Formation of the Earth: The Hadean
Formation of the Earth
(4.6-4.0 billion years ago)
The solar system form
During the Hadean, the solar system formed. The sun and all the planets, including the Earth, were assembled from a large cloud of gas and dust. Small particles of dust collided to form bigger and bigger particles until planet-sized objects formed. At this time, the Earth and other planets were molten due to the heat generated from these collisions. At some time between 4.6-4.0 billion years ago, the Earth’s crust cooled and solidified, but very little remains of this original crust. The oldest rocks on Earth are meteorites and moon rocks, from which we get much of what we know about the Hadean. Meteors are rocks traveling through space and are sometimes caught in the Earth’s gravitational field. Most of these are small and burn up in the Earth’s atmosphere, but those that survive and crash into the Earth are called meteorites. Radiometric dating of meteorites reveals that most of them were formed about 4.5 billion years ago. Samples of moon rocks brought to Earth by the Apollo moon missions have also been dated to about 4.5 billion years ago. The oldest dated Earth material is 4.4 billion years old and is a zircon crystal found in sandstone from Western Australia. The zircon crystal was once part of another rock that was broken down by erosion; only the zircon crystal survives today.