buried under sand and mud and gradually changed into coal.
We often get drinks in glass bottles, or drink out of a glass.
Glass is made from a special sand made up of pure quartz grains mixed with limestone and other chemicals which are added to give the glass its colour, strength and brightness. For example, lead oxide makes glass heavier and gives it a sparkle; cobalt gives glass a lovely blue colour.
Sand is very common – we can find it on the beach, in sand dunes and in sandpits.
To make glass, very pure quartz sand is needed; ordinary sand is not pure enough to make clear strong glass because it has impurities such as iron, which makes the glass dark brown or green.
If you get some sand from a beach or a builders’ merchant and look at it through a magnifier, you’ll see that most of the grains are likely to be made of quartz and will appear clear (glassy) or white. Some grains may be stained red or brown by a thin layer of iron oxide (rust).
Beach sands may also contain salt, shell fragments or small pieces of other rocks. It is these other materials which make the sand unsuitable for making glass.
Items used at a meal table may also be made from plastic rather than glass, metal or pottery.
Plastics are made from oil (together with other chemicals and ‘fillers’ such as china clay and limestone), and oil comes from rocks.
When grains of sand and mud from rivers are carried out to sea they settle on the sea floor to form layers of sediment. The remains of any plants (especially algae) and animals that live in the sea become trapped between the sediment grains. When the sediments of ancient seas become buried and form rocks, the plant and animal remains break down into a brown, sticky liquid called crude oil.
The oil can become trapped in the rocks, and we can take it out of the ground by sinking boreholes to find it. In Britain, we get much of our oil from boreholes sunk into the floor of the North Sea. The oil is found in rocks a kilometre or more under the sea floor. It is used to make petrol and many other everyday items, including plastics.
Find out more at www.geolsoc.org.uk
Geology Rocks is supported by the Geological Society of London