Here's a map of rock bands in the metal genre per capita:Immigrant Song" about the Viking exploration of the North Atlantic was on the Top 40 charts in 1970:
We come from the land of the ice and snow,
From the midnight sun where the hot springs flow.
The hammer of the gods will drive our ships to new lands,
To fight the horde, singing and crying: Valhalla, I am coming!
And I recall as an eleven-year-old, thinking that, boy, they were on to something by combining heavy metal with Viking imagery. Apparently, a lot of lads in the North countries felt that fit even more strongly.
This sounds obvious today, but in 1970, heavy metal was all about riffing off Mississippi Delta Blues, so that its fate would end up at the polar opposite culture was quite startling.