Page 22 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

The top or base of a section is the most obvious but not very densely welded pyroclastic deposits (2.1-5, 20,
always the easiest place to begin logging. A far better 17.2);
approach is to review quickly the entire section, in order Pumice and scoria ─ produced in abundance by both
to find the least complicated parts, and to start there. effusive and explosive eruptions (6.1-7, 20);
Having logged these parts, it is usually possible to
progress to adjacent, more difficult intervals until the Glass shards ─ found in hyaloclastite, primary
syn-eruptive
resedimented
deposits,
pyroclastic
log is complete. Complex contacts or relationships can
be deciphered by constructing supplementary logs at volcaniclastic and volcanogenic sedimentary deposits
(7.1-3,12.4,23,30. IB);
larger scales. The accompanying description consists of
lithological and lithofacies terms that summarize Fiamme—found in diagenetically compacted, non-
essential features and complement the graphic log with welded, primary and secondary pumiceous deposits, and
additional information; for example, percentage in welded primary pyroclastic deposits (both fallout and
abundances of important components such as flow deposits) (24.1-3, 26.5, 45.7, 46.2);
phenocrysts, or types of lithic clasts present. In general, Pseudo-fiamme ─ found in a wide variety of deformed
each depositional or emplacement unit is logged then and altered volcanic deposits (44.5).
described more or less in turn, so it is imperative to
devote time to locating contacts. For altered rocks, the A summary of the essential features of the
description also includes a summary of alteration
mineralogy and textures. Graphic logging is very sea-floor massive sulfide environment
versatile and can be adapted to cater for any special As presently understood, volcanic-hosted massive
features and relationships. However, it is important to sulfide mineralization forms in "relatively deep" sea
be consistent. water and is at least spatially associated with volcanic
sequences. Although absolute water depths are not
Important textures and structures easily constrained, appropriate environments are well
below storm wave base and are here simply referred to
A small number of textures and structures are as "deep". The sedimentary and volcanic processes that
particularly important in deciphering genetic processes operate in deep marine settings differ from those that
and/or setting of volcanic deposits. Early identification operate in shallow marine and subaerial settings.
of these features thus greatly accelerates progress
toward volcanological interpretations; for example:
Porphyritic texture ─ found in lavas, syn-volcanic
intrusions, lava-like ignimbrites and clasts derived from
these deposit types (1.1, 1.2);
Spherulites, lithophysae and micropoikilitic texture ─
indicating high-temperature devitrification of coherent
volcanic glass (3, 4, 25.1-2);
Perlite ─ indicating hydration (or quenching?) of
coherent volcanic glass (5, 27.2, 42.6-7);
Accretionary lapilli ─ formed by subaerial explosive
eruptions but may be redeposited and reworked (7.6-8,
22.6, 38.1, 39.6, 40.1);
Flow foliations ─ found in lavas, syn-volcanic
intrusions, and rheomorphic and lava-like ignimbrites
(8, 26.3-5);
Columnar joints ─ found in lavas, syn-volcanic
intrusions and primary volcaniclastic deposits (mainly
pyroclastic) that are emplaced hot (9.1-3, 26.1-2);
Pillows ─ found in lavas emplaced subaqueously and
intrusions emplaced into wet sediment (15, 16, 17);
Graded bedding ─ indicating sedimentation from mass
flows or suspension (18.1, 30, 31.1, 32.1, 34.2-3);
Planar thin bedding ─ indicating suspension or traction
current deposition (38.1, 38.8, 39.2, 39.5, 40.5-7);
Cross stratification ─ indicating traction current
deposition (38, 40.8, 41.3).

Many components and textures are not strongly
diagnostic of particular origins. The most frequently
misinterpreted are: Fig. 7 Example of a drillcore graphic log drawn directly
in a field notebook. J. McPhie (unpub. data).
Vesicles —found in lavas, intrusions and non-welded or

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