Publisher Summary Volcanic ash or tephra is commonly unconsolidated, comminuted materials containing a large quantity of volcanic glass which shows the least resistance to chemical weathering. Therefore, tephras weather rapidly resulting in formation of large amounts of noncrystalline materials. This process occurring preferentially in tephras was first called “andosolizatiod" by Duchaufour. Development of A horizons by andosolization is characterized by accumulation of organic matter, organic matter stabilization by active Al and Fe, carbonic acid weathering (allophanic) versus organic acid weathering (nonallophanic), andformation of laminar opaline silica. Formation of B horizons proceeds primarily by carbonic acid weathering with no significant translocation of Al, Fe, and dissolved organic carbon. Therefore, preferential formation of noncrystalline materials such as allophane, imogolite, laminar opaline silica, ferrihydrite, and Al/Fe humus complexes is a characteristic feature of the process of andosolization. Such formation of noncrystalline materials is not specific to Andisols. It is also widely observed for tephra-derived Spodosols making it difficult to separate Andisols and tephra-derived Spodosols according to solid-phase chemical criteria. Andisols can form in tephras by andosolization in a relatively short time under most climates throughout the world.