The crust never is at rest. She then falls, then rises, then winces, then gathers in folds. As a result, mountains, valleys and hills. Of course, it takes a lot of time - a few dozen million years.
Sometimes mountains are born as a result of volcanic eruptions. From the crater of the volcano magma escapes, flying stones, ashes. It all falls to the ground. Gradually, the lava hardens and forms a hill, which with each new eruption is growing. Mountains of volcanic origin have the form of a pyramid with a truncated top.