degrade
v.v. to break down or wear away slowly over time. You use this when talking about rocks, soil, or materials that get weaker or smaller because of nature.
v. to break down or deteriorate into simpler components or a lower state. In geological contexts, it refers to the wearing down of landforms through erosion or weathering.
The plastic will slowly degrade in the sun.
Heavy rainfall caused the cliffside to degrade, eventually leading to a dangerous landslide near the coastal road.
Over millions of years, the relentless action of wind and water will degrade even the most formidable mountain ranges into rolling hills and sediment.
From Middle French dégrader.
The verb is both transitive and intransitive; it can describe an external force breaking something down or the object breaking down itself.