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G12a-Origin of the Mississippi A mountain range once separated the continental interior of the U.S. from the Gulf of Mexico. Some clever geologic sleuthing has revealed how that barrier was breached, allowing the river to reach the Gulf n examining a map of the world, many schoolchildren notice that the continents surrounding the Atlantic Ocean can be neatly fit together like pieces of a gigantic jigsaw puzzle. Just Osnug West Africa up against the East Coast of the U.S. and shove the northern end of South America into the Gulf Coast. That is indeed how these continents were arranged a few hundred million years ago, a fact geologists know, in part, because the plate tectonic movements that created this great landmass left their marks. In the eastern U.S., the collision with Africa raised the Appalachian Mountains to heights that probably once rivaled the Rockies. Similar- ly, the impact of South America created the Oua- chita (pronounced “WAH-shi-TAH”) Moun- tains, which run west to east across Oklahoma and Arkansas and formerly blended smoothly into the southern Appalachians. Yet somehow the once continuous Ouachita-Appalachian range was cleaved in two, leaving room for the SOUTHERN PORTION of the Mississippi River (blue) flows within a tongue-shaped region of low-lying land (purple) that separates the Ouachita Mountains to the west from the Appalachians to the east. This distinctive feature, called the Mississippi Embayment, owes its existence to the passage of North America over a geologic “hot spot”— a concentrated source of heat located, most probably, close to the earth’s core. 76 SCIENTIFIC AMERICAN COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. The Mississippi’s Curious Origins By Roy B. Van Arsdale and Randel T. Cox COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. chain. For the next 70 million years or so, this east-west-trending mountain The Mississippi Embayment constitutes range divided the interior of one vast continent. Rivers that began as tiny one of the largest and least understood streams in these mountains flowed either to the south, onto the future South Amer- landforms in the central U.S. ican continent, or to the north, onto what would become North America. Mississippi River to flow into the Gulf would be incorrect. As geologists have About 230 million years ago (which of Mexico. The explanation for the split, known for quite a while, the base of these happens to be when the first dinosaurs which the two of us have been investi- sediments sits as much as 2.6 kilometers appeared), Pangaea started to break up. gating for most of the past decade, below current sea level. Clearly, this part Not surprisingly, it ripped apart where touches on many other mysteries of of the midcontinent must have dropped it had previously been sutured together: North American geology, too—such as down considerably, allowing whatever just outside the arc of the Ouachita- why you can find diamonds in Arkansas river had previously drained the vast con- Appalachian mountains. This opening and why the largest earthquake that was tinental interior toward the north or west formed both the Atlantic Ocean and the ever recorded in the contiguous U.S. oc- to turn around and flow south, thus be- Gulf of Mexico. The Atlantic is still ex- curred not in California or Washington coming the familiar Mississippi. But why panding today, but the Gulf continued but in Missouri, of all places. did the land there sink? to widen only until about 145 million The break that now separates the years ago (about when the first birds Ouachita Mountains from the Appala- The Earth Moved evolved). chians, a feature known as the Missis- t h e a nsw e r to this particular geo- For tens of millions of years, the sippi Embayment, constitutes one of the logic riddle begins with some well- Ouachita-Appalachian mountains stood largest and least understood landforms understood events that took place while as a continuous range along the northern of the central U.S. This huge horseshoe- trilobites were still scurrying around and border of the Gulf of Mexico. While shaped lowland is underlain by massive long before the Mississippi Embayment such a barrier was in place, the continen- quantities of sand, silt and mud depos- formed. Throughout most of the Paleo- tal interior could not have drained south- ited within a bay of the ancestral Gulf of zoic era (545 million to 245 million years ward into the Gulf. The geology of the Mexico, which first washed over this re- ago), the region destined to become the Gulf of Mexico indicates as much. Over gion some 85 million years ago (during U.S. was surrounded by ocean. But 300 this lengthy interval, the northern fring- the late Cretaceous, when dinosaurs still million years ago (about when the first es of the Gulf hosted a bank of coral roamed the earth), and did not recede reptiles evolved), the landmasses that reefs, which later became the thick pe- PALEOMAP Project until tens of millions of years later. would later turn into North