The term 'crumble over time' can be described by the word 'ERODE', which has five letters. Erosion is a natural process that occurs when wind, water, and other elements wear away at the surface of the earth. Over time, this process can cause rocks and other materials to break down and crumble. This process is often seen in areas with high levels of precipitation, such as near rivers or oceans. Erosion can also occur due to human activities, such as deforestation or construction.