Privileged Planet: formation of continents (Introduction)

by David Turell , Saturday, June 27, 2015, 18:19 (3438 days ago) @ David Turell

Perhaps 3 billion years ago. Therefore all life developed in the oceans and only come up on land when land was available:-http://www.livescience.com/51359-continents-formed-3-billion-years-ago.html?cmpid=NL_LS_weekly_2015-06-27-"The continents are composed of a thick, buoyant crust that's about 21 miles (35 km) deep, on average, whereas the comparatively thin, dense crust of the ocean floor is only an average of about 4 miles (7 km) thick. Because the continents are so thick and buoyant, they are less likely to get dragged downward. That's why so many ancient continental rocks have survived in the Earth's crust. Still, much about the earliest days of continents, and when and how they formed, remains hotly contested. -"The researchers found that modern, silica-rich continental crust first appeared about 3 billion years ago. The thick, buoyant nature of these chunks of crust would have made them rise high above what became the seafloor, Dhuime and his colleagues note online June 22 in the journal Nature Geoscience.-"'They are showing when continents actually emerged from the oceans," said Lee, who wrote an accompanying news article in Nature Geoscience. "Continents certainly existed early in Earth's history, but perhaps many were submerged."-"It remains uncertain why continental crust made its first appearance about 3 billion years ago. One possibility is the onset of plate tectonics — when the plates of rock making up the planet's exterior began moving slowly over the Earth's mantle layer. Plate tectonics would have resulted in wet rock getting shoved down into Earth's interior, eventually helping to form silica-rich magmas that make up much of the continental crust."