Final answer:
Mountains become rounded over time due to the natural force of erosion, particularly as a result of water and ice. The process of glaciation by ice is a significant factor in sculpting and smoothing mountain ranges. Other planetary bodies without water and ice, like the Moon, feature smoother mountainous landscapes because they lack these erosive forces.
Step-by-step explanation:
The natural force responsible for the rounding of mountains over time is erosion, primarily caused by water and ice. Once a mountain range is formed, typically through tectonic forces such as the upthrusting of Earth's crust, it begins to experience erosion. Water can cause erosion as it flows over the rock, carrying away small particles. Ice, however, acts as a particularly effective sculptor through the process of glaciation, where glaciers carve and shape the land as they move.
Comparatively, on other planets like the Moon or Mercury, where there is no running water or moving ice, mountains do not experience the same erosion and thus retain smoother shapes. The Alps and the Rocky Mountains on Earth are examples of ranges formed by different tectonic processes, but both are subjected to the universal process of erosion, leading to the rounding of their peaks over time. The Rocky Mountains, for example, were formed during the Laramide Orogeny and have since been glaciated and eroded.
Ultimately, the rounded appearance of a mountain range is the combined result of the mountain-building forces and the relentless wear of erosion. It's important to understand that the same forces that create these stunning natural features are also actively shaping and changing them through erosion.