Page 71 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Fig. 27 Cross-section through remnants of a rhyolite lava lobe grading into in situ hyaloclastite (A), and a well-
preserved, texturally zoned, lava lobe (B) in sharp contact with the enveloping hyaloclastite. Modified from Kano et
al. (1991).

Fig. 28—Schematic volcanic fades associated with subaqueous emplacement of rhyolite lava; Miocene, Ushikiri
Formation, Japan. Modified from Kano et al. (1991).

In ancient volcanic sequences, the best criterion for partly extrusive cryptodomes are largely subsurface, the
establishing a subaqueous setting for silicic lava flows best descriptions come from dissected ancient examples
and domes is the close spatial association of coherent (Allen, 1992). They exhibit the characteristics of both
lava, in situ hyaloclastite and resedimented high-level intrusions and extrusive domes (Fig. 29). The
hyaloclastite. Another constraint is the presence of coherent core of a cryptodome passes gradationally
lobes, pods and pillow-like masses of coherent lava. In outward into in situ hyaloclastite that is overlain locally
most cases, the character of associated sedimentary by stratified, mass-flow resedimented hyaloclastite
facies is of prime importance in determining the setting. generated during emergence of the dome. Resedimented
hyaloclastite transported farthest from the emergent
Subaqueous partly extrusive cryptodomes portions of the dome is intercalated with other
contemporaneous sedimentary or volcanic deposits. In
Cryptodomes are high-level intrusions that cause up- situ hyaloclastite and peperite commonly develop along
doming of the overlying sediments or rocks (Minakami the intrusive margins of the dome and separate the
et al., 1951). Partly extrusive cryptodomes locally break coherent interior from the intruded host sequence. Dome
through the cover and emerge at the surface. emplacement can be responsible for local deformation
Cryptodomes mostly involve intermediate or silicic of any bedding present in the host sequence. In cases
composition magmas, and their emplacement appears to involving poorly consolidated host sediments,
be especially favored in subaqueous, mixed heterogeneous compaction around the rigid dome may
sedimentary-volcanic settings. In these settings, the also cause irregularities in bedding nearby (Baldwin,
water column contributes to the confining pressure, 1971).
allowing the magma to rise to very high levels, where it
is likely to encounter a substantial interval of "weak",
poorly consolidated, less dense sediments. Because

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