Age of the Earth
The question of the origin of the earth and the world already occupied mankind since prehistoric times. In different cultures many creation myths, such as the Bible exist in the first book of Moses (Genesis). Since the mid-19th century predominate in natural science naturalistic explanations, which astrophysical processes may have led to the formation of the earth.
In a wider context, the legislative history of the earth is linked with the history of the universe and our home galaxy, the Milky Way, in general, and the history of our solar system in particular.
Science now believes that the universe is constantly expanding since the Big Bang about 13.8 billion years ago. In the course of this, the matter distributed in space in it, but irregular. The irregularities in the density of matter led by gravity to the first major gas accumulations. Only in the course of billions of years and after many transformations formed from the us today known order of galaxies and planetary systems by matter bundled by gravity and centrifugal force (centrifugal force ) in certain systems and ordered.
It is believed that as a precursor of our solar system was a solar nebula, a compression in a large cloud of gas and dust that collapsed through its gravitational pull in about 10,000 years (see star formation ). In the accretion disk around the protostar, the dust fell from the mid-plane and clumped together. As was found with the help of the uranium - lead dating, this happened just before 4.57 billion years. The larger clumps collected by their gravity on other dust and grew into planetesimals, which then formed by collisions protoplanets, see planet formation. These and partly also the planetesimals were already differentiated into a metallic core (mostly iron ) and a rocky mantle, which froze repeatedly between collisions.
Earliest stage ( Hadean )
The proto- Earth grew mainly by collisions with not much smaller protoplanets. After the collision theory, the moon was created as a result of the last large impact event. The hypothetical Proto planet or comet is called Theia and must have possessed between Moon and Mars size. Theias iron core has been connected to the earth and parts of the mantle of the proto- Earth and Theia were hurled into orbit from which the moon was formed. That happened sometime between 30 to 50 million years after the dust phase. The bombardment smaller body had already subsided; in any case can then hardly iron seeped through the jacket (less than 1 % of the core mass ), as analyzes of the hafnium - tungsten - decay chain of Early Archaic rocks revealed.
The partially melted by the impact of mantle solidified within a few million years, from the inside out. Thus it was not much hotter inside than on the outside, at least not hot enough for mantle convection, so that in the first half of Hadaikums the mantle was stably stratified. In addition, a thick oceanic crust was in the final stages of solidification, no longer cared as convection for mixing formed. Thus the earth was geologically quiet for a few 100 million years: There was no earthquake nor volcanoes. The planet was mostly covered in water and relatively cool and possibly icy in places because of the time still weak, young sun. Under the then frequently impacting small bodies was every few 100,000 years time, a larger, with a diameter of some 100 km - not big enough to wipe out global life, if it already existed.
Black smokers, where possibly life emerged there has been only toward the end of the Hadaikums (arbitrarily to 4 billion years (4 Ga) fixed ): Deep in the Earth's mantle, the temperature was increased slowly by radioactive decay heat until the mantle began and the temperature gradient near the surface increased greatly. The thick oceanic crust was partially melted and partially incorporated into the coat again, which resulted from leaving behind residual material first pieces of continental crust, see TTG complex.
The onset of geological activity and the existence of liquid water is indicated by the formed before 4.2 to 4.0 Ga oldest preserved minerals, zircons. In this time probably falls the transition from chemical to biological evolution, certainly is found in the oldest preserved crustal parts, called cratons, from the end of Hadaikums, sometimes the typical life depletion of C -13 over C- 12th
A little later, at the beginning of the Archean life invented oxygenic photosynthesis, whereby elemental oxygen was produced, the iron is bound to be found today as Bändererz. However, in the primordial atmosphere, the oxygen content increased only in the last 0.05 Ga of the Archean slowly until it suddenly rose before about 2.5 Ga, which is called the Great oxygen catastrophe.
The climate was much warmer throughout the Archean than today and very humid, see paradox of the weak young sun.
Approximately in the middle of the Archaic period falls the maximum sheath temperature. The area of the continental crust is increasing rapidly.
In the Proterozoic, the atmosphere continued to develop in a favorable direction for higher life composition. Due to the increase in biological activity (photosynthesis, methane emissions ), the oxygen content increased.
At the end of the Proterozoic was the Cambrian explosion of life, before about 500 million years ago.
For further development see also:
- Biological Evolution
- Chemical Evolution
- Development of the earth's atmosphere
- Origin of terrestrial water
- Volcanism, plate tectonics
- Geological time scale
Overview: Phases of Earth's history
( Dates shown in million years before present)