Geology Matters: The Complex Geology of Ecuador


Ecuador, wedged on the western side of South America between Columbia on the north and Peru on the south, straddling the equator, is WC’s favorite place on the planet to look for and photograph birds. Ecuador has an area of about 109,500 square miles; by contrast, Colorado has an area of about 114,000 squares miles. Colorado has about 400 bird species; Ecuador has at least 1,600 species. Ecuador, smaller than Colorado, has more than four times as many bird species.

Overview of the geologic forces around Ecuador, from “Ecuadorian Andes volcanism: A review of Late Pliocene to present activity,” by M.L. Hall, P. Samaniego, J.L. Le Pennec and J.B. Johnson, Journal of Volcanology and Geothermal Research 176 (2008)

There are lots of reasons for that immense avian diversity. One of them is geology. The Andes Mountains in Ecuador, especially northern Ecuador, are an expression of the subduction of the Nazca Plate under the South American Plate. The plate tectonics dance also involves the Pacific Plate and the Cocos Plate, all interacting in complex ways. As if that weren’t enough, there is a hot spot, much like the one under the Hawaiian Islands, that has created the Galapagos Islands and a long, underwater mountain range called the Carnegie Ridge on the Nazca Plate that is lifting up the southwestern corner of Ecuador as it subducts under the continental margin. Underwater mountain ranges don’t subduct gracefully. In this case, the tectonic pressures are shoving northwestern Ecuador, parts of Columbia and parts of Venezuela northeasterly into the Caribbean Plate.

Volcanoes and volcanic field in Ecuador, from “Ecuadorian Andes volcanism: A review of Late Pliocene to present activity,” by M.L. Hall, P. Samaniego, J.L. Le Pennec and J.B. Johnson, Journal of Volcanology and Geothermal Research 176 (2008)

It’s called the Dolores-Guayquil Megashear – trips off the tongue, doesn’t it? – and it has created a high valley in the middle of the northern Ecuadorian Andes, the Interandean Valley. It’s DGM in the first illustration. West of the DGM, Ecuador is being pushed northeasterly. East the the DMG, the rest of South America is being pushed west-southwest. The subducted Nazca Plate has created immense amounts of volcanism, s the second illustration shows.

For a birder, the division of the and in the Western and Eastern Cordillera – separate mountain ranges, divided by a valley 4,000 to 6,000 feet lower, is critical to avian diversity. That’s two different major ecozones, each with its own avifauna; three, really, because the Interandean Valley has its own set of birds. Of course, as you climb up each of the cordillera, you get different habitats, isolated from other habitats, with still more speciation of the birds. Add in the Pacific Plain and the Amazon Basin and you have an extraordinary set of diverse, geographically and biologically isolated habitats. And birds have evolved for each of them.

Geologists joke that if you could iron Ecuador out flat, it would be bigger than Texas. It’s an exaggeration, of course, but carries enough truth to illustrate the remarkable verticality of this small country. Consider: the lowest pass through the Eastern Cordillera is Papallacta Pass, is at 13,500 feet above sea level. And that, in turn, is highly important to the avifauna diversity.

All of which is another reason why geology matters. Even to birders and bird photographers.