Volcanic processes more complex than originally thought
MBARI mapped the 2011 eruption at Axial Volcano with our AUV. Subtracting bathymetry of the entire summit of Axial mapped between 2006 to 2009 from the 2011 bathymetry revealed the extent, volume, fissures and channel systems of the flow at high resolution. Surprises included how most of the fissures had been pre-existing, and were reused (even widened and deepened) and fed lava into pre-existing channels.
A mechanism for how lava is injected at mid-ocean ridges was proposed from evaluation of the seismic and bathymetric data from the 1998 eruption at Axial Volcano. Lava propagated for many kilometers along the rift before extruding at the sea floor.
More about Axial Volcano:
A small portion of the Axial 2011 flow is depicted in this map of the difference between before and after bathymetry from MBARI's Mapping AUV. Color ramp (blue to orange) is 0 to 15 m thickness, vertical precision is 0.2 m, horizontal resolution is 1 m, and the area shown is about 2x3 km. Eruptive fissures are visible at right, flow channels in the middle, and inflated flow margins at left and lower middle.
Map © MBARI 2012
Perspective view from the southeast of Axial Volcano on the Juan de Fuca Ridge. The map at top is part of the caldera floor, in the upper right of this map.
Map © MBARI 2001
Our research on volcanic processes at mid-ocean ridges
Axial Seamount 2011 eruption mapped with AUV
AXIAL SEAMOUNT - At sites with frequent submarine volcanic activity, it is difficult to discern between new and pre-existing lava flows. In particular, the distribution of the fissures from which lava erupts, the routes taken by lava flows and the relationship between the new flows and the pre-existing seafloor bathymetry are often unclear. The volcanic and hydrothermal systems of Axial Seamount submarine volcano in the Pacific Ocean have been studied intensively since eruptions were detected in 1998 and 2011. Here we combine pre- and post-eruption bathymetric surveys, with 1-m lateral resolution and 0.2-m vertical precision, to precisely map the extent and thickness of the lava flows, calculate the volume of lava and unambiguously identify eruptive fissures from the April 2011 eruption. Where the new lava flows extend beyond the boundaries of the repeated surveys, we use shipboard multibeam surveys to map the flows with lower resolution. We show that the eruption produced both sheet and lobate flows associated with high eruption rates and low eruption-rate pillow mounds. We find that lava flows erupted from new as well as existing fissures and tended to reoccupy existing flow channels. This reoccupation makes it difficult to map submarine flows produced during one eruption without before-and-after bathymetric surveys.
Reference: Caress, D.W., D.A. Clague, J.B. Paduan, J.F. Martin, B.M. Dreyer, W.W. Chadwick Jr, A. Denny, D.S. Kelley (2012) Repeat bathymetric surveys at 1-metre resolution of lava flows erupted at Axial Seamount in April 2011. Nature Geoscience, 5(7): 483-488. doi: 10.1038/NGEO1496
Deep-sea volcanic eruptions, case studies
MID OCEAN RIDGES AROUND THE GLOBE - Volcanic eruptions are important events in Earth's cycle of magma generation and crustal construction. Over durations of hours to years, eruptions produce new deposits of lava and/or fragmentary ejecta, transfer heat and magmatic volatiles from Earth's interior to the overlying air or seawater, and significantly modify the landscape and perturb local ecosystems. Today and through most of geological history, the greatest number and volume of volcanic eruptions on Earth have occurred in the deep ocean along mid-ocean ridges, near subduction zones, on oceanic plateaus, and on thousands of mid-plate seamounts. However, deepsea eruptions (> 500 m depth) are much more difficult to detect and observe than subaerial eruptions, so comparatively little is known about them. Great strides have been made in eruption detection, response speed, and observational detail since the first recognition of a deep submarine eruption at a mid-ocean ridge 25 years ago. Studies of ongoing or recent deep submarine eruptions reveal information about their sizes, durations, frequencies, styles, and environmental impacts. Ultimately, magma formation and accumulation in the upper mantle and crust, plus local tectonic stress fields, dictate when, where, and how often submarine eruptions occur, whereas eruption depth, magma composition, conditions of volatile segregation, and tectonic setting determine submarine eruption style.
Reference: Rubin, K.H., S.A. Soule, W.W. Chadwick Jr., D.J. Fornari, D.A. Clague, R.W. Embley, E.T. Baker, M.R. Perfit, D.W. Caress, and R.P. Dziak (2012) Volcanic eruptions in the deep sea. Oceanography 25(1):142–157, doi:10.5670/oceanog.2012.12. [Article]
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. [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.
Next: Explosive eruptionsQuestions? Comments? Please contact Jenny Paduan