Pāhoehoe, ‘a‘ā, and block lava: an illustrated history of the nomenclature
Résumé
Lava flows occur worldwide, and throughout history,
various cultures (and geologists) have described flows
based on their surface textures. As a result, surface
morphology-based nomenclature schemes have been proposed
in most languages to aid in the classification and distinction
of lava surface types. One of the first to be published
was likely the nine-class, Italian-language description-based
classification proposed by Mario Gemmellaro in 1858. By far,
the most commonly used terms to describe lava surfaces today
are not descriptive but, instead, are merely words, specifically
the Hawaiian words ‘a‘ā (rough brecciated basalt lava) and
pāhoehoe (smooth glassy basalt lava), plus block lava (thick
brecciated lavas that are typically more silicic than basalt).
‘A‘ā and pāhoehoe were introduced into the Western geological
vocabulary by American geologists working in Hawai‘i
during the 1800s. They and other nineteenth century geologists
proposed formal lava-type classification schemes for scientific
use, and most of them used the Hawaiian words. In
1933, Ruy Finch added the third lava type, block lava, to the
classification scheme, with the tripartite system being formalized
in 1953 by Gordon Macdonald. More recently, particularly
since the 1980s and based largely on studies of lava flow
interiors, a number of sub-types and transitional forms of all
three major lava types have been defined. This paper reviews
the early history of the development of the pāhoehoe, ‘a‘ā, and
block lava-naming system and presents a new descriptive classification
so as to break out the three parental lava types into
their many morphological sub-types.