[Widdekind]

About 2 billion years ago, Earth's Atmosphere became Oxygenated*. Atmospheric concentrations of Molecular Oxygen reached 5-18% of present levels**.

* Peter Cattermole. Building Planet Earth: Five Billion Years of Earth History, pg. 5.
** Nick Lane. Oxygen, pg. 49 ; J.F. Luhr. Earth, pp. 26-29.

About the same time (~2 Ga), Continental Crusts evolved from primitive, to modern, compositions:

The rate of continental growth can be traced by studying the changing composition of the sediments which were won from them and deposited onto the sea-floor. Had all of the continents appeared early on, one would anticipate little change in the composition of sediments since that time. In reality we find that during the Archaean, sediments were mainly won from oceanic lavas (or a more primitive type of crust), a condition which prevailed until around 2000 million years ago*.

This coincides exactly with the change in composition of Continental Sediments.

* Peter Cattermole, ibid., pg. 106.

CONCLUSION:

Atmospheric Oxygenation caused the change in composition of Continental Crust, about 2 billion years ago.

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[Widdekind]

About this time, Craters begin getting preserved in Continental Crusts:

2.4 billion years ago -- Suavjärvi Crater, northwestern Russia*
2.02 billion years ago -- Vredefort Crater, South Africa**
1.85 billion years ago -- Sudbury Crater, central Canada***

* http://en.wikipedia.org/wiki/Suavj%C3%A4rvi_crater
** http://en.wikipedia.org/wiki/Vredefort_crater
*** National Geographic Naked Science -- Dino Meteor (TV) ; http://en.wikipedia.org/wiki/Sudbury_Basin

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[Widdekind]

Early Earth's Oceans may have been 40 km deep; and, indeed, Archaean Continental Crusts were created underwater*. Perhaps Global Oceans only dropped down below the tips of protruding island-tops at this time.

* http://www.geosciences-forum.com/ftopic598.html

[Widdekind]

Two billion years ago, oceanic basalts composed most Continental Crusts :

...during the Archaean, sediments were mainly won from oceanic lavas (or a more primitive type of crust), a condition which prevailed until around 2000 million years ago*.

This could be explained, by assuming, that 2 Gya, Earth's internal temperature was much hotter than it is today, and much more like modern Venus. On Venus, to this day, the Crust is thin, and does not get dragged down by Mantle convection currents. Thus, on Venus, there are "Hot Spot" volcanoes above rising Mantle convection currents, and "Himalaya-like" jumbled domes of piled up Crust above sinking Mantle convection currents**.

* Peter Cattermole, ibid., pg. 106.
** Ron Miller. (Worlds Beyond) Venus, pg. 51.

CONCLUSION:

Until about 2 Gya, early Earth's Crust did not sink back down into the interior Mantle. Instead, early Earth's Continents -- those regions of dry land jutting up above the water -- were comprised of either (1) "Hot Spot" volcanoes; or (2) "Himalaya-like" jumbled domes of piled up sea floor.

Then, beginning about 2 Gya, Earth's oceanic crust started to sink down into the interior, wherever sinking Mantle convection currents occurred. If so, whole "Continents" of jumbled up sea floor would have been pulled -- like legendary Atlantis -- down into the deep heart of this world*.

* It is remotely within the realm of possibility -- not necessarily of plausibility, much less probability -- that (1) land lifeforms had already evolved, upon this planet, back before 2 Gya; and (2) the "Atlantean" subduction, of Earth's early continents, back into the hot heart of this world, eradicated all evidence of those higher lifeforms; thus (3) forcing Earth life to "start over" from the simple surviving microbes, and re-evolve back into the more complex lifeforms with which we are familiar today. Thus, Earth's present Biosphere may be "version two" (as it were) -- we might be this world's "second life".

This coincides with the appearance of Banded Iron Formations, and the deposition of copious quantities of heavy & dense iron, onto the sea floors*.

* http://www.geosciences-forum.com/htopic676-iron.html

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[Widdekind]

4.5 Gya, the early Earth's atmosphere was thick, heavy, dense and dark, and full of greenhouse gases:

Volcanoes were pouring gases, such as carbon dioxide and water vapor, in to the sky, in vast quantities. Even today, water vapor forms 60 to 95 percent of the gases emitted by volcanoes. The volcanic gases of 4 to 4.5 billion years ago may have been even richer in water vapor. Because o the presence of such large quantities of heavy gases — that is, gases heavier than hydrogen — this early atmosphere may have been very dense, perhaps as much as 70 times as dense as it is today. This is similar to the present-day atmosphere of Venus, which is almost totally made up of carbon dioxide. It has a surface pressure 90 times that of Earth's — the equivalent of being under 3000 feet (914 meters) of water (Ron Miller. Earth & Moon, pp. 22-32).

Therefore, the early Earth was hot:

As long as the crust remained hot, water condensing from the atmosphere simply boiled away again as steam. But as the crust cooled, liquid water began to collect in pools and lakes. Torrential rains poured from the cooling clouds, and soon the low-lying surface was covered in a worldwide ocean. The landscape of Earth 4.4 billion years ago would have seemed like an alien planet... The air would be thick and humid, the pressure crushing, and the temperature near boiling (ibid.).

Greenhouse gases, including carbon dioxide, dominated Earth's atmosphere for over 2 billion years. But, beginning about 2.3 billion years ago, the accumulated efforts of vast fields of photosynthetic lifeforms had drawn down carbon dioxide levels, and oxygen levels began to rise in their stead:

Until 2 to 3 billion years ago, the atmosphere of Earth was mostly carbon dioxide and water vapor. But, by 2 to 2.3 billion years ago, life had become so abundant [on Earth] that it started to affect the environment [of Earth]. Like the process of photosynthesis in today's plants, blue-green algae used sunlight to break down atmospheric carbon dioxide in order to obtain the carbon it needed to build organic molecules. The excess oxygen, which the algae didn't need, was released into the atmosphere... By 2 billion years ago, the proportion of oxygen [in the atmosphere] was about 1 percent (ibid.).

The loss of greenhouse gases plunged this planet into an ice age:

The first ice age we know of occurred 2.3 billion years ago. It may have lasted as long as a hundred million years (ibid.).

Thus, this ice age ended about 2.2 Gya. By 2 Gya, Earth's "continents" were no longer composed of jumbled up sea floor, but had begun to become granitic in composition, because Earth's crust and upper mantle had cooled enough for Plate Tectonics and Subduction to start (see above).


CONCLUSION:

Early Earth's atmosphere was dominated by greenhouse gases, like carbon dioxide and water vapor. But, by 2.3 Gya, Earth's photosynthetic lifeforms had consumed that carbon dioxide, plunging this planet into an ice age. This allowed the crust, and upper mantle, to cool down. In turn, this allowed Plate Tectonics & Subduction to start, by about 2.0 Gya.