during its ascent towards the sea floor surface (Fig. 5.13). Prior to eruption, the
basaltic source material for these explosive, volcanic rocks from the MAR, had
crystallized less than 15 % solids within the parental liquid.
Hence, pyroclasts and hyaloclastites resulting from submarine explosion could
occur on both fast and slow spreading ridge systems and seamounts. Finding these
pyroclasts and hyaloclastites at some distance away from emerged landmasses
indicates that underwater explosion can occur at less than 3,000 m depth, and this
type of explosive volcanism occurs more frequently than had been thought.
Growth of a Volcano
The construction of an underwater volcano is related to the eruption of molten lava
as well as to the propagation of sub-crustal melt forming a magma reservoir and its
conduits such as dykes and sills. From the reservoir, magma will flow through both
vertical and lateral fissures to form the skeleton of a volcanic edifice (Figs. 5.15
and 5.16). If the magma solidifies within the edifice during its intrusion, it will
form a series of vertical dykes representing the conduits of ascending melt. These
magmatic conduits supply the lava flow that will be extruded on the volcano’s
surface. Each eruption is the result of magmatic injection taking place at different
levels within the volcanic edifice. As a volcano grows, its mass will increase,
therefore it will rise taller and as a result, less magma will erupt from the summit
while more lava will erupt from lateral vents causing the creation of adventive or
parasite cones. In order for the extrusive flow to continue piling up, it also has to
overcome the surrounding lithostatic pressure.
During crystal-liquid fractionation, a melt in the magma reservoir will evolve
and become more enriched in volatiles. The volatiles and the lighter elements will
tend to rise towards the roof and/or the cooler borders of the magma chamber,
while the heavier components will sink. Also, the replenishment of a magma
reservoir is an important consideration after a sequence of eruptive events since if
there is an arrival of new melt further eruptions will eventually take place.
Cycles of volcanism, and the repetitive nature of volcanic cycles, will contribute to build a volcanic edifice. Volcanic stratigraphy of successive lava flows is
easier to observe on intraplate volcanoes of hotspot origin and on slow spreading
ridges where magmatic activity builds tall volcanic cone structures. (See Chap. 9).
On fast spreading ridge segments, volcanic cycles and successive magma
upwelling takes place during fissural eruption along the strike, so the resulting
structure seems to be flatter, and forms somewhat continuous dome-shaped features. The cyclical nature of volcanic events is observed on sub-aerial volcanoes
where there is evidence of short, sequential volcanic activities (over periods of
days) as well as longer sequences (over periods of thousands of years).
The successive accumulation of lava poured out on top of previous flows will
build a volcanic edifice that could eventually reach above sea level. However, it is
obvious that the growth of a submarine volcano will depend on numerous factors.
Submarine Volcanic Explosion
133
basaltic source material for these explosive, volcanic rocks from the MAR, had
crystallized less than 15 % solids within the parental liquid.
Hence, pyroclasts and hyaloclastites resulting from submarine explosion could
occur on both fast and slow spreading ridge systems and seamounts. Finding these
pyroclasts and hyaloclastites at some distance away from emerged landmasses
indicates that underwater explosion can occur at less than 3,000 m depth, and this
type of explosive volcanism occurs more frequently than had been thought.
Growth of a Volcano
The construction of an underwater volcano is related to the eruption of molten lava
as well as to the propagation of sub-crustal melt forming a magma reservoir and its
conduits such as dykes and sills. From the reservoir, magma will flow through both
vertical and lateral fissures to form the skeleton of a volcanic edifice (Figs. 5.15
and 5.16). If the magma solidifies within the edifice during its intrusion, it will
form a series of vertical dykes representing the conduits of ascending melt. These
magmatic conduits supply the lava flow that will be extruded on the volcano’s
surface. Each eruption is the result of magmatic injection taking place at different
levels within the volcanic edifice. As a volcano grows, its mass will increase,
therefore it will rise taller and as a result, less magma will erupt from the summit
while more lava will erupt from lateral vents causing the creation of adventive or
parasite cones. In order for the extrusive flow to continue piling up, it also has to
overcome the surrounding lithostatic pressure.
During crystal-liquid fractionation, a melt in the magma reservoir will evolve
and become more enriched in volatiles. The volatiles and the lighter elements will
tend to rise towards the roof and/or the cooler borders of the magma chamber,
while the heavier components will sink. Also, the replenishment of a magma
reservoir is an important consideration after a sequence of eruptive events since if
there is an arrival of new melt further eruptions will eventually take place.
Cycles of volcanism, and the repetitive nature of volcanic cycles, will contribute to build a volcanic edifice. Volcanic stratigraphy of successive lava flows is
easier to observe on intraplate volcanoes of hotspot origin and on slow spreading
ridges where magmatic activity builds tall volcanic cone structures. (See Chap. 9).
On fast spreading ridge segments, volcanic cycles and successive magma
upwelling takes place during fissural eruption along the strike, so the resulting
structure seems to be flatter, and forms somewhat continuous dome-shaped features. The cyclical nature of volcanic events is observed on sub-aerial volcanoes
where there is evidence of short, sequential volcanic activities (over periods of
days) as well as longer sequences (over periods of thousands of years).
The successive accumulation of lava poured out on top of previous flows will
build a volcanic edifice that could eventually reach above sea level. However, it is
obvious that the growth of a submarine volcano will depend on numerous factors.
Submarine Volcanic Explosion
133