I’ve always been told that the world’s great mountain ranges were created where two tectonic plates rammed into each other, causing the earth to buckle and uplift. The classic and most dramatic example is the Himalayas, created when the Indian plate collided with the Eurasian plate. So far, makes sense enough.
But a quick look at the tectonic plate map shows that many of the world’s great mountain ranges are no where near plate boudaries. The Appalachians, the Urals, the Alps, and the Pyrennes are prime examples. Even the Rockies seem to be at a moderate remove from the actual junction of the North American and Pacific plates.
The Appalacians, IIRC, were created during a much earlier collision. They’re severely worn down compared to the Rockies/Andes. as for the Rockies – the edge of the plate has been crumpling up for some time, but the craxcks and the rifts are right there – the San Andreas fault and others go right through mountain territory.
I actually did some work on the Himalayas as an undergrad – we measured the waveforms of earthquakes that had traversed the Himalayan plateau and compared the dispersion data against finite-element models to try and figure out what was going on there – Deep Subduction, Shallow Subduction, Mutual Crumpling, whatever. Ity seemed Deep Subduction fit the model best.
As CalMeacham says, most of the mountain ranges that are not near plate boundaries today are in the fact the site of ancient collisions between plates. (Besides folded mountains, some isolated ranges are due to volcanism in rifting zones or over drifting hot spots.)
The origins of the Rockies are still rather enigmatic. This Wikipedia article on the Laramide orogeny (the mountain-building episode in which the Rockies originated) presents some ideas. However, this abstract says that their origin is still not well-explained.
The formation of the Alps is apparently related to the collision of the African and Eurasian plates, although that is not where the plate boundary is at present. Africa and Europe have basically been playing bumper cars for tens of millions of years, and the history of the Mediterranean region is very complex.
Which just goes to show that there are no simple answers to questions involving complex processes. Why for instance is there a major fault in Missouri and why was there an earthquake in New England recently.
Becauser faults don’t only exist on the edges of plate boundaries, and they still exhibit motion. There’s a major fault running through Manhattan. Both NYC and Boston have experienced pretty large quakes, despite not being anywhere near a plate boundary. But if you think of the plates as geologically thin and ephemeral skins of cooled magma floating on a convectively moving globe of magma, it’s not hard to imaghne that there’d be disturbances even away from the much more active edges.
I’m under the impression that the eastern ranges, Allegheney, Appalachian, Blue Ridge, etc. result from the pushing back of the continental plate by the Atlantic sea floor spreading.
On a flight to Washington, D.C. we came over those ranges in the evening when the sun was low and shadows long. The landscape resembled nothing other than the folds in a rug that is pushed back from one edge while the other edge is held firm. The effect was quite startling.
No, this is not how it happened. The whole North American continent (with the exception of California and a few other areas) is being carried westward along with the rest of the plate, including the western Atlantic seafloor. The seafloor is not exerting any pressure against eastern North America; they are both on the same plate and are moving at the same rate.
When an oceanic plate pushes against a continental margin, it generally subducts, producing a deep trench along the edge of the continent and a volcanic chain on land. These features can clearly be seen in western South America along the Andes; they are absent in the eastern US.
As has been said, the Appalachians were formed in ancient collisions with Europe in the remote Paleozoic. These collisions ultimately resulted in the formation of Pangaea. Pieces of crust that were once part of North America now form parts of the British Isles.
Most earthquakes in New England are due to “post-glacial rebound.” The terrain was heavily weighted down by the great ice caps in the Pleistocene, and is still rebounding now that they have melted.
Interesting. Apparently the formation of these mountains was a lot more complicated than the article I read, or my memory of it, indicated. In any case it seems to be a combination of plates colliding, volcanism, accretion, and sea floor spreading (but not necessarily the Atlantic).
“All three orogenies are interpreted in terms of collisions during closing of the Iapetus Ocean between North America, Europe and Africa (Gondwana). Seafloor spreading, subduction-zone volcanism, and accretionary wedges were all part of a long, drawn-out sequence of events.”
For anyone interested in this topic, I would highly recommend Annals of the Former World ,by John McPhee, a highly readable account of the geology and formation of North America and of plate tectonics in general.
