Proterozoic Eon

This was the period of early life on the Earth, which lasted from about 2.5 billion years ago to about 542 million years ago. The record of this period is much better preserved than the Archean. Unlike Archean rocks, which have been greatly degraded due to their great age and metamorphosis (most strata was laid down in deep oceans); the Proterozoic strata has been much better preserved. Land had grown and become stable enough to allow large inland seas, and strata deposited in these areas is relatively well preserved.

Several important things happened during the Proterozoic, including:

The Earth was much more geologically active during the Proterozoic than it is today. The rocks beneath the crust were hotter, the crust itself was less stable and moved more quickly above the mantle than it does today. The mid-ocean ridges formed, and new ocean floor was generated at a rapid rate. As a result, the early continental plates moved and bumped against each other frequently, leading to active formation of surface features such as mountains, rifts, etc.

Very little is known about ancient landmasses. Continental drift and modern plate tectonics can be used to extrapolate the state of continents as far back as 650 million years, or perhaps even 750 million years ago. Our picture of what the Earth looked like earlier than that is more sketchy. We can infer the presence of a large land mass (Rodinia) as far back as 1.1 or 1.2 billion years ago, from several lines of evidence, mostly contiguity of similar rocks and fossils over geographically separated areas. Strata that is even older occurs in isolated areas of present day continents, such as remnants of Archean strata in some of the older cratons. A brief description of the ancient continents can be found here.

Modern plate tectonics is shown in the diagram above. The solid rocky crust of the Earth (lithosphere) overlies a region of hot, semi-molten viscous rock (the asthenosphere), in which heat-generated currents flow as shown. At points where two such convection current cells meet, the rocks of the lithosphere is torn apart, creating the mid ocean ridges. Upwelling of lava in these areas creates new ocean floor (shown in dark brown). Forcing the lithosphere apart in this manner creates enormous lateral pressures, which creates subduction zones. Subduction zones are areas where the thinner crust of the oceanic floors is forced beneath the much thicker continental crust. As it is forced in deeper, it becomes hot and viscous in turn, merging into the asthenosphere.

A side effect of this process is continental drift, since the continental plates are being constantly pushed away on one side (away from the mid oceanic ridges). Over long periods of time, the lateral motion of the continental plates is very significant, and can entirely change the appearance and configuration of the continents.

 

Further description of the history of these changes can be found in the following pages describing the three Proterozoic eras: the Paleoproterozoic, Mesoproterozoic and the Neoproterozoic. Click to read more about them, or return to the index.