Late Precambrian Super-continent and Ice House World

4.5 billion years ago, the Earth was formed. Nearly 400 million years passed after the Earth's inception before the first animals left their traces. This stretch of time is called the Precambrian. To speak of "the Precambrian" as a single unified time period is misleading, for it makes up roughly seven-eighths of the Earth's history.

During the Precambrian, the most important events in biological history took place. Consider that the Earth formed, life arose, the first tectonic plates arose and began to move, eukaryotic cells evolved and the atmosphere became enriched in oxygen. Just before the end of the Precambrian, complex multi-cellular organisms, including the world's first animals, evolved.

The Precambrian can be divided into three different periods.

• Hadean - 4.5 to 3.8 billion years ago. Hadean time is not a geological period as such. No rocks on the Earth are this old - except for some meteorites. During Hadean time, the Solar System was forming, probably within a large cloud of gas and dust around the sun, called an accretion disc. The relative abundance of heavier elements in the Solar System suggests that this gas and dust was derived from a supernova, or supernovas - the explosion of an old, massive star. Heavier elements are generated within stars by nuclear fusion of hydrogen, and are otherwise uncommon. We can see similar processes taking place today in so-called diffuse nebulae in this and other galaxies.
  
  The sun formed within such a cloud of gas and dust, shrinking in on itself by gravitational compaction until it began to undergo nuclear fusion and give off light and heat. Surrounding particles began to coalesce by gravity into larger lumps, or planetesimals, which continued to aggregate into planets. "Left-over" material formed asteroids and comets.
  
  Because collisions between large planetesimals release a lot of heat, the Earth and other planets would have been molten at the beginning of their histories. Solidification of the molten material into rocks happened as the Earth cooled. The oldest meteorites and lunar rocks are about 4.5 billion years old, but the oldest Earth rocks currently known are 3.8 billion years old. Sometime during the first 800 million or so years of its history, the surface of the Earth changed from liquid to solid. Once solid rock formed on the Earth, its geological history began. This most likely happened prior to 3.8 billion years, but hard evidence for this is lacking. Erosion and plate tectonics have probably destroyed all of the solid rocks that were older than 3.8 billion years. The beginning of the rock record that is currently present on the Earth is the inception of a time known as the Archaean.
  
  
• Archaean - 3.8 to 2.5 billion years ago. If you were able to travel back to visit the Earth during the Archaean, you would likely not recognize it is the same planet we inhabit today. The atmosphere was very different from what we breathe today; at that time, it was likely a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. Also during this time, the Earth's crust cooled enough that rocks and continental plates began to form.
  
  It was early in the Archaean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. In fact, all life during the more than one billion years of the Archaean was bacterial. Stromatolites were colonies of photosynthetic bacteria which have been found as fossils in Early Archaean rocks of South Africa and Western Australia. They increased in abundance throughout the Archaean, but began to decline during the Proterozoic. They are not common today.
  
  
• Proterozoic - 2.5 billion to 543 million years ago. The period of Earth's history that known as the Proterozoic can be divided into three sub-periods. Paleoproterozoic - 2.5 to 1.6 billion years ago, Mesoproterozoic - 1.6 billion to 900 million years ago and Neoproterozoic - 900 to 543 million years ago.
  
  Many of the most exciting events in the history of the Earth and of life occurred during the Proterozoic -- stable continents first appeared and began to accrete, a long process taking about a billion years. Also coming from this time are the first abundant fossils of living organisms, mostly bacteria and archaeans, but by about 1.8 billion years ago eukaryotic cells appear as fossils too.
  
  With the beginning of the Middle Proterozoic comes the first evidence of oxygen build-up in the atmosphere. This global catastrophe spelled doom for many bacterial groups, but made possible the explosion of eukaryotic forms. These include multi-cellular algae, and toward the end of the Proterozoic, the first animals.

The above map illustrates the break-up of the super-continent, Rodinia, which had formed 1100 million years ago. The Late Precambrian was an "Ice House" World, much like the present-day.

The absence of fossils of hard-shelled organisms and the paucity of reliable paleo-magnetic data make it difficult to produce paleo-geographic maps for much of the Precambrian. With available data, 650 million years is about as far back as we can go.

The late Precambrian, however, is an especially interesting time because the continents were colliding to form ancient super-continents, and because the Earth was locked in a major Ice Age.

About 1100 million years ago, the super-continent of Rodinia was assembled. Though its exact size and configuration are not known, it appears that North America formed the core of this super-continent. At that time, the east coast of North America was adjacent to western South America and the west coast of North America lay next to Australia and Antarctica.

Rodinia split into 2 halves approximately 750 million years ago, opening the Panthalassic Ocean. North America rotated southwards towards the ice-covered South Pole. The northern half of Rodinia, composed primarily of Antarctica, Australia, India, Arabia, and the continental fragments that would one day become China, rotated counter-clockwise, northwards across the frigid, North Pole.

Between the two halves of Rodinia lay a third continent - the Congo craton, made up of much of north-central Africa. It was caught in the middle as the two halves of Rodinia came crashing down on it. By the end of the Precambrian, about 550 million years ago, the three continents collided to form a new super-continent called Panotia. The mountain-building event associated with this collision is called the Pan-African orogeny.

As mentioned previously, the global climate was cold during the Late Precambrian. Evidence of glaciation is found on nearly every continent. Why cold conditions were so widespread has puzzled geologists. Several hypotheses, have been proposed. One explanation suggests that the Earth was tilted sideways so that the North Pole faced towards the Sun, and the South Pole faced away from the Sun. This would have created a situation where 1/2 of the Earth would have broiled under the Sun for 6 months, while the other 1/2 of the Earth would have frozen solid. Though tantalizing, no mechanism can be found that would have produced such a drastic tilt of the Earth's axis.

A second hypothesis proposes that the Earth was encircled by a rocky and icy ring, much like the rings of Saturn and Uranus. These rings would have cast a shadow on the Earth, cooling the climate. No trace of this ring, however, has been found.

A third, and the most popular hypothesis, suggests that the Earth was completely frozen - oceans and all - like a giant snowball. The "Snowball Earth" hypothesis also explains anomalous isotopic signatures in overlying rocks.

All of these hypotheses were put forward before there were accurate paleogeographic maps. The mystery of the Late Precambrian Ice House World can be better explained by the fact that during times of continental collision and super-continent formation the world enters a global Ice House (like the present-day). The Late Precambrian Ice House world was very severe, because it just so happened that many continents were near either the North Pole or the South Pole. (The occurrence of ice on Australia which was near the Equator is an interesting exception.) It should be pointed out that though much of the Earth was glaciated, there were regions near the equator that were ice free and enjoyed warm - if not balmy climates!