Do you know how are mountain chains built? Let’s find the answer. Most of the major mountain chains of the world have been constructed by periods of accumulation of unusually thick deposits of sediments in elongated basins followed by uplift, folding, thrusting of huge masses of rock called nappes, and faulting.
These processes, known collectively as tectogenesis are usually accompanied by intense volcanic activity as well as the forcing into the position of huge masses of granite which may continue to rise for a long time after the main movements have ceased and the processes of weathering are active.
These regions in which Earth movements are concentrated are usually accompanied by intense volcanic activity as well as the forcing into the position of huge masses of granite which may continue to rise for a long time after the main movements have ceased and the processes of weathering are active.
These regions in which Earth movements are concentrated are usually given the name of orogenic belts and the whole process is known as an orogeny.
You can see that the process of orogeny involves the sinking of a part of the earth’s crust, followed by sedimentation in the basin that has been formed, and finally, uplift of the whole area.
It is interesting to note that mountain chains such as the Juras that we have already mentioned are formed by a different process. In this case, certain parts of the nearby Alps had been uplifted by progeny so that the area now occupied by the Juras was tilted.
It so happened that the rocks which now compose the Juras were lying on a huge bed of salt. In effect, this salt acted like a layer of lubricating oil so that the rocks on the top slid very slowly downhill folding and faulting as they went. This is known as gravity tectonics.
In 1859, the geologist, James Hall, working in the northern Appalachian mountains of America discovered that the sediments that made up this range were of a type that could only have been deposited in quite shallow water in the sea.
At the same time, he recorded that the sediments were in some places as much as 12,000 metres thick. How, then, did such a thick deposit accumulate? It is clear that the older rocks underneath must have been sinking as fast as the new sediments were being laid down.
This type of elongated, sinking basin has been called a geosyncline. At first, it was thought that the sinking was caused by the weight of the sediments being deposited on top but it was soon realized that this was not so. In fact, it was the famous mineralogist, Dana, Who first suggested in 1873 that the sinking of the basin was the cause of and not the result of the sedimentation.
Nowadays a new, all-embracing theory seems to provide us with all the answers to the problems of how and why mountain building takes place in the way it does. This is the theory of plate tectonics.
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