The geology of North America, like most topics of scientific study, is undergoing progressive investigation by numerous public- and private-sector earth scientists, academicians, and students. In that regard, the detailed picture is subject to revision and change as knowledge advances.
Contents |
The lower 48 U.S. states can be divided into roughly five physiographic provinces:
The geology of Alaska is typical of that of the cordillera, while the major islands of Hawaii consist of Neogene volcanics erupted over a hot spot.
The American cordillera extends roughly from the Great Plains westward to the Pacific Ocean, narrowing somewhat from north to south. It includes the Cascades, Sierra Nevada, and Basin and Range province; the Rocky Mountains are sometimes excluded from the cordillera proper, in spite of their tectonic history. The geology of this region is complex, having gone through numerous orogenies, with their associated deformation, faulting, volcanic activity, and periods of uplift separated by intervals of erosion. Much of the cordillera consists of terranes, ancient microcontinents and island arcs that were "welded" onto the North American craton during the Paleozoic and Mesozoic eras. Such a convoluted history is typical of convergent plate boundaries, as has characterized the cordillera through most of the Phanerozoic. Although the Colorado Plateau is near the cordillera, it has remained tectonically stable, with little deformation.
The Canadian Shield consists of surficial, deeply eroded Precambrian rocks, exhumed by past glaciations. Consisting of a variety of rocks from igneous to ancient sedimentary, it is well-exposed only in the Great Lakes region.
A large part of the center of the lower 48 consists of the stable (or continental) platform. Here, the Precambrian rocks of the Shield are buried beneath sedimentary Phanerozoic strata. Tectonic activity is minor to nonexistent, with occasional broad domes and basins that reveal mild epeirogenic deformation. The coastal plain extends from the southern tip of Texas across the northern Gulf of Mexico and into the Mississippi embayment, and northeast through the Mid-Atlantic states. A classic passive continental margin, it consists of a deep clastic wedge of sediment eroded from the platform and mountain belts; it first formed during the opening of the Atlantic Ocean and Gulf of Mexico during the Jurassic and Cretaceous periods.
The Appalachian orogenic belt extends from well into New England south into Mississippi and Alabama. The Ouachita Mountains of Arkansas, and even the Marathon Uplift of Texas are also part of the same province, having all formed in the Alleghenian orogeny that took place when Pangea assembled during the late Paleozoic. Once lofty, they have been heavily weathered since the opening of the Atlantic Ocean. The Appalachians proper consist of deformed sedimentary rocks, cut through by numerous thrust faults; as in the western cordillera, the Appalachians experienced several orogenies over the course of the Paleozoic, making their geologic history difficult to interpret.
|
|