Summary, characteristics and origin of the planet Earth. Composition, motion, heat flow, magnetism, electricity, currents and curiosities.
What is Planet Earth?
The Earth is the third planet in the solar system , in order of distance from the Sun ; it is the only known planet that hosts forms of life. Since the Earth’s surface has different curvatures, the shape of the Earth cannot be identified with that of a defined geometric solid. However, ignoring the reliefs and surface irregularities, it can be traced back to that of a rotation ellipsoid, that is, the geometric figure obtained by rotating an ellipse around its minor axis. Recent calculations, based on the perturbations of the orbits of artificial satellites, have shown that the Earth has a slightly piriform (pear-shaped) shape: in fact, the difference between the minimum equatorial radius and the polar radius (distance between the center of the Earth and the North Pole) is about 21 km, furthermore the North Pole “protrudes” by about 10 m, while the South Pole is depressed by 31 m.
The motion of the Earth
The position of the Earth in space is not fixed but is the result of a complex series of movements that occur with different characteristics and periods. Together with the Moon , our planet orbits the Sun, at an average distance of 149,503,000 km and with an average speed of 29.8 km/s, completing a complete revolution in 365 days, 6 hours 9 minutes and 10 seconds (sidereal year); the trajectory is an ellipse of small eccentricity, therefore the orbit is almost circular with a length approximately equal to 938,900,000 km. Furthermore, the Earth is also affected by a rotation motion around its axis, which occurs from west to east, that is, in the opposite direction to the apparent motion of the Sun and the celestial sphere, and takes place with a period of 23 hours, 56 minutes and 4.1 seconds (sidereal day).
The Earth follows the motion of the entire solar system and moves in space at a speed of about 20.1 km/s in the direction of the constellation of Hercules; it also participates in the recession motion of the galaxy and together with the Milky Way moves towards the constellation of Leo.
In addition to these principal movements, there are other secondary ones, including the precession of the equinoxes and nutations. The latter consist of a periodic variation in the inclination of the Earth’s axis, due to the gravitational attraction of the Sun and the Moon.
The composition of the Earth
The Earth can be divided into five parts:
- the atmosphere, gaseous: it is the gaseous shell that surrounds the solid body of the planet: although it has a thickness of over 1100 km, about half of its mass is concentrated in the first 5.6 km.
- Hydrosphere, liquid : is the layer of water that, in the form of oceans, covers approximately 70.8% of the globe’s surface. The hydrosphere includes the oceans but also inland seas, lakes, rivers and groundwater. The oceans have an average depth of 3,794 m, about five times the average height of the continents, and a mass approximately equal to 1,350,000,000,000,000,000,000 (1.35 × 1018) tons, or about 1/4400 of the total mass of the Earth.
- The lithosphere: The rocks of the lithosphere have an average density 2.7 times greater than that of water and are almost entirely made up of eleven elements, which together represent approximately 99.5% of their mass. The most abundant of these is oxygen (which represents approximately 46.60% of the total), followed by silicon (approximately 27.72%), aluminum (8.13%), iron (5.0%), calcium (3.63%), sodium (2.83%), potassium (2.59%), magnesium (2.09%), titanium, hydrogen and phosphorus (in total quantities less than 1%). In addition, there are traces of other elements such as carbon, manganese, sulfur, barium, chlorine, chromium, fluorine, zirconium, nickel, strontium and vanadium. These substances are found in the lithosphere in the form of chemical compounds and are rarely found in their elemental state.
The lithosphere comprises two shells , the crust and the upper mantle , which are divided into a tectonic plate structure delimited by constructive margins, where new lithospheric material is formed, destructive margins, where subduction processes (ocean ridges) or conservative margins occur, characterised by transform faults along which sliding occurs between adjacent lithospheric layers; some plates are delimited by a combination of the three types of margins.
There are two types of crust, which differ both in the nature and structure of the rocks that constitute them, and in their age, and in the average level of their surface. The continental crust includes the continental shelf and part of the adjacent continental slope and is composed of magmatic, metamorphic and sedimentary rocks of average chemical composition similar to that of granite and of extremely variable age: the oldest rocks, formed about 4 billion years ago, are adjacent to very young rocks. The oceanic crust, on the other hand, forms the base of the ocean basins and has a very regular layered structure: under a relatively thin layer of sediments there is a layer of basaltic rocks, followed at greater depths by a layer of gabbro. The age of these rocks is no more than 190 million years.
The average surface level of the continental crust is more than 4,000 meters higher than that of the oceanic crust, although oceanic reliefs and depressions constitute only a small percentage of the land and ocean floors. According to a classification due to the German geologist A. Wegener and now almost completely abandoned, the Earth’s crust is made up of an upper platform of sial (from the initials of silicon and aluminum), made up of rocks with a granitic composition, which “floats” on a dense layer of sima (from silicon and magnesium), formed essentially by basalts. According to this theory, sialic rocks constitute the continents while simatic rocks emerge on the ocean floors. - The mantle : extends from the base of the crust to a depth of about 2900 km. Except for some areas, such as the asthenosphere, it is solid and its density increases with depth, varying between 3.3 and 6. The upper mantle is composed of iron and magnesium silicates, and is significantly represented by the mineral olivine; the lower part probably consists of a mixture of magnesium, silicon and iron oxides organized in crystalline structures capable of withstanding conditions of high temperature and pressure.
- The core : The upper mantle is separated from the overlying crust by a seismic discontinuity, called the Moho, and from the lower mantle by a more plastic zone, known as the asthenosphere. The upper mantle, sliding laterally over the partially molten rocks of the asthenosphere, which is 100 km thick, allows the continents to drift and the ocean floors to spread.
The transition between the mantle and the core is marked by a surface of seismic discontinuity, known as the Gutenberg discontinuity. Seismic studies show that the core has an outer shell of fluid matter, about 2,225 km thick, with an average density of 10. Research results show that its outer surface has depressions and peaks that probably form where hot material rises. In contrast, the inner core, which has a radius of about 1,275 km, is solid. Both layers of the core are thought to be composed of iron, with a small percentage of nickel and other elements. The temperature in the inner core is estimated to be around 6650 °C and the average density is estimated to be around 13.
