A very, very long time ago before there was an Earth, a Supernova occurred in our solar system. This explosion sent raw material scattered across space. Eventually this debris collected into a mass again due to gravity. From the remaining material, our planets and their moons were formed, due to Accretion. The early earth was a molten ball of lava, constantly being impacted by large chunks of space rock and debris. In the very beginning of earth's history, this planet was a giant, red hot, boiling sea of molten rock, a magma ocean. The heat had been generated by the repeated high speed collisions of much smaller bodies of space rocks that continually clumped together as they collided to form this planet. As the collisions off the earth began to cool, forming a thin crust on its surface. The oldest rocks geologists have been able to find are 3.9 billion years old. Using radiometric dating methods to determine the age of rocks means scientists have to rely on when the rock was initially formed. When Earth was molten, the denser materials sank to the bottom and lighter elements to the top and this transformed into the distinct layers themselves : Crust / Mantle / Outer core (liquid) / Inner core . The reason why Earth had so much internal heat was because of impact energy, gravitational energy, and radioactive decay of elements.
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