Laurentia

a name chosen by geologists because Atlas gave his name to the Atlantic, and the Iapetus was, in a sense, the Atlantic's predecessor, occupying roughly the same position between roughly the same landmasses before those landmasses collided and destroyed it.

The Iapetus Ocean separated the ancient continents of Laurentia and Baltica what would eventually become North America and Scandinavia, respectively.

And its floor was geologically active, rifted by mid-ocean ridges and dotted with volcanic island arcs, a seascape of submarine mountains and hydrothermal vent fields, Not unlike the mid-Atlantic ridge that exists today, though in a different location and in a different configuration, the rocks beneath Hestalen were formed on the floor of that ocean, in conditions that bear no resemblance to the frozen valley that exists today.

They were formed by submarine volcanic activity, by hydrothermal vents on the ocean floor, chimneys of superheated water rising through cracks in the seafloor basalt, carrying dissolved metals upward from the hot rock below and depositing them as sulphide minerals in the surrounding sediments.

The vents on the floor of the Iapetus Ocean would have looked similar, Columns of dark, mineral-laden water rising into the cold ocean, depositing copper, zinc, iron, sulphur and other metals in the surrounding rock, building up layer by layer over millions of years, creating the massive sulphide deposits that would eventually become the ore bodies of central Norway.

Then the Iapetus Ocean closed.

Laurentia and Baltica converged, driven by the same tectonic forces that drive continental plates today.

The convection of the Earth's mantle, the slow enormous circulation of semi-molten rock beneath the rigid crust, powered by the radioactive decay of elements in the planet's interior.

There is Gondwana, the landmass that will later include South America, Africa and Australia.

Laurentia, to which the largest part of North America belongs, as well as Siberia and the Baltics, which extend over what you might recognize as later Scandinavia, Eastern Europe and parts of Russia.

However, you would not really see anything.

No familiar mountain peaks or lush green.

Let's talk a little bit about the broader tectonic context that resulted in the rocks that we see in the Western Gneiss region of Norway.

Here, the Caledonian orogeny involved the collision of Laurentia to the west and Baltica to the east, closing the Iapetus Ocean between them.

So what are these ultra high pressure rocks we see? Were they part of Laurentia or part of Baltica?

The Western Gneiss Complex, the gneisses and the mafic rocks sitting in them, they were definitely part of Baltica.

All the age characteristics are typical for the rest of Baltica.

I'm trying to piece together a picture of how this collision and the subduction zone took place.

So if the ultra-high pressure rocks are Baltican rocks, were they subducting towards the west under Laurentia, or how did that happen?

Everybody agrees that during closure of the Iapetus Ocean, the subduction direction has been almost always towards the west.