(Text for this page excerpted from "Decade of North American Geology: Geologic Map of North America - Perspectives and explanation," published by The Geological Society of America, Inc., 2005)
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.
Index map (above) showing the area covered by the two sheets that comprise the Geologic Map of North America (right).
In May 1980, the Decade of North American Geology (DNAG) project members approved the creation of a new geologic map of the continent, with a planned completion date of 1989. By May 1981, the compilers and principal cartographers had been selected, the base map chosen, the essential features of the explanation agreed upon, and compilation was under way. The progress of the compilation was much slower than had been optimistically projected. This was partly the result of the size and complexity of the compilation, partly due to evolution of cartography from traditional pen and ink compilations and scribing to modern digital methods, but most importantly, due to the diversion of each of the compilers to other activities and responsibilities within their respective supporting institutions. Hence, the actual completion of the map came in 2005, more than 15 years after the original projection.
The resulting Geologic Map of North America is designed to depict bedrock and less indurated sedimentary units at or near the surface throughout all parts of the North American continent, as well as selected parts of adjacent continents and islands. In addition, the map shows similar units across the seafloor. The emphasis is on pre-Quaternary geology, so Quaternary surficial deposits are shown only where they completely conceal the underlying units in significant areas. The extensive glacial deposits of the continental interior and Canadian shield are not shown, but Quaternary surficial deposits are shown in the intermontane basins in the Cordillera, in parts of the Atlantic and Gulf coastal plains, and in the Orinoco basin. By far the most extensive areas of Quaternary deposits depicted on the map are on the seafloor.
The base map was compiled by the U.S. Geological Survey (USGS) for use in all of the continental-scale maps published as part of the DNAG project. The map was prepared with data from World Data Bank II and other sources, including the Department of Energy Mines and Resources (EMR) of Canada and the Dirección General de Geografía del Territorio Nacional (DGGTN) of Mexico. During compilation of the geologic map some corrections were made to the base, chiefly in northeastern Greenland and in the overlap area between the northern and southern sheets. A 1:10,000,000 version of the base map was published by the USGS in 1982.
The projection is a spherical form of the Transverse Mercator. Conceptually, the projection may be pictured as formed by passing a cylinder through a spherical Earth with its axis perpendicular to the plane of the 100th meridian. The radius of the cylinder is taken so that it intersects the sphere in two small circles, each 2,343 km away from the central meridian. Points on the surface of the sphere are then projected on to the cylinder from the center of the sphere, as in a conventional Mercator projection. On the map the small circles formed by the intersection of the cylinder with the sphere are represented by two straight lines parallel to the central meridian and halfway between the central meridian and the sides of the map. Along these lines the map scale is exactly 1:5,000,000; along the central meridian the scale is about 1:5,400,000 (5,000,000/0.926) and near the sides of the map the scale is about 1:4,240,000. The scale remains constant along any line on the map parallel to the 100th meridian. Thus the distortion is minimal within North America because the central meridian is chosen to bisect the continent as nearly as possible.
On the previous Geologic Map of North America, the only information depicted in submarine areas was bathymetric contours. Over subsequent years, seafloor geological maps have been constructed in some detail in a few limited, heavily sampled areas, or where constraints are imposed by extrapolation from nearby or surrounding land areas. In some cases larger-scale maps that necessarily are quite generalized have been compiled (for example, Okulitch and others, 1989). The kinds of geological characteristics that these maps document are highly variable, and few attempt to map geology in the "classic" fashion of normal geologic maps.
With the inception of the Decade of North American Geology in the early 1980's, it was decided that the time was ripe to map the seafloor geology around North America as part of the new Geologic Map of North America. Not only had a well defined conceptual geological framework been established from plate-tectonic theory, but enough geological information also had been or was being acquired to justify efforts to map the seafloor geology in a manner consistent with classical mapping of continental geology. This project proceeded over the next two-plus decades. The seafloor geology was mapped largely from primary data rather than being compiled from pre-existing maps such as those available for most of the continental geology. The seafloor geology in most respects is depicted in the same manner as the land geology. As a practical matter, however, the lesser control by hard data required certain compromises. In addition, the map shows some features that either are peculiar to the seafloor (for example, hydrothermal vents, iron-manganese nodules) or are uncommon enough there that they merit documentation (for example, ultramafic rocks).