Andisols develop from volcanic parent materials. Volcanic deposits have a unique process of weathering that causes the accumulation of allophane and oxides of iron and aluminum in developing soils.
Andisols form in material that has recently ejected from volcanoes (ash) and are commonly found on land surrounding the Pacific Rim ( the root and comes from the Japanese word ando meaning black). Since volcanoes have erupted several times during the past hundred to million years the soils often have distinct layers. The soils have high porosity, particle surface area and water-holding capacity. The also have a very low bulk density or are very light in weight. On steep slopes they are forested but on more gently slopes they are a prized agronomic soil because of their innate fertility and water holding capacity. The diagnostic feature is the low bulk density of volcanic ash soils.