Final answer:
Mountains are narrower at the top due to erosion and weathering, particularly from ice and water. The peaks are more exposed to severe weather conditions and glaciation, which leads to increased erosion at higher elevations, resulting in sharper and more defined peaks.
Step-by-step explanation:
Mountains are generally narrower at the top than at the bottom due to the processes of erosion and weathering. When mountains are formed, typically by the collision of continental masses such as tectonic plates, they are subjected to immense internal pressures that cause the Earth to buckle and fold, with some rock being pushed up while other portions are dragged below the surface. These tremendous forces cause the initial shape of a mountain range. After formation, mountains are continually shaped by external forces, chiefly erosion by water and ice. Ice, in the form of glaciers, carves out the landscape more effectively at higher elevations where temperatures are lower, hence sculpting the sharp peaks and serrated edges we see in many mountain ranges.
For example, in the formation of the Alps, the collision between the African and Eurasian plates created large folds of rock. Over time, however, the natural processes of erosion have gradually worn down the mountains. The peaks experience more severe weather conditions and are also subject to glaciation, which intensifies erosion at higher altitudes. The result is a more pronounced and narrower peak compared with the base of the mountain, which is subject to less intense erosive processes.
The effect of ice and water erosion is visible when comparing mountains on Earth with those on celestial bodies such as the Moon or Mercury, where, in the absence of an atmosphere and thus water and ice, mountains remain more smooth and less defined. Additionally, this phenomenon can be observed in the distribution of sediments around mountain ranges, with deep-water sediments such as shales often being found in the concave basins created by the weight of the mountains pressing down on the lithosphere.