Oceanfloor volcanism

byRoger Hékinian (Ifremer - Géosciences Marines) -2000

 

The Planet Ocean, with its 70 thousand-km long and 2-4 km wide spreading ridge system is volcanically and seismically themost active part of our Planet Earth. It is the area where about 70 % of new oceanic crust are created (Figure 1).

 

 Figure 1 - The cycle of accretion and subduction. The oceanic lithosphere created on the ridge axis is transported in the basins as it gets older and cooler. The transported lithosphere eventually sinks into the Mantle underneath island arcs and/or continents.

Other active regions of the Earth are where the oceanic platesconverge underneath a lighter (less dense) plate generally associatedwith island arcs and back arc basins. The region between the diverging and converging plates comprises the ocean basins or intraplate regions. This area is the site of intense volcanic activities,contributes to more than 20 % of our planet volcanism. While spreading ridges at diverging plate boundaries have been extensively studied since the early 50's, it is only recently that comprehensive submarine explorations of intraplate volcanism were performed.The first European project on intraplate volcanism were initiated in 1986 in collaboration between several French and German institutes and universities. The project led to a more global approach to magma genesis related to deep mantle convection and upwelling systems responsible for transferring energy (heat) and matter to the surface.

In fast (> 5 cm/yr) diverging plate boundaries,lateral extension gives rise to quiet and fluid (low viscosity) volcanism through fissure types of eruptions (basaltic volcanism) (Figure 2). Instead in intraplate regions and slow spreading ridges (< 2 cm/yr) where magmatism is more focused in central volcanic conduits, a great diversity of lavas with more variable compositions, degrees of viscosity and degrees of vesicularity are erupted. During fracturing and fissuring of the oceanic crust seawater penetrates into the lithosphere and it is heated in the vicinities of a magmatic reservoir. The heated seawater changes composition and becomes an acid fluid:

·  Seawater + heat + basaltic rock = corrosive fluid (H2S + disolve metals) + altered rock

·  Corrosive fluid - heat = metal sulfides+2H+.

 

Figure 2 - A large (about 1 m) fissure at a divergent plate boundary of a fast spreading ridge. The fissuring and fracturing of the oceanic crust take place along the axis of the ridge; it is through such fissures that eruption of lava takes place. This photo was taken by CYANA at 2606 m inside an axial graben of the East Pacific rise near 13°N.

This hydrothermal fluid charged with metals (Mn, Cu, Zn, Fe) and volatiles rise to the surface where sulfide minerals, which are the modern equivalent of ancient ore deposits, are formed (Figure 3).

 

Figure 3 - A black smoker on the East Pacific Rise at 13°N (2620 m) spewing out hydrothermal fluid charged with sulfides containing Cu, Fe, Zn, Mn and volatiles. Hydrothermal fluid is generated when seawater is heated in the vicinities of a hot magmatic reservoir and transformed into a corrosive fluid which has leached the basaltic crust. The hydrothermal fluid forming an acid solution transports the extracted metals towards the surface in the form of geysers.

The styles of volcanic eruptions responsible for building tall edifices with central conduits are:

1) The volcanic edifice starts with relatively high eruptive rate forming flat flows (sheet flows) of very fluid lava at more than 2500-4000 m depth (Cf. Figure 2). More viscous pillow lavas (bulbous, tubular and blocky) follow (Figure 4).

 

Figure 4 - Pillow lava tubes flowing over a cliff along a rift zone located at the flank of an intraplate volcano (Teahitia) in the Society hotspot (South Pacific).

2) Many of these flows form rift zones on the flank of evolved edifices at 1000-2500 m depth. At shallower depths (< 2000 m) intermittent volcanism gives rise to highly vesicular lavas due to explosive eruptive events (Figure 5). The scattered nature of eruption is due to lower rate of lava discharge on the sea floor.

 

Figure 5 - Bottom photograph taken of a pyroclastic deposit (NAUTILE dive, Oceanaut cruise) located at the rim of a crater on a volcanic cone in the axis of a slow spreading ridge near 34°50'N (Mid-Atlantic Ridge). This deposit is overlaid by a thin (<40 cm thick) hyaloclastites (white continuous line). The deposit is at least 15 m thick and consists of unsorted irregularly fragmented rocks emplaced during an explosive eruption (Cf. Fig. 6).

