Pāhoehoe, ‘a‘ā, and block lava: an illustrated history of the nomenclature

Co-auteur étrangerInternational audienceLava flows occur worldwide, and throughout history,various cultures (and geologists) have described flowsbased on their surface textures. As a result, surfacemorphology-based nomenclature schemes have been proposedin most languages to aid in the classification and distinctionof lava surface types. One of the first to be publishedwas likely the nine-class, Italian-language description-basedclassification proposed by Mario Gemmellaro in 1858. By far,the most commonly used terms to describe lava surfaces todayare not descriptive but, instead, are merely words, specificallythe Hawaiian words ‘a‘ā (rough brecciated basalt lava) andpāhoehoe (smooth glassy basalt lava), plus block lava (thickbrecciated lavas that are typically more silicic than basalt).‘A‘ā and pāhoehoe were introduced into the Western geologicalvocabulary by American geologists working in Hawai‘iduring the 1800s. They and other nineteenth century geologistsproposed formal lava-type classification schemes for scientificuse, and most of them used the Hawaiian words. In1933, Ruy Finch added the third lava type, block lava, to theclassification scheme, with the tripartite system being formalizedin 1953 by Gordon Macdonald. More recently, particularlysince the 1980s and based largely on studies of lava flowinteriors, a number of sub-types and transitional forms of allthree major lava types have been defined. This paper reviewsthe early history of the development of the pāhoehoe, ‘a‘ā, andblock lava-naming system and presents a new descriptive classificationso as to break out the three parental lava types intotheir many morphological sub-types