Volcanic processes more complex than originally thought
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A mechanism for how lava is injected at mid-ocean ridges is proposed from evaluation of the seismic and bathymetric data from an eruption at Axial Volcano. Lava propagated for many kilometers along the rift before extruding at the sea floor. Go to the MBARI mapping program for more maps and information on Axial Volcano. |
 Perspective view from the southeast of Axial Volcano on the Juan de Fuca Ridge Map © MBARI 2001 |
Our research on volcanic processes at mid-ocean ridges
The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.
Evidence in a lava pillar: assimilation of seawater into molten lava
JUAN DE FUCA RIDGE - A lava pillar formed during the 1998 eruption at Axial Seamount exhibits compositional and textural evidence for contamination by seawater under magmatic conditions. Glass immediately adjacent to anastomosing microfractures within 1 cm of the inner pillar wall is oxidized and significantly enriched in Na and Cl and depleted in Fe and K with respect to that in glassy selvages from the unaffected outer pillar wall. The affected glass contains up to 1 wt % Cl and is enriched by ~2 wt % Na2O relative to unaffected glass, consistent with a nearly 1:1 (molar) incorporation of NaCl. Glass bordering the Cl-enriched glass in the inner pillar wall is depleted in Na but enriched in K. The presence of tiny (<10 μm) grains of Cu-Fe sulfides and Fe sulfides as well as elemental Ni, Ag, and Au in the Na-depleted, K-enriched glass of the inner pillar wall implies significant reduction of this glass, presumably by hydrogen generated during seawater contamination and oxidation of lava adjacent to microfractures. We interpret the compositional anomalies we see in the glass of the interior pillar wall as caused by rapid incorporation of seawater into the still-molten lava during pillar growth, probably on the time scale of seconds to minutes. Only one of seven examined lava pillars shows this effect, and we interpret that seawater has to be trapped in contact with molten lava (inside the lava pillar, in this case) to produce the effects we see. Thus, under the right conditions, seawater contamination of lavas during submarine eruptions is one means by which the oceanic crust can sequester Cl during its global flux cycle. However, since very few recent lava flows have been examined in similar detail, the global significance of this process in effecting Earth's Cl budget remains uncertain.
Reference: Schiffman, P., Zierenberg, R., Chadwick, W.W., Clague, D.A., Lowenstern, J. (2010) Contamination of basaltic lava by seawater: Evidence found in a lava pillar from Axial Seamount, Juan de Fuca Ridge. Geochem., Geophys., Geosyst., 11(4): Q04004, doi:10.1029/2009GC003009. [Abstract] [Article]
Characteristics of submarine basaltic eruptions
MID-OCEAN RIDGES, NEAR-RIDGE SEAMOUNTS, HOT SPOT VOLCANOES, CALIFORNIA MARGIN SEAMOUNTS - Basaltic volcanism in the deep oceans has long been thought to consist of quietly effusive discharge of lava to form pillow, lobate, and sheet flows. However, new high-resolution mapping tools and exploration and sampling using submersibles and remotely operated vehicles are revealing a more diverse array of volcanic processes operating in the deep sea. These processes include upbiquitous pyroclastic activity in all volcanic settings and at all depths, emplacement of sills into sedimentary sections, construction and drainage of lava ponds, construction of circular flat-topped cones, emplacement of >100 km long tube-fed flows on gentle slopes, formation of pit craters and craters in small circular cones, and collapse of calderas on larger volcanoes near the ridge system, on seamounts formed near the ridges, and on hot-spot volcanoes like Loihi Seamount. The larger caldera collapse events appear to be accompanied by energetic pyroclastic and hydromagmatic activity, even at depths >1600 m. These diverse volcanic processes have implications for the formation and distribution of hydrothermal activity and deposits.
Clague, D.A., Paduan, J.B. (2009) Submarine basaltic volcanism, In: Submarine Volcanism and Mineralization: Modern through Ancient, B. Cousens and S.J. Piercey (eds.), Geological Association of Canada, Short Course 39-30 May 2008, Quebec City, Canada, p. 41-60.
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