The Rise of the American Cordillera

Described as the 'backbone' of the Americas, the American Cordillera is series of dramatic mountains stretching all the way from North America as far south as Antarctica.

The surface of Earth is broken up into large and small 'tectonic plates' – landmasses that drift across our planet over geological time. When they collide, they unleash the most powerful force on Earth. Nothing withstands this movement; oceans are closed, whole continents lifted. Whether we’re looking at oceanic or terrestrial plates, almost all known mountains owe their existence to these collisions.

A striking example is the west coast of the Americas, where we find a variety of mountain chains created by such a convergence. The relentless if very slow movement of the plates underlying the Pacific Ocean towards the continental mass of North and South America has created a huge topographic rise - the American Cordillera (or mountain chain).

North America

When we think of the North American Cordillera, immediately the Rocky Mountains spring to mind. But there is more: the Pacific Coastal Belt, which spans from Alaska to Mexico, is also part of it, just as are several other coast-parallel mountain chains, like the Southern Alaska Ranges and the Yukon Ranges.

To compare the mountain-building process of the Rockies with the Andean chain is only partly possible: both were created by the powerful push of one of the Pacific plates (the Nazca Plate in the South and the Pacific Plate & Juan de Fuca Plate in the North) towards America, but this is where the similarity ends. Only in the immediate coastal areas (e.g., the Cascade Range) was the mountain build-up the result of subduction, melt, and subsequent rise of the magma. Further inland, the collision translated into a sheer 'head-on' collision of enormous land masses, folding up the land like a table cloth being pushed from two sides.

At the time of the onset of the convergence, which was much earlier than in the South, there was a huge basin where we find the Rockies today. The mighty push from west to east first closed the basin completely, then lifted up the compressed rock, resulting in an 18,000-ft-high plateau. Only in the millions of years that followed did erosion by ice and water carve out the picturesque landscape we know today. Like in the South, the convergence is ongoing, still slowly changing the topography.

Photo: Daniel Allen

The Aleutian Arc

The youngest member of the subduction-generated mountain chains in the Americas is the Aleutian Arc in Alaska. In terms of geology, quite recently, i.e., 55-50 million years ago, the northernmost part of the Pacific Plate thrust north and down under the shelf of the North American Plate. Although this process started entirely underwater - the principle remains: one plate is pushed deep down under another until it melts. The molten rock, like the bubbles in a lava lamp, makes its way to the surface where it emerges in a volcanic eruption. Once the volcanoes are high enough to get to the ocean surface and above, they form so-called island arcs, structures only occurring in the oceans (not to be confused with volcanic arcs).

The Andes and Antarctic Peninsula

When we sail to Chile, Tierra del Fuego and Antarctica, we see these mountains first-hand. About 180 million years ago, following the breakup of the supercontinent Pangaea, the Nazca Plate began wedging itself powerfully under the South American Plate, pushing deeper and deeper. This is called subduction. At a depth of several hundred kilometers, the subducted material started to melt and move back up again through the overlying crust, thus creating the longest mountain chain on the planet - the Andes. The thrusting is still going on, accompanied by volcanic eruptions and earthquakes.

We can even look much further South to the Antarctic Peninsula, which was created at the same time by the same process as Tierra del Fuego to the north. At that time, Antarctica and southern South America were joined, but eventually, many millions of years later, they separated.

So, the longest mountain assembly on the planet has been created by just one gigantic force, which has been driving our planet since the very beginning. While sailing with us on our expedition cruises, you’ll get the chance to see some of the fascinating mountain ranges that form part of the mighty American Cordillera.