Why are the two major mounain chains in the form of a great circle?

You ask a question and then tell us to disregard the only answer? As a matter of fact, erosion works much, MUCH slower than tectonic activity. With mountain ranges you are almost always talking about solid bedrock, some of the hardest rock on the planet, and it can take thousands of years for that to erode any significant amount. The only mountains I could see being eroded faster than that would be volcanic peaks, since some types of volcanic rock erode rather rapidly. Some tectonic plates are moving at over 2 in per year, and the converging plate boundary running along the west coast of South America is the singular reason for the existence of the Andes mountains (where the Nasca oceanic plate is running into under the South American continental plate). The same thing is true for the Himalayas (where the Indian plate is running into the Eurasian plate) and almost all other mountain ranges on the planet. Some mountains may have been created by a hot spot volcano (such as is forming the Hawaiian Islands), but the rest were formed by tectonic plate collisions either in the geologically recent past or during the time of Pangaea. Although I haven't studied African plates much, I'm pretty sure there is a diverging boundary somewhere in the Sahara that is making it expand similar to the Mid-Atlantic Ridge in the middle of the Atlantic Ocean. As for these mountain ranges appearing in a circle, that's mostly because they ring the Pacific Ocean. Oceanic plates are much more dense than continental plates, so when they collide the oceanic plate almost always pushes under the continental plate and forces it to rise, creating a mountain range. I don't know what would have made you think that tectonic movements are "garbage", but I strongly urge you to take a good hard look at the facts before making another such radical umption.