# The age of tectonic plates

There is information which says that East European Platform sticks out at the Baltic Sea and, therefore, we can see the ordovican layer in Estonia whereas crystalline Paleoproterosoic basement crops up in the Finland.

What does that mean? I see that other layers are cambrian and ordovican, which means that they were formed 500 and 400 mln years ago. Does it mean that East European Platform has formed 2 giga years ago when we had the Paleoproterozoic era? I ask because it is hard to believe that we could have the magma cristallization at that scale to solidify over the whole continents. I expect that these processes that shield formation has finihsed in the range 5-4 giga years ago and pretty solidified Earth existed for ~ca 2 bln years before Paloproterozoic ever begun. Why is that platform called Paleoproterozoic?

## 1 Answer

The process of forming continental crust is indeed thought to have started in the early Archean (i. e. between ca. 4 Ga and ca. 3.5 Ga). But it did continue later: Hawkesworth & Kemp showed in their 2006 review that while much of the continental crust had formed by 2 Ga, i. e. until the middle of the Paleoproterozoic, some of it still continued forming later.

As far as the Fennoscandian shield is concerned, it seems that though it started forming in the Meso-Archean, and that much of the eastern and northern part of it are Archean (see Slabunov et al. 2006), the southern and western part is indeed mostly Paleo- and Mesoproterozoic (see e. g. Lahtinen et al. 2008). In Figure 1 of Lahtinen et al. 2008, it seems that the southern part of Finland, just north of the Gulf of Finland (the Uusimaa Belt) is Paleoproterozoic.

Sources:
Hawkesworth & Kemp, 2006. Evolution of the continental crust. Nature, 443: 811-817.
Lahtinen et al., 2008. Paleoproterozoic evolution of Fennoscandia and Greenland.Episodes, 31: 20-28. Slabunov et al., 2006. The Archaean nucleus of the Fennoscandian (Baltic) Shield. In Gee & Stephenson (eds): European Lithosphere Dynamics, Geological Society of London, Memoirs, 32: 627-644.

• Do you mean that for 2 bln of years the Earth existed without any crust 0_0? – Little Alien Aug 2 '16 at 9:42
• The earth was formed at ca. 4.5Ga, so at most it would be half a billion years. And we're talking continental crust here, not oceanic crust. – plannapus Aug 2 '16 at 13:55
• Do you mean that new cratons can born and die? I do not see what continetnal vs. oceanic gives here. Oceans are only 5 km lower than the continents and hardly affect anything that is 100 km thick. Moreover, the go up and down, which means that ocean and continent exchange their roles. The illustration demonstrates that ordocican sediments, which are currently at the surface, been on the floor of the ocean some 400 mln years ago. – Little Alien Aug 2 '16 at 17:45
• Ok, I have got it. You have the platform collisions on the continents because rock goes up, forming the mountain belts, and subdues there. At the same time, your plates diverge in the ocean and new lava enters the fracture. It gives rise to the new new plate when solidified. I see now. – Little Alien Aug 2 '16 at 18:22