<P> Earth was initially molten due to extreme volcanism and frequent collisions with other bodies . Eventually, the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere . The Moon formed soon afterwards, possibly as a result of the impact of a planetoid with the Earth . Outgassing and volcanic activity produced the primordial atmosphere . Condensing water vapor, augmented by ice delivered from comets, produced the oceans . </P> <P> As the surface continually reshaped itself over hundreds of millions of years, continents formed and broke apart . They migrated across the surface, occasionally combining to form a supercontinent . Roughly 750 million years ago, the earliest - known supercontinent Rodinia, began to break apart . The continents later recombined to form Pannotia, 600 to 540 million years ago, then finally Pangaea, which broke apart 200 million years ago . </P> <P> The present pattern of ice ages began about 40 million years ago, then intensified at the end of the Pliocene . The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40,000--100,000 years . The last glacial period of the current ice age ended about 10,000 years ago . </P> <P> The Precambrian includes approximately 90% of geologic time . It extends from 4.6 billion years ago to the beginning of the Cambrian Period (about 541 Ma). It includes three eons, the Hadean, Archean, and Proterozoic . </P>

Most of earth's history occurred during which of these times