Page 68 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

intrusions. In contrast, sediment that occurs between gravitational collapse. Spaces between pillows are
pillows at the top of pillow lava flows is usually bedded, commonly occupied by granular hyaloclastite generated
because it either settles from the water column or else is by spalling of the brittle glassy pillow rind, and large
washed in by currents, and is no more indurated or amounts of pillow fragment breccia can be formed by
altered than adjacent parts of the enclosing sequence. disintegration of pillow lobes (Carlisle, 1963; Dewit and
Stern, 1978; Staudigel and Schmincke, 1984;
Subaqueous basaltic lava flows Yamagishi, 1987; 1991). Some examples of basaltic
hyaloclastite retrieved from seamounts on the East
The best documented features of subaqueous basaltic Pacific Rise occur in thin, sheet-like, graded beds
lavas are pillows (Carlisle, 1963; Jones, 1969; Moore, (Smith and Batiza, 1989). Talus breccia accumulates
1975; Ballard and Moore, 1977; Dimroth et al., 1978; adjacent to fault scarps in submarine basalt sequences
Ballard et al., 1979; Lonsdale and Batiza, 1980; and within lava tubes that have collapsed. Larger scale
Staudigel and Schmincke, 1984; Yamagishi, 1985; gravitational collapse of the flanks of pillow cones and
1987; 1991; Yamagishi et al., 1989; Tribble, 1991; seamounts also produces fragmental deposits, but their
Walker, 1992) (Fig. 23; 15, 16, 17). Basaltic lavas in internal textures and structures are poorly known.
modern oceans also exhibit other flow morphologies:
thick (at least several metres) massive tabular flows, In both ancient and modern submarine basaltic
thin (<1 m) sheet flows, lobate flows, block lavas, and sequences, some deposits include fluidally shaped clasts
flows with surface features (buckles, folds, coils) similar to subaerial bombs and spatter, and thought to be
similar to those of subaerial pahoehoe (Lonsdale, 1977; generated by lava fountains akin to subaerial Hawaiian-
Basaltic Volcanism Study Project, 1981). Tube systems or Strombolian-style eruptions. Such lava fountains are
and channels identified in lavas of the East Pacific Rise probably restricted to relatively shallow-water settings
(Fornari, 1986) are apparently similar to their subaerial (Staudigel and Schmincke, 1984; Dimroth and
counterparts. Tumuli and other structures due to lava Yamagishi, 1987; Dolozi and Ayres, 1991) or, in deeper
flow inflation, that are common in subaerial pahoehoe water, to brief periods of exceptionally vigorous
lava fields, have also been identified in submarine basalt discharge (Smith and Batiza, 1989). In some ancient
lava fields; for example, at the summit of Axial Volcano sequences of subaqueous basaltic lava flows, interpreted
and in the axial valley of the Juan de Fuca Ridge proximal sections are dominated by massive lava flows,
(Applegate and Embley, 1992). whereas in more distal sections, pillow lava and pillow
fragment breccia predominate (Dimroth et al., 1978;
Subaqueous basaltic lava sequences can include Waters and Wallace, 1992) (Fig. 25).
significant volumes of lava-derived clastic aggregates
that are mainly produced by quench fragmentation and

Fig. 24 Details of the top (A), base (B) and tip (C) of a single, 5 m thick, pillowed, basaltic andesite sill that
intruded wet, unconsolidated sediments, Josoji Formation, Miocene, Japan. Modified from Kano (1991).

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