Discovery Science: The Earth – Origins
The stages of the Earth’s development can be derived from hypotheses based on both good evidence and speculation about the origins of the solar system.
However, one thing is certain: it was a long journey from a glowing ball to the blue planet, and at the beginning there was no hint that life would one day flourish there.
Once upon a time, 4.56 billion years ago
In its early phase, the Earth was a place of extremes, with fire-spewing volcanoes, innumerable comet impacts, lightning, severe thunderstorms, and a toxic atmosphere. These conditions improved as the Earth cooled down.
The Earth was created-along with the sun and the other planets-about 4.6 billion years ago from an interstellar gas and dust cloud. Presumably triggered by a shock wave from a supernova explosion, this cloud collapsed, started to rotate, and gradually flattened. Its matter became increasingly concentrated in the center, and eventually the increasing density triggered nuclear fusion, forming the protosun.
The volatile components of the original cloud were blown toward the edges, while the firm dust particles in the interior of the cloud agglomerated into small clumps of matter. These clumps collided with each other and formed larger bodies, the planetesimals. These in turn grew into the inner planets of our solar system in a process called accretion. One of these planets was the proto-Earth.
The process beginning with the fusion of the first cosmic particles and continuing on until the formation of the Earth-a body around 8,000 miles (13,000 km) in diameter-took place very quickly in astronomical terms, probably in no longer than 30 million years.
Crust formation
The cosmic dust that formed the Earth primarily contained aluminum, magnesium, iron and nickel compounds, and silica. Under the pressure of the growing mass, the interior of the Earth heated up and became a liquid, igneous mass. The decay of radioactive elements added to the heating process. This liquidation caused a separation of sub- stances: the heavier elements, especially nickel and iron, sank and formed a metallic core.
Lighter substances, such as silicates, were displaced toward the surface and formed the crust. The formation of a shell, which is known as differentiation, was probably complete about 100 million years after the birth of the Earth. The moon was also formed during this phase, after a collision between the Earth and another protoplanet.
RADIOMETRIC DATING
The age of the Earth was unknown until the French physicist Henri Becquerel (1852-1908) discovered radioactivity in 1896.
Every radioactive isotope decays at a characteristic rate (called the half-life). The remaining amount of isotopes present in rocks can therefore be used to calculate the age at the rocks and the age of the Earth.