THE ORIGINAL EARTH

Formation

The Earth's surface has been comprehensively remodelled from its initial state as it has evolved through time, with new rock systems replacing and covering those of greater age. As a consequence, the early stages of the Earth's development are poorly known, through progressive lack of evidence backwards to the beginning. The most accepted scientific theory is that following the big bang and the origin of the universe, and after the contraction of the matter that formed the Sun, a huge rotating disc of dust and gas, spread across billions of kilometres of space, was left behind. This disc of debris gradually collapsed due to gravity, coalescing into a series of variously sized lumps, or planetesimals. The planetesimals, which ranged in size from a few metres to Mars-like proportions, then accreted by gravity over a period estimated to have been 29–100 million years. They aggregated to form spinning balls of rock, gas and ice – the planets – and the Earth is one of them.

The most widely accepted hypothesis is that the Moon originated from a collision between the Earth and a similar-sized body, with the mass now forming the Moon ejected as a consequence. Moon rocks can be matched with igneous rocks of the Earth and are between about 4.5 and 3.1 Ga old, overlapping substantially with the age of the early Earth's rocks. The early Earth and Moon were bombarded with meteoroids, asteroids and comets, probably the debris left from formation of the Solar System.