What are Rocks?
Rocks form the hard outer crust of our planet, The Earth. Rocks are made up of minerals, and minerals themselves are made up of elements.
Unlike animals and plants, rocks are not alive. But that does not mean they always stay the same – in fact, they are changing all the time. Rocks can dissolve, melt, fall apart and even change into other rocks!
Yes – you could say the Earth is one gigantic rock! But the kind of rock we are used to seeing is only on the outside, or crust.
What are Minerals?
People use the word mineral to mean different things. True minerals are pure, solid substances made up of crystals. some minerals, like gold and carbon, are made of one element. Others, like salt and quartz, are made of a combination of elements. only natural substances are true minerals.
So if you find some salt in a salt mine, it is a mineral, but if you make salt in a science laboratory, it is not – even though they are exactly the same!
Elements are simple chemical substances such as gold and oxygen. They are the building blocks of most other substances. In fact, most things, including rocks, minerals, animals, plants, people, water and air are made up of combinations of elements.
What’s Inside the Earth?
Just under the Earth’s crust is a layer of incredibly hot, melted rock called magma. As you get nearer to the centre, or core, of the Earth, the rock gets hotter and hotter. Scientists think there is a metallic ball at the core of the Earth made of two elements, iron and nickel.
Sometimes bits of rock from space, called meteors, get so close to the Earth that they get sucked in by gravity. Most burn up as they zoom through the Earth’s atmosphere. But that land can make a huge crater when they hit the ground.
Where this crust forms the Earth’s continents, the most common rock is granite. The main difference is that a mineral is ‘homogeneous’ – it is the same all the way through. if you look at a lump of salt under a microscope, every part of it has the same structure. But rock is not homogeneous, it is a mixture. For example, granite, a rock, is made up of three minerals, mica, feldspar and quartz.
Substances like chalk, coal and oil were formed millions of years ago when plants or tiny sea creatures got squashed under layers of mud and water, and eventually turned into stone. Some scientists say that things that were once alive are not true minerals. But you will find them in most books on rocks and minerals.