The word “karst” comes from the name of an enormous limestone plateau located along the border between the border Yugoslavia and Italy, where Austrian speleologists studied and described the phenomenon. Karst or karstic formations refer to the portions of the earth’s surface made up mainly of limestone, which contains calcium carbonate (CaCO3) and dissolves due to acidic water eroding and breaking the rock down.
About one third of the land of Puerto Rico is karstic (Figure 1).
|Descripción: Mapa que muestra dónde están ubicadas las regiones cársticas de Puerto Rico.|
The largest region is the northern edge of Puerto Rico, between Aguadilla and Loíza. This region is characterized by large-scale processes of breaking down and dissolution of rock. A second broad karstic region is in the southwestern area, between Ponce and Cabo Rojo. Here these same processes are hardly apparent. There are also outcroppings of limestone scattered through the mountainous volcanic region in the center of the island.
The phenomenon of karstification in Puerto Rico basically formed on limestone made up primarily of calcium carbonate, though it also developed on other kinds of rock to a much lesser degree. Some 95 percent of the carbonate rock is of biological origin. This is a phenomenon that also occurs in Cuba and other Caribbean islands. Carbonate rock derives from the skeletal remains of marine organisms deposited on the bed of shallow seas (continental and island shelves) millions of years ago. Most of the organisms involved were coral reefs and accumulations of coral colonies that contain the bones of fish, conch shells, urchins and other organisms that consolidated over time.
These sedimentary strata on the bottom of the sea broke up and crumbled as the tectonic plates of the earth’s crust moved and converged. Eventually geological movement pushed the seabed up and formed limestone mountains. This took place during the mid tertiary period of the Cenozoic era. The constant movement of the surface of the earth over millions of years produced faults and fractures in this limestone rock. At the same time, several strata of rock were exposed to the elements. The corrosive and erosive action of such natural elements as rain and wind degraded the foundations of the weaker parts of the limestone crust. That is why karst has its very particular and varied forms in different parts of the world.
The most important and largest karst region, where karstic processes are most notable, is the northern edge of Puerto Rico, between Aguadilla and Loíza. Among the most respected publications of the US Geological Survey (USGS) on this zone of limestone rock are the very detailed studies by Watson Monroe (1976), Ennio V. Giusti (1978), and Jesús Rodríguez Martínez (1995).
The second largest karst region is that in the southwest, between Ponce and Cabo Rojo, which is fragmentary in character. It occupies a smaller area and has outcroppings with other characteristics for geological, climatic, and evolutionary reasons. In this southwestern karst region, dissolution is hardly to be noticed, since for climatic reasons, it being a dry area, precipitation is much scarcer and, as a result, the effect of karstic processes is limited. The prevalent types of karst are limestone pavement and covered karst. There are caves, but they are not very large; they are mostly dry; and they do not have large underground rivers as in the northern area, with the exception of the Convento Caves in Peñuelas.
The intermittent outcropping of karst in the mountainous volcanic region at the center of the island, from the Cretaceous, are exploited as quarries from Caguas and Cidra to Cayey. Here runs the famous Aguas Buenas cave system, connected to the Río Cagüitas. These are the oldest karst outcroppings in Puerto Rico, as they were exposed to the elements at the same time as areas of Puerto Rico’s central mountain range, the Cordillera Central.
The northern karst, which runs from the town of Río Grande to Aguadilla and covers 1,760 km2 (680 mi2), approximately 19 percent of the surface area of Puerto Rico. north to south, the northern karst extends from the Atlantic Ocean toward the south, where it meets the Cordillera Central running East to West. The North to South outcropping of the zone is approximately 18 km (11 mi) wide, near Camuy, and narrower near San Juan at some 3.62 km (2.25 mi).
Eight principal rivers pass through the northern karst zone, originating in the volcanic Cordillera Central and flowing mostly to the north to pour into the Atlantic Ocean. These rivers are the Río Guajataca, Río Camuy, Río Grande de Manatí, Río Grande de Arecibo, Río Cibuco, Río la Plata, Río Bayamón and Río Grande de Loíza. There are also lagoons (such as the Laguna Tortuguero Natural Reserve), wetlands, swamps, and above all, major springs. The largest herbaceous wetlands in Puerto Rico are in the Caño Tiburones, which is found on the north coast between the Río Grande de Arecibo and the Río Grande de Manatí. It is fed by the outcroppings of aquifers. In 1998, almost half of the area included in this ecosystem of 1,310 [hectáreas (ha)] was made a nature reserve by the Commonwealth of Puerto Rico, given its great ecological value.
Puerto Rico’s northeast karst zone, which includes the municipalities of San Juan, Carolina, Loíza and part of Río Grande, has been highly eroded by the runoff of rains and changes in the sea level over the course of millions of years. In modern times, the dissolution of the limestone rock has been minimal due to the high clay content of the subsoil. That is why, in the northeast, the karst is characterized by very little topographical relief and by surface rather than subterranean drainage.
Types of Karst Topography
The types of karst topography are defined by features in which chemical, physical, and hydro-geological matters play a part, as well as by the type of soil. There are eight types of karst topography. Of these, the dominant form in Puerto Rico, according to White (1988), is cones and towers. But there are also karst plains, coastal karst, marshy karst, and limestone pavement, as is explained below. In Puerto Rico, for example, the humid northern karst is very different from the southern karst that developed in a much drier climate.
Cone and tower karst
This is the dominant type of karst in Puerto Rico. In the limestone of northern Puerto Rico, the cones and towers are quite close together, with deep ravines between them (Río Abajo in Utuado, for example). In lower areas, there are isolated conical hills, commonly called mogotes, that rise in alluvial valleys (Barceloneta and Vega Baja, for example).
The requirements for the development of cone-shaped and tower-shaped karst seems to be a thick layer of limestone with many deep fractures in the rock. Dissolution in these fractures separates the limestone at higher elevations into massive blocks. The liquid corridors that separate the blocks grow deeper and wider with the process over time. The tops of the limestone promontories are generally round. The channels or trenches then widen and transform themselves into alluvial plains or valleys, which separated the cones and towers even more.
Cone and tower karst is found in tropical regions. That is why they are also found in Cuba, Central America and the South Pacific. In Central America they are common in Mexico and Belize. They are also found in southern China and in Malaysia, Java, New Guinea, Borneo and Sarawak.
Limestone pavement is found in areas where there is denuded limestone rock, sculpted into lapies or rock teeth of various kinds. Limestone pavement occurs in northern countries where continental glaciation during the Pleistocene removed the soil and denuded the limestone rock, which was then exposed to dissolution by the active agents of the environment. Many karst areas in the Rocky Mountains in the United States and Canada contain limestone pavement. As well as being typical of the higher latitudes, it can be seen in arid and semi-arid tropical climates. In the south of Puerto Rico, there are expressions of this kind of karst in places such as the Bosque de Guánica, Punta Guaniquilla, and Cerro Las Cuevas in Juana Díaz. They can even be observed in places in the interior of the island where accelerated erosion, brought on by the activities of human beings, has removed the soil cover from the rock.
Author: María A. Juncos Gautier
Published: August 27, 2014.
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