Page 15 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

An approach to the genetic interpretation useful, particularly in ancient sequences in the early
of textures in volcanic deposits stages of mapping or logging and in strongly altered
rocks. Apparent volcaniclastic textures are very
Identification and interpretation of volcanic textures common in altered coherent lavas and intrusions, as a
involves a blend of process-oriented volcanology and result of patchy or domainal alteration or fracture- and
sedimentology, and primarily depends on careful joint-controlled alteration (Part 5). The apparent
observations made at a range of scales. Proficiency textures in many cases superficially resemble welded
improves dramatically with an awareness of genetic ignimbrite or coarse lithic breccia. Some unaltered
processes and by adopting a systematic approach to microspherulitic or micropoikilitic lavas and intrusions
description. We therefore emphasize: also display apparent volcaniclastic textures, and
(1) the use of appropriate terminology for the accurate resemble well-sorted, massive sandstone. Apparent
field description of volcanic deposits; coherent textures are best developed in some very
(2) recording outcrop and drillcore sections by means of densely welded primary pyroclastic deposits, especially
graphic logs; rheomorphic and lava-like ignimbrites (Part 4). In these,
(3) identification of original volcanic textures, and the glassy pyroclasts have completely coalesced and are
discrimination of these from textures attributable to no longer separately distinguishable, and crystal
alteration, deformation and/or metamorphism; particles are dominantly complete euhedra. Fine-
(4) recognition of textures and structures diagnostic of grained, massive or planar laminated volcaniclastic
emplacement processes, in particular, coherent facies deposits, such as shard-rich mudstone, may display
(lavas, intrusions) versus volcaniclastic facies apparent coherent textures as a result of recrystallization
(autoclastic, pyroclastic, resedimented, volcanogenic during diagenesis and alteration.
sedimentary);
(5) recognition of outcrop features diagnostic of In order for the field geologist to proceed further to
particular depositional settings, in particular, subaerial appropriate descriptive terminology and, thereafter, to
versus subaqueous environments, and relatively deep genetic interpretation, it is necessary first to make a
versus relatively shallow subaqueous settings. decision regarding the textural category, that is, whether
volcaniclastic or coherent. As more information
Two textural categories: coherent volcanic becomes available from mapping or thin-section studies,
and volcaniclastic and understanding increases, that initial decision should
be reviewed and evaluated.
The diverse genetic processes involved in the formation
of volcanic deposits result in original textures that can Descriptive nomenclature for coherent
be classified into either of two categories: volcaniclastic volcanic and volcaniclastic deposits
or coherent. The term "volcaniclastic" is descriptive and
applies to deposits composed predominantly of volcanic Isolated outcrops and hand specimens of ancient
particles (Fisher, 1961). The particles may be any shape volcanic rocks rarely exhibit clear and unambiguous
and size. No specific clast-forming processes, transport evidence of their origins. Because uncertainty is part of
and depositional processes, or settings are implied. the practical reality of working on volcanic sequences, it
Textures in volcaniclastic deposits encompass enormous is advisable to begin with lithological and lithofacies
variation but, in general, are characterized by the terminology, until there is adequate justification for
presence of separate particles or fragments, of mixtures applying terms that have genetic implications.
of a few or many different particle shapes, sizes and Lithological terminology provides information on
types, or, in many cases, of bedding or other composition, components and grain size. Lithofacies
sedimentary structures indicating particulate transport terminology provides information on facies
and deposition. The four main genetic categories of characteristics evident at outcrop scale in the field, such
volcaniclastic deposits (autoclastic, pyroclastic, as structures, internal organization and geometry.
resedimented and volcanogenic sedimentary) each have Genetic terminology provides information on eruption
sets of distinguishing features and numerous further and emplacement processes for primary volcanic and
subdivisions (Parts 3 and 4). volcaniclastic deposits, and on subsequent redeposition,
erosion, transport and depositional processes for
Coherent volcanic textures form from cooling and resedimented and volcanogenic sedimentary deposits. It
solidification of molten lava or magma. The most also takes into account facies geometry and facies
ubiquitous hallmark is porphyritic texture, especially the associations at a range of scales, from single eruptive or
presence of evenly distributed, euhedral crystals that sedimentation units to entire volcanic centers.
have narrow size ranges. Aphyric, aphanitic and totally
glassy textures are also coherent. Vesicles, flow Existing descriptive lithological classifications actually
foliations, spherulites and lithophysae are common in involve implicit distinctions between deposits with
deposits with coherent textures, though not coherent and volcaniclastic textures and, in this respect,
independently diagnostic, and also occur in are also genetic. Having decided whether the texture is
volcaniclastic deposits. Coherent textures occur coherent or volcaniclastic or, in difficult cases, apparent
principally in lava flows and in intrusions. coherent or apparent volcaniclastic, it is then possible to
build descriptive names using combinations of
There are two additional textural categories that can be lithological and lithofacies terms (Tables 1, 2). The

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