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The islands that make up Gulf Islands National Seashore are made of quartz sand eroded and washed down rivers from the Appalachian Mountains. A sand dune today was perhaps once, thousands of years ago, a mountain top near the Georgia-South Carolina border. Swept into the Gulf of Mexico, powerful currents pushed the quartz sand westward. Currents gradually built long, thin ridges of sediment on the shallow sea floor. Sand continued to deposit, the ridges grew higher and eventually islands were created. Alongshore currents wear away the islands on their eastern ends while building them up on the west. As a result, the islands shift steadily westward. More changes are made by violent storms that overwash the islands and rearrange the sand, and by winds that shift and sculpt the dunes. Such changes are constant. They are slowed only by the protective covering of plants that grow on the dunes and in the island interiors. The sea oat, which has an elaborate stem and root system, plays a vital role in holding the islands together. Barrier islands protect natural and human communities against ocean storms. Heavy seas are dampened by impacting against island beaches. Less forceful waves wash by the islands, but do not tear out valuable mainland saltmarsh. Millions of dollars in damage to homes and property are avoided. Large vessels and small boats seek shelter behind the islands for safe passage. Young sea creatures find shelter in the coast's extensive marshlands. Due to protection from barrier islands, mainland salt marsh nurseries add millions of dollars to the economy through commercial and sport fishing opportunities. (Excerpts from "Gulf Islands National Seashore: Geology Training Manual" by Dr. Robert J. Lillie, Oregon State University, January 27, 1999) Gulf Islands National Seashore lies along the passive continental margin of the southern United States. The nature of the margin and its position, however, are only the current product of a long history of continental margin development that is still occurring. The development is in part the result of continental rifting, the opening of the Atlantic Ocean and Gulf of Mexico. The rifting and continental drift that opened the Gulf of Mexico ceased about 150 million years ago, so that current activity involves subsidence, sedimentation, and sea level change that shift the coastline through time. As the Southern Appalachians eroded material was deposited in the northern Gulf coastal plain and offshore areas, accounting for enormous thickness of sediment observed today. The type of sedimentary layers deposited along a passive continental margin depend on the nature, height, and proximity of the source area, the climate of the margin, openness of the ocean basin, and depth of water. When the Gulf of Mexico originally developed it was a narrow trough between North and South America, with only a small opening to the Atlantic and Pacific oceans. It was also at very low latitude, near the equator. Those two factors meant that there was restricted water circulation; lot of evaporation. Consequently, an enormous thickness of salt precipitated out of the water. The salt was overlain by sand and mud deposited by streams from the flanking Ouachita and Appalachian mountains, which were much higher than they are today. In places lacking streams to bring in continental sediment, such as the shallow shelf surrounding the Florida peninsula, shell fragments from marine animals were dissolved then deposited as lime mud. The layers of sand, mud, and lime compacted and hardened into the rocks sandstone, shale, and limestone, respectively. The thick salt layer below, however, could not compact so easily. Through time the overlying sedimentary layers compacted to higher density material than the salt below. Like a "lava lamp" (or oil rising through water), the less-dense salt rose up through the overlying layers, forming salt domes. Gulf Islands National Seashore contains beaches along the coastlines of Florida and Mississippi, as well as barrier islands on the low-lying continental shelf. The coastlines and barrier islands are features that, geologically speaking, change positions rapidly, depending on global sea level, sediment supply, and rate of subsidence of the continental margin. In fact, all nine of the national seashores lie along the very gentle slope that extends hundreds of miles from the landward edge of the coastal plain to the seaward edge of the continental shelf. Over this large distance, elevation changes by only about 1000 feet (300 meters), from 400 feet (120 meters) above sea level to 600 feet (180 meters) below. The average slope is thus only about 0.03, or 3 feet per mile (0.6 meters/kilometer)! The type of sediment deposited along a coastline depends of the source of the sediment and the water currents that move and deposit the sediment. The sizes and locations of the drainage areas for the streams that supply the sediment heavily influence deposition along the northern Gulf Coast. The sediment that reaches Gulf Islands National Seashore comes from a very small drainage area. In fact, virtually all the particles are from erosion of the southernmost portion of the Appalachian Mountains. The fast moving streams bring mostly grains of clear white quartz the short distance across the coastal plain to the Gulf. The westward longshore current deposits it as pure white sand along the coast and barrier islands of the seashore. To the west and east the sedimentary supply is quite different. The Mississippi River system drains an enormous portion of the North American continent, through slow-moving streams that bring in a lot of mud and silt, in addition to sand. Beaches