Dry Tortuga: Physical Geography of the Dry Tortugas National Park - Coursework Example

www.academia-research.com Sumanta Sanyal Dated: 8th December, 2005 Dry Tortuga: Physical Geography of the Dry Tortugas National Park Abstract The essay introduces the physical geography of the Dry Tortugas, a set of small islets off the keys of mainland Florida. The climate, water bodies with precipitation characteristics, temperature, humidity, wind patterns and wave mechanics are studied in detail. The geology of the islets together with how and when they were formed is discussed. Weathering characteristics together with other formations are also discussed. The essay ends with a conclusion that, no matter how small an area the islets encompass, their fragile ecosystem of coral reefs and tropical vegetation should be protected from the hazards of human influence on this Earth. Introduction The Dry Tortugas are a cluster of ten low-lying islands located about 140 miles (252 km) from the Florida mainland and 61 miles (110km) from Key West. They lie within N and W. The islands are very small, islets almost, and are composed of coral reefs and sand and some may be exposed only at high tide or at certain times of the year when the water levels are low. The islets, together with the surrounding shoals (shallow water 12-20 m deep), compose the Dry Tortugas National Park. The park is stretched across 100 miles and is of 64,657 acres of which 99% is water and submerged land. The area is well-protected, specifically the part that lies within the Florida Keys Marine Sanctuary (Dry Tortugas and Sustainable Seas Expedition) The park’s geography and flora and fauna attract popular activities like diving, snorkeling fishing and bird watching. Climate The Dry Tortugas lie just North of the Sub-tropical belt and, as such, receive quite a high amount of energy from sunlight. The Dry Tortugas, together with the Florida Keys, receive an average of 3300 hrs of sunshine annually (Florida Biodiversity Project, 2000). Day length is long and the sun is at a relatively high average altitude at this latitude and Christopherson (91) estimates that the cluster of islets and the region receive approximately 220W/ with south facing surfaces receiving averages of 5.5 kWh/ per day annually. The Tortugas Ecological Reserve has very little land and the contained waters border those of the Atlantic Ocean and the Gulf of Mexico. The relatively light-colored surfaces of the land with sand and coral have a slightly higher albedo than the surrounding waters and the overall albedo of the area is about 18-20 (Christopherson, 92-93). Key West, Florida, lies on the same latitude as the region and it may be deduced that the regions have similar temperature incidence taking into account that the Tortugas region have a higher albedo than Key West as the Key has human habitation with darker surface areas and probably more vegetation. The temperatures range from highs of F in July-August and no less than F in the colder months (Florida Biodiversity Project, 2000). Thus, it can be seen that the variation in temperature is not extreme, as can be expected for a location so near to the subtropical belt (Christopherson, 132), especially for a location that is under direct influence of the Gulf Stream, which considerably reduces cold conditions. Conditions are humid with 50 5 year round humidity. On days at F, the humidity can be as high as 60-70 % and even up to 90 % at dawn (edis ifas). The Dry Tortugas are relatively uninhabited and the climate remains unaffected by human interference. The islets’ temperature, precipitation and wind speeds are affected by the seasonal changes in positions and interactions between the combination of the Bermuda-Azores high-pressure belt and the Caribbean and other air masses (Florida Biodiversity Project, 2000). The two main seasons are the rainy one from May to October and the dry one from November to April. The rainy season is characterized by warm winds from the east-south-east and the dry season, in addition to less precipitation, has fast-dissipating cold fronts prevailing from the east-north-east. Both wind systems, Southeast Trades and Northeast Trades, are generated by air moving towards the Bermuda-Azores subtropical high, the warm air coming in from the south in the rainy season and the cold air moving in from the north in the dry season. The wind patterns in the rainy season are sometimes disrupted by cyclonic disturbances and in the dry season by cold fronts with strong winds (Florida Biodiversity Project, 2000). The warm and shallow waters of the Gulf of Mexico which feeds the Gulf Stream which, in turn, ultimately meanders off towards the West shores of Europe and creates milder climatic conditions there, especially in winter, has a strong influence on the climate in the Dry Tortugas. The warm waters generate a lot of energy and create low-pressure fronts that often culminate in hurricanes, category three and above, that make the Florida Keys a target for more tropical depressions and hurricanes than anywhere else in North America (Dry Tortugas National Park, 2003). Water Bodies The general distribution of water on the Earth’s surface goes something like this: salt water in the oceans – 97.22 %, and freshwater – 2.78 % The freshwater breakdown distribution is important as it is a