[Widdekind]

Above a certain thickness, Glaciers depress the underlying Crust about 1 meter, for every 3 meters of ice. Today, in the Great Lakes Region of North America, long overlain by the vast Laurentide Ice Sheet, the Crust is rising about 3 milimeters per year*.

* National Geographic Channel Naked Science -- The Great Lakes (TV)

CONCLUSION: Billions of years ago, the Earth's Mantle was much hotter, and much more fluid than it is today. Therefore, the Earth's Mantle was much more compressible. So, during the catastrophic Global Glaciations ("Snowball Earths") of the Protozeroic Eon (~2300 to 600 million years ago*), the Earth's much more compressible Mantle would have sagged deeper, and more rapidly, than during the much more recent Pleistocene Epoch (1.8 to 0.01 million years ago**). Likewise, Crustal Rebounds would have occurred more rapidly as well.

* http://en.wikipedia.org/wiki/Snowball_earth
** http://en.wikipedia.org/wiki/Pleistocene

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[Widdekind]

The volcanic island of Hawaii sits atop a high-temperature "Hot Spot", beneath the Pacific Tectonic Plate. As the Pacific Plate slides past, the Hot Spot repeatedly melts through that Oceanic Crust, creating Volcanoes that have formed the Hawaiian Island Chain. The largest such Volcano, Mauna Loa, is a massive Shield Volcano, which juts up about 9 km above the Ocean Floor. But, the massive weight of the Volcano also pushes down into the Oceanic Crust, making a huge dent about 8 km deep. Indeed, Mauna Loa makes the whole Oceanic Crust sag all around it*.

* History Channel Mega-Disasters -- Hawaiian Volcano (TV)

CONCLUSION: Billions of years ago, Earth's Mantle and Crust were much hotter, than they are today*. As such, the Mantle was more compressible, and the Crust more pliable, and prone to sag, compared to today. Thus, the weight of early islands and Continents would have pushed down deeper into the Earth, submerging more of the early landmasses under water. This could explain, why early Earth Crusts appear to have formed underwater**.

* http://www.geosciences-forum.com/htopic618-.html
** http://www.geosciences-forum.com/ftopic587.html

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