Russell 1Thomas RussellGEO 111-50026 July 2012 Geomorphology: The Great Sand Dunes National Park Declared a national monument by the National Parks and Preserve Act of 2000, the GreatSand Dunes are the largest dunes in North America, rising a staggering 750 feet; the dunes arelocated at the base of an alcove in the Sangre de Cristo Mountain Range in Colorado. Forgeographical clarity, it is important to note that both the San Juan and Sangre de Cristo MountainRanges are part of the larger Rocky Mountains and are separated by the San Luis Valley. Building the Great Sand Dunes While it is commonly accepted that the Great Sand Dunes are a young landform, aprecise age has not been determined. “U.S. Geological Survey geologists estimate that the dunesare between a couple of thousand years and 12,000 years old….” 1 What is lucidly understood,however, is the process and structure by which the sand dunes are created and maintained. Building the Great Sand Dunes National Park requires four predominant factors—plentiful loose sediment deposits, strong winds, a permanent sand trap structure, and a method bywhich to control the expansion or migration of the dunes. Loose sediment in the form of sand,silt, and gravel exists in abundance across the floor of the San Luis Valley; “[t]he sand is blownby prevailing southwesterly wind from alluvial sediments of the San Luis Valley. Sandmineralogy indicates that much of the sediment was derived from volcanic rocks of the San JuanMountains, transported by the Rio Grande, and deposited in the rivers huge alluvial fan on thewestern side of the valley.”2 Sand carried by the wind is directed by the Sangre de CristoMountains north and funneled into a naturally occurring sand trap—an alcove in the mountain1 National Park Service. “Geology Fieldnotes: Great Sand Dunes National Park and Reserve.”http://nature.nps.gov/geology/parks/grsa/. (Accessed July 18, 2012).2 James Arber. “San Luis Valley, Colorado.” Emporia State University.http://academic.emporia.edu/aberjame/field/rocky_mt/zapata.htm#dune. (Accessed July 18, 2012).
Russell 2range whose concave side opens westward. Anchored to the mountains, the depositedsedimentary material is bounded on all other sides by geologic forces; more specifically,prevailing winds blow from the west and the south while Sand Creek boarders the north andMedano Creek boarders the south. Collectively, these provide the methods by which expansionof the dunes can be moderated; evidently, it would prove impossible for the dunes to expand eastor west, and the creeks to the north and south progressively erode the dunes as they attempt tomigrate across. Crucial to the region‟s process of sedimentrecycling, the Sand and Medanocreeks return sediment to the plains of the San Luis Valley, where the process is free torepeat.The aggregate result is a relatively stagnant dunefield composed of fine-grained sedimentsand whose individual dunes change shape daily with the winds. Geologic Structure of the Great Sand Dunes National Park While it may appear counter-intuitive at face value, the sand dunes have a structure morecomplex than simply “piles of sand.” Beginning with the watershed to the east—Sangre deCristo mountains—this is the primary source of water for the sand dunes; water originates asrainfall and snowmelt which then flows down the west side of the mountains towards the sanddunes. Infiltration, “[t]he movement of surface water into porous soil,”3occurs where water fromthe watershed meets the dunefield and sinks underground. The dunefield pertains to an areacovering about forty square miles and containing the largest dunes. “For a place formed of sand,the Great Sand Dunes National Park is surprisingly wet. You can actually reach wet sand just bydigging a few inches in the dunes, even on their tops.”4 Water underneath the dunefieldcontinues flowing westward, gradually approaching the surface.3 “Geological Terms Beginning with „I‟.” http://geology.com/dictionary/glossary-i.shtml. (Accessed July 18,2012).4 “The Great Sand Dunes National Park.” http://denvercolorado.org/history-places-of-interest/the-great-sand-dunes-national-park/. (Accessed July 18, 2012).
