Hydrated halloysite was formed by the low temperature alteration of trachytic tuff in onion-like spherical or curly platy forms. The Si content was higher than that of ideal kaolin minerals, indicating the possible presence of amorphous materials. The high Fe content is responsible for the platy morphology of halloysite. The leached ions precipitated as halloysite filling the interparticle pores, while trachytic glass was replaced by Si-rich amorphous materials in an hemispherical form, which was further evolved into spherical halloysite. Halloysite is one of the major alteration products of the volcanic ash in Ulleung Island underwent intense trachytic volcanism.