PRIMORDIAL ARCHAEAN EARTH
A primordial Archaean crustal assemblage is shown below along with the location of the Archaean equator and poles. The named remnants of the present-day modern continents are also highlighted as black outlines. It should be noted that this figure represents an assemblage of the most ancient continental crusts that are currently preserved on the present-day Earth. This model represents the first time that the ancient supercontinental crusts have been assembled back to the beginning of geological time and are therefore unique.
The primordial Archaean Expansion Tectonic small Earth model. The model shows the assemblage of ancient early-Precambrian continental crusts. The cratons are shown as pink and red, Proterozoic basement rocks are shown as khaki and the location of the present-day Antarctic and Greenland ice-sheets (covering Precambrian crustal rocks) are shown as pale blue. The ancient equator is shown as a horizontal red line and the poles are shown as dots. Ancient remnants of each of the present-day continents are outlined in black.
As shown elsewhere in the Geological Map of the World, approximately 66 percent of the present-day exposed continental crusts are sedimentary rocks which once formed the seafloor of the ancient continental seas. Geological evidence shows that between 10 to 40 kilometres of rock material has been eroded from each of the ancient crusts. All that has been done to create this primordial small Earth model is to then return these eroded sediments back to the ancient land surfaces, as well as return any intruded or extruded volcanic and magmatic rocks back to the mantle or lower crustal regions where they originally came from.
After returning the eroded sediments back to the ancient lands, the remaining sedimentary basins are then progressively closed off until each of the most ancient crusts dock together as a complete primordial supercontinent. In essence, these ancient continental crustal fragments are analogous to assemblage of the modern continents into the Pangaea supercontinental assemblage after returning the seafloor crusts back to the mantle.
The size of the primordial Archaean supercontinent was about 27 percent of the present Earth radius, about the size of the present Moon. Because of the extremely low expansion rates existing during the Precambrian times–microns per year–the Archaean small Earth model represents the period of time extending from the early-Archaean through to the middle-Proterozoic Era. This one model then represents about 2,400 million years, or two thirds of all known geological history.
Modern geological mapping of the ancient Archaean crusts also show that the vast majority of these ancient crusts comprise granite and volcanic rocks, with lesser sedimentary rocks. These crusts are often referred to as granite-greenstone terranes—where the term greenstone refers to the distinctive dark green colour of the volcanic rocks. Similarly, modern evidence shows that the atmospheric conditions existing during the early-Archaean times were vastly different to those of today, suggesting that it may have been quite acidic and devoid of oxygen.
During the very earliest Archaean times it is envisaged that once the primitive granite and volcanic crust had cooled and solidified, onset of Earth expansion was first initiated as global-scale cracking and fracturing of the crust to form a network of crustal weakness surrounding relatively stable lands. As time very slowly progressed the influence of changing surface curvature, although minimal during that time, assisted in maintaining a distinction between the elevated land surfaces and the low-lying areas located around the margins. Eroded sediments were then deposited in these low-lying areas. It is considered that this elevation contrast was instrumental in establishing a distinction between the first primordial supercontinental lands and the first ancient seas and sedimentary basins.
The emergent primordial Archaean land surface would have been vastly different to what is familiar to us now. It would have been a barren, possibly windswept rocky landscape devoid of all forms of life and exposed to erosion of rock and dust particles by winds and maybe some acidic rain. There would have also been strong chemical weathering and erosion of the rocks themselves, in particular erosion by hot and potentially acidic waters from volcanic eruptions localised along the newly established network of crustal weakness. This weathering and erosion then gave rise to deposition of the very first sediments, deposited in low-lying regions, along with any extruded volcanic lava.
Remnants of this network of low-lying regions can be seen in the picture above by the distribution of khaki coloured, mainly sedimentary rocks. These early sedimentary basins would have been small and isolated, becoming progressively more extensive after a very long period of time as the low-lying basins progressively filled with sediment. This early granite-greenstone and sedimentary crust now represents the oldest preserved crustal remnants on Earth today. The prevailing acidic atmosphere and waters also enabled metals rich in sulphur as well as elemental carbon to accumulate within the sedimentary basins, which in turn may have formed the basis for bacterial life forms to emerge and evolve during the later Precambrian times.
The geological evidence suggests that during the latter part of this Archaean supercontinent phase, very large stable sedimentary platforms—very large flat basins filled with sediments—covered with relatively shallow seas had formed. These had a very low elevation contrast between the dry lands and the seas. In other words, throughout these times there was a relatively flat landscape covering the entire ancient Earth. These large platforms simply represent sedimentary basins that had been filled to capacity over the extremely long period of time operative during the ancient times. Any further erosion of the lands and deposition of sediments was limited to mainly chemically-precipitated siliceous chert, carbonate, and banded iron formation rocks within the relatively shallow seas. Remnants of these chemically-precipitated rocks, along with the aerially extensive granite, volcanic, and associated sedimentary rocks making up the ancient lands are now preserved on many of the modern continents.
During this extended period of time, the ancient North and South Poles are shown above to have been located within what are now Northern China and West Africa respectively. Similarly, the ancient equator is shown to have passed through what is now North America, East Antarctica, Australia, Greenland, and Scandinavia; a strong contrast to where it is located now. Over this extended length of time the primordial Archaean supercontinent also underwent many changes to its network of seaways and sedimentary basins. These geological changes eventually evolved imperceptibly into the better known supercontinent called Rodinia, some 1,000 million years ago.
It is emphasised that on an Expansion Tectonic Earth it is the changes to sea-level and the distributions of the network of sedimentary basins that define the progression from one supercontinent to the next; not as a result of fragmentation or re-assemblage of prior supercontinental crustal fragments.
