Final answer:
Crevasses form in the upper part of a glacier where tensile stress causes the ice to crack, often due to the glacier moving over varied terrain.
Step-by-step explanation:
Crevasses tend to form in the upper part of a glacier, and form where the glacier flows over a slope or obstruction, causing the ice to crack due to tensile stress. Since ice is a brittle material, it can fracture when it experiences stress beyond its tensile strength. This is similar to how mudcracks, also known as desiccation cracks, form when wet sediment dries out and shrinks, creating cracks. Crevasses can be quite deep and are often hidden by a layer of snow, which makes them particularly dangerous for mountaineers and adventurers on glaciers. They generally form perpendicular to the flow of the glacier, opening up as the glacier moves and closes up as it passes the stress point. Understanding where and how crevasses form is crucial for safely navigating glaciated terrain.