Russell 3 Outside the dunefield is a wide perimeter of sandy grasslands which actually contains themajority of the sand attributed to the Great Sand Dunes National Park, though it containsrelatively few and miniscule dunes. Known as the sand sheet, it contains the majority ofsediment because of its vegetation; “[w]ith just 10 percent plant cover, movement across thesand sheet drop by 50 percent. With 50 percent coverage by vegetation, movement ceasesaltogether.”5 Continuing outward from the dunefield and passed the sand sheet, the finalstructural component of the park is the Sabkha. “When the water evaporates away in latesummer, minerals similar to baking soda cement sand grains together into a hard, white crust.Areas of sabkha can be found throughout western portions of the sand sheet, wherever the watertable meets the surface.”6 Weathering and Erosion of the Great Sand Dunes Having established a firm background regarding the structure and processes underlyingthe sand dunes, consider a closer analysis of weathering and erosion related to the sand dunes.Beginning with mechanical weathering processes in the San Juan Mountains to the west, largerocks can be cut down by frost wedging; this process occurs when liquid water enters small holesand cracks in the rock and is later frozen by cold temperatures. Since water expands in volumenearly ten percent when frozen, it widens the openings in the rock and will eventually cause afracture. Additionally, biological weathering in the form of lichens may weaken and break rocksas the organisms extract minerals. Fluvial forces such as rainfall or snowmelt will often displace smaller rock fragments intoa stream or river. This same result can also be accomplished through a mass wasting process5 Stephen Trimble. Great Sand Dunes National Monument: The Shape of the Wind. (Tucson: Western NationalParks Association, 2000), 6.6 National Park Service. “Great Sand Dunes System.” http://www.nps.gov/grsa/naturescience/sand_system.htm.(Accessed July 18, 2012).
Russell 4such as a mudslide, landslide, or rockfall. Once part of a stream, it is likely that the sedimentwill be flushed into the Rio Grande River, which bisects the San Luis Valley and contains abroad shoreline upon which fine sediments can be deposited and retrieved by the wind. Destroying the Great Sand Dunes Despite the awesome beauty and seeming immobility of some billions of tons of sand, theGreat Sand Dunes are ultimately doomed. Responsible for their fate millions of years in thefuture is the Rio Grande Rift. “Over the last 25 million years, a gigantic rift has been opening upat the southern end of the rocky mountains. It stretches over 160,000 square miles and is knownas the Rio Grande Valley.”7 The rift continues spreading north while widening from the south asthe surrounding mountains erode into the valley. Rifting is caused by a sinking and thinning ofthe earth‟s crust by “…lava from a source deep in the mantle [that] periodically spreads acrossthe surface. In the near geologic future, several million years or so, a youthful ocean basin mayoccupy this area.”8 As the Rocky Mountains—and its Sangre de Cristo Mountains—erode and collapse intothe rift, shifting wind patterns will displace and ultimately destroy the Great Sand Dunes. In thedistant geologic future, the same sediments that compose the Great Sand Dunes today may berelocated to the bottom of the ocean only to be later subducted into the mantle. Produced by a rare conglomeration of geologic features, the San Luis Valley in Coloradoboasts what is, without doubt, one of the most striking landforms on the continent—The GreatSand Dunes—and will continue to so until the whole of the Rocky Mountains erode into the RioGrande Rift.7 “How the Earth was Made: The Rocky Mountains.” http://www.history.com/shows/how-the-earth-was-made/episodes#slide-9. (Accessed July 18, 2012).8 “A Tapestry of Time and Terrain: The Rio Grande Rift.” http://tapestry.usgs.gov/features/28riogrande.html.(Accessed July 18, 2012).
Russell 5 BibliographyArber, James. “San Luis Valley, Colorado.” Emporia State University. http://academic.emporia.edu/aberjame/field/rocky_mt/zapata.htm#dune.“Geological Terms Beginning with „I‟.” http://geology.com/dictionary/glossary-i.shtml.“The Great Sand Dunes National Park.” http://denvercolorado.org/history-places-of-interest/the- great-sand-dunes-national-park/.“How the Earth was Made: The Rocky Mountains.” http://www.history.com/shows/how-the- earth-was-made/episodes#slide-9.National Park Service. “Geology Fieldnotes: Great Sand Dunes National Park and Reserve” http://nature.nps.gov/geology/parks/grsa/.National Park Service. “Great Sand Dunes System.” http://www.nps.gov/grsa/naturescience/sand_system.htm.“A Tapestry of Time and Terrain: The Rio Grande Rift.” http://tapestry.usgs.gov/features/28riogrande.html.Trimble, Stephen. Great Sand Dunes National Monument: The Shape of the Wind. Tucson: Western National Parks Association, 2000.