The rocks of Devils Tower are very hard compared to the surrounding sedimentary rocks, but they are not completely resistant to erosion. The cracks and columnar joints are the weak spots.
During the winter months, water from rain and snowmelt enters the cracks between the columns. When the temperatures fall below freezing, the resulting ice expands and pries the columns apart.
Other agents of erosion include lichen, trees, and other plants that grow on the tower. These plants physically and chemically break down the rocks of the tower. Organic acids secreted by the plants dissolve the rock chemically. The roots of the plants wedge themselves into the cracks and physically pry the rocks apart.