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Question: How was the Earth created?
Most scientists these days subscribe to the nebula hypothesis for the origin of our solar system, including planet Earth. The nebula hypothesis was originally proposed by the German philosopher Immanuel Kant in 1755, but it has evolved considerably recently as a result of data collected using spacecraft and modern telescopes.
Although the nebula hypothesis is based on numerous careful observations made by many scientists, it remains a hypothesis because we do not know for sure: we were not around to see the Earth forming. The nebula was a diffuse, roughly spherical, slowly rotating cloud of gas and dust. The gases were mostly hydrogen and helium, two elements that make up most of the Sun, and the dust was chemically similar to material that makes up the Earth.
According to the hypothesis, the nebula contracted due to gravitational attraction between the bits of matter because of their mass. This caused the rotation rate to increase and flattening of the nebula into a disc shape. Most of the matter drifted towards the center of the nebula, eventually forming the Sun. Compression of matter under its own weight caused tremendous increase in temperature in the central part of the nebula. The disc-shaped nebula then started to cool and many of the gases condensed into liquid or solid. Gravitational attraction caused the dust and condensed matter to stick together (accrete) into kilometer-sized chunks called planetesimals, and these in turn collided and stuck together to formed the nine planets.
The inner planets (Mercury, Venus, Earth and Mars) are relatively small and made of dense minerals and rocks. Light material, such as hydrogen and helium, was blown away by radiation and matter streaming from the Sun. Meteorites that regularly strike the Earth are thought to be remnants of the planetesimal stage, and their age (determined by radiometric dating) suggests that the inner planets began to accrete about 4.56 billion years ago. Planetary accretion could have happened in less than 100 million years.
The Earth of 4.5 billion years ago was quite different from what we see today: a living planet with continents, oceans and an oxygen-rich atmosphere. Violent impacts from planetesimals were common on the early Earth, and the kinetic energy of these impacting masses was converted to heat. Apparently, one particularly large mass impacted the Earth about 4.5 billion years ago and ejected a large amount of debris into space that ultimately accreted to form the Moon.
The Earth's rotation rate and axis of rotation was changed by this major impact, and an enormous amount of heat was generated. Additional heat was generated by radioactive decay of certain elements in the Earth (e.g., uranium). Perhaps 50% of the earth was either molten or in a plastic state at this time. This molten and plastic state allowed relatively heavy material to drift towards the center of the earth to form the core and lighter material to move towards the surface to form the mantle and crust (i.e., the Earth became concentrically zoned, or differentiated). The core is made mainly of iron and other heavy elements, whereas the mantle and crust are made mainly of silicates (compounds of silicon, oxygen, aluminum, iron, magnesium, calcium, sodium and potassium). The rising molten material brought heat to the surface and radiated out into space. Thus, the Earth cooled down and became mostly solid, apart from the liquid outer core and isolated patches of hot molten material in the mantle and crust.
During the differentiation stage, the lightest material escaped from the solid Earth as gas and liquid to form the oceans and atmosphere. This escape process continues today during volcanic eruptions. The main gases released from volcanoes today are hydrogen, carbon dioxide, nitrogen and water vapor but no oxygen. Oxygen only formed a substantial part of the atmosphere once photosynthetic organisms had evolved.
The present Earth and its oceans and atmosphere have evolved considerably over the past 4.5 billion years. This evolution has involved the formation and movement of a number of 100-km-thick solid plates over the surface of the earth, associated with earthquakes and volcanic eruptions, and giving rise to continents, mountains and ocean basins. The interaction between the atmosphere, hydrosphere and the surface of the earth has caused erosion of the land and deposition of the eroded material in low places on the Earth. The composition of the atmosphere and oceans has changed over time, and life has evolved somewhat episodically.
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