Historical Geology/Orogeny< Historical Geology
In this article we shall discuss the concept of orogeny, and investigate how plate tectonics causes the formation of mountains. It will be helpful to the reader to recall the facts discussed in the articles on subduction, terranes, and ophiolites.
Note on the word "orogeny"Edit
The term orogeny means, literally, "the formation of mountains". It is sometimes used to mean just that; so, for example, we may use the term "subduction orogeny" to refer to the volcano- and island-arc-forming process described in the article on subduction.
When I use the term "orogeny" without any further qualification (i.e. unless I specifically say that I am talking about subduction orogeny) I shall be using the word in this last sense.
In orogeny, mountains are formed when a continent meets another continent, a micro-continent or an island arc, and instead of one subducting beneath the other, they push together and buckle, forming mountains. Hence if we wish to distinguish this mechanism from subduction orogeny, we may call it "collisional orogeny".
Now, this mechanism for mountain-forming is at least plausible, for the following reasons:
(1) When (for example) two continents collide, each will be too thick, and too low in density compared to the athenosphere, for one to simply subduct under the other.
(2) There is no reason why, when the two continents meet, the forces that brought them into collision should then regard their job as done and knock off: whatever propelled one continent into another will, it seems safe to suppose, continue to impel the continent.
(The reader should note, however, that the continents do not continue to move because of their "momentum", as some people carelessly say. The momentum even of something with the mass of a continent is negligible when it's only moving at a speed that can be measured in centimeters per year. Rather, as we have said, they continue to move because the forces that brought them together continue to push them together.)
(3) We know that rocks can deform under stress; in fact, geologists perform experiments with plasticine to gain insights into orogeny, on the grounds that two slabs of plasticine in collision should behave on a small scale over a small period of time much as two continents in collision will behave on a larger scale over a longer period of time.
But it is one thing to be plausible, another thing to be right. In order to show that the concept of collisional orogeny is more than just the thought of an idle hour, it is necessary to look at the details of mountain ranges.
How do we know?Edit
We are used to the fact that mountains come in belts (the Urals, the Himalayas, the Appalachians, and so forth) but when you think about it the fact is rather striking. Why do mountains come in long and relatively thin strips rather than being arranged in big circles (for example) or being dotted randomly here and there about the landscape?
Well, a subduction orogeny will produce belts (such as the Andes) because the boundary between two plates is necessarily a line; and collisional orogeny will produce belts (for example the Himalayas) for exactly the same reason: the boundary (the suture zone) between two adjoining pieces of continental crust must in the nature of boundaries be linear.
When we look in detail at orogenic belts, we see that the nature of the folds they contain is consistent with the theory of their origin. Consider a piece of deformable material such as a floor rug. If you push it from one end so as to shorten the area it occupies, it must necessarily ruck up in a series of parallel folds; if this process continues some of the folds will become recumbent: they will fall over sideways.
This is just what we see in orogenic belts. The photograph to the right shows a fine example of a recumbent fold belonging to the Caledonian orogeny.
Then there is the matter of sedimentology. Before the discovery of plate tectonics, geologists recognized that non-volcanic mountain ranges tended to consist largely of prisms of marine sediment (often metamorphosed); they just didn't know why. It was simply the fact that these great accumulations of sediment would form in the ocean and then climb out of the sea and become mountains, and the most that geologists could say about it was that this was just the sort of thing that tended to happen from time to time.
But this can be explained in terms of collisional orogeny: before a continent meets another continent (or microcontinent, or island arc) there must be sea floor between them; this will contain the sediment of the continental shelf and slope. Naturally when the two landmasses meet this sediment will be squeezed between them.
Because in order for the two landmasses to meet the oceanic crust between them must be subducted, we might well expect to find signs of subduction orogeny accompanying collisional orogeny: we would expect to see volcanoes on at least one landmass or the other, or volcanic island arcs between them which would then be trapped when they meet to form a terrane. And this is in fact what we find.
The idea that orogenies are collisional also explains why ophiolites are found in mountain belts. Recall that these are sections of oceanic crust: what, then, are they doing in the middle of the Alps, the Appalachians, the Himalayas, the Urals, the Apennines, the Klamath Mountains, the Cascade Mountains ... ?
But this is all explained on the collisional theory of orogeny. These ophiolites are there because that's where the ocean was, before two landmasses collided; they remain as clear evidence of the suture zone.
Case study: the HimalayasEdit
As an example of all that we have been discussing, consider the Himalayas.
Note first of all that they do in fact form a mountain belt; they have a long thin topography. Along the north edge, we have the Transhimalayas, which look just like they were produced by subduction volcanism; further south we have 10 - 17 kilometers of marine sedimentary rock and metamorphosed sediments; at the Indus Suture Zone where the two plates meet we have ophiolites.
The whole Himalayan range has very evidently been smashed, mashed, folded and faulted, and contains many recumbent folds.
To the south of the Himalayas is of course India, which may be considered one vast terrane, having its own distinct geological, paleomagnetic, and fossil history up until the point where it crashed into south Asia; if, as this evidence suggests, they were once separate, they must at some point have collided.
Finally, we may note that in this case the collision is still going on: India is measurably still moving north by about 5cm/year, the Himalayas are still measurably undergoing uplift at about 1cm/year, and Asia north of the Himalayas is still measurably stretching and deforming as a result of the collision. So when we find something that looks similar to the Himalayas but where the motion has ceased, we are entitled to infer motion in the past.