Page 106 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

eruptions (e.g. lava flows, pyroclastic flow deposits). commonly hydrothermally altered and may include
Phreatomagmatic eruptions are common where ground hydrothermal mud or sinter fragments (Hedenquist
water has access to vents, such as in marshy areas or on and Henley, 1985).
shorelines, and where vents are submerged in shallow
water. Typical activity ranges from relatively dry, Resedimented syn-eruptive volcaniclastic deposits
rapidly repeated explosions, generating ash-laden
eruption columns and laterally directed base surges In studies of volcanic sequences, especially ancient
(Moore, 1967; Waters and Fisher, 1971; Kokelaar, 1986), sequences, the importance of distinguishing
to relatively wet jetting and continuous uprush of between primary volcaniclastic (pyroclastic,
water-pyroclast slurries (Kokelaar, 1983; Moore, autoclastic) deposits and reworked volcanogenic
1985). Phreatoplinian eruptions involve large volumes of sedimentary deposits is widely recognised. In addition, it
vesiculating rhyolitic magma affected by interaction with is just as important to distinguish resedimented,
water (Self and Sparks, 1978; Self, 1983; Wilson and essentially syn-eruptive volcaniclastic deposits from
Walker, 1985, Orsi et al., 1992), in circumstances where reworked, post-eruptive volcaniclastic deposits that
vents are located in caldera lakes or in calderas have no genetic connection with active volcanism.
inundated by the sea. Secondary or rootless Resedimented syn-eruptive pyroclastic or autoclastic
phreatomagmatic explosions may also occur at sites other deposits have immense significance in establishing the
than the eruptive vent; for example, where lava flows or composition, setting, eruptive style and proximity of
hot pyroclastic deposits are emplaced on snow or wet source volcanic centres, and may also be sensitive
ground. Littoral cones composed of indicators of the depositional environment. The
phreatomagmatic pyroclastic deposits can be resedimented syn-eruptive category (3) is an attempt to
constructed at the shoreline where lava meets the sea take account of this important class of volcaniclastic
(Fisher, 1968). deposits.

Phreatomagmatic eruptions produce a high proportion Some criteria that distinguish resedimented, syn-
of ash pyroclasts, and eruption columns and plumes are eruptive pyroclast-rich deposits are listed below:
steam-rich. Suspended fine, moist ash commonly 1. Sedimentation units are composed of texturally
aggregates into small clumps or spherical accretionary unmodified pyroclasts.
lapilli, or forms a coating around larger pyroclasts. 2. Each sedimentation unit, and successions of such
Steam condensation produces ash-laden rain that units, are compositionally uniform in containing a
results in deposition of fine ash at the same time as narrow range of pyroclast types and compositions.
coarser pyroclasts. Pyroclasts in phreatomagmatic 3. Only non-welded clasts can be resedimented, so
deposits can range widely in vesicularity (Houghton resedimented deposits are invariably non-welded.
and Wilson, 1989). If the magma is largely degassed at 4. Bedforms indicate rapid (commonly mass-flow)
the time of interaction with water or if vesiculation is deposition. However, sedimentation units are
prematurely arrested by quenching, poorly vesicular, different in internal organisation from primary
angular, juvenile pyroclasts dominate the ejected pyroclastic depositional units.
mixture. If vesiculation is advanced at the time of 5. Single mass-flow depositional units are typically
interaction, pumiceous or scoriaceous pyroclasts and very thick (several tens to more than 100 m), reflecting
bubble-wall shards can be abundant. Significant rapid influx of large volumes of pyroclastic debris
amounts of accessory lithic pyroclasts are also generated and the exclusion of non-volcanic "ambient"
in each case. sedimentation, as occurs only during and
immediately after a major eruption.
6. The resedimented pyroclastic deposits may be
Phreatic or steam eruptions
associated with primary pyroclastic deposits of the
Steam-driven explosions that do not directly involve same composition.
fresh magma are called phreatic. Magma may be the
heat source for steam generation but does not Many of the above criteria also apply in the case of
participate further in eruptive processes. Phreatic resedimented hyaloclastite but the volumes, thicknesses
explosions are common in active geothermal systems of sedimentation units and transport distances are much
(e.g. Muffler et al., 1971; Nairn and Wiradiradja, 1980; smaller, and the associated primary volcanic facies are
Hedenquist and Henley, 1985) and in or around active coherent lava flows or lava domes.
vents between eruptions (e.g. Jaggar and Finch, 1924;
McPhie et al., 1990). The explosions occur when There are several situations in which pyroclasts are
subsurface, superheated water flashes to steam as a delivered more or less directly to sedimentary transport
result of rapid reduction in confining pressure. and depositional systems, bypassing initial deposition
as primary pyroclastic deposits altogether, or else being
Phreatic eruptions typically emit large amounts of briefly stored prior to redeposition. For example:
steam that contain only small volumes of solid ejecta. The (1) fallout of pyroclasts onto lakes, rivers, shorelines and
explosions are comparatively weak and most pyroclasts the sea;
are deposited close to the eruptive site by fallout. The (2) entry of pyroclastic flows into rivers and
deposits are dominated by non-juvenile lithic pyroclasts. transformation to subacrial water-supported
In cases involving geothermal systems, pyroclasts are volcaniclastic mass flows and hyperconcentrated flows

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