 Seventy percent of North America is underlain by the Laurentia craton, which is exposed as the Canadian Shield in much of central and eastern Canada around the Hudson Bay, and as far south as the U.S. states of Michigan, Wisconsin, and Minnesota.The continental crust started to form 4 billion years ago (Ga), and six of the microcontinents collided to form the craton about 2 Ga.This core has been enlarged by plate tectonics, most notably between 1.8 and 1.65 Ga when a piece currently stretching from Arizona to Missouri fused with the south and west portion of the craton.