The Earth

Our planet on which we live, the earth seems to be a very large body. But compared with quite an ordinary star, like the sun the earth is only a tiny speck in the universe. The earth is not a true sphere but a spheroid flattened at the poles so that the axis on which it rotates is shorter by 27 miles than the equatorial diameter. The outer layers of the earth are the atmosphere, the hydrosphere, and the lithosphere. The atmosphere, the outermost layer, is the envelope of gases that surrounds the earth. The hydrosphere includes all the bodies of water on the earth in gaseous, liquid, and solid states. It makes up only 0.02 per cent of the mass of the earth.

The three major parts of the earth are the crust, the mantle, and the core. The outermost more or less rigid shell covering the earth constitutes the earth’s crust. It includes the rocks of the continents as well as the rocks beneath the ocean floor. The lower boundary of the crust is 20 to 25 miles beneath the surface of the continents and only about 4 miles beneath the ocean. The continents are made up of a rock type known as granite covered by a thin layer of sedimentary rocks, and underlain by basalt, while the floor of all ocean basins is mostly underlain by basalt with a thin layer of sedimentary rocks above.