History Of EarthThe history of the Earth covers approximately 4.5 billion years (4,540,000,000 years), from Earth’s formation out of the solar nebula to the present. This article presents a broad overview, summarizing the leading, most current scientific theories. The first eon in the Earth's history is called the Archaean. It lasted until 2.5 billion years ago. The oldest rocks found on Earth are about 4.0 billion years old. The timespan between the age of those oldest rocks and the formation of the Earth is sometimes seen as a separate eon, called the Hadean. Because no material from this time is preserved, little is known about Hadean times. The Earth's surface must have been under an intense bombardment of meteorites and volcanism must have been severe due to the large heat flow and geotherm. Sometimes sporadic detrital zircon crystals are found older than 4.0 billion years, and they show evidence of having been in contact with liquid water 4.3 billion years ago. This is proof that the planet already had oceans or seas at that time. From crater counts on other celestial bodies it is known that the intense meteorite bombardment (Late Heavy Bombardment) came to an end about 3.8 billion years ago. At the beginning of the Archaean eon, the Earth had cooled considerably. Due to the composition of the atmosphere, life would have been impossible for most present life forms, because of the lack of oxygen and absence of an ozone layer.
Origin of the solar system
The Solar System (including the Earth) formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. About 4.6 billion years ago, the solar nebula began to contract, possibly due to the shock wave of a nearby supernova. Such a shock wave would have caused the nebula to gain angular momentum. As the cloud began to accelerate its rotation, gravity and inertia flattened it into a protoplanetary disk oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small perturbations due to collisions and the angular momentum of other large debris created the means by which protoplanets up to several kilometres in size began to form.
The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star ignited to create the Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside the gravitational grasp of the new sun, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately becoming protoplanets. These included one collection approximately 150 million kilometers from the center: Earth. The planet formed about 4.54 billion years ago (within an uncertainty of 1%), and the planet was largely completed within 10–20 million years.The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies.
Computer simulations have shown that planets with distances equal to the terrestrial planets in our solar system can be created from a protoplanetary disk.
The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star ignited to create the Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside the gravitational grasp of the new sun, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately becoming protoplanets. These included one collection approximately 150 million kilometers from the center: Earth. The planet formed about 4.54 billion years ago (within an uncertainty of 1%), and the planet was largely completed within 10–20 million years.The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies.
Computer simulations have shown that planets with distances equal to the terrestrial planets in our solar system can be created from a protoplanetary disk.
