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UNITED STATES MINERAL RESOURCES 



their crystalline counterparts ; scoria and cinder are 

 like basalts (and basaltic andesites), and pumice, 

 pumicite, and perlite are like granites and rhyolites. 

 Contained water in basalts is little different from 

 that in scoria and cinder, but the difference between 

 that in the granitic rocks and that in silicic glassy 

 rocks is significant. It is this excess of water that 

 allows the perlites and pumicites to be thermally 

 expanded. In a general way, the viscosity of the 

 extruded rock melt varies directly with the Si02, 

 NasO, and K2O content and inversely with the FcaOs, 

 FeO, CaO, and MgO content ; thus, whereas the mafic 

 rocks are more fluid, the silicic rocks are more likely 

 to retain gases. 



SCORIA AND VOLCANIC CINDER 



Scoria and volcanic cinder are cellular, frothy 

 products of explosive volcanism — erupted as tephra 

 (blocks, bombs, lapilli, and gravel) or lava flows 

 from mafic magmas. Much of the material is force- 

 fully ejected from the vent, vesiculating in the air 

 as the volcanic gases (mostly water) expand and are 

 entrapped by the rapid cooling of the relatively fluid 

 lava. Scoria also develops where the gases are en- 

 trapped by rapid cooling at the surface of the flows ; 

 it may then be broken into blocks and irregular 

 fragments by continued movement of the flows. 

 There is a fairly wide range in vesicle size (particu- 

 larly in the ejecta, most of which have dense fused 

 surfaces) and in the thickness of the cell walls, 

 which results in a considerable variation in the 

 density of the rocks. The distinction between scoria 

 and volcanic cinder is an arbitrary one of size; 

 cinder is the material less than 1 inch (about 2.5 

 cm) in largest dimension. Characteristically, mafic 

 ejecta are fairly heavy and are not thrown long dis- 

 tances from the vent; they form cinder cones 

 around the vent that are generally a few thousand 

 to several thousand feet in diameter and several 

 hundred feet in height. In contrast, mafic lava flows 

 (basalt or basaltic andesite) tend to be fluid, several 

 hundred feet to tens of miles in lateral extent, and 

 a few feet to several tens of feet thick. The flows are 

 seldom abundantly scoriaceous throughout; most of 

 vesicular zones are confined to the upper few feet of 

 the flow and are highly variable in their develop- 

 ment. 



Most of the deposits of scoria and volcanic cinder 

 are geologically youthful, in Quaternary or upper 

 Tertiary terranes (<1-11 m.y. old), and some are 

 only a few thousand years old. Many maflc extru- 

 sive rocks are latecomers in the volcanic sequence 

 and, thus, are less likely to be buried deeply enough 

 to survive prolonged erosion. In addition, the ejecta 



seldom make well-consolidated resistant lithologic 

 units because the individual blocks, bombs, and 

 lapilli do not weld together consistently. They are 

 easily eroded and, in humid climates, are subject to 

 rapid chemical weathering. Although the ejecta 

 enter readily into the sedimentary cycle, they are 

 not persistent in it. They lack suflScient resistance 

 to abrasion to survive reworking for very long in 

 the moderate-energy environment needed to move 

 them to protected sites where they can be preserved. 



Mafic vesicular volcanic rocks are widely distrib- 

 uted throughout the Western United States, includ- 

 ing Hawaii and Alaska, but are missing from the 

 Central and Eastern States. Deposits of scoria and 

 cinder are known in every State west of the 104th 

 meridian, and production was recorded in 1969 or 

 1970 from all States but Alaska. Arizona, Oregon, 

 California, Hawaii, and New Mexico are the 'largest 

 producers, generally in that order ; however, in 1970 

 Oregon was first. The ranking in production does 

 not always reflect the relative amounts of scoria 

 and cinder available, in part because production is 

 determined by competition and the size of the 

 market within economic range of the deposits and 

 in part because the reporting of production includes 

 pumice and pumicite. In the States west of 104°, 

 nearly all the metropolitan districts with a core 

 city of at least 50,000 people and most cities of at 

 least 25,000 are within about 100 miles of deposits 

 of scoria or cinder, but not all can be served eco- 

 nomically from them. 



Estimating the recoverable reserves of scoria and 

 cinder is an exercise in very short term prediction, 

 for production is extremely sensitive to the state of 

 the construction industry, and pits can be and are 

 shut down quickly. For example, Meisinger (1972) 

 reported the closing in 1970 of 48 operations of the 

 171 that were open in 1969. His figures are for all 

 pumice, pumicite, scoria and cinder operations, but 

 inasmuch as 85 percent of the total production was 

 cinder, it seems reasonable to assume that most of 

 the closed operations were also for scoria or cinder. 

 Although the more than 25 percent drop in opera- 

 tions resulted in only a 15 percent drop in output 

 from 1969, it might represent either a larger or 

 smaller decrease from the 1969 recoverable re- 

 serves, for when an operation closes, all its recov- 

 erable reserves, by definition, become at least para- 

 marginal. 



For scoria and cinder, the economic criteria that 

 separate recoverable from paramarginal and sub- 

 marginal reserves are primarily the level of activity 

 of the construction industry and competition from 

 expanded clays, shales, and slates. Other materials, 



