White and Houghton (2006) propose to expand broad terms like “ash” and “lapilli” into the detail of classic Wentworth (1922) sedimentary grain-size terms. Creating a scheme where none has existed is laudable, but this one is flawed.
Use of terms like ash and lapilli for volcanic sediments in place of standard grain-size terms like sand and gravel stems from historical accident. Principles of objective grain-size measurement and terminology developed in nonvolcanic terrains (Udden, 1914; Wentworth, 1922; Krynine, 1948; Pettijohn, 1949; Folk, 1954; Folk and Ward, 1957), while volcanoes were studied by petrologists little interested in details of fragmental deposits. The term ash has for decades included the whole grain-size range from coarse sand to clay. Even when divided into fine and coarse components (e.g., Wentworth and Williams, 1932; Fisher, 1961, 1966), “coarse ash” has encompassed the entire sand range, −1 to 4 φ, only two volcanic terms defining eight grain-size grades (very coarse sand to clay). Mount St. Helens' 1980 eruptions distributed sediment by surge, flow, fall, lahar, debris avalanche, etc. The descriptive grain-size terminology sufficiently divided to classify all of these was the Wentworth scale—one numerically defined name (e.g., coarse sand) for each full phi-size interval readily identified in the field. As we discussed erupted fragmental ejecta in the field at Mount St. Helens in the mid-1980s, Richard Fisher said, “they're volcanic on the way up but sediment on the way down.” And the textbook he co-authored with H.-U. Schmincke presents volcaniclastic grain size in Wentworth terms (Fisher and Schmincke, 1984). Problems arise only if genetic implication is incorrectly read into terms like “medium sand” or “pebble gravel” that have always described grain size alone.
In place of Robert L. Folk's (1954, 1964–1980) 15 terms describing bodies of mixed grain sizes that are readily distinguished in the field, White and Houghton's (2006, their Fig. 1) ternary grain-size diagram contains four broad terms that are gross lumpings, inadequate even for field description.
Most scientists acknowledge a mental discipline: objectively describe first, interpret later, and keep the two separate. A classification restricted to “primary” volcaniclastic deposits needs to infer genesis first, though often one cannot know this. White and Houghton define primary as “not stored at any time” (p. 677). What about secondary pyroclastic flows; loose sand held on proximal steep slopes before flowing to valley floors? What about lahars shed off volcanoes as swiftly as pyroclastic flows? What about hybrid deposits that are half snow? Having tentatively distinguished primary from secondary, is one to apply two sets of descriptive terms to deposits differing mainly by inferred genesis? A deposit formed by pyroclastic surge is “medium ash” but where redeposited as lahar is instead “medium sand” (White and Houghton, Table 1, p. 671)? A primary fall deposit (“medium lapilli”) becomes “pebble” gravel on the stream fan below—though grain size and texture are identical (White and Houghton, Table 1, p. 671)? This seemed silly in 1980 and still does. A standard grain-size term like large-pebble gravel works as well for a pumicious-fall deposit as it does for a beach deposit.
White and Houghton refer to standard sedimentologic texts but misapply many definitions in their Table 1. For grain size −1 to 0 φ, White and Houghton 's Table 1 is missing the adverb “very” from columns 5 and 6. The column 5 terms are not those for “unconsolidated sedimentary deposits” as claimed, but rather for particle-size class. “Granule” is a particle size between −2 φ and −1 φ; the equivalent term for a deposit is “granule gravel” (Wentworth, 1922; Folk, 1954,Folk, 1980). Ditto for other terms at the bottom of column 5—where “pebble,” “cobble,” and “boulder” are misdefined. The traditional boundary between pebble and cobble is −6 φ, and between cobble and boulder is −8 φ. The numerical limits on the bottom three rows of Table 1 are off. And at the top of Table 1, column 5 omits silt (4 to 8 φ). For the composite size class finer than sand (finer than 4 φ), the usual term is “mud,” not clay (Wentworth, 1922; Folk, 1980), and for sediment bodies the term is also mud. Clay designates only the mud fraction finer than 8 φ.
Since the 1950s, angularity of grains has disassociated from grain-size terms such as “sand” or “cobble.” I disagree with White and Houghton's (2006, p. 678) assertion that “genetic connotations also attach to terms derived from sedimentary geology, such as ‘mudstone,’ ‘sandstone,’ and ‘conglomerate.’” These terms have long defined grain size alone, implying nothing about genesis. Folk is clear on this in his primary papers (Folk, 1954; Folk and Ward, 1957) and in each edition of Sedimentary Rocks (Folk, 1964–1980). Like his predecessors Went-worth, Krynine, and Pettijohn, Folk clearly intends size only—and makes no implication whatever to inferred genesis.
Wentworth's terms, employed almost universally to describe grain size of terrestrial or marine particles or of bodies of particulate rock, work as well for the description of primary pyroclastic sedimentary deposits. The need for a separate terminology is dubious despite its long practice by some volcanologists focused on specialized topics. In a comprehensive field project like mapping, which examines all deposits on a volcanic cone regardless of inferred genetic origin, two parallel and duplicative sets of terms to describe something so basic as grain size is needlessly arcane. Superfluous size terms like “ash” and “lapilli” are better abandoned than amplified into a duplicative nomenclature.