Page 104 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Part 4. Pyroclastic, resedimented volcaniclastic

and volcanogenic sedimentary deposits

Active volcanic terranes, whether subaerial or fragmentation of lava or magma (autobrecciation and
subaqueous, combine primary volcanic and sedimentary quench fragmentation (Part 3);
processes, both of which have the potential to generate Pyroclastic — primary volcaniclastic deposits made of
diverse volcaniclastic deposits. Primary volcaniclastic particles (pyroclasts) generated by explosive eruptions
deposits involve volcanic processes of clast formation, and deposited by primary volcanic processes (fallout,
transport and deposition. Clasts that are generated and flow, surge); Resedimented syn-eruptive volcaniclastic ─
initially deposited by primary volcanic processes may be volcaniclastic deposits from rapid resedimentation of
rapidly resedimented, or subsequently eroded and texturally unmodified, freshly erupted pyroclastic or
reworked by non-volcanic surface processes, and autoclastic particles;
incorporated in a variety of resedimented and Volcanogenic sedimentary — volcaniclastic
volcanogenic sedimentary deposits. aggregates containing particles derived by erosion of pre-
existing volcanic deposits and that were subject to
Studies of volcaniclastic deposits, therefore, require significant reworking prior to final deposition, and/or
information on: that were redeposited long after eruption.
(1) the clast-forming processes;
(2) the clast transport and depositional processes; This part concentrates on pyroclastic deposits,
(3) post-emplacement syn-volcanic processes (welding, resedimented syn-eruptive volcaniclastic deposits
high-temperature devitrification, hydration, and volcanogenic sedimentary deposits. The principal
vapour-phase crystallisation, diagenetic and components in these volcaniclastic deposits are juvenile
hydrothermal alteration). magmatic clasts that range in vesicularity from highly
vesicular pumice and scoria to variably vesicular lava
Clast type, shape, composition and texture help bombs and blocks, glass shards, free crystals and crystal
constrain the clast-forming processes, whereas fragments, lithic fragments and accretionary lapilli (Part
lithofacies character (bedforms, geometry, structures, 2). Non-volcanic components, such as biogenic and
internal organisation, nature of contacts, relationships to epiclastic sedimentary particles, can be significant,
enclosing units) helps constrain the clast transport and especially in volcanogenic sedimentary deposits.
depositional processes. Textural evidence of hot
emplacement (welding, thermal oxidation, columnar Explosive eruptions and pyroclastic deposits
jointing, baked contacts, vapour-phase crystallisation,
degassing structures) is especially important in Explosive eruptions involve the rapid release and
recognition of some primary volcaniclastic deposits. decompression of gas which results simultaneously in
fragmentation and ejection of magma and/or wall
Correct information on these facets is extremely rocks. The three main sorts of explosive eruptions -
powerful, and can be used to: explosive magmatic, phreatomagmatic, and phreatic -
(1) distinguish among the different genetic differ in the source of the gas and the extent of direct
categories of volcaniclastic deposits; involvement of magma. Phreatomagmatic and phreatic
(2) constrain the depositional setting (subaerial versus eruptions are hydrovolcanic phenomena, that is, they
shallow subaqueous versus deep subaqueous); both involve steam generated from external water. All
(3) assess the proximity of the deposits relative to the three of these explosive eruption styles are capable of
source of volcanic components; generating abundant pyroclasts ranging from fine ash (<
establish the character, composition and setting of the 1 /16 mm) to blocks a few metres across. The pyroclasts are
volcanic source. dispersed by:
(1) injection into the atmosphere followed
These should be the aims of textural interpretation of eventually by fallout from suspension;
volcaniclastic deposits, even though in reality, especially (2) ground hugging, relatively high particle
in the exploration context, limited time, poor exposure concentration pyroclastic flows;
and poor preservation of original textures interfere and (3) relatively low particle concentration pyroclastic
greatly restrict the yield. surges.

Genetic classification of volcaniclastic Transportation is regarded as a primary volcanic
deposits process because it follows directly from eruption,
and involves pyroclasts suspended or entrained in
Volcaniclastic deposits are classified on the basis of volcanic gas, even though gravity and mixing with
clast-forming processes and transport and depositional the atmosphere play fundamental roles.
processes. The principal genetic categories are (Fig. 1):
Autoclastic ─ primary volcaniclastic deposits made of
particles (autoclasts) generated by in situ, non-explosive

89