sentences:  Minnesota contains some of the oldest rocks on Earth, granitic gneisses that formed some 3,600 mya (million years ago) — roughly 80% the age of the planet.About 2,700 mya, the first volcanic rocks that would later underlie Minnesota began to rise up out of an ancient ocean, forming the Superior craton.This craton later assembled into the Canadian shield, which became part of the North American craton.