Alabama's diverse geology has produced great variation in terrain and physiography, ranging from the coastal lowlands of the southern part of the state to rugged mountainous areas of the north. Physiographically, the state is divided into five sections: the Cumberland Plateau, Highland Rim, Valley and Ridge, Piedmont Upland, and East Gulf Coastal Plain. Each of these is characterized by rocks of specific geologic age and composition, and the resultant landforms reflect these rock types. The Cumberland Plateau is characterized by Paleozoic sandstone, shale, and limestone underlying the valleys, whereas more resistant sandstone supports the ridges. The Highland Rim to the north has moderate relief and primarily consists of Paleozoic limestone and chert. The Valley and Ridge physiographic section in central and northeastern Alabama is characterized by diverse Paleozoic sedimentary rocks and consists of a series of folded and faulted ridges and valleys that run generally northeast-southwest. The Piedmont Upland section is composed of faulted crystalline metamorphic and igneous rocks that represent the oldest rocks in the state, dating back to the Precambrian. The East Gulf Coastal Plain section is an area of Mesozoic and Cenozoic sediments that occupies the southern part of the state and curves northwesterly up into Lauderdale County, where generally unconsolidated sediments overlap rocks of the Highland Rim, Cumberland Plateau, Valley and Ridge, and Piedmont Upland sections.
Through much of Alabama's geologic past, the state was covered by ancient oceans, swamps, and other near-shore environments. The sediment left behind in these environments composes most of the state's bedrock: limestone, sandstone, shale, and chalk. All of these deposits are economically very important to Alabama. For example, quarrying of limestone is the largest nonfuel mineral industry in the state. Sedimentary rocks are not only important for economic use, but also make up the 20 major aquifers (groundwater reservoirs) in the state. These sedimentary rocks have also enabled agricultural development throughout the state's history. For example, the chalks and marls of the Black Belt and limestones of the Tennessee Valley are rock types that develop rich, fertile soils, allowing cotton and other crops to thrive.
Igneous and Metamorphic Rocks
Most of the state's igneous and metamorphic rocks lie within the Piedmont Upland and range in age from Precambrian to Devonian (2.5 billion years to 360 million years old). These rocks are subdivided further into two major districts: the Northern and Southern Piedmont Upland. Each district is bounded by a major regional fault, and each includes characteristic rock units, which are groups of rocks that have similar properties. The grade (an indication of the amount of temperature and pressure to which the rock was subjected) of metamorphic rock generally increases across the Piedmont Upland from low-grade in the northwest to high-grade in the southeast. Several varieties of metallic and nonmetallic minerals have been successfully mined from Alabama's igneous and metamorphic rocks, including gold, lead, zinc, mica, talc, asbestos, and kaolin. Marble (the official state rock) and granite have also been quarried successfully from the Piedmont Upland, and gemstones, such as star blue quartz (the official state gemstone), garnet, beryl, smoky quartz, tourmaline, and kyanite, have been collected as well.
The sedimentary rocks across the state are rich in fossils, which are the preserved remains of ancient animals and plants. Common fossils in Alabama include a wide variety of invertebrate and vertebrate marine and non-marine organisms. These fossils, as well as the characteristics of the sedimentary rocks in which they are preserved, provide paleontologists and geologists with important insights into our prehistoric past and the diverse environments and ecosystems, many of them quite different from those of today, that have characterized the area that is now Alabama. Stromatolites (fossil bacterial colonies), trilobites, corals, crinoids, brachiopods, and many other organisms have been preserved in Cambrian through Mississippian limestones, shales, and sandstones and can be found in the Cumberland Plateau, Highland Rim, and Valley and Ridge. A variety of plants, including ferns and prehistoric trees (Calamites and Lepidodendron bark and logs), as well as footprints or trackways of amphibians and horseshoe crabs are found primarily in shales associated with Pennsylvanian coal in the Cumberland Plateau and Highland Rim. Oysters, sea urchins, ammonites, shark teeth, Basilosaurus and other marine animal bones, terrestrial dinosaur bones, and marine bird bones can be found in Cretaceous chalks and sandstones in the Coastal Plain section. Bivalves, sea urchins, sand dollars, shark teeth, whale bones, and bones of other vertebrates, such as mastodons, have been preserved in Tertiary clays, marls, and sands in the East Gulf Coastal Plain in southern Alabama.
