Many disciplines have developed for the study of Earth: geological sciences study the composition, structure, chemistry, and minerals; hydrologic sciences study the behavior of water on the planet; and atmospheric sciences study meteorology, climatology, and planetary atmosphere. Since ancient time, geometry and trigonometry have been developed to measure the vital data such as size, distances and locations. Thus we have some rather precise data about the planet. The information is often tabulated in many handbooks, and some have been provided here for your interest.
Structure of Earth provides some hint about its origin and evolutionand its somewhat erratic motion allows us to calculate positions of other stars and planets.
Aside from the atmosphere and oceans, the outer surface of Earth is a crust made up of mostly silicates, which has a simple chemical formula of SiO2. However, the silicates have many interesting forms and they appear as quartz, rose quartz, smoky quartz, amethyst, and other precious stones, the color of which are due to the presence of impurities. Other abundant oxides are Al2O3, TiO2, Fe2O3, CaCO3, etc. The Earth crust has all the stable chemical elements, but they form compounds with each other. Coal, diamond (carbon) and gold are some ]natural elements. During prehistoric times, people did find native silver and copper as collectibles, but these are depleted resource. Thickness of the Earth crust varies from some 30 to 50 km for continents to 5 km below the deep ocean floor. The crust has an average thickness of 15 km, and its density 3 g/mL
Many minerals are present in the earth crust, and we are interested in their formation and distribution whereas geologists, mining engineers and business people are interested in their exploitation and utilization. Table II summarizes some characters of the various shells of the planet earth, and the features of the shells are further described later.
Below the crust is a 3650-kilometer thick shell called mantle mostly made up of silicates. The second major compound in the upper mantle is aluminum oxide, Al2O3, whereas in the lower mantle is magnesium oxide, MgO. The density increases from 3.5x103 kg m-3 in the upper to 5.5 or 6x103 kg m-3 in the lower mantle, due to pressure increase with depth.
The Earth core is mostly iron and nickel, which ordinarily has a density of 7.9x103 kg m-3. The upper portion is mostly molten iron and nickel but the center is a solid iron and nickel core. The density increases from 12x103 kg m-3 in the molten shell to 13x103 kg m-3 or higher in the center core. The high density is again due to high pressure experienced by material in the center.
Between the inner core, outer core, mantles, and the crust are transition zones. Due to their differences in chemical composition and density, we describe them as different shells or layers. The methods used to determine the properties and thickness of these layers are interesting, and they can be found in books describing the planet Earth. Other properties such as magnetic field, gravitational field, mechanical motions, heat flow, continental movement, elemental and chemical distributions are also studied in specific fields.
In summary, the shell structure is given in a table form below:
|Shell name||Major chemicals||Density |
|0-15||Crust||SiO2, Al2O3, MgO
TiO2, Fe2O3, FeO etc
|3||30-50 km for continents
5 km under ocean floor
0.35 % of Earth mass
|15-900||Upper mantle||SiO2, Al2O3||3.5||Higher density than crust|
|900-2700||Lower mantle||SiO2, MgO||5-6||The mantles:
84 % of Earth by volume
67 % of Earth by mass
|SiO2, MgO, Fe||6||Transition region|
|2900-5000||Outer core||Molten Fe and Ni||12-13||The cores:
15 % of Earth by volume
32 % of Earth by mass
|Fe and Ni||13||Transition region|
|5150-6371||Inner core||Solid Fe and Ni||13-14|