4. Fragments with prominent tubular vesicles loosely
resembling tube pumice (Figure 3d).
5. Various lithics ranging from basaltic rock clasts to mineral fragments from a hydrothermal stockwerk may
occur.
Deep-sea pyroclastic deposits are in tendency finer
grained than subaerial Strombolian deposits. Although
larger fragments and spatter are known, grain sizes typically range between <63 mm and 2–4 mm. This is diagnostic of increased fragmentation efficiency.
General eruption mechanism
Although much if not most about the nature of explosive
submarine eruptions is still puzzling, a general and simplistic view may be described by the following:
Hydromagmatic activity is largely ruled out as the main
force to trigger fragmentation and formation of pyroclastic
deposits, in particular as the violence of water-magma
interaction diminishes with increasing water pressure
(cf. Zimanowski and Büttner, 2003). Instead the
volcaniclastic record is often considered to represent
explosive volcanic activity and magma fragmentation
driven primarily by exsolved magmatic gas. Further fragmentation due to rapid quenching upon contact with seawater is thought to reduce the primary clast size
(Potuzak et al., 2008; Helo et al., 2013).
At MOR, pyroclastic eruptions are widely envisioned
to occur as Strombolian bubble bursts accompanying effusive activity. In exceptional cases, more energetic eruptions and fragmentation levels deeper within the conduit
seem possible. The extent of pyroclastic discharge has
been proposed to depend broadly on the rise rate of the
magma within the conduit. Sheet lavas forming under high
extrusion rates have been associated with intensive formation of pyroclastic fragments, whereas the opposite
appears to hold true for the slow extrusion of pillow lavas.
Direct observations at NW Rota-1 and W-Mata (Figure 1)
have confirmed both the transient eruption character, i.e.,
discrete eruption pulses, and the coeval occurrence of
explosive and effusive activity. The eruptive events at
NW Rota-1 were Strombolian in style and characterized
by the repeated ascent of gas-rich pockets. This gave rise
to a cycle of individual eruptive bursts sustained for several minutes with short response intervals (Chadwick
et al., 2008). Explosive activity on W-Mata ranges from
lava fountaining to bubble bursting and vigorous active
degassing. Its eruptions produced a variety of Limu
o Pele and Pele’s hair (Clague et al., 2009b).
After fragmentation, pyroclasts may be entrained into
an eruption plume that mainly rises by thermal buoyancy
(Deardorff et al., 2011). Constraints on how effective the
entrainment of clasts into a rising plume aids to their dispersal are still poor. Field observations and modeling suggest that small Limu o Pele might advect up to few
hundreds of meters within buoyant plumes and can be dispersed up to a few kilometers from the vent (e.g., Barreyre
et al., 2011).
Role of magmatic volatiles
The principle role of volatiles is to form gas bubbles that
drive the eruption and ultimately cause fragmentation.
Formation of bubbles typically happens when magma
rises from deep, high-pressure levels to shallow,
low-pressure levels. Since volatile solubility decreases as
pressure is reduced, the concentration of one or more volatile compounds will at some point exceed the solubility
and starts to exsolve.
At MOR, CO 2 is the crucial volatile to drive explosive
eruptions (Clague et al., 2003b; Helo et al., 2011;
Pontbriand et al., 2012). Due to its low solubility (e.g.,
Dixon et al., 1995; Newman and Lowenstern, 2002;
Papale et al., 2006; Witham et al., 2012), it starts exsolving
from the magma at early stages during the ascent from the
mantle into a shallow magmatic reservoir. This situation is
broadly analogous to subaerial volcanoes. When the
magma eventually starts to erupt from the shallow reservoir, the two cases differ. Under subaerial pressure condition, vigorous exsolution of various volatiles and fast
bubble growth is characteristic for this stage, while high
ambient water pressure at the seafloor hampers
syn-eruptive CO 2 and H 2 O exsolution (Shaw et al.,
2010; Helo et al., 2011). Eruptions are then merely driven
by gas bubbles that formed in the magmatic reservoir prior
to the onset of eruption. This is the key to resolve the
apparent paradox of dense pyroclasts forming during
explosive activity: at the onset of eruption, the free gas
phase already present becomes concentrated in larger
potentially coalesced bubbles rising in the conduit, with
the interstitial melt remaining fairly dense as only marginal amounts of new bubbles grow at this point. In the
pyroclastic record, the interstitial dense melt is then
represented by angular clasts, while Limu o Pele fragments represent remnants of the larger pockets of bubbles.
The bulk vesicularity (effectively the large bubbles)
becomes thereby completely obscured in the deposits.
Magmatic H 2 O is considerably more soluble in basaltic
melts (cf. Moore, 2008) than CO 2 . Exsolution of H 2 O
therefore requires very low pressure and/or a high amount
of H 2 O dissolved in the magma. Consequently magmatic
H 2 O qualifies as a driving force for submarine explosive
eruptions merely at volcanic settings with sufficiently high
concentrations of dissolved H 2 O (e.g., Schipper et al.,
2010).
The initial volatile budget of many deep-sea volcanic
systems, in particular MOR, may be too low or just adequate to drive efficiently explosive activity at the given
water depth (Head and Wilson, 2003; Shaw et al., 2010).
This obstacle is solved effectively when magmatic foam
collapse models are invoked (e.g., Vergniolle and Jaupart,
1990). Magmatic gas that exsolves from magma arriving
in the reservoir progressively accumulates at the top section of the magma body. This way, a high degree of vesicularity can build up, culminate in a sudden collapse of the
foam, and trigger explosive activity. Hence, the capacity to
erupt explosively becomes less dependent on the intrinsic
EXPLOSIVE VOLCANISM IN THE DEEP SEA
245
resembling tube pumice (Figure 3d).