direct source of sustenance for land animals and it goes somewhat like this: surface water – 77.78 %, and 99.357 % solidified in ice and glaciers, and 00.643 % distributed in lakes, rivers, streams, other freshwater bodies and water vapor in air; groundwater and soil moisture – 22.22 %. Precipitation in the Tortugas Ponce de Leon, The Spanish explorer, found the small set of islets and named them ‘Las Tortugas’ after the large turtles he found on them. Early nautical maps titled them ‘Dry Tortugas’ to indicate the lack of freshwater on the islets (Dry Tortugas and Sustainable Seas Expedition). The Tortugas Keys are relatively narrow, often less than 1/4 mile, and rainfall events allow the water to run off quickly into surrounding waters making it impossible for it to be retained on the islets. There are subsequently no fluvial systems on the islets either (Gardening guide for the Florida Keys). The Tortugas get their rain in fits through the tropical storms both in the rainy season, when it is frequent, and during the dry season when the storms are less frequent. It is not unusual for the islets to have 10-12 inches of rain within a 12 hr period (Gardening guide for the Florida Keys). There is some amount of flooding during these intense rain events but because the islets are uninhabited no-one comes to harm. The Dry Tortugas has two kinds of lifting mechanisms – convectional and convergent. The mechanisms depend upon the warm climate to work. As is seen from the graph (Appendix 1) the warmer months from July to November generate the maximum rains. The convectional system generates normal rains while the convergent system, which depend upon winds attracted by the warmth of the nearby Atlantic Ocean and Gulf of Mexico waters, generates more powerful systems like tropical storms and hurricanes that may have winds raging at over 200 km/hr (Christopherson, 211-212). Wave mechanics The Tortugas do not fall in an area where tsunamis are common. The Dry Tortugas are low-lying (high vulnerability at less than 1 m) in the sea and tidal range is defined as both permanent and episodic inundation hazards. Very low and low vulnerability waves (mean significant waves) are determined to be 0.5 and 0.8 m respectively (USGS Open File Report, 2004 - 1416). Geology Except for the coral which is a biological addition to the landmass of the Tortugas the islets are mostly made of rocks formed during the Pleistocene Epoch (Scott, M. T. et al). The Epoch lasted from 1.6 million years ago to 10,000 years ago. The Tortugas have rock that is related to those found around Miami but the Florida Upper Keys have different geological composition. This is surprising but does indicate the islets may have broken off from the mainland some time during the Pleistocene or may have risen out of the ocean floor due to some upheaval during that period. The rock is mainly oolite, as surmised from the composition of the Florida Lower Keys. Oolite is a medium to hard limestone that forms on the floors of shallow and warm seas. The rock is thus a local phenomenon. Oolite is formed when the higher temperatures and high evaporation rates of the subtropics causes excessive evaporation and concentrates calcium carbonate particles in the water which grow heavy and are gradually deposited on the ocean floor. Over time, due to pressure, the particles solidify into oolite. While the Upper Keys are mostly coral which is quite porous with large solution spaces oolite is porous but not as much as coral because of smaller solution spaces. Oolite is often called ‘Miami Limestone’ to indicate its point of origin. Plate Tectonics Cuba is a result of the Caribbean Plate pushing at the North American Plate but the Tortugas are firmly placed on the North American continental shelf. There are no fault lines or any other fissures in the Tortugas area. The closest fold in the system under the Tortugas is Cuba. There are almost no phenomena of earthquakes in Florida and the same can be said for the nearby Tortugas. The most recent record of any strong tectonic activity is in January 1880, when Cuba felt two strong quakes which were quite strongly perceived in Key West, Florida. The tremors occurred within a day of each other and since the area has been relatively free of any such activity (USGS, Earthquake History of Florida).More recent activities are almost insignificant and not mentioned here. Land Formation The Tortugas together with mainland Florida emerged out of the sea very recently but sedimentary deposits have been forming under the seas for a long time. The Key islands sit on 20,000 feet of marine sediments that began depositing several million years ago. For example, the present-day Rebecca Shoals began its career as a young barrier reef 140 million years ago. Nevertheless, it was only 25 million years ago that mainland Florida began emerging from the sea and terrestrial deposits began forming. Since then the Florida coastline has been changing with the rise and fall of sea levels. During the ice age 20 million years ago, the accretion of the Wisconsin Glacier lowered sea levels 450 feet below present-day levels and the Key islands together with probably the Tortugas islets were left high and dry with no water body nearby (Gardening Guide for the Florida Keys). There is evidence of Aeolian processes on the beaches of the