THE STRUCTURE OF THE EARTH

The earth's structure

     

Structure of the mantle

The earth is a sphere, slighly flattened at the poles, with an average diameter of 12 800 km. It consists of a series of concentric spheres overlaid by a rocky crust. These are

The temperature increases the deeper one gets into the earth. The earth's core is estimated to have a temperature of around 5000 K. A large portion of the heat contained deep in the earth is due to the heat produced by the radioactive decay of certain elements.

2. The earth's crust

The solid outer layer of the earth, the crust, consists of the CONTINENTAL CRUST, about 20-70 km thick, and the OCEANIC CRUST, which is thinner (5-10 km thick) and overlaid by the oceans.

3. Rocks and minerals

Rocks are solid pieces of the earth's crust, formed over very long periods of time. Geologists classify rocks into three basic types:

IGNEOUS ROCKS: These are derived from the magna, which can either be expelled in volcanos as lava, in which case we call such rocks EXTRUSIVE, or rise close to the surface where weathering exposes them. The huge granite boulders that make up the top of the Paarl mountain (shown on the right) are typical igneous rocks. Another example are the spectacular basalt cliffs of the Drakensberg.

SEDIMENTARY ROCKS: These rocks are formed by an initial weathering of other rocks. The fine particles and pepples found their way, through rivers, into sedimentary basins, such as ancient lakes and seas, normally forming layers over long periods of time. High pressures slowly converted these layers into rocks of various hardness. The Cape Folded Mountains are made largely of sandstone, which is a typical sedimentary rock, and frequently display these rock layers, as in the photo on the right.

METAMORPHIC ROCKS: These rocks arise by the action of high temperatures and pressures on sedimendary rocks. Marble, slate and mica are examples of metamorphic rocks.

Silicon dioxide (SiO2) and metallic silicates make up an estimated 87% of the earth's crust.

Rocks are made up of MINERALS. Minerals may be defined as any solid compound or element found naturally in the earth's crust, that is, in the upper part of the lithosphere. Some minerals are elements such as diamond or gold, others may be relatively simple oxides, such as bauxite (Al2O3) or haematite (Fe2O3) carbonates, such as limestone (CaCO3).

4. Additional questions