While we know a good deal about the composition and structure of the interior of the planet it is much more difficult to understand how we can gain such knowledge. In earthquakes we see what happens to the waves from them, these waves may be bent or even blocked altogether by the different materials making up the depths. A very important piece of evidence of having a thorough understanding of the Earth is its density.
Density simply means the weight of an object divided by its volume. The density of the surface rocks can easily be measured at an average of about 2.7 grams per cubic centimetre, but the density of the Earth as a whole seems to be about 5.5 grams per cubic centimetre. This clearly means that the materials which make up the interior of the Earth are very much more dense than the rocks on the surface.
There are two other clues. Firstly, the lavas which are spewed out from volcanoes and other types of rocks which arise from greater depths are an indication of what is going on at quite shallow depths. Finally, meteorites are thought to be composed of materials which are representative of planetary composition. But it has been the coming of earthquake studies that has given us such a clear picture of the Earth.
The Earth is generally divided into three main zones. The surface zone is the crust which has been found to average 6 kilometres thick under the ocean floor and about 45 kilometres thick under the continents. Bellow this there is a zone which is usually called the mantle and may also be called the zone of heavy rock.
The heavy rock is called peridotite and is composed of minerals containing mostly elements such as silicon, magnesium and iron. This zone continues to a depth of about 2900 kilometres and has a density of 3.4 grams per cubic centimetre. In fact the crust and topmost layer of the mantle has often been given the names sial and sima. The sial is composed of light rocks such as granite and its name comes from the chemical symbols for silica and alumina (silicon and aluminium oxides) which makes up these rocks. The sima is composed of dark heavy rocks such as basalt containing silica, iron oxides and magnesia. The centre zone of the Earth is called the core which has metallic properties probably composed of metals such as iron and nickel under very great pressures.
