Geologic history of Earth, evolution of the continents, oceans, atmosphere, and biosphere. The layers of rock at Earth’s surface contain evidence of the evolutionary processes undergone by these components of the terrestrial environment during the times at which each layer was formed. By studying this rock record from the very beginning, it is thus possible to trace their development and the resultant changes through time.
The Pregeologic Period
From the point at which the planet first began to form, the history of Earth spans approximately 4.6 billion years. The oldest known rocks—the faux amphibolites of the Nuvvuagittuq greenstone belt in Quebec, Canada—however, have an isotopic age of 4.28 billion years. There is in effect a stretch of approximately 300 million years for which no geologic record for rocks exists, and the evolution of this pregeologic period of time is, not surprisingly, the subject of much speculation. To understand this little-known period, the following factors have to be considered: the age of formation at 4.6 billion years ago, the processes in operation until 4.3 billion years ago, the bombardment of Earth by meteorites, and the earliest zircon crystals.