The singing/chanting in Kalevala is definitely the source of magic. The runos
that you mention are basically spells that are sung or chanted, and there
doesn't seem to be much of a limit to what a skilled minstrel-wizard like
Vainamoinen can do with them.
From the intro of the public domain text:
The songs of Wainamoinen
disarm his opponents; they quiet the angry sea; they give warmth to the new sun
and the new moon which his brother, Ilmarinen, forges from the magic metals;
they give life to the spouse of Ilmarinen, which the "eternal metal-artist"
forges from gold, silver, and copper.
The public domain version of
the text (in English) is at
Project Gutenberg. It's lengthy
(I know, shocking for an epic poem to be lengthy), but an interesting
read.