Fossil fuels (coal, oil, and natural gas) associated with geologic strata in Alabama have played an important part in the state's development and growth. Alabama ranks 14th in the United States in coal production, with three main coal fields in the state: the Warrior coal field (which contains 90 percent of the state's coal reserves), the Cahaba coal field, and the Coosa coal field. Since the 1944 discovery of oil near Gilbertown in Choctaw County, nearly 400 oil and natural gas fields, producing from geologic strata that range in age from Cambrian to Miocene, have been discovered in Alabama. These include massive natural gas reserves in the Jurassic Norphlet Formation in Alabama's offshore waters; rich oil-producing areas such as the Citronelle Oil Field in Mobile County, and various fields developed in the Jurassic Smackover Formation across southwest Alabama; and prolific oil and natural gas production in the Black Warrior basin of west-central Alabama. Alabama is also a world leader in the development of coalbed methane gas as an energy resource, being home to the first commercial production in 1980. Most coalbed methane is produced from coal beds that occur in the Pennsylvanian Pottsville Formation in Tuscaloosa and Jefferson counties. Alabama ranks 10th nationally in the production of natural gas and 15th in the production of liquid hydrocarbons (oil and condensate).
Other Important Mineral Resources
Alabama currently ranks 20th among the states as a producer of nonfuel minerals. Of these, limestone quarrying is the largest industry, and the state is ranked third in the nation in this commodity. Much of the limestone quarried in the state comes from Paleozoic strata and is used to produce cement, but some is used for other purposes, such as agricultural lime and crushed stone. Sand and gravel production is also important, especially in southern Alabama. The state is a major producer of various clays (bentonite, common clay, fireclay, fuller's earth, kaolin, and shale), as well as mica. Economically important metals occur largely in the Piedmont Upland and include gold and chalcopyrite, both of which have been produced in limited quantities from small scattered deposits since about 1830. Deposits of hematite (the official state mineral) are found in the Birmingham District and along with nearby coal and limestone deposits led directly to the iron and steel industry boom and the subsequent development of Birmingham during the late nineteenth century.
More than 33 trillion gallons of fresh water flow every year through Alabama in 77,000 miles of stream channels coursing through 14 river basins. Approximately 18 percent of all surface water flowing through the lower 48 states flows through Alabama. Much of this surface water has been harnessed for manufacturing centers and hydroelectric facilities, as well as for water transportation for various commodities, such as coal and forest products. Across the state, some 550 trillion gallons of water are stored in underground geologic reservoirs (aquifers). Approximately 40 percent of public water supplies in Alabama come from ground-water sources, and these are particularly important in the Coastal Plain section, where many counties and cities obtain all of their public supply from ground water. Alabama has 20 major aquifers that supply water from near the land surface to depths approaching 3,000 feet.
Although most of the earthquakes in Alabama are small in magnitude, Alabama has experienced multiple events of moderate intensity within the past century. The majority of earthquakes in Alabama are associated with the Southern Appalachian Seismic Zone (SASZ) in northern and central Alabama and the Bahamas Fracture Seismic Zone (BFSZ) in southern Alabama. The largest Alabama earthquake on record, a magnitude 5.1 associated with the SASZ, occurred in 1916 near Birmingham. An additional SASZ earthquake of magnitude 4.9 occurred in the Fort Payne area in 2003, and a 4.9 magnitude quake associated with the BFSZ occurred in Escambia County in 1997.
Geology's Influence on Tourism and Recreation
Geology has an important relationship to tourism and recreation in Alabama. Many of the more than 20 national and state parks in Alabama, which are visited by some 5 million people each year, have geologic features of interest. Locations such as Bladon Springs and Blount Springs have attracted tourists since the nineteenth century for their mineral-rich waters. Other areas, such as Cathedral Caverns State Park, Rickwood Caverns State Park, and DeSoto Caverns State Park, are popular with people interested in caving and karst geology. For those interested in hiking and climbing, Cheaha State Park provides scenic overlooks, cliffs, and canyons carved from weathering-resistant metamorphic rocks, with Mt. Cheaha being the highest point in the state at more than 2,400 feet above sea level. One of the most significant geologic contributions to tourism and recreation in Alabama are the beaches in Mobile and Baldwin counties. Coastal geology and sea surface currents provide more than 6,000 acres of Alabama beaches that are famous for their white sands. These beaches bring in more than 4 million tourists and generate more than $2 billion in travel-related expenditures annually. The state’s fossils, minerals, coastal geology, economic geology, and planetary geology are an important part of natural history and science exhibits in museums across the state, as well. Museums such as the Alabama Museum of Natural History, Anniston Museum of Natural History, McWane Science Center, U.S. Space and Rocket Center, Iron and Steel Museum of Alabama, Alabama Mining Museum, Vulcan Park and Museum, Dauphin Island’s Estuarium, and others showcase exhibits that help more than one million residents and tourists each year learn more about geology, geologic processes, and the geologic history of Alabama.
Adams, G. I., Charles Butts, L. W. Stephenson, and Wythe Cooke. Geology of Alabama. Alabama Special Report No. 14, 1926.
The Geological Survey of Alabama and State Oil and Gas Board, http://www.gsa.alabama.gov