America, troleum-bearing limestone deposits that ) It is tempting to imagine that the de- South America, Africa and Eurasia came geologists have traced in the subsurface posits formed because Gulf waters flood- together to form the supercontinent that all the way from Florida to northern ed a slightly low-lying area of the conti- geologists call Pangaea. These titanic Mexico. Corals cannot tolerate being opposite page nent, leaving behind a veneer of sediment collisions raised several mountain rang- smothered by silt and clay; hence, a ma- ) ( when they receded. But such a conclusion es, including the Ouachita-Appalachian jor river such as the Mississippi could not have been dumping sediment into ); CHRISTOPHER R. SCOTESE Overview/Making Way for a River the northern Gulf during that era. For this great breadth of time, the ■ The Ouachita Mountains of Arkansas and Oklahoma once connected with the Gulf Coast remained geologically sta- southern end of the Appalachians, forming a continuous range that forced ble, with no continents plowing into it preceding pages rivers draining the continental interior to flow north or west into the sea. or rifting away. But the calm was broken ) ( ■ Passage of North America over a temporarily reinvigorated mantle hot spot about 95 million years ago: beginning caused uplift of a section of this mountain range just north of the Gulf of then, and continuing for the next 10 Mexico in the mid-Cretaceous, forming an arch. Weathering soon eroded million years, the crust warped upward the newly formed highlands, reducing them to the level of adjacent lands. over a large area, from southern Louisi- ■ As this region moved away from the hot spot, the land subsided, allowing ana north into southeastern Missouri base satellite image the ocean to flood a large tongue-shaped area into which rivers draining the and from the present-day Tennessee continent could then flow. The modern Mississippi continues to meander River west to Little Rock, Ark.—the south through sediments that fill this depression. footprint of what is now the Mississippi ■ Passage of North America over mantle hot spots also accounts for the Embayment. earthquakes that have struck some areas of the midcontinent. The result was a broad, northeast- paleogeographic globes for Triassic, early Cretaceous and late Cretaceous trending arch, which rose to an impos- NASA/THE( VISIBLE EARTH ( 78 SCIENTIFIC AMERICAN JANUARY 2007 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. ing altitude, perhaps two or three kilo- meters above sea level. (In comparison, Snapshots the Himalayan Plateau currently stands about five kilometers high.) Evidence from the Past that such a vast uplifted region once Plate tectonic reconstructions existed over the Mississippi Embayment show the evolution of the is indirect—but quite solid. For exam- mountainous barrier that long ple, geologists have found telltale grav- prevented the rivers draining els at various sites to the east. Detailed the interior of North America studies of these deposits indicate that from flowing south to the sea, energetic streams flowed at this time as the Mississippi does today. out of the newly created highlands, car- About 200 million years ago rying gravel with them as they cascaded (top) the continents of the eastward. 200 million world abutted one another, years ago forming a giant super- Even more compelling proof of wide- continent, which geologists spread uplift comes from what is not call Pangaea. The collisions found in the Mississippi Embayment: up that brought the continents to three kilometers of rock formed be- into this configuration raised fore the late Cretaceous. That pile of various mountain ranges, rock is conspicuously missing along the including what was then a axis of the embayment. We would ex- continuous line of mountains pect the missing strata to have been that ran along the southern eroded away if they were raised far above Ouachita- and eastern flanks of the sea level during this 10-million-year pe- Appalachian North American plate. Rivers riod—high-standing terrain being espe- Mountains draining the interior of cially susceptible to the ever present in- North America could not have fluence of weathering, which over geo- Gulf of crossed this formidable logic time tends to grind even great Mexico Atlantic Basin barrier and thus must have mountains down to near nothingness. flowed northward or westward The next major event was the subsid- into the sea. ence of the embayment, which geolo- The Ouachita-Appalachian gists have determined must have taken mountains remained a con- place from 85 million to 24 million tinuous barrier even after the 140 million years ago. During this interval, the area Yucatán block shifted years ago that had previously been lifted two to southward and the African three kilometers into the air headed in plate separated from North the other direction, dropping below sea America, plate tectonic motions that by the early Cretaceous level by about the same amount.
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