LIGHTWEIGHT AGGREGATES 



343 



like expanded slag and fly ash, are not significant 

 competitors in the Western States. The stumbling 

 block is usually the cost of transportation, and this 

 cost almost invariably cannot be decreased. For 

 most commodities the grade of the ore enters into 

 the criteria, and new techniques of recovery may 

 convert protore into ore. These criteria are not 

 particularly applicable to any of the nonmetallic 

 deposits, where the whole rock mass is the ore. Vol- 

 canic ejecta have already undergone a crude form 

 of aerial classification that has substantially sepa- 

 rated vesiculated from dense rock. Only a small 

 additional tonnage could be won at any deposit by 

 separating vesiculated rock from predominantly 

 dense rock. In deposits that are scoriaceous zones 

 in flowrock, more intermingling of vesiculated and 

 dense rock occurs, but even here the additional ton- 

 nage to be gained is small, and the cost of winning 

 it comparatively high. In summary, these para- 

 marginal resources are not significant. 



An order of magnitude can be established, al- 

 though not from strictly geologic evidence, for both 

 the recoverable reserves and the identified resources. 

 For example, many cinder cones have basal diam- 

 eters greater than 2,000 feet, and angles of slope 

 (determined by the angle of repose for fragmental 

 volcanic material) that range from 30° to 40° (Col- 

 ton, 1964). For a cone that has a 2,000-foot base 

 diameter, a crater lip 425 feet above the base, and 

 a crater 500 feet in diameter at the lip, the angle 

 of slope is less than 30°, well within the angle of 

 repose. Depending on the density of the scoria, 

 which can range from 1,000 to 1,500 lb per cu yd 

 (pounds per cubic yard), such a cone could contain 

 from 10 to 15 million tons. One such cone, then, can 

 contain from three to six times the entire U.S. 

 production of volcanic cinder, 2,656,000 tons for 

 1970 (Meisinger, 1972, p. 965) . An assumption seems 

 reasonable that 25-30 of the operating pits in 1970 

 (there were more than 100 in all) were in cinder 

 cones of this size in the 10 or more States where 

 scoria was produced ; as a result, recoverable re- 

 serves that year were between 250 and 450 million 

 tons in just those deposits. In the category of para- 

 marginal resources, economically recoverable at 1.5 

 times the present price (McKelvey, 1972) , there are 

 probably a similar number of deposits and tonnages 

 and, also, a similar quantity in the submarginal re- 

 sources. Identified resources can be thought of as 1 

 billion tons, plus or minus 25 percent. 



Let us put it in another kind of perspective. If we 

 assume that 3 million tons of scoria and cinder was 

 produced in 1970 and that an annual growth rate 

 of 5 percent persists through the year 2000, we 



shall have need for a cumulative output of about 200 

 million tons. That appears to be significantly less 

 than the recoverable reserves in the cinder cones 

 being mined in 1970. 



The category of hypothetical resources requires a 

 different approach, for basically the search is not 

 for hidden deposits but for unmapped ones. By 1972 

 the location of most of the cinder cones and mafic 

 lava flows in the conterminous United States prob- 

 ably had been depicted on some sort of geologic 

 map at some kind of scale. Knowledge of the sizes 

 and volumes of these cones and flows and of the 

 ratio of vesicular to dense rock within them is what 

 is required to convert their status to one of the 

 reserve classes. The distribution of the deposits that 

 make up the identified resources suggests that there 

 may be at least an equal number for which informa- 

 tion on size is lacking, and, on that basis, the hypo- 

 thetical resources can be estimated to be on the 

 order of 1 billion tons, as well. 



Speculative resources require a further exercise 

 of the imagination. Higgs (1949) estimated that 

 Cenozoic extrusive rocks in the conterminous United 

 States covered 8.368 percent of the land surface. 

 In the absence of other estimates on the ratio of 

 mafic to silicic extrusives at the surface, and for 

 ease of estimation, let us assume that mafic rocks 

 cover only 1 percent of the surface and that they 

 average about 50 feet thick. Let us further assume 

 that 1 percent of this volume is scoriaceous. This is 

 equivalent to about 3 cubic miles of scoria, or about 

 9 billion tons. The potential reserves and hypo- 

 thetical resources listed above can be adequately 

 accommodated within this total resource. 



PUMICE AND PUMICITE 



Pumice and pumicite are the silicic extrusive 

 equivalents of scoria and volcanic cinder but are 

 lighter in color, of lower density, and more highly 

 cellulated. In pumice the voids may be spherical, 

 ovoid, tubular or irregular; the cell walls are com- 

 monly very thin; and the rock is light enough to 

 float. If sufficient gas and steam are available dur- 

 ing explosive extrusion, the pumiceous froth is very 

 commonly shattered, and the shards and finer frag- 

 ments form pumicite (volcanic ash). The range in 

 composition (table 68) is from rhyolitic to dacitic, 

 andesitic, and even basaltic, but most pumices are 

 in the rhyolite-dacite range. The higher viscosity of 

 the silicic rock melts generally leads to more violent 

 extrusion, and the lower density pumice and pum- 

 icite are thrown much farther from the vents than 

 the scoria and cinder; many pumicite deposits are 

 several hundred miles from their sources. 



