trachy-andesite. Indeed, there was no evidence of any high temperature basaltic
lava, such as was seen on the larger main edifice. This observation became
important, and pushed us to sample other small volcanic edifices.
During our survey, we found that all the other small volcanic edifices less than
500 m tall, less than 2 km in diameter, and located at a distance from the larger
cones, were made up of silica-enriched flows similar to that of the adventive cone
of the Bounty Volcano. Thus an important question arose from our findings: Is
such silica-rich and viscous lava formed as a result of crystal-liquid fractionation
taking place during the channeling of magma in a sub-crustal environment at a
distance from its source? Or, are these silica-enriched lavas produced directly from
the partial melting of a low temperature component of the mantle source material?
The channeling of subcrustal magma is similar to a plumbing system. It could
supply magma from either a main reservoir underneath the larger edifices and/or
directly from the mantle plume piercing the lithosphere. It is believed that the
shallow magma reservoir underneath the large volcanic edifices is supplied from
the partial melting of the upper mantle located underneath the hotspot. Then, a
more fractionated magma, the silica enriched melt, could flow laterally through
crustal weaknesses, fractures and fissures, to supply small eruptive cones. This also
agrees with the Sr and Pb isotopic data published and interpreted by Colin Devey
and his co-workers (2003), which suggested that the volcanics on the smaller
edifices have their origin directly from the partial melting of a mantle plume
source. It is likely that repeated melt injection within the main magma reservoir
underneath the large edifices and then subsequent crystal-liquid fractionation will
facilitate the lateral flow of the magma supplying the small edifices.
Volcanic Cyclicity
On the ocean floor it is difficult to be able to observe the sequential nature of
volcanic events. The most reliable way is to conduct in situ detailed submersible
and/or ROV observations and to sample the different types of material exposed
(Fig. 9.18a, b). The dives made on the Bounty and Adams volcanoes helped us to
understand the succession of volcanic events constructing the edifices. The most
obvious sequential variability is observed when there is a drastic change in the
mode of extrusion and/or related to a major tectonic disruption of the volcanic
terrain. For example, explosive events generating collapsed structures and other
types of forceful injections (i.e. dykes) exposed during faulting will give rise to
contrasting volcanic landscapes, such as intrusive rocks overlying extrusive flows.
Examples of fracture zones and uplifted blocks during tectonic events are given in
Chap. 8 which deals with the Hess Deep and the Garrett transform fault.
Other evidence of sequential variability is due to the compositional changes of
the extruded material. In fact, the compositional changes observed among the
different rock types helped us to recognize the existence of volcanic stratigraphy
and determine the sequential eruptive events. Following the logic of crystal-liquid
338
9 Hotspots
lava, such as was seen on the larger main edifice. This observation became
important, and pushed us to sample other small volcanic edifices.
During our survey, we found that all the other small volcanic edifices less than
500 m tall, less than 2 km in diameter, and located at a distance from the larger
cones, were made up of silica-enriched flows similar to that of the adventive cone
of the Bounty Volcano. Thus an important question arose from our findings: Is
such silica-rich and viscous lava formed as a result of crystal-liquid fractionation
taking place during the channeling of magma in a sub-crustal environment at a
distance from its source? Or, are these silica-enriched lavas produced directly from
the partial melting of a low temperature component of the mantle source material?
The channeling of subcrustal magma is similar to a plumbing system. It could
supply magma from either a main reservoir underneath the larger edifices and/or
directly from the mantle plume piercing the lithosphere. It is believed that the
shallow magma reservoir underneath the large volcanic edifices is supplied from
the partial melting of the upper mantle located underneath the hotspot. Then, a
more fractionated magma, the silica enriched melt, could flow laterally through
crustal weaknesses, fractures and fissures, to supply small eruptive cones. This also
agrees with the Sr and Pb isotopic data published and interpreted by Colin Devey
and his co-workers (2003), which suggested that the volcanics on the smaller
edifices have their origin directly from the partial melting of a mantle plume
source. It is likely that repeated melt injection within the main magma reservoir
underneath the large edifices and then subsequent crystal-liquid fractionation will
facilitate the lateral flow of the magma supplying the small edifices.
Volcanic Cyclicity
On the ocean floor it is difficult to be able to observe the sequential nature of
volcanic events. The most reliable way is to conduct in situ detailed submersible
and/or ROV observations and to sample the different types of material exposed
(Fig. 9.18a, b). The dives made on the Bounty and Adams volcanoes helped us to
understand the succession of volcanic events constructing the edifices. The most
obvious sequential variability is observed when there is a drastic change in the
mode of extrusion and/or related to a major tectonic disruption of the volcanic
terrain. For example, explosive events generating collapsed structures and other
types of forceful injections (i.e. dykes) exposed during faulting will give rise to
contrasting volcanic landscapes, such as intrusive rocks overlying extrusive flows.
Examples of fracture zones and uplifted blocks during tectonic events are given in
Chap. 8 which deals with the Hess Deep and the Garrett transform fault.
Other evidence of sequential variability is due to the compositional changes of
the extruded material. In fact, the compositional changes observed among the
different rock types helped us to recognize the existence of volcanic stratigraphy
and determine the sequential eruptive events. Following the logic of crystal-liquid
338
9 Hotspots