To produce the sedimentary rock there had to have been weathering and erosion as well as deposition in a marine environment. Thus the atmosphere and ocean must have been developed on earth by 3.8 b.y. The large central area of the United States and Canada between the Rocky Mountains and the Appalachian mountains is a craton. Most of the craton in the United States is covered by layers of Paleozoic and younger rock, but in much of Canada, the Precambrian rocks are exposed in a shield. A shield is a large region where the exposed bedrock is Precambrian igneous and metamorphic rocks.
Archean rocks differ somewhat from later rocks.
This probably reflects a hotter outer part of the earth. Plates were thin, plate motion faster. There were probably more plates and more rapid destruction than at present. Two types of terranes developed, gneiss terranes and greenstone belts. The gneisses are strongly metamorphosed granites and sedimentary rocks.