fractionation, pyroclasts and silica-enriched lava should erupt prior to basalt. As
differentiation takes place, the volatiles forming the lighter components of the
molten material will tend to aggregate and concentrate in the higher levels of the
reservoir or magma column. When the pressure exercised by the volatiles exceeds
the lithostatic pressure, violent eruptions giving rise to pyroclasts will occur. Thus,
during subaerial volcanism, it is often reported that an eruptive cycle starts with
the most evolved and fractionated material and terminates with a quieter event
extruding basaltic lava. In the presence of sequential units of composite lavas, the
explosive events giving rise to the pyroclasts and hyaloclasts are believed to
precede the quieter eruptions of basaltic flow. Indeed, in a magma conduit, the less
viscous mafic melts such as basalt ascend more rapidly. If they are mixed with the
more viscous silica-rich melt, this will lower the bulk density of melt and facilitate
its extrusion (Gibson and Walker 1963; Yoder 1973). This is true when both the
silica-rich and the mafic magma coexist together.
Generally, the melting of evolved basalt (i.e. basaltic andesite) inside a magma
chamber produces silica–enriched minerals during the transit of a hotter mafic melt
(see Chap. 5). Hence, during an eruption, it is possible that pyroclasts and hyaloclasts could mark the end of volcanic events (Batiza 1989; Hekinian et al. 2000)
rather than their commencement. This is mainly observed for edifices whose
summits are built by only pyroclasts and hyaloclasts (Fig. 9.18a, b). Whether the
volcanism terminates with either explosive pyroclastic material or a quieter
basaltic flow, knowing what is, in fact, the final stage is very useful for defining
volcanic cyclicity.
The changes in lava morphology and rock composition are often difficult to
interpret without a continuous observation conducted on the sea floor. Sequential
differences in morphology could be seen only when deep-towed cameras and/or
submersible explorations have been able to conduct detailed and continuous
geological observations. Unfortunately, there are still very few visual observations
on the deep-sea environment that have been able to map the stratigraphy of
sequential eruptions. Thus, the direct field observations on Bounty and Adams
have enabled us to recognize several volcanic events that are responsible for
building tall ([3000 m) and small (\500 m high) volcanoes. It was inferred that
these edifices were built on a geological time scale of less than one million years,
and more likely in just a few thousand years.
The two volcanic edifices on which we were able to identify the cyclicity of
sequential volcanism are the Bounty and the Adams volcanoes constructed on the
Pitcairn hotspot. As shown from the geological profiles made during the submersible dive observations on these edifices, we can say that the exposed lava units
have different morphologies and composition. The least evolved lava of basaltic
composition occurs essentially as pillows and giant tubes. Some dull-looking,
older flows are partially covered by more recent glassy flows (PN3-03). Occasionally, basaltic flows form small (\20 m high) rootless mounds (e.g. haystacks
and hornitos) supplied by short tubular lava-channels along the slopes of the
edifice, disturbing the sequential variability. This is an indication of a replenished
magma chamber giving rise to a new eruptive event.
Volcanic Cyclicity
339
differentiation takes place, the volatiles forming the lighter components of the
molten material will tend to aggregate and concentrate in the higher levels of the
reservoir or magma column. When the pressure exercised by the volatiles exceeds
the lithostatic pressure, violent eruptions giving rise to pyroclasts will occur. Thus,
during subaerial volcanism, it is often reported that an eruptive cycle starts with
the most evolved and fractionated material and terminates with a quieter event
extruding basaltic lava. In the presence of sequential units of composite lavas, the
explosive events giving rise to the pyroclasts and hyaloclasts are believed to
precede the quieter eruptions of basaltic flow. Indeed, in a magma conduit, the less
viscous mafic melts such as basalt ascend more rapidly. If they are mixed with the
more viscous silica-rich melt, this will lower the bulk density of melt and facilitate
its extrusion (Gibson and Walker 1963; Yoder 1973). This is true when both the
silica-rich and the mafic magma coexist together.
Generally, the melting of evolved basalt (i.e. basaltic andesite) inside a magma
chamber produces silica–enriched minerals during the transit of a hotter mafic melt
(see Chap. 5). Hence, during an eruption, it is possible that pyroclasts and hyaloclasts could mark the end of volcanic events (Batiza 1989; Hekinian et al. 2000)
rather than their commencement. This is mainly observed for edifices whose
summits are built by only pyroclasts and hyaloclasts (Fig. 9.18a, b). Whether the
volcanism terminates with either explosive pyroclastic material or a quieter
basaltic flow, knowing what is, in fact, the final stage is very useful for defining
volcanic cyclicity.
The changes in lava morphology and rock composition are often difficult to
interpret without a continuous observation conducted on the sea floor. Sequential
differences in morphology could be seen only when deep-towed cameras and/or
submersible explorations have been able to conduct detailed and continuous
geological observations. Unfortunately, there are still very few visual observations
on the deep-sea environment that have been able to map the stratigraphy of
sequential eruptions. Thus, the direct field observations on Bounty and Adams
have enabled us to recognize several volcanic events that are responsible for
building tall ([3000 m) and small (\500 m high) volcanoes. It was inferred that
these edifices were built on a geological time scale of less than one million years,
and more likely in just a few thousand years.
The two volcanic edifices on which we were able to identify the cyclicity of
sequential volcanism are the Bounty and the Adams volcanoes constructed on the
Pitcairn hotspot. As shown from the geological profiles made during the submersible dive observations on these edifices, we can say that the exposed lava units
have different morphologies and composition. The least evolved lava of basaltic
composition occurs essentially as pillows and giant tubes. Some dull-looking,
older flows are partially covered by more recent glassy flows (PN3-03). Occasionally, basaltic flows form small (\20 m high) rootless mounds (e.g. haystacks
and hornitos) supplied by short tubular lava-channels along the slopes of the
edifice, disturbing the sequential variability. This is an indication of a replenished
magma chamber giving rise to a new eruptive event.
Volcanic Cyclicity
339