Clay most commonly forms due to erosion or weathering; both methods involve rocks coming into contact with something, such as air or water, to form the clay from existing minerals on the ground. It is common for clay to form in specific geological environments, such as where there are volcanic deposits, marine sediments or soil horizons.
Erosion and weathering both result in the breaking down of rocks, and this can occur because of natural elements, such as ice and wind, or because of chemical elements, such as acid. The processes result in either primary or secondary clay deposits, with primary deposits originated in the soil where they were formed and secondary deposits moving from where they originated. More rarely, hydrothermal activity can cause clay to form.