Scott M. White

Volcanic eruptions on mid‐ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°–19°S

uniform sediment cover were recovered from lava that buries older faulted terrain. The boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector, consistent with magmas from two separate reservoirs being erupted in the same event. Chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments. A comparison of our results to global data on submarine mid-ocean ridge eruptions suggests consistent dependencies of erupted volume, activated fissure lengths, and chemical heterogeneity with spreading rate, consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 8414 Volcanology: Eruption mechanisms; 8439 Volcanology: Physics and chemistry of magma bodies; 3655 Mineralogy and Petrology: Major element composition; KEYWORDS: lava flow, chemical heterogeneity, erupted volume, lava morphology, side-scan sonar

Crustal fissuring on the crest of the southern East Pacific Rise at 17°15′–40′S

S with the near-bottom DSL-120 andArgo II imaging systems. We observe that the youngest lava flows (on a relative age scale) aresparsely fissured and that there is a cumulative increase in fissure abundance with time thatproduces a strong positive correlation between fissure density and relative age of lava flows.Average fissure widths were used to estimate fissure depths. In the 17 15

Basaltic lava domes, lava lakes, and volcanic segmentation on the southern East Pacific Rise

Meter-scale DSL-120 sonar mapping and coregistered Argo II photographic observations reveal changes in eruptive style that closely follow the third-order structural segmentation of the ridge axis on the southern East Pacific Rise, 17°11′–18°37′S. Near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology. The ubiquity of pillow lava suggests low effusion rate eruptions. The abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices. Near segment centers we observe fewer but larger lava domes, voluminous drained and collapsed lava lakes, and smooth lobate and sheet lava flows with very little pillow lava. The abundance of sheet flows suggests that high effusion rate eruptions are common. Fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources. This pattern was observed on eight third-order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale. The third-order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across-axis width of the region of seismic layer 2A thickening along the one segment where sufficient cross-axis seismic lines exist. The geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries, although rock sampling density is not adequate to constrain the variation across all the third-order volcanic segments that we identify. These observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading (>140 km/Myr full rate) mid-ocean ridges.

High-resolution surveys along the hot spot–affected Galápagos Spreading Center: 1. Distribution of hydrothermal activity

The spatial density of hydrothermal activity along most mid-ocean ridges is a robust linear function of spreading rate (or magmatic budget), but extreme crustal properties may alter this relationship. In 2005–2006 we tested the effect of thickened crust on hydrothermal activity using high-resolution mapping of plumes overlying the hot spot–affected Galapagos Spreading Center from 95° to 89°42′W (∼560 km of ridge crest). Plume mapping discovered only two active, high-temperature vent fields, subsequently confirmed by camera tows, though strong plume evidence indicated minor venting from at least six other locations. Total plume incidence (ph), the fraction of ridge crest overlain by significant plumes, was 0.11 ± 0.014, about half that expected for a non–hot spot mid-ocean ridge with a similar magmatic budget. Plume distributions on the Galