USGS Geologic Map of North America

The Geologic Map of North America (published February 2005) portrays the grand architecture of the continent as we understood it in the closing years of the 20th century. As the final product of the Geological Society of America's Decade of North American Geology project, it was compiled by John C. Reed, Jr. (USGS), John O. Wheeler (Geological Survey of Canada), and Brian E. Tucholke (Woods Hole Oceanographic Institution). The map is at a scale of 1:5,000,000 and covers about 15% of the Earth's surface. The previous geologic map of North America (North American Geologic Map Committee, 1965) was published before general acceptance of plate tectonics, before radiometric dates were widely available, and when the geology of the sea floors was largely unknown. The old map distinguished about 100 rock units, all of them onshore. The new map distinguishes 939 units, of which 142 are offshore. It also depicts many geologic features not shown on the previous map, including volcanoes, calderas, impact structures, axes of submarine canyons, spreading centers, transform faults, magnetic isochrons, and subduction zones. For the first time it portrays the relationships between the geology of the continent and the geology of the ocean basins that flank it. The map was compiled over an interval of almost 25 years and its assembly spanned the technological change from traditional cartography to digital cartography. Although the map is not yet available in digital form, plans are underway for construction of a digital database and ultimately for the release of GIS compatible files. [Summary provided by the USGS.]