Chapter 11, Problem 1RQ

To determine

The changes that the rocks undergo during the formation of an orogenic belt.

Answer to Problem 1RQ

The rocks at orogenic belts undergo a thrust faulting followed by the metamorphism, passive folding, and eventual upliftment which give a rise to the mountain belt.

Explanation of Solution

Orogenic belt is formed at the continental plate boundaries where the two plates collide with each other. There occurs a large number of thrust faults in which the edge of one block slips up and over the margin of the other. This boundary with the separate blocks is known as a suture.

During the thrusting process at or near the suture, the rocks of footwall, goes tens of kilometers deep into the earth but as they are less dense than the mantle, they gradually stop moving down. These rocks undergo high grade of metamorphism and initiates the passive flow folds and tectonic foliation.

At the same time, blocks of the hanging wall go several kilometers upward. Due to folds and reverse faults, the rocks that are metamorphosed at the depth of orogen are eventually brought up to the Earth’s surface. The crust here is around 70km thick.