Final answer:
Vertical crater retreat is a geological process where a crater gradually deepens and expands vertically due to erosion or collapse of its walls. This process is commonly observed on planetary bodies with loose or weak surface materials.
Step-by-step explanation:
Vertical crater retreat is a geological process where a crater gradually deepens and expands vertically due to erosion or collapse of its walls. As the walls of the crater erode or collapse, the crater floor moves down, creating a deeper and wider crater. This process is commonly observed on planetary bodies with loose or weak surface materials, such as the Moon and Mars.