What would likely happen to a fence on a hill over time if the fence were built perpendicular to the hill's slope?

The fence built on the hill's slop over time will fall. It will not be a sudden process though, but slowly, the fence will bend over till it falls. This will be caused by the erosion, over time little by little the hill will be eroding because of the natural phenomenons like the wind and the rain (depending on the place where the hill is, maybe ice as well), so there will come a point where the basis of the fence that were once in the ground will be uncovered and there will be nothing to hold it, so it will fall.

Mantle convection is the process that drives plate tectonics. the theory of plate tectonics states that the crust is divided into plates that float on the semi molten mantle below. the circular motion of the magma as it slowly rises and falls or move though convection cells drags the plates along causing them to move. the heat source in the lamp would represent the earth's core. the core is the source of energy that drives convection in the mantle. the lava represents molten magma under the earth's surface. if the lava lamp had floating pieces they would move as the circular motion of the lava in the lamp as it rises and falls would move them along the surface. these floating pieces would represent pieces of the earth's crust.