The Story Of South America

A previous posting on this blog was "The Story Of North America". If anything, South America is even more geologically interesting and today I would like to explain my version of South America. I had this posting in the pipeline, but with the recent earthquake in Chile I decided to post it today.

Here is a map link http://www.maps.google.com/

THE FORMATION OF SOUTH AMERICA

The first thing that becomes obvious while looking at South America on a map is that the east coast of the continent forms a perfect fit with the west coast of Africa. This applies to not just the coastlines of the two continents, but their continental shelves. Not only that, if the map shows the depths of the oceans we can see that the Mid-Atlantic Ridge follows the same line as the two coasts and is equidistant between the two.

This is no mystery. South America and Africa were once one continent until being split apart by the volcanic activity of the Mid-Atlantic Ridge. The story of this split is very similar to the split between Africa and North America, as I described in The Story Of North America.

However, the motion in the split of Africa and South America was more complex than that of the split of North America and Africa. The reason is the great arc in the Mid-Atlantic Ridge between the area around the equator and about 40 degrees north latitude. This arc functioned as a vast cookie cutter which cut out the bulge of northwest Africa, which is solid underlying rock known as the West African Craton.

The factor which complicates the split of Africa and South America is the location of the arc in the Mid-Atlantic Ridge. The north coast of South America is south of this arc. So after the volcanic activity of the ridge split the two continents, South America was actually pushed southward. Africa was pushed northward in the same way.

This southward push to the new continent of South America put it in contrast to the rest of the continents, which were all drifting northward after having broken off Antarctica, in the far south. South America would be blocked from drifting back northward later by collision with the Caribbean Plate.

This complex motion caused a frictional collision between Africa and South America during the process of being split. Africa was being pushed northward due to it's position relative to the arc in the Mid-Atlantic Ridge, while South America was being pushed southward. It is easy to see on a map of the world today how far south South America really is relative to the other continents.

The results of this frictional collision are the mountains of Brazil and the range of mountains along the west coast of southern Africa in Namibia and Angola, as well as further inland in southern Africa. If Africa has been moving northward and eastward, then what else could have formed a range of mountains along the continent's southwest coast? The region is not volcanic, although I believe the high rocky lands of eastern Brazil to be from one of the Continental Asteroids described in "The Story Of Planet Earth" on this blog.

In The Story Of North America, on this blog, I described how the vast area of lowland, shaped like an upside-down V, that comprises much of America's midwest, was formed by seafloor being pulled upward when the eastern portion of what was then North America was torn away by a collision with what was then Africa. But this was before those two continents were split apart by the volcanic activity of the Mid-Atlantic Ridge. This collision was also at a low angle and so involved more friction than a direct collision. The direct result of this collision that we see today is the Appalachian Mountains.

This collision between the forerunners of North America and Africa happened before the scenario that I am describing involving the split between South America and Africa because the former collision happened before the continental divisions caused by the volcanic activity of the Mid-Atlantic Ridge.

THE COLLISION LOWLAND REGION OF SOUTH AMERICA

However, the same phenomenon of seafloor being pulled upward to form a wide area of lowland also happened in South America. The similarities between the two are remarkable. Both areas of lowland are vast and both are shaped like an upside-down V with the peak facing north. Both areas of lowland are swampy in places.

In South America, this area of lowland created by frictional collision encompasses northern Argentina, Paraguay and, much of southern Brazil. Part of this South American lowland is called the Gran Chaco or Chaco Plain. You can read more about it on wikipedia if you like.

The Parana River drains this lowland region of South America in a way that is very similar to that of the Mississippi River in North America. I believe the wide continental shelf off Argentina to have once been part of this seafloor that was pulled upward but has since been eroded back into sea.

The long coastal mountain range along the southwest coast of Africa tells us that, unlike in North America, the split caused by the Mid-Atlantic Ridge happened before the frictional collision that formed this area of lowland, as the two separated continents rubbed against each other. Of course, all of this was happening long before South America's most prominent mountain range, The Andes, were formed by collision with the Nazca Plate, adjacent to the Pacific Plate.

THE COMPLEX TECTONIC MOVEMENT OF SOUTH AMERICA

The thing making this situation in the formation of South America more complex than the congruent situation with North America is the number of movements involved. All of the continents were drifting northward after splitting off from Antarctica. Both Africa and South America were being pushed apart by the volcanic Mid-Atlantic Ridge, South America was being pushed westward and Africa eastward. To complicate things, the great arc in the Mid-Atlantic Ridge, described above, was pushing South America southward and Africa northward.

The final result was a vector of these three forces pushing South America to the west southwest. You can see on a map that the greatest concentration of peaks in the Andes Mountains is in Peru and Bolivia because these mountains were formed by tectonic collision and that represents the direction of impact.

If there had been no northward movement, all of South America would today be south of the equator because that was where it's east coast was cut. An interesting illustration of South America's northward momentum in relation to it's southwestward momentum created by the Mid-Atlantic Ridge is the relatively short distance from the northeast coast of South America to the Mid-Atlantic Ridge compared to the much further distance from the east coast of Brazil to the ridge.

This complex vector of motions for South America does solve the mystery of why the lowland area of frictional collision in South America is smaller than the congruent area in North America. The volcanic activity of the Mid-Atlantic Ridge was pushing South America and Africa in opposite directions so that the frictional collision that created the lowland by pulling seafloor upward was limited in it's duration. The Appalachian Mountains of North America are long and this shows that the frictional collision that produced them went on for quite some time and the lowland area of North America that was thus created is larger.

As I pointed out, South America was blocked from moving further north by collision with the Caribbean Plate, while Africa continued northward until colliding with Europe.

THE MID-ATLANTIC RIDGE AND THE SOUTH AMERICAN PLATE

I have saved what I think is the most interesting geological feature of South America for last. Both coasts of South America are cut from the Mid-Atlantic Ridge and this can easily be seen on any map showing the ocean depths. The east coast of the continent was obviously cut apart from Africa by the volcanic activity of the ridge. The coasts of the two continents fit neatly together and the ridge, from which the seafloor spreads outward, is midway between them.

It is the west coast of South America that I find really intriguing. Has anyone ever noticed how the west coast of South America forms a nearly perfect equidistant line with the Mid-Atlantic Ridge? I find this to be truly amazing and unlike anything else in the world.

The virtually straight line formed by the coast of Chile reflects the ridge from the equator southward and the great arc in the Mid-Atlantic Ridge from around the equator to about 40 degrees north latitude is reflected in the curve of South America's west coast in Peru, Ecuador and, Colombia. Even the curve in the west coast in the southernmost tip of the continent is reflected in a matching curve in the Mid-Atlantic Ridge.

The reason for this is that the spreading of the seafloor from the ridge literally created the surface upon which South America rests. The spreading westward meets the Nazca Plate, which is in the process of sliding beneath the plate hosting South America. This is why the coast of South America remains essentially equidistant from the Mid-Atlantic Ridge.

This sliding of one plate beneath another is what is known as a subduction zone. The process forced the coast of South America upward, creating the Andes Mountains. Last week, the world got a reminder of the subduction process with the extremely powerful quake that struck Chile. These tectonic movements take place in increments, which we experience as earthquakes.

The west coast of North America is not shaped in the same way because there is no subduction zone there. Two plates are sliding relative to each other, but one is not sliding beneath the other.