variable due to the presence of islands formed by plumes (see chapter on Azores
Islands hotspots). The crust can reach about 9 km in the FAMOUS area (MAR,
37°N (Witmarsh 1975). At 35°N on the MAR, the LVZ is located at 3 km depth
and has a bulls-eye shaped negative gravity anomaly suggesting the presence a
magma reservoir (Detrick et al. 1987).
Based on seismic data (Perfit and Chadwick 1998) and using thermal calculations, we can conclude that when the sea floor spreading rates are low (with half
spreading rates of less than 20 mm/year), there will be a ‘‘steady state’’ magma
chamber. This means that most of the upwelling melt will solidify at depths within
less than 10 km of the sea floor (Sleep and Barth 1997). On the other hand, only
ridge segments with intermediate, fast and ultrafast rates of spreading (total
rates [40 mm/year), will sustain a transient, non-steady state magma reservoir
since magma is extruded as soon as it rises through the crust. These types of
magma chambers are mainly found underneath fast spreading ([60 mm/year)
ridges such as the EPR. In slow spreading ridges, magma lenses are absent or
ephemeral and there is only sporadic volcanism, with the exception of slow
spreading ridges that are associated with mantle plumes (a hotspot), which could
result in voluminous but localized volcanism (such as in the region of the Azores).
Volcanic Landscape
The extrusion of lava flows has shaped the sea floor’s volcanic landscape.
Spreading ridge systems and intraplate volcanoes constitute the primary places
where volcanic landscapes are formed and where they vary the most. The general
appearance of a spreading ridge segment is that of an elongated dome-shape
feature, whose topography becomes lower as volcanism decreases.
Comparisons of the morphological and compositional variability of lava
observed on the sea floor along the ridge segments have been made in order to
detect magma that has been channeled towards the segments’ end in sub-crustal
environments as opposed to the magma released on topographic highs. Rift
propagation and dyke injection implies that as magma moves away from its main
upwelling zone, it will change its physical properties due to a drop of its initial
temperature, which will influence the magma’s composition, its degree of crystallinity and its viscosity. This is observed by a decrease in crustal thickness, an
increase in pillow/sheet flow ratios and an increase in the degrees of magmatic
differentiation along the strike of a ridge. For example, the lava composition
towards the segment-end, located at some distance away from the source of
magma, tends to become more viscous and contains higher contents of incompatible elements (i.e. K, Na, Ba, Sr) and silica.
We must also take into consideration that the rising mantle material is dispersed
within the lithosphere and is not simply limited to extrusion on the sea floor
surface. A large amount of melt can stagnate in the lithosphere forming separate
units such as vertical dykes, horizontal sills or magma ponds. The melt will follow
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5 Earth’s Mantle Melting and Volcanism
Islands hotspots). The crust can reach about 9 km in the FAMOUS area (MAR,
37°N (Witmarsh 1975). At 35°N on the MAR, the LVZ is located at 3 km depth
and has a bulls-eye shaped negative gravity anomaly suggesting the presence a
magma reservoir (Detrick et al. 1987).
Based on seismic data (Perfit and Chadwick 1998) and using thermal calculations, we can conclude that when the sea floor spreading rates are low (with half
spreading rates of less than 20 mm/year), there will be a ‘‘steady state’’ magma
chamber. This means that most of the upwelling melt will solidify at depths within
less than 10 km of the sea floor (Sleep and Barth 1997). On the other hand, only
ridge segments with intermediate, fast and ultrafast rates of spreading (total
rates [40 mm/year), will sustain a transient, non-steady state magma reservoir
since magma is extruded as soon as it rises through the crust. These types of
magma chambers are mainly found underneath fast spreading ([60 mm/year)
ridges such as the EPR. In slow spreading ridges, magma lenses are absent or
ephemeral and there is only sporadic volcanism, with the exception of slow
spreading ridges that are associated with mantle plumes (a hotspot), which could
result in voluminous but localized volcanism (such as in the region of the Azores).
Volcanic Landscape
The extrusion of lava flows has shaped the sea floor’s volcanic landscape.
Spreading ridge systems and intraplate volcanoes constitute the primary places
where volcanic landscapes are formed and where they vary the most. The general
appearance of a spreading ridge segment is that of an elongated dome-shape
feature, whose topography becomes lower as volcanism decreases.
Comparisons of the morphological and compositional variability of lava
observed on the sea floor along the ridge segments have been made in order to
detect magma that has been channeled towards the segments’ end in sub-crustal
environments as opposed to the magma released on topographic highs. Rift
propagation and dyke injection implies that as magma moves away from its main
upwelling zone, it will change its physical properties due to a drop of its initial
temperature, which will influence the magma’s composition, its degree of crystallinity and its viscosity. This is observed by a decrease in crustal thickness, an
increase in pillow/sheet flow ratios and an increase in the degrees of magmatic
differentiation along the strike of a ridge. For example, the lava composition
towards the segment-end, located at some distance away from the source of
magma, tends to become more viscous and contains higher contents of incompatible elements (i.e. K, Na, Ba, Sr) and silica.
We must also take into consideration that the rising mantle material is dispersed
within the lithosphere and is not simply limited to extrusion on the sea floor
surface. A large amount of melt can stagnate in the lithosphere forming separate
units such as vertical dykes, horizontal sills or magma ponds. The melt will follow
112
5 Earth’s Mantle Melting and Volcanism