Formation of Earth.
The Earth, which is incredibly old, was not even present at the beginning of the universe. The universe began without human beings some 10 billion years earlier than Earth. The universe started out with only two elements, hydrogen and helium gas, which formed stars that burned these elements in nuclear fusion reactions. Generations of stars were born in gas clouds and died in explosive novas.
The Earth was formed some 5 billion years ago after the initial phase took place for the sun. After the sun was formed we know from observations and other indirect evidence that there were left over gases and heavier elements. The gravity of the sun helped to flatten these left over gases into a disk and started to fuse them together. This created the planetesimals and planetoids which would later make up the planets. Over time, these planetesimals would collide creating even bigger masses. Following this evolution, the Earth was eventually formed.
While the sun grew in size and energy, beginning to ignite its nuclear fires, the hot disk slowly cooled. This took millions of years. During that time, the components of the disk began to freeze out into small dust-size grains. Iron metal and compounds of silicon, magnesium, aluminum, and oxygen came out first in that fiery setting. Bits of these are preserved in chondrite meteorites. Slowly these grains settled together and collected into clumps, then chunks, then boulders and finally bodies large enough to exert their own gravity - planetesimals.
As time went by, planetesimals grew by collision with other bodies and as their mass grew larger, the energies involved did too. By the time they reached a hundred kilometre or so in size, planetesimal collisions produced a lot of outright melting and vapourization and the materials - rocks and iron metal, began to sort themselves out. The dense iron settled in the centre and lighter rock separated into a mantle around the iron, in a miniature of Earth and the other inner planets today.
The fusion eventually creates heavier elements such as carbon and iron. These elements were to compose a significant part of young Earth. The pressure and heat from radioactive decay of elements and the aftershocks of massive collisions caused the Earth to be molten. Over time the surface of the Earth cooled and became the crust. However the molten layers that remained became our mantle and the core. The currents of this massive underground ocean of magma caused volcanic activity that released gases. This led to the creation of atmosphere and oceans starting the water cycle. The formation of the Earth is only the beginning and we will still witness the Earth changing year after year through erosion and plate tectonics.
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