Dry Tortugas. Winds blowing in from the sea create sand ripple effects that are similar to those in a desert. Sand dunes can be found on an inland side of a beach. Beach berms/dunes are not automatic here. Only where the current is able to breach the reefs and carry large amount of sand onto the beaches are dunes found. Sea vegetation like sea oats, salt marsh grasses, etc. holds done the dunes so that they survive depredation from wind and water (Gardening Guide for the Florida Keys) Volcanic Activity The Dry Tortugas islets are a part of the Florida plateau that separates the Atlantic Ocean from the Gulf of Mexico. The plateau is based on volcanic mountain rocks that were buried in the Paleozoic Era 600-225 million years ago under Central Florida. These volcanic rocks form the base of the plateau and are at present 13,000 feet below the plateau. Sand and other sedimentary deposits form a thick layer 4000 feet over the volcanic rocks (4 to 40 Kids Portal for Parents). Reef Structures About 6,000 reef structures are to be found between Key Biscayne and Dry Tortugas. These are the most prolific in the Caribbean. The most common reef structures found there are bank reefs, patch reefs and thickets of staghorn corals (FKNMS, Tortugas Ecological Reserve). Weathering Weathering in this area is mainly chemical – mainly carbonation and solution-induced ones. The moist air dissolves carbon dioxide and erodes the limestone gradually. There are no sinkholes as the water is ubiquitous. Rainwater solutions also react with the coral. There are no karst problems as the Tortugas are relatively uninhabited. There are also no known caves in the Tortugas. Land Movements The islets and the surrounding continental shelf are relatively flat and no land mass movements take place though the interface between the islets’ land and sea is extremely dynamic. This dynamism extends from the interface towards the land in the forms of erosion, deposition and landsliding (mass wasting), etc. while it also extends seawards over tidal flats, mangrove swamps and other low-profile lands. Within this zone the shoreline is being constantly shaped by meteorological, hydrological and oceanic processes that work on the surface and bedrock geology. Shoreline erosion and accretion takes place in the Dry Tortugas using Digital Shoreline Analysis System (DSAS). The shoreline change rate (m/yr) is calculated at 200 m intervals (transects) along the shoreline. The change rates for each transect within each grid cell with positive numbers gave the rate of erosion rate while negative numbers gave the accretion rate. The erosion rate at high vulnerability at 2 m/yr and accretion rate at low vulnerability at 2 m/yr is found to be the norm (USGS Open File Report, 2004 – 1416, Geological Variables) Conclusion The Dry Tortugas are a subtropical chain but the influence of the warm waters around gives it a tropical climate. Wherever on the islets there is enough topsoil there is lush vegetation with exotic fauna. Also, the coral reefs in the waters are rich in marine. The ecosystem of these exotic islets are constantly threatened by pollution. The flow of effluents from the mainland threaten fish and other aquatic organisms’ lives. The phenomenon of global warming threatens the reefs (solcomhouse, Coral Reefs). The effluents cause euttrophication which leads to damage to other species. These have to be monitored and minimized if the Tortugas are to retain their natural beauty. Reference 4 to 40 Kids Portal for Parents. Extracted from: http://www.4to40.com/earth/history/print.asp?article=earth_history_florida Christopherson, R. W., 2002. Geosystems: An Introduction to Physical Geography. Coastal Vulnerability Report of Dry Tortugas National Park to Sea-Level Rise, USGS Open File Report 2004 – 1416. Extracted on 6th December, 2005, from: http://pubs.usgs.gov/of/2004/1416/html/ppvariables.htm Coastal Vulnerability Report of Dry Tortugas National Park to Sea-Level Rise, USGS Open File Report 2004 – 1416, Geological Variables. Extracted on 6th December, 2005, from: http://pubs.usgs.gov/of/2004/1416/html/gvariables.htm Dry Tortugas and Sustainable Seas Expedition. Extracted on 5th December, 2005, from: http://www.marine.usf.edu/pjocean/packets/sp00/sp00u1le1.pdf#search='Dry%20Tortugas%20and%20Sustainable%20seas' Dry Tortugas National Park, 2003. Extracted on 6th December, 2005, from: http://www.nps.gov/applications/parks/drto/ppdocuments/drtolrip.pdf Earthquake History of Florida, USGS Earthquake Hazards Program. Extracted on 6th December, 2005, from: http://neic.usgs.gov/neis/states/florida/florida_history.html Florida Biodiversity Project, 2000. Extracted on 6th December, 2005, from: http://planning.nps.gov/document/drtofgmpeis.pdf Florida Keys National Marine Sanctuary, Tortugas Ecological Reserve. Extracted on 6th December, 2005, from: http://floridakeys.noaa.gov/tortugas/studyarea/welcome.html Gardening Guide for the Florida Keys. Extracted on 6th December, 2005, from: http://monroe.ifas.ufl.edu/gardening_guide.htm Scott, M. T., et al, Geologic Map of the State of Florida. Extracted on 6th December, 2005, from: http://sofia.usgs.gov/publications/maps/florida_geology/Florida_geology-med.jpg SolcomHouse, Coral Reefs. Extracted on 6th December, 2005, from: http://www.solcomhouse.com/coralreef.htm Appendix Graph 1: Read More