Rock Classification Scheme (BGS).pdf - The Water, Sanitation and ...

to 75% are angular, in a block-bomb-tephra 25 to 50%are angular and in a bomb-tephra less than 25% areangular.The name ash-breccia should be used where the tephracontains more than 25% of each of the main fragment sizedivisions (i.e. block/bomb, lapilli and ash).The names lapilli-tephra or lapilli-ash or ash should beused where the average size of more than 75% of the pyroclasticfragments is less than 64 mm. The relative proportionsof lapilli- and ash-grade fragments is indicated in thefollowing way: in a lapilli-tephra more than 75% of thepyroclastic fragments are lapilli (i.e. in the range 64 to2 mm), in a lapilli-ash between 25 and 75% of thefragments are lapilli, and in an ash less than 25% of thefragments are lapilli (i.e. > 75% are ash-grade).4.2.1.2 Pyroclastic rocksThe following root names should be used for unimodal andwell-sorted pyroclastic rocks, (see Figures 5 and 8a).The names agglomerate or pyroclastic-breccia shouldbe applied to a rock in which more than 75% of the pyroclasticfragments exceed 64 mm. The relative proportionsof angular (i.e. block) and rounded (i.e. bomb) fragmentsexceeding 64 mm is indicated in the following way: in anagglomerate more than 50% of the fragments are rounded(i.e. bombs). This is the consolidated equivalent of abomb-tephra or block-bomb-tephra. In a pyroclasticbrecciamore than 50% of the pyroclastic fragments areangular (i.e. blocks). This is the consolidated equivalent ofa block-tephra or bomb-block-tephra.The name tuff-breccia should be used where a pyroclasticrock contains more than 25% of each of the mainfragment size divisions (i.e. block/bomb, lapilli and ash).The names lapillistone or lapilli-tuff or tuff should be usedwhere the average size of more than 75% of the pyroclasticfragments is less than 64 mm. The relative proportions oflapilli- and ash-grade fragments is indicated in the followingway: in a lapillistone more than 75% of the fragments arelapilli (i.e. in the range 64–2 mm), in a lapilli-tuff between 25and 75% of the fragments are lapilli, and in a tuff less than25% of the fragments are lapilli (i.e. > 75% are ash-grade).The term lapillistone, as defined above, comes from theclassification scheme of Fisher and Schminke (1984) forpyroclastic rocks. It is preferred to the term ‘lapilli-tuff’,which is recommended by the IUGS for the same rocktype. The term lapilli-tuff is used in this classification fora poorly sorted pyroclastic rock containing lapilli-sizedand ash-sized fragments (see Section 4.2.1.4 Poorly sortedpyroclastic rocks and tephra).4.2.1.3 Special qualifier terms for pyroclastic rocks andtephraEach of the root names listed above may be qualifiedfurther on the basis of fragmental composition, as shown inFigure 7. Thus, a lithic lapillistone is comprised predominantlyof rock fragments, a crystal lapillistone iscomprised predominantly of crystal fragments and a vitricblock-tephra is comprised predominantly of glassy andpumiceous fragments. Vitric components include ‘shards’,which are glassy fragments, and highly vesiculated, frothy(i.e. pumiceous) material which can be separated into‘scoria’ (dark-coloured) and ‘pumice’ (light-coloured).Note that the terms ‘lithic’, ‘vitric’ and ‘crystal’ are usedas qualifiers and are not part of the root name.Special qualifier terms may be used to divide tuffs andashes into coarse tuff and coarse ash, which have anaverage pyroclast size of between 2 mm and 0.032 mm,and fine tuff and fine ash which have an average pyroclastsize of less than 0.032 mm (Figure 5).The qualifier term reworked can be used as a prefix to astandard root name for pyroclastic rocks and tephra whichconsist of more than 50% reworked pyroclastic fragments,as defined in Section 4.1 Types of volcaniclastic fragmentsabove, for example reworked lapillistone, reworked ash.Most pyroclastic rocks and tephra are composed dominantlyof pyroclasts. Where no other indication is given inthe name assigned to a pyroclastic rock or to tephra it shouldbe assumed that pyroclasts are volumetrically the dominanttype of pyroclastic fragment. Where other types of pyroclasticfragment (hydroclasts, autoclasts) are identified thegeologist should decide whether their presence is sufficientlyimportant to merit inclusion in the name assigned to the rockor sediment. For example, a rock or sediment in which mostor all of the pyroclastic fragments are hydroclasts could beassigned a root name in the normal way, that is according tothe origin (pyroclastic versus epiclastic) and granulometricstate of the fragments, to which the qualifier term hydroclasticis added. In this way, names such as hydroclastic lapillituffare generated. Where hydroclasts form only a small proportionof the total volume of pyroclastic fragments, but theirpresence is still considered important, the qualifier termcould be modified to reflect this, for example hydroclastbearinglapilli-tuff.The name ‘hyaloclastite’ has been used traditionally todescribe rocks or sediments composed entirely of shattered,angular, glassy fragments created by quenching of lavasduring subaqueous or subglacial extrusion. As such, thename implies only a process and gives no information aboutfragment size. In keeping with the root name and qualifierapproach, names such as hyaloclastite tuff and autoclastiteblock-tephra, should be used in preference to terms such as‘hyaloclastite’ and ‘autoclastite’.4.2.1.4 Poorly sorted pyroclastic rocks and tephraThe current IUGS recommendations suggest that wherepyroclastic rocks and tephra are poorly sorted the nameassigned to them should be composed of an appropriatecombination of the terms described above (for example‘ash-lapilli-tuff’ where lapilli > ash, or ‘lapilli-ash-tuff’where ash > lapilli). However, this approach is not usedwidely because it is unwieldy and can result in rock nameswith ambiguous meaning. The scheme proposed by Fisherand Schminke (1984), in which poorly sorted volcaniclasticrocks and sediments are named using the ternarydiagrams shown in Figures 8a and 8b, is more popular andis generally considered more practical; for these reasons itis recommended here. According to this scheme, a pyroclasticrock or sediment can be defined as poorly sortedwhere no one glanulometric class of pyroclastic fragment(i.e. ash, lapilli or blocks and bombs) exceeds 75% of thevolume. The root names tuff-breccia, and lapilli-tuff aregenerated for poorly sorted pyroclastic rocks, and ashbrecciaand lapilli-ash for poorly sorted tephra.4.2.2 TUFFITESThe general term tuffite can be used for volcaniclasticrocks and sediments which consist of between 25% and75% by volume of pyroclastic fragments (see Figures 4and 6). The prefix ‘tuffaceous’ should be used withstandard root names for clastic sediments and sedimentaryrocks to produce root names such as tuffaceous-sand andtuffaceous-mud for unconsolidated deposits, for which theconsolidated equivalents would be tuffaceous-sandstoneand tuffaceous-mudstone. Figure 3 shows the grain size9