Page 67 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

been interpreted to indicate waning discharge rates absence of multiple-rind structure, is not, however,
during emplacement (Dimroth et al., 1978; Staudigel indicative of emplacement in relatively deep water.
and Schmincke, 1984). [Note that multiple-crust structure (Yamagishi, 1985) is
distinct from multiple-rind structure and has no
Pillows are diagnostic of the subaqueous emplacement recognized significance with regard to water depth).
of lavas, especially those of basaltic composition. The Finally, although pillowed lava flows are good
emplacement setting, however, is not necessarily the indicators of subaqueous emplacement, not all
same as the eruption setting. Subaerially erupted lava subaqueous lava flows are pillowed.
flows, especially tube-fed, basaltic pahoehoe flows,
commonly reach coastlines several to tens of kilometers Ancient examples of subaqueous lava flows of
from source (Moore et al., 1973; Tribble, 1991). compositions other than basalt (intermediate and silicic)
Subaerial lava flows that enter water can continue to are also organized into pillow lobes and pods (e.g.
advance, building a lava delta, the foresets of which are andesite ─ Cousineau and Dimroth, 1982; Yamagishi,
composed of dispersed, elongate pillow lobes and 1985; 1987; 1991; rhyodacite ─ Bevins and Roach,
hyaloclastite with appreciable primary dips (Jones and 1979; rhyolite ─ Kano et al., 1991; trachyte-
Nelson, 1970). Pillows can be useful in distinguishing trachyandesite ─ Yamagishi and Goto, 1991). Studies of
"relatively deep" from "relatively shallow" water ancient submarine volcanic sequences show that pillow
depths. For example, vesicle size and abundance in lobes also form when magma intrudes or invades water-
pillow lobes may be used to interpret relative water saturated hyaloclastite or sediments (Fig. 24). Intrusive
depth of emplacement of pillowed flows (Moore, 1965; examples range from basaltic to silicic in magma
Jones, 1969; Moore and Schilling, 1973; Yamagishi et composition (e.g. basalt, basaltic andesite ─ Kano,
al., 1989; Dolozi and Ayres, 1991). In pillow lava 1991; Snyder and Fraser, 1963b; Yamagishi, 1987;
sequences comprising more or less consistent andesite, dacite ─ Snyder and Fraser, 1963a; Hanson
compositions, including original volatile contents, larger 1991; silicic ─ Hanson, 1991). Distinguishing pillowed
and more abundant vesicles are expected to occur in intrusions from extrusive pillow lava flows rests on
pillows emplaced at shallower water depths. Kawachi careful examination of the top contacts and on the
and Pringle (1988) considered the presence of multiple- character of the inter-pillow sediment. Peperite
rind structure (16.1) at the margins of pillows to be a sometimes occurs along the upper contacts of pillowed
feature limited to flows emplaced in shallow water (less intrusions but does not occur above pillowed flows. The
than 1-2 km, depending on the initial dissolved H 2O and sediment in contact with pillowed intrusions is usually
CO 2). Multiple-rind structure is thought to form by the locally indurated or altered, and any pre-existing
implosion and rupture of the pillow skin, processes bedding is disturbed or else destroyed. Irregular
which are probably limited to low-pressure enclaves of host sediment can be completely detached
environments (that is, relatively shallow water). The and incorporated deep into the interior parts of pillowed

Fig. 23 Characteristic surface structures of pillow lobes and model for pillow growth. Multiple crusts form at the
end of the pillow toe by repeated surges of liquid lava. Two pillow lobes diverge from the original single pillow lobe
by formation of a symmetrical longitudinal spreading crack, and each pillow lobe advances from transverse
spreading cracks. Open and closed arrows indicate spreading and flow direction respectively. Modified from
Yamagishi (1985).

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