of Louisiana and east Texas are therefore a "chocolate brown" color. Along the west coast of Florida streams bring in dissolved limestone from the peninsula, but very little sand, silt, or mud. The material deposited is mostly fragments of seashells; the beaches are white, but not nearly as sparkling as those of Gulf lslands National Seashore! Ten Geological Questions about the Northern Gulf of Mexico (Dr. Robert J. Lillie, Oregon State University, January 27, 1999) 1. How did the Gulf of Mexico form? About 300 million years ago Europe, Africa, and South America collided with North America, form the supercontinent of Pangea. As Pangea ripped apart 200 million years ago, South America and the Yucatan Peninsula pulled away from the southern part of North America, opening the Gulf. 2. Why is North America above sea level, the Gulf of Mexico below? Like other continents, North America has thick crust, compared to surrounding ocean basins, like the Gulf of Mexico. Crust is light material that "floats" over the underlying, heavier mantle. Continental crust is thick and buoyant like a beach ball; it therefore sticks up higher than the surface of thin oceanic crust. 3. Why don't large earth quakes or other tectonic activity occur along margins of the Gulf of Mexico? Tectonic activity commonly occurs along the boundaries of large, moving plates of Earth's outer shell. The Pacific coast of the United States coincides with plate boundaries and is thus an active continental margin, with earthquakes, volcanoes, and developing mountain ranges. The Atlantic and Gulf coasts, however, are passive continental margins, far away from plate boundaries and accompanying tectonic activity. 4. What is the overall topography of the northern Gulf of Mexico? The Gulf Coast shows the classic morphology of a passive continental margin: a low-lying coastal plain, broad continental shelf, then a steep continental slope, gentle continental rise, and flag abyssal plain. This topography is a consequence of the transition from continental to oceanic crust. Like an iceberg sticking out of water, thick crusts rises up higher out of the mantel than thin crust. This high areas (coastal plain and shelf) are thus underlain by continental crust, while relatively thin continental and oceanic crusts underlies low regions (continental rise and abyssal plain). 5. Why is the coastline of the northern Gulf of Mexico so far from the edge of the continental shelf? Widths of continental shelves are sensitive to changes in sea level. When worldwide temperatures are low, much of the water is frozen in the polar ice caps, so that sea level drops. During those times the northern Gulf shoreline is near the edge of the continental shelf, more than 100 miles (160 kilometers) to the south of its current position. If Earth's atmosphere were to warm up sea level would rise, flooding coastal areas as far as 70 miles (110 kilometers) to the north, to the cities of Baton Rouge, LA, Hattiesburg, MS and Augusta, GA. 6. Besides sea level change, what other factors influence the position of the coastline? The north-south position of the coastline is also sensitive to subsidence and sediment supply. When the continents ripped apart their crust was stretched and thinned, eventually subsiding below sea level as the Gulf opened. Sediment eroded from North America was continuously deposited in the depression. The supply of sediment is enormous compared to the rate of subsidence, so that over the very long term the northern Gulf Coast and continental shelf have been building southward. 7. How thick are the sedimentary deposits of the Gulf coast plain and continental shelf, and why are they such thickness? In places the sedimentary layers approach a thickness of 10 miles (16 kilometers). Subsidence accompanying crustal thinning created a basin as the Gulf of Mexico opened 200 million years ago. Shortly before Atlantic and Gulf basins developed, the flanking Appalachian mountains were very high, perhaps as high as the Alps or Himalayas are today. Erosion of the mountains to their current, more modest heights supplied tremendous volume of sediment as the basin continued to subside. 8. Why are the beaches in Gulf Islands National Seashore so white? Sediment reaching the Gulf along the Florida Panhandle, Alabama, and Mississippi coasts is almost entirely from erosion of hard rocks in the southern Appalachian Mountains. Quartz, a hard, durable mineral, survives transport by fast-moving streams, while softer minerals break down and dissolve, or remain suspended in the water. Sedimentary particles available for deposition along the northeastern Gulf Coast are thus pure white quartz. 9. Why are barrier islands so elongated? As rivers bring in sediment from the eroding Appalachians, the ocean current sweeps the sand particles westward, parallel to the coast. Wave action from the Gulf keeps the sand concentrated along a ridge, so that the islands are long and narrow. 10. How do the barrier islands change through time? The westward longshore current erodes islands on their east sides and deposits sand on the west. The barrier islands are thus temporary features that migrate gradually westward through time. One a longer time scale the islands also migrate north-south directions, due to the changes in subsidence, sediment supply, and sea level discussed above. The islands that make up Gulf Islands National Seashore are made of quartz sand eroded and washed down rivers from the Appalachian mountains. A sand dune today was perhaps once, thousands of years ago, a mountain top near the Georgia-South Carolina border. |
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Last Updated:
1/6/03
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