1 880 million years – 1.9 kilometres

Micro-continents and island arcs collide in Finland. The collision is called the Fennian orogeny, and it forms the mountain chain of Karelides.

Image: Zhen Zhao | Wikimedia

A schematic picture of the geological history of the southern Finland. a) 1900 million years ago island arcs (green) formed, when two oceanic plates collided. Sediments are formed from the eroded material (blue). b) 1880 – 1860 million years ago the island arcs collide with a microcontinent (yellow) at the Fennian orogeny.  Rocks were subjected to high pressure and temperature, and were metamorphosed. c) 1850-1810 million years ago temperature and pressure rises as a result of new collisions and the lower part of the crust starts to melt (pink). Image: Ari Brozinsky and Olav Eklund.

Fennian orogeny
Svekofennic main area (Finland, eastern Sweden and northern Estonia) was formed, when island arcs collided with the microcontinent at the north. Rocks from the island arcs and ocean sediments were folded and pushed upwards forming a new mountain range. This formation of mountains is called as Fennian orogeny.

The crust formed in such collisions is typically composed on various different components that are pushed together, deformed and undergo metamorphism. Heterogeneous tensions align minerals, which form cleavages and foliations. Rocks are folding. Fault zones form; these weaker zones are important also for the later development of the crust. The crust also gets thicker and the roots of the mountains are pushed towards the hot mantle, where rock melts and forms magmas

Baltic shield. Rocks formed at svekofennic orogeny are shown with dark orange. Image: Adapted from Geologia.fi and Geophysics, Uppsala University