Mid-ocean ridge volcanic processes
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.
Our research on volcanic processes at mid-ocean ridges
Historical Eruptions at Axial Seamount
AXIAL SEAMOUNT – The lava flows produced by eruptions at Axial Seamount in 1998, 2011, and 2015 were mapped at 1 m resolution from autonomous underwater vehicles (AUVs) developed at the Monterey Bay Aquarium Research Institute (MBARI). A portion of the flows erupted in 2011 and 2015 are defined by pre- and posteruption AUV surveys. Data processing software, also developed at MBARI, precisely coregisters pre- and post-eruption surveys to allow construction of difference maps by subtracting a pre-eruption grid from a post-eruption grid. Such difference maps are key to extracting detailed information about eruptive processes and emplacement of the lava flows. All three eruptions began on the east side of the caldera, and each produced ~25 × 106 m3 of thin channelized flows (with sheet lava channels, lobate lava interiors with pillars, and distal inflated pillow lobes) in the caldera and on the upper south or north rifts. The 1998 and 2011 eruptions propagated down the south rift, and the 2015 eruption propagated down the north rift. The 2011 and 2015 eruptions formed shallow grabens surrounding new non-eruptive open fissures on the east rim of the caldera and produced thick hummocky flows on upper to mid rifts, and the 2011 eruption also produced a thick hummocky flow on the lower south rift. Future eruptions at Axial Seamount will likely follow this pattern, regardless of which rift is the locus of the eruption.
Reference: Clague D.A., Paduan, J.B., Caress, D.W., Chadwick Jr, W.W., Le Saout, M., Dreyer, B.M., Portner, R.A. (2017). High-resolution AUV mapping and targeted ROV observations of three historical lava flows at Axial Seamount, Oceanography, 30(4), 82-99, doi: 10.5670/oceanog.2017.426.
Geologic history of Axial Seamount
AXIAL SEAMOUNT – Multibeam (1-m resolution) and sidescan data collected from an autonomous underwater vehicle, and lava samples, radiocarbon-dated sediment cores, and observations of flow contacts collected by remotely operated vehicle were combined to reconstruct the geologic history and flow emplacement processes on Axial Seamount’s summit and upper rift zones. The maps show 52 post-410 CE lava flows and 20 pre-caldera lava flows as old as 31.2 kyr, the inferred age of the caldera. Clastic deposits 1-2 m thick accumulated on the rims post-caldera. Between 31 ka and 410 CE, there are no known lava flows near the summit. The oldest post-caldera lava (410 CE) is a pillow cone SE of the caldera. Two flows erupted on the W rim between ~800 and 1000 CE. From 1220-1300 CE, generally small eruptions of plagioclase phyric, depleted, mafic lava occurred in the central caldera and on the east rim. Larger post-1400 CE eruptions produced inflated lobate flows of aphyric, less-depleted, and less mafic lava on the upper rift zones and in the N and S caldera. All caldera floor lava flows, and most uppermost rift zone flows, postdate 1220 CE. Activity shifted from the central caldera to the upper S rift outside the caldera, to the N rift and caldera floor, and then to the S caldera and uppermost S rift, where two historical eruptions occurred in 1998 and 2011. The average recurrence interval deduced from the flows erupted over the last 800 years is statistically identical to the 13 year interval between historical eruptions.
Reference: Clague, D.A., B.M. Dreyer, J.B. Paduan, J.F. Martin, W.W. Chadwick, D.W. Caress, R.A. Portner, T.P. Guilderson, M.L. McGann, H. Thomas, D.A. Butterfield, R.W. Embley (2013) Geologic history of the summit of axial seamount, Juan de Fuca Ridge. Geochem., Geophys., Geosyst., 14(10): 4403-4443. doi: 10.1002/ggge.20240 [Abstract] [Article]
Mapping the 1998 eruption at Axial Seamount
AXIAL SEAMOUNT – Axial Seamount, an active submarine volcano on the Juan de Fuca Ridge at 46°N, 130°W, erupted in January 1998 along 11 km of its upper south rift zone. We use ship-based multibeam sonar, high-resolution (1 m) bathymetry, sidescan sonar imagery, and submersible dive observations to map four separate 1998 lava flows that were fed from 11 eruptive fissures. These new mapping results give an eruption volume of 31 × 106 m3, 70% of which was in the northern-most flow, 23% in the southern-most flow, and 7% in two smaller flows in between. We introduce the concept of map-scale submarine lava flow morphology (observed at a scale of hundreds of meters, as revealed by the high-resolution bathymetry), and an interpretive model in which two map-scale morphologies are produced by high effusion-rate eruptions: “inflated lobate flows” are formed near eruptive vents, and where they drain downslope more than 0.5–1.0 km, they transition to “inflated pillow flows.” These two morphologies are observed on the 1998 lava flows at Axial. A third map-scale flow morphology that was not produced during this eruption, “pillow mounds,” is formed by low effusion-rate eruptions in which pillow lava piles up directly over the eruptive vents. Axial Seamount erupted again in April 2011 and there are remarkable similarities between the 1998 and 2011 eruptions, particularly the locations of eruptive vents and lava flow morphologies. Because the 2011 eruption reused most of the same eruptive fissures, 58% of the area of the 1998 lava flows is now covered by 2011 lava.
Reference: Chadwick, W.W., D.A. Clague, R.W. Embley, M.R. Perfit, D.A. Butterfield, D.W. Caress, J.B. Paduan, J.F. Martin, P. Sasnett, S.G. Merle, A.M. Bobbitt (2013) The 1998 eruption of Axial Seamount: New insights on submarine lava flow emplacement from high-resolution mapping. Geochem., Geophys., Geosyst., 14(10): 3939-3968. doi: 10.1002/ggge.20202 [Abstract] [Article]
Pillow mounds offer clues about dike propagation
JUAN DE FUCA AND GORDA RIDGES – Linear, hummocky pillow mound volcanism dominates at slow and intermediate spreading rate mid-ocean ridges. Volcanic hummocks are thought to be formed by low effusion rates or as a result of flow focussing during effusive fissure style eruptions and in which the initial dike intercepts the seafloor and erupts along its entire length.
In this study, high-resolution autonomous underwater vehicle (AUV) bathymetry is used to accurately map the extents of four historical fissure eruptions of the Juan de Fuca and Gorda ridges: on the North Gorda, North Cleft and CoAxial ridge segments. The four mapped eruptions take the form of pillow mounds, which are similar in both lithology and dimension to hummocks on the Mid-Atlantic Ridge. Pillow mounds may be isolated, or coalesce to form composite mounds, aligned as ridges or as clustered groups. In two of the four mapped sites, the eruptions were discontinuous along their lengths, with pillow mounds and composite mounds commonly separated by areas of older seafloor. This style of discontinuous eruption is inconsistent with typical en echelon fissure eruptions and is probably due to a mildly overpressured, fingering dike intersecting the seafloor along parts of its length.
Reference: Yeo, I.A., D.A. Clague, J.F. Martin, J.B. Paduan, D.W. Caress (2013) Pre-eruptive flow focussing in dikes feeding historical pillow ridges on the Juan de Fuca and Gorda Ridges. Geochem. Geophys. Geosyst., 14(9): 3586-3599. doi: 10.1002/ggge.20210 [Abstract] [Article]
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.