Minnesota has some of the earth's oldest rocks, gneisses that are about 3.6billion years old (80% as old as the planet). About 2.7billion years ago basaltic lava poured out of cracks in the floor of the primordial ocean; the remains of this volcanic rock formed the Canadian Shield in northeast Minnesota. The roots of these volcanic mountains and the action of Precambrian seas formed the Iron Range of northern Minnesota. Since a period of volcanism 1.1billion years ago, Minnesota's geological activity has been more subdued, with no volcanism or mountain formation, but with repeated incursions of the sea, which left behind multiple strata of sedimentary rock.