The great mountain ranges that extend more than 65,000 km across the ocean bottoms are formed at:

Spreading centers

Explanation: The formation of mountain ranges is associated with the theory of plate tectonics which explains the movement and interaction of continental plates at the plate boundaries. In summary, two major zones are clearly identifiable, the convergent plate boundary or subduction zones whereby continental plates moves towards one another, hence causing one to slide below the other resulting in a shortened crust. On the other hand, the at the divegeht plate boundaries, plates move away from one another, allowing magma to penetrate through cracks and new oceanic crust which extends over several kilometers, these activity is called sea floor spreading which occurs at spreading centre's usually the mid oceanic ridges.

81 is a composite number.