Over the last three million years, the Central American land bridge has driven biotic exchange between the continents. In part, the unusual biodiversity observed in Costa Rica (topographical map left) has been due to the high frequency of biological migration, between the continents, through environmentally diverse locations.Given converging plates in the Pacific’s Middle American Trench, just 40 miles west of the coast, volcanic activity has stretched a line from Guatemala to Panama. In Costa Rica, there are more than a hundred volcanoes, five of which are still active. The majority of the volcanism in Costa Rica ended around 5-8 million years ago and much of the region is covered by intrusive basaltic formations from past eruptions (image lower right). The plate subductions are expected to have resulted in metamorphism, but there is reported to be little indication of metamorphic rock as it's thought to be buried. The oldest rocks are around 180 million years old and were found on the western coast of the Nicoya peninsula. These old rocks, called ophiolites, have been identified just north and south of CIRENAS and are result of broken chunks of uplifted ocean floor.
