344 



UNITED STATES MINERAL RESOURCES 



Deposits of pumice and pumicite occur in a larger 

 variety of forms than do scoria and cinder. A very 

 common form is an air-fall blanket, semiconcentric 

 to a crater source vent or broadly fan-shaped lateral 

 to fissure vents and more extensively developed 

 along the prevailing wind directions. Pumice cones 

 along fissure vents are also numerous. Very large 

 volumes occur in ash-flow tuffs and in the upper 

 parts of rhyolite and "obsidian" domes. Any of 

 these deposits is subject to rapid erosion, reworking, 

 and consequent redeposition as stream or lake sedi- 

 ments. Thicknesses range from a tenth of an inch 

 to many tens of feet, and areal extents range from 

 a few square feet to tens of square miles ; therefore, 

 volumes may be a few tons or many hundred 

 thousand tons. Ash-fall deposits are more likely to 

 be interbedded with other sediments than are ash 

 flows, and reworked and redeposited materials very 

 commonly are interbedded. 



In contrast with ash falls, most ash flows are 

 unsorted, heterogeneous mixtures of pumice and 

 ash, the pumice occurring in fragments and blocks 

 as much as perhaps 8 or 10 feet in maximum dimen- 

 sion. Commonly, ash flows were hot enough and 

 cooled slowly enough to weld and to cause the 

 pumice to soften to the point of collapse, so that 

 dense glassy zones in the central or lower part of 

 the flow were formed. Ash flows are crudely sorted, 

 with most of the larger blocks and fragments close 

 to the source and successively finer materials at 

 greater distances ; the finest ash may be carried for 

 hundreds of miles. Some ash falls also retained 

 enough heat to weld and to form glassy zones. Where 

 ash and pumice fall directly on standing bodies of 

 water, and where reworked material is deposited in 

 quiet water, an inverted graded bedding may occur. 

 The finest material has the greatest density and 

 settles first, whereas coarser material is vesicular, 

 floats longer, and is deposited last. 



Pumice deposits occur in all the Western States 

 from the Rocky Mountains to the Pacific Ocean, 

 and pumicite deposits are in Texas, Oklahoma, Kan- 

 sas, North and South Dakota, and Nebraska nearly 

 as far east as the Missouri River. In 1970 pumice or 

 pumicite was produced in every Western State but 

 Montana, and in Hawaii as well. Oregon and Cali- 

 fornia were probably the leading producers of 

 pumice, and Kansas was the leader in pumicite. 

 Production statistics are so grouped that accurate 

 data for each rock type cannot be determined. 



Prediction of the resources base for pumice and 

 pumicite presents many of the same problems as 

 for scoria and volcanic cinder. Any figure for re- 

 coverable reserves reflects wide annual variations 



in the number of operating mines and no accurate 

 census has ever been kept of producers grouped by 

 product. In part, the extreme increases and de- 

 creases in annual tonnage (fig. 40) reflect this im- 

 precise classification, but they also suggest that 

 many of the operations were marginal and that 

 what were recoverable reserves one day were 

 paramarginal the next. Since 1957, except for 

 1963 and 1964, the overall trend has been toward 

 smaller production. But because most pumice and 

 pumicite is used in the construction industry, the 

 long-term-consumption trend will be a growth rate 

 similar to that for construction. On the basis of the 

 1960-70 average annual output of slightly more than 

 600,000 tons and the previously used annual growth 

 rate of 5 percent, cumulative production through 

 the year 2000 will be about 40 million tons. 



Recoverable reserves can be estimated only within 

 very broad limits. In 1970 some 30 mines operated 

 in the conterminous United States. Although the 

 range in deposit size seems to be fairly large, a 

 conservative estimate suggests that many occur- 

 rences have dimensions of 500 by 1,000 by 10 feet. 

 Bulk densities range from 850 lb per cu yd for 

 pumice to 1,750 lb per cu yd for pumicite; tonnage 

 for the sample deposit lies between about 80,000 and 

 160,000 tons. Thus, the 1970 level of operations 

 might entail between 2V2 and 5 million tons of rock 

 as recoverable reserves, ignoring all past production. 

 Estimating paramarginal and submarginal resources 

 each at twice the recoverable reserves is acceptable 

 (see estimate of the total resource below) so that 

 the identified resources can be thought of as a mini- 

 mum of 25 million tons, more than half the amount 

 needed through the year 2000. 



The total resource can be approximated on the 

 same basis as that for scoria and volcanic cinder, 

 using Higgs' (1949) estimate (8.368 percent) of 

 extrusive rocks at land surface in the Western 

 States. Higgs' method excludes all volcanic ash in 

 the States east of the Rocky Mountains, however. 

 Again no reliable estimates of the relative amounts 

 of silicic, intermediate, and mafic volcanic rocks are 

 available, but a general feeling among geologists 

 indicates that intermediate rocks predominated in 

 the early and middle Cenozoic and that both silicic 

 and mafic rocks were abundant in the late Cenozoic, 

 the mafiic rocks possibly having been more abun- 

 dant. Basic rocks were assumed to occupy 1 percent 

 of the surface for the scoria estimate; here, let us 

 assume 0.1 percent coverage and an average thick- 

 ness of 10 feet for the silicic rocks; let us further 

 assume that 1 percent of this volume is pumice or 

 pumicite. These parameters would seem to be almost 