5. Various lithics ranging from basaltic rock clasts to mineral fragments from a hydrothermal stockwerk may
occur.
Deep-sea pyroclastic deposits are in tendency finer
grained than subaerial Strombolian deposits. Although
larger fragments and spatter are known, grain sizes typically range between <63 mm and 2–4 mm. This is diagnostic of increased fragmentation efficiency.
General eruption mechanism
Although much if not most about the nature of explosive
submarine eruptions is still puzzling, a general and simplistic view may be described by the following:
Hydromagmatic activity is largely ruled out as the main
force to trigger fragmentation and formation of pyroclastic
deposits, in particular as the violence of water-magma
interaction diminishes with increasing water pressure
(cf. Zimanowski and Büttner, 2003). Instead the
volcaniclastic record is often considered to represent
explosive volcanic activity and magma fragmentation
driven primarily by exsolved magmatic gas. Further fragmentation due to rapid quenching upon contact with seawater is thought to reduce the primary clast size
(Potuzak et al., 2008; Helo et al., 2013).
At MOR, pyroclastic eruptions are widely envisioned
to occur as Strombolian bubble bursts accompanying effusive activity. In exceptional cases, more energetic eruptions and fragmentation levels deeper within the conduit
seem possible. The extent of pyroclastic discharge has
been proposed to depend broadly on the rise rate of the
magma within the conduit. Sheet lavas forming under high
extrusion rates have been associated with intensive formation of pyroclastic fragments, whereas the opposite
appears to hold true for the slow extrusion of pillow lavas.
Direct observations at NW Rota-1 and W-Mata (Figure 1)
have confirmed both the transient eruption character, i.e.,
discrete eruption pulses, and the coeval occurrence of
explosive and effusive activity. The eruptive events at
NW Rota-1 were Strombolian in style and characterized
by the repeated ascent of gas-rich pockets. This gave rise
to a cycle of individual eruptive bursts sustained for several minutes with short response intervals (Chadwick
et al., 2008). Explosive activity on W-Mata ranges from
lava fountaining to bubble bursting and vigorous active
degassing. Its eruptions produced a variety of Limu
o Pele and Pele’s hair (Clague et al., 2009b).
After fragmentation, pyroclasts may be entrained into
an eruption plume that mainly rises by thermal buoyancy
(Deardorff et al., 2011). Constraints on how effective the
entrainment of clasts into a rising plume aids to their dispersal are still poor. Field observations and modeling suggest that small Limu o Pele might advect up to few
hundreds of meters within buoyant plumes and can be dispersed up to a few kilometers from the vent (e.g., Barreyre
et al., 2011).
Role of magmatic volatiles
The principle role of volatiles is to form gas bubbles that
drive the eruption and ultimately cause fragmentation.
Formation of bubbles typically happens when magma
rises from deep, high-pressure levels to shallow,
low-pressure levels. Since volatile solubility decreases as
pressure is reduced, the concentration of one or more volatile compounds will at some point exceed the solubility
and starts to exsolve.
At MOR, CO 2 is the crucial volatile to drive explosive
eruptions (Clague et al., 2003b; Helo et al., 2011;
Pontbriand et al., 2012). Due to its low solubility (e.g.,
Dixon et al., 1995; Newman and Lowenstern, 2002;
Papale et al., 2006; Witham et al., 2012), it starts exsolving
from the magma at early stages during the ascent from the
mantle into a shallow magmatic reservoir. This situation is
broadly analogous to subaerial volcanoes. When the
magma eventually starts to erupt from the shallow reservoir, the two cases differ. Under subaerial pressure condition, vigorous exsolution of various volatiles and fast
bubble growth is characteristic for this stage, while high
ambient water pressure at the seafloor hampers
syn-eruptive CO 2 and H 2 O exsolution (Shaw et al.,
2010; Helo et al., 2011). Eruptions are then merely driven
by gas bubbles that formed in the magmatic reservoir prior
to the onset of eruption. This is the key to resolve the
apparent paradox of dense pyroclasts forming during
explosive activity: at the onset of eruption, the free gas
phase already present becomes concentrated in larger
potentially coalesced bubbles rising in the conduit, with
the interstitial melt remaining fairly dense as only marginal amounts of new bubbles grow at this point. In the
pyroclastic record, the interstitial dense melt is then
represented by angular clasts, while Limu o Pele fragments represent remnants of the larger pockets of bubbles.
The bulk vesicularity (effectively the large bubbles)
becomes thereby completely obscured in the deposits.
Magmatic H 2 O is considerably more soluble in basaltic
melts (cf. Moore, 2008) than CO 2 . Exsolution of H 2 O
therefore requires very low pressure and/or a high amount
of H 2 O dissolved in the magma. Consequently magmatic
H 2 O qualifies as a driving force for submarine explosive
eruptions merely at volcanic settings with sufficiently high
concentrations of dissolved H 2 O (e.g., Schipper et al.,
2010).
The initial volatile budget of many deep-sea volcanic
systems, in particular MOR, may be too low or just adequate to drive efficiently explosive activity at the given
water depth (Head and Wilson, 2003; Shaw et al., 2010).
This obstacle is solved effectively when magmatic foam
collapse models are invoked (e.g., Vergniolle and Jaupart,
1990). Magmatic gas that exsolves from magma arriving
in the reservoir progressively accumulates at the top section of the magma body. This way, a high degree of vesicularity can build up, culminate in a sudden collapse of the
foam, and trigger explosive activity. Hence, the capacity to
erupt explosively becomes less dependent on the intrinsic
EXPLOSIVE VOLCANISM IN THE DEEP SEA
245