The differences between the volcanics from intraplate regionsand those from fast spreading ridge axes reside in their degreeof rock vesicularity (> 0.1-80 %) and their composition. The intraplate volcanics contain more volatiles (up to > 3 % of CO2, H2O and K) than most Mid Oceanic Rocks (MOR). It is believed that the intraplate magmas have their origin in a deeper part of the mantle than that giving rise the MOR.

 

Submarine explosive events

 

 Figure 6 - Schematic representation showing the conditions of volcanic eruptions for a volcanic cone forming the Median Ridge in the Mid-Atlantic rift valley near 34°50'N.
(a) A stage of volcanic cone constructed essentially of vesicular (15-30 vol.) pillows of E-MORB and alkali basalt types. This stage is related to a volcanic cycle with lower melt extraction (low extent of melting) than that giving rise to the N- and T-MORBs (<15% vesicles) found in the rift valley NVZ and/or forming the main MR structure underlying the volcanic cones.
(b) During intermittent volcanism, the upwelling of new alkali enriched melts will segregate underneath the conduit plugged from a previous eruption. Gas bubbles accumulate at the top of the magmatic column and dissolved water in the melt starts to degas increasing the pressure within the conduit.
(c) The bubble concentration and the pressure increase will enhance the bursting of the vesicle walls, which will trigger the volcanic explosion and the release of CO2.

 

Underwater volcanic explosions are inferred from the presence of Pyroclastic (layered volcanic ejecta) and hyaloclastite (volcanic glassy ash-like material mixed with sediment and/or hydrothermal product). They are found in association with focussed volcanism forming individual cones with central conduits. They are observed mainly in intraplate regions as well as on slow spreading ridges (Mid Atlantic Ridge, MAR) up to 1500-3000 m (Figure 6).

Volatile-rich magmas may form significant pockets of gas, which expand and burst in the confined environment of a magmatic conduit. Also, magmatic crystallization could concentrate the volatiles in the residual melt, and eventually causes large amounts of exhaled bubbles to migrate to the top and the sides of a magmatic conduit, thus participating in a fluid or gas pressure increase (Figure 6). Vesicular lavas are normally formed by the exsolution of volatiles due to magma decompression as it rises through the lithosphere. In Mid-Oceanic Ridge basalts (MORB), dissolved volatiles are mainly CO2 and H2O. The concentration of volatiles in upwelling melt reflects the source composition and partial-melting rate. In addition the melting of altered crust (seawater contaminated) could also be a source of volatiles. The degree of vesicularity is mainly governed by the original enrichment in carbon and its exsolution during magma ascent. The calculated carbon and the water concentrations associated with the depth of eruption imply, for the most enriched alkali lavas, the nucleation of CO2 bubbles early after partial melting of the mantle source. As soon as these alkali magma have formed their bubbles, they will ascend towards the surface due to their melt reduced density linked to the continuous CO2 exsolution. The water in the ascending alkali enriched magma is exsolved a few hundred meters before reaching the sea floor.

Volatiles added to seawater may have consequences in the processes involved of matter exchange with the atmosphere.

 

Bibliographie

  • Hekinian, R., J. Francheteau, V. Renard, R.D. Ballard, P. Choukroune, J.L. Cheminée, F. Albarède, J.F. Minster, J.C. Marty, J. Boulègue and J.L. Charlou, 1982, Intense hydrothermal activity at the axis of the East Pacific Rise near 13°N: Submersible witnesses the growth of sulfide chimney. Marine Geophys. Res., Vol 6, pp 1-14
  • Hekinian, R., Pineau, F., Shilobreeva S., D.Bideau, E. Gracia, and M. Javoy, 2000 , Deep Sea Explosive Activity on the Mid-Atlantic Ridge near 34°50'N: Magma Composition, Vesicularity and Volatile Content, J. Volc. Geotherm. Res. 98, 49-77.
  • Binard N., R. Hekinian, J.L. Cheminée, and P. Stoffers, 1992, Styles of Eruptive Activity on Intraplate Volcanoes. Jour. Geophys Res., Vol 97, pp 13,999-14,015
 
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