Hispaniola

Long ago, lines of volcano islands formed in the sea. Pieces of them were pushed together.

The island sits where two huge pieces of Earth's crust meet.

Hispaniola sits where the Caribbean plate meets the North American plate. Long ago, chains of volcanic islands and pieces of ocean crust formed in the ocean. Later they were pushed together and joined into one island.

In the north, the island is still pushing against the edge of the Bahamas Platform, an area of thick crust that resists sliding under it. In the middle of the island, the Cordillera Central holds volcanic rocks from an ancient island arc.

Núñez and colleagues describe the Caribbean-North American boundary near Hispaniola as a seismically active zone 100 to 250 km wide, mostly of left-lateral strike-slip deformation, running more than 2,000 km along the northern edge of the Caribbean Sea. The Cordillera Central includes volcaniclastic rocks of the Late Cretaceous-Eocene Circum-Caribbean Island Arc, whose units, the authors note, formed and were accreted in an intra-oceanic setting from the Late Jurassic-Early Cretaceous onward.

Northern Hispaniola is still colliding with the southern edge of the Bahamas Platform, a process under way since the late Miocene along a restraining bend. Because the platform's crust is thicker than average, it resists subduction beneath the eastward-moving island. The authors note that the tectonics of the Hispaniola region remain poorly understood.

Long cracks in the rock, called faults, cross the island. Along one of them lies a long valley.

In a low valley lies Lake Enriquillo. Its water is very, very salty.

Two major east-west faults run through Hispaniola, one in the north and one in the south. Along them, the Caribbean plate slides east past the North American plate. NASA shows that the southern fault, the Enriquillo-Plantain Garden Fault, is marked by a long, straight valley across southern Haiti. In January 2010, a strong earthquake struck near this fault.

Farther east, a deep basin between raised blocks of rock holds Lake Enriquillo, a lake that is saltier than the sea. NASA satellite images showed it doubling in size between 2004 and 2011.

NASA's Earth Observatory describes the Enriquillo-Plantain Garden Fault as the southernmost of two major east-west faults that take up the stress where the Caribbean and North American plates converge; it is a strike-slip fault, with the Caribbean plate moving east relative to North America, and its trace shows as a long, straight valley in southern Haiti. A magnitude 7.0 earthquake struck near it on 12 January 2010. In the north, the Septentrional-Oriente Fault Zone runs about 320 km onshore and helped uplift the Cordillera Septentrional.

The Enriquillo Basin is about 5 km deep, set between fault-raised blocks; Núñez and colleagues place Lake Enriquillo 42 m below sea level. NASA's ASTER images show the hypersaline lake growing from 164 km2 in 2004 to 350 km2 in 2011, with more rainfall, sediment raising the lakebed and milder temperatures reducing evaporation among the reasons given.

Two kinds of iguanas from one special group live on Hispaniola. One of them is very rare.

American crocodiles live in salty Lake Enriquillo.

Hispaniola has two iguanas of the group called Cyclura: Ricord's iguana and the rhinoceros iguana. IRCF, a reptile conservation group, says it is the only island with two Cyclura species. Ricord's iguana lives only in the dry south-central part of the island. IRCF reports that the IUCN ranks it critically endangered, and one of its groups lives on Isla Cabritos, an island in the middle of Lake Enriquillo.

The lake is also home to American crocodiles. NASA says it has the largest population of them in the Caribbean.

IRCF reports that Hispaniola is the only island with two Cyclura species. Ricord's iguana (Cyclura ricordii), ranked critically endangered by the IUCN, is restricted to the arid Neiba Valley and the driest coastal lowlands of the Barahona Peninsula, with a range under 100 km2 split into four subpopulations: on Isla Cabritos in Lake Enriquillo, on the lake's southern shore, near Pedernales, and a small group in southeastern Haiti found in 2007. IRCF calls its estimate of 2,000 to 4,000 adults somewhat outdated. IRCF describes the rhinoceros iguana (Cyclura cornuta) as still widely distributed throughout Hispaniola.

NASA describes Lake Enriquillo as home to the largest population of American crocodiles in the Caribbean.

Crocodiles are shy. Watch them from far away, and never feed them.

Swim only with a grown-up. If water pulls you out, swim along the beach.

Florida's wildlife agency describes American crocodiles as shy. A crocodile basking in the sun may splash into the water if a person comes near, because it is frightened. A crocodile lying with its mouth open is controlling its body temperature, not acting aggressively. PlayOwl's advice: give crocodiles and iguanas plenty of space, and never feed them.

At the beach, NOAA warns about rip currents. If one catches you, don't fight it. Swim parallel to the shore, then swim back to land at an angle.

Florida's Fish and Wildlife Conservation Commission describes the American crocodile as shy and reclusive: a basking animal surprised by an approaching person may enter the water quickly and noisily, which signals fear, not aggression, and open-mouthed 'gaping' helps regulate body temperature. PlayOwl's own advice is to keep well back from crocodiles and iguanas, never feed wildlife, and stay on paths in protected areas.

NOAA's National Ocean Service notes that rip currents can move at up to eight feet (about 2.4 m) per second, faster than an Olympic swimmer. The advice is not to fight one: swim parallel to the shore, then return at an angle.