Page 65 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

breccia; bedded basaltic granule breccia; poorly sorted magma are lensoidal, bulbous or lobate (14.4). The
dacitic breccia; rhyolitic jigsaw-fit breccia. textural differences between globular and blocky
peperite were considered by Busby-Spera and White
The abundance of glass and fractures, and the water- (1987) to be strongly influenced by the host sediment
saturated setting that characterize hyaloclastite character. Globular clasts are thought to form in cases
formation combine to promote its alteration. where a water-vapour film is established and maintained
Modification of clast shapes and packing results from at the interface of the magma with the sediment. The
alteration concentrated along fractures and has the effect vapour film insulates the magma from direct contact
of converting the original jigsaw-fit texture of in situ with the wet sediment, so both quench fragmentation of
hyaloclastite to an apparent matrix-supported fabric the magma and steam explosions are suppressed
(42.2-4, 43.1-2; Part 5). Variable devitrification and (Kokelaar, 1982). Furthermore, sediment in the vicinity
alteration of glassy clasts can transform a strictly of the vapour film is transported laterally along the
monomict aggregate into an apparent polymict contact zone, ultimately resulting in bulk displacement
aggregate. of the sediment and concomitant penetration by magma.
Globular peperite is more likely to develop if the host
Peperite (14) sediments are fine grained, well sorted and loosely
packed, because these are more permissive to fluid flow
Peperite is a rock generated by mixing of coherent lava (either steam or hot pore water) and more easily
or magma with unconsolidated wet sediment (Fisher, fluidized.
1960; Williams and McBirney, 1979), and characterized
by a clastic texture in which either component may form Poorly sorted, coarse-grained sediments, however, are
the matrix (14). Peperite occurs at the contacts between associated with blocky peperite. In these, the greater
intrusions and wet sediments (Hanson and Schweickert, permeability of the host sediment interferes with
1982; Hanson and Wilson, 1993), and along basal development of a vapour film and, in any case, only a
contacts of lava flows that override or burrow into small part of the sediment size range is amenable to
unconsolidated sediments (Schmincke, 1967; Bull and fluidization. In the absence of an insulating vapour film,
Cas, 1989). Contacts are commonly complex in detail, magma-sediment interaction is dominated by quenching
involving intricate interpenetration between the and phreatomagmatic fragmentation of the magma,
intrusion or lava flow and the sediment, and mixed accompanied by the steam-driven mixing and dispersal
contacts can occur together with sharp, planar, unmixed of clasts. Peperite in which magma fragmentation is
contacts. Magmas involved in peperite formation range largely the result of quenching is a variety of
from basaltic to rhyolitic in composition, and from hyaloclastite (intrusive hyaloclastite or peperitic
aphanitic to strongly porphyritic textural types. hyaloclastite). Thus, any one magma or lava body can
Sediments involved in peperite are also texturally be surrounded by different types of peperite that reflect
diverse, ranging widely in grain size and in changes in interaction processes controlled by natural
composition. In some cases, the host sediments are variations in the host sediment properties (especially
volcaniclastic deposits genetically related to the grain size, porosity and permeability).
magmatism responsible for the intrusions.
The mechanisms of peperite formation are also
The presence of pore water and the unconsolidated controlled by the external confining pressure. For sills,
nature of the host sediment have important effects on the confining pressure is exerted by the overlying
processes occurring at magma-wet sediment contacts. sediments and, in cases where the sediments are
Expansion of intensely heated pore fluid can initiate subaqueous, by the water column above as well. The
stationary fluidization of adjacent sediment, resulting in confining pressure along bases of lava flows depends on
entrainment of sediment particles away from the the thickness of the flow and, for subaqueous flows, the
contact. If pore fluid is flashed to steam, it can expand depth of the water column. If the confining pressure
explosively. Both processes profoundly disrupt the exceeds the critical pressure of water (about 312 bars
coherence of the sediments adjacent to the contact and for sea water, 221 bars for pure water — Kokelaar,
promote rapid, unconfined, and irregular penetration by 1982), the degree of expansion of heated pore fluid is
magma (Kokelaar, 1982; 1986). Bedding in the impeded, steam explosions are suppressed and
sediments is commonly destroyed or else broken up and fluidization may be inhibited. On the other hand, for
contorted. Parts of the magma may founder into the high-level sills emplaced into relatively shallow-water
sediment and be partially detached or completely sediments, the low confining pressure can allow
separated. The magma can also be disrupted by a explosive vaporization of pore fluid sufficiently
combination of quench fragmentation on contact with disruptive to cause breaching of the sediment-water
the wet sediments and shattering by steam explosions interface above and "eruption" of the sediment-magma-
(Wohletz, 1986; Kokelaar, 1982). steam mixture (White and Busby-Spera, 1987). Local
"rootless" phreatomagmatic eruptions that break through
Busby-Spera and White (1987) identified two textural lava flows are also generated by explosive vaporization
types of peperite: in blacky peperite, the clasts derived of trapped pore fluid.
from the magma have sharply angular, blocky shapes
and commonly exhibit jigsaw-fit texture (14.1), whereas Peperite is an important component of mixed
in globular (fluidal) peperite, clasts derived from the sedimentary-volcanic sequences, especially those in

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