Posted by: Rob Viens | February 3, 2013

The “Backbone of Tierra del Fuego”

Over the next several days, Darwin and his crewmates cruised the Beagle Channel continuing to survey the coastline. Darwin’s comments were brief and ranged from awe at the beauty of his surroundings to complaints about the weather, natives, and sleeping conditions:

“The scenery was very grand, we were sailing parallel, as it were to the backbone of Tierra del; the central granitic ridge which has determined the form of all the lesser ones: It was a great comfort finding all the natives absent; the outer coast during the summer is on account of the seals, their chief resort. — At night we had miserable quarters, we slept on boulders, the intervals being filled up with putrefying sea-weed; & the water flowed to the very edge of the tent.” (Jan 30)

“The channel now ran between islands; & this part was entirely unknown; it rained continually & the weather well became its bad character.” (Jan 31)

“The countryside was most desolate, barren, & unfrequented: we landed on the East end of Stuart island, which was our furthest point to the West being about 150 miles from the ship” (Feb 1/2)

Though I have never been to Tierra del Fuego, I can’t help but feel that the character of the land is very similar to Southeast Alaska, where I have spent time.  It’ not surprising – both regions are located at approximately 55° latitude (on opposite sides of the equator), and on the western/southwestern edge of a major mountain range. In fact, the mountain range itself is all part of the great Cordillera that borders the eastern Pacific – with Alaska and Tierra el Fuego forming its end points. Starting in Alaska in the Brooks Range, you pass through the Alaska Range, the Coast Mountains, the Rockies, Cascades, Sierra Nevada, Sierra Madres, and the Andes – a nearly continuous wall of rock that separates the Americas from the Pacific Ocean. In Darwin’s words – a figurative “backbone” of the Western Hemisphere.

The mountain ranges of western North and South America (modified from Google Maps)

Western Cordillera

It is not a coincidence that lofty mountains form the western edge of the Americas – it is a result of plate tectonics. Here is their story…

Both the North and South American plates (including the two continents plus half the Atlantic seafloor) are traveling westward. This motion is slow – about 5 cm per year – but it adds up over millions of years of geologic time to move these plates thousands of kilometers across the surface of the Earth. The western edge of these two moving plates happens to be located on right at the edge of the two American continents – hence this edge marks a boundary between these plates and the Pacific and Nazca plates to the west. And at this boundary, the relatively less dense, westward-moving continents are riding over the denser, eastward-moving ocean plates creating a subduciton plate boundary.  (The name comes from the fact that the ocean plate is “subducted” back down in the Earth – ultimately recycled back into the Earth’s mantle.)

This subduction zone does not extend as one continuous boundary all the way from Alaska to Tierra del Fuego today (that would be too easy). The complicated geometry of moving plates around on a sphere means that in some places the plates are sliding past one another rather than converging. However, subduction boundaries have existed virtually everywhere along this coast at some time in “recent” geologic history (i.e., in the last 40 million years or so).  So mountain ranges formed by “recent” or modern subduciton can be found everywhere along the west coast.  Where that subduction is still active today – such as in the Andes Mountains or the Cascades in the Pacific Northwest US, you can find active volcanoes and some of the most dramatic mountain ranges in the world.

Plate map of the western hemisphere. Note the North American Plate in brown and the South American Plate in purple. (From US Geological Survey)

Western Palte Map

Granted, this is a bit of an oversimplification – the westward moving Americas also “collide” with islands and small continental fragments resulting in “collisions” and even more lofty mountain ranges (this, for example, is the story of the Alaska Range). These collisions result in new material being added on to the “leading edge” of the continent and  result in the growth of the Earth’s landmass through time.  Unfortunately for Darwin, the exact cause of these mountain ranges, and the formation of continents was still about 130 years in the future.  For Darwin, today at least, he would have to settle for basking in the beauty of the Cordillera.

By the end of the weekend, the Beagle turned around and headed back to see how Matthews and the three Fuegians where holding up:

“Miserable weather: we proceeded by the outside coast to the Southern entrance or arm of the Beagle Ch. & thus commenced our return.” (Feb 3)

In a couple of days they would the fate of their friends. (RJV)

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