Galactic Fountains

When a star goes supernova, the rapidly expanding bubble of hot, metal-rich gas from it hits the edge of the galactic disk where it suddenly encounters much less resistance from a much lower density of the interstellar medium, allowing it to form a chimney perpendicular to the disk. After arcing well outside the disk, this material then cools and rains down over a large portion of the galaxy, seeding stellar nurseries with metals that will later become planets.