Not long ago, geologically speaking, Costa Rica did not exist. A mere 50 million years ago, this narrow mountainous strip of land was part of the ocean floor, along with the rest of the Central American isthmus between Guatemala and Colombia. Although that may sound like a long time ago in earth's history, the dinosaurs had already disappeared some 15 million years earlier.

Currently, most geologists believe that from 40 million years ago until within only the past three million years what are now Costa Rica and its neighboring countries were nothing more than a volcanic archipelago. Using the theory of plate tectonics, which suggests that the earth's crust is fragmented in a number of sections that fit together like pieces of a jigsaw puzzle, though they are not stationary, the formation of Costa Rica's land mass is explained by its position at the western edge of the so-called Caribbean plate, which is moving westward and overriding the Cocos plate located in the Pacific Ocean.

As these two chunks of crustal material collide, the rocks of the Cocos plate are pushed downward, subjecting them to increased heat and pressure that eventually turns them into molten rock under great pressure. Because the downward thrust is at an angle to the east, when the pressure build-up finally becomes too much and the lava and steam move back upward towards the surface, they do so beneath the western edge of the Caribbean plate. The result is volcanism.

The earliest volcanoes in the area were submarine ones. As layer after layer of cooled volcanic material collected, the peaks finally emerged above the ocean's surface. In this manner, a chain of volcanic islands formed in a line parallel to and east of the zone of contact between the two plates.

As the millennia passed and eruptions continued to throw more material down the slopes of the rising volcanoes, the land area filled in around their bases. Only as recently as three million years ago was the uninterrupted land bridge completed between northern Central America and South America, giving rise to movements of plant and animal species both northward and southward. This land bridge between two great continents is in large part responsible for the incredibly high biodiversity found in an area as small as Costa Rica.

The other prominent factor that explains the country's tremendous variety of flora and fauna is the range of climatic conditions that result from the changes in temperature and rainfall as one goes up and over the mountains.