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

older, consolidated deposits. While it is possible to know agreat deal about the depositional history and fragment originof modern deposits, it is often the case that relatively littlecan be said with confidence about the depositional historyand fragment origin of older, usually compacted and altered,volcaniclastic rocks. Where a volcaniclastic rock hasundergone compaction and/or alteration to an extent that thenature of the original constituent fragments cannot be determinedwith confidence, it should be classified and named asa tuffaceous sedimentary rock (see Section 4.2.2 Tuffites) oras a volcaniclastic sedimentary rock (see Section 4.2.3Volcaniclastic sedimentary rocks and sediments), as appropriate.4.1.1 PYROCLASTIC FRAGMENTSPyroclastic fragments are generated by disruption as adirect result of explosive volcanic action. Three principaltypes of pyroclastic fragment can be distinguished.i Juvenile fragments. These form directly from coolingmagma during transport prior to primary deposition.Juvenile pyroclastic fragments may be subdivided, asfollows:pyroclasts—form directly from cooling magma duringaerial and subaerial transport. They may be individualcrystals, crystal fragments, glass fragments or rockfragments, or they may consist of a combination ofthese.hydroclasts—are chilled glass particles that form bymagma–water interactions during subaqueous orsubglacial extrusion.autoclasts—are fragments formed by mechanical frictionof moving lava flows, or through gravity crumbling ofspines and domes.ii Cognate fragments. These are fragments of rock whichformed during earlier volcanic activity, but which havebeen detached and ejected with other pyroclastic debrisduring a later eruption. Although these are most likely tohave an ‘igneous’ composition they can, in theory, becomposed of any rock type.iii Accidental fragments. These are fragments of rockgenerated by disruption as a direct result of volcanicaction, but were not formed by previous activity of thevolcano. They are generally derived from the subvolcanicbasement and they can, therefore, be composed ofmany rock types.The genetic basis of the definitions presented above does notconform to the stated principle of using descriptive attributesonly to classify igneous rocks, however the approach and terminologyare so deeply engrained in, and relevant to, the fieldof volcaniclastic research that the terms have been retainedhere. More detailed definitions and descriptions of pyroclasticfragments are presented in Fisher and Smith (1991), inHeiken and Wohletz (1985; 1991) and in Marshall (1987),among other publications.Despite the genetic basis for distinguishing fragmenttypes, the origin of pyroclastic fragments is not taken intoconsideration in the classification and nomenclature of volcaniclasticrocks and sediments. For the purposes of classification,pyroclastic fragments are distinguished by theirsize (Figures 3 and 5) and shape in the following way:bombs—are pyroclastic fragments whose mean diameterexceeds 64 mm and which have a shape (generallyrounded) or texture (e.g. ‘bread-crust’ surface) whichindicates that they were in a wholly or partly molten stateduring their formation and subsequent transport.blocks—are pyroclastic fragments whose mean diameterexceeds 64 mm and which have an angular or subangularshape indicating they were solid during transport.lapilli—are pyroclastic fragments of any shape with amean diameter of 2 mm to 64 mm.ash—grains are pyroclastic fragments with a meandiameter of less than 2 mm. These are subdivided intocoarse ash grains (0.032 mm to 2 mm) and fine ash grains(less than 0.032 mm).In many pyroclastic rocks and sediments, pyroclasts aredominant volumetrically and the above terms, where they areused alone, can be assumed to refer to pyroclasts. However,the terms can be modified to identify other types of pyroclasticfragments, for example ‘ash-grade hydroclast’, ‘lapilligradeautoclast’ and ‘block-grade cognate clast’.If pyroclastic fragments have been moved and redepositedprior to lithification they may be termed ‘reworkedpyroclasts’.4.1.2 EPICLASTIC FRAGMENTSEpiclasts are fragments in volcaniclastic rocks andsediments that have not been generated by disruption as adirect result of volcanic action. They are defined, accordingto Schmid (1981), as ‘crystals, crystal fragments, glass androck fragments that have been liberated from any type ofpre-existing consolidated rock (volcanic or non-volcanic)by weathering or erosion and transported from the site oforigin by gravity, air, water or ice’. Thus, epiclasts can bevolcanic or non-volcanic, and they can be non-igneous.4.2 Classification of volcaniclastic rocks andsedimentsRoot names used in the classification describe only thegranulometric state of the rocks and sediments. Grain sizelimits used for defining volcaniclastic fragments matchthose used in the classification scheme for Sedimentaryrocks and sediments (Figure 3). These should be regardedas provisional until there is international agreement on thegranulometric divisions of sedimentary materials. Thehierarchical scheme for classifying Volcaniclastic rocksand sediments is shown in Figure 4.4.2.1 PYROCLASTIC ROCKS AND SEDIMENTS (TEPHRA)Pyroclastic rocks and sediments contain more than 75% byvolume of pyroclastic fragments, the remaining materialsbeing generally of epiclastic, organic, chemical sedimentaryor authigenic origin. If predominantly consolidatedthey should be classified as pyroclastic rocks, and if predominantlyunconsolidated they should be classified astephra (Figure 4). The term ‘tephra’ is synonymous with‘pyroclastic sediment’.4.2.1.1 TephraThe following root names should be used for unimodal,well-sorted sediments comprised of more than 75% pyroclasticfragments (see Figures 5 and 8a).The names block-tephra or bomb-tephra or blockbomb-tephraor bomb-block-tephra should be usedwhere the average size of more than 75% of the pyroclasticfragments exceeds 64 mm. The relative proportions ofangular (i.e. block) and rounded (i.e. bomb) fragmentsexceeding 64 mm is indicated in the following way: in ablock-tephra more than 75% of all pyroclastic fragmentsexceeding 64 mm are angular, in a bomb-block-tephra 508