Fe–Mn crust could subsequently cover the cemented shards. At the same time, the
coarser fractions (found inside the extruded lava) are made up of fragmented rocks
stripped from the magma chamber’s walls or conduits. These larger pieces will
solidify more slowly than the finer shards. Because of size differences between the
small millimeter-size of the shards and centimeter size of the larger fragments, they
are sorted in the water column during eruption and will settle and solidify at different
rates to form hyaloclastites (fine-grained glass shards and rocks) and pyroclasts
(coarser debris). The pyroclasts, which have most of their vesicles preserved,
probably got their vesicle-forming gas bubbles from a deeper, higher-pressure zone
in the upwelling magma (Pineau et al. 1998). The highly vesicular pyroclasts found
in lava are easily fragmented during volcanic explosion and extrusion.
The observed lithological sequence shows pyroclasts lying under hyaloclasts
(on the Pitcairn hotspot volcanoes, at the MAR at 34°54
0 N, and in other locations)
due to the different rate of sedimentation in the water column related to the size,
morphology (shape) and density of the particles projected into seawater. Such
types of explosive events are directly linked to the magmatic volatile content and
are similar to the ‘‘lava fountaining phenomenon’’ observed in subaerial Hawaiian
volcanoes during a high rate of magmatic upwelling in a narrow conduit where
exsolved volatiles are concentrated in large bubbles (Jaupart 1996).
Deep-sea explosive events can also take place in a conduit within a fractured
and collapsed volcanic cone filled with fragmented debris. Seawater circulating in
such an environment could be trapped after hydrothermal precipitates and/or lava
sealed the voids of the fractured edifice. The arrival of a new magma batch within
the edifice will enhance the formation of a vapor phase caused by heating the
trapped seawater leading to a subsequent increase in pressure, which could trigger
an explosive event. This can explain the presence of mixed hydrothermal precipitates and explosive volcanic rock debris, which have been found on the sea
floor after volcanic activity.
Sub-Aerial Versus Submarine Explosion
Sub-aerial (above the surface of seawater) eruption gives rise to explosive events
that are more dramatic than volcanism taking place in an underwater environment
because they are frequently devastating for nature and for human beings. Many
sub-aerial volcanoes have their roots below the ocean floor while others, created
on continental crust, have nevertheless undergone similar processes to what takes
place inside submarine volcanoes. Numerous examples of the ‘‘fire-fountaining’’
eruptions associated with Hawaiian volcanoes have occurred during sub-aerial
eruptions (Macdonald 1982) as well as in undersea environments.
The observed ‘‘fire-fountaining’’ is due to explosive eruptions ejecting volcanic
ash (small size debris) and rock fragments that vigorously flow down slope in subaerial terrain as well as in submarine environments. This style of eruption is also
called ‘‘strombolian’’ (the name is given after the Stromboli Volcano on Lipari
Hyaloclasts and Pyroclasts: Their Origin
129
coarser fractions (found inside the extruded lava) are made up of fragmented rocks
stripped from the magma chamber’s walls or conduits. These larger pieces will
solidify more slowly than the finer shards. Because of size differences between the
small millimeter-size of the shards and centimeter size of the larger fragments, they
are sorted in the water column during eruption and will settle and solidify at different
rates to form hyaloclastites (fine-grained glass shards and rocks) and pyroclasts
(coarser debris). The pyroclasts, which have most of their vesicles preserved,
probably got their vesicle-forming gas bubbles from a deeper, higher-pressure zone
in the upwelling magma (Pineau et al. 1998). The highly vesicular pyroclasts found
in lava are easily fragmented during volcanic explosion and extrusion.
The observed lithological sequence shows pyroclasts lying under hyaloclasts
(on the Pitcairn hotspot volcanoes, at the MAR at 34°54
0 N, and in other locations)
due to the different rate of sedimentation in the water column related to the size,
morphology (shape) and density of the particles projected into seawater. Such
types of explosive events are directly linked to the magmatic volatile content and
are similar to the ‘‘lava fountaining phenomenon’’ observed in subaerial Hawaiian
volcanoes during a high rate of magmatic upwelling in a narrow conduit where
exsolved volatiles are concentrated in large bubbles (Jaupart 1996).
Deep-sea explosive events can also take place in a conduit within a fractured
and collapsed volcanic cone filled with fragmented debris. Seawater circulating in
such an environment could be trapped after hydrothermal precipitates and/or lava
sealed the voids of the fractured edifice. The arrival of a new magma batch within
the edifice will enhance the formation of a vapor phase caused by heating the
trapped seawater leading to a subsequent increase in pressure, which could trigger
an explosive event. This can explain the presence of mixed hydrothermal precipitates and explosive volcanic rock debris, which have been found on the sea
floor after volcanic activity.
Sub-Aerial Versus Submarine Explosion
Sub-aerial (above the surface of seawater) eruption gives rise to explosive events
that are more dramatic than volcanism taking place in an underwater environment
because they are frequently devastating for nature and for human beings. Many
sub-aerial volcanoes have their roots below the ocean floor while others, created
on continental crust, have nevertheless undergone similar processes to what takes
place inside submarine volcanoes. Numerous examples of the ‘‘fire-fountaining’’
eruptions associated with Hawaiian volcanoes have occurred during sub-aerial
eruptions (Macdonald 1982) as well as in undersea environments.
The observed ‘‘fire-fountaining’’ is due to explosive eruptions ejecting volcanic
ash (small size debris) and rock fragments that vigorously flow down slope in subaerial terrain as well as in submarine environments. This style of eruption is also
called ‘‘strombolian’’ (the name is given after the Stromboli Volcano on Lipari
Hyaloclasts and Pyroclasts: Their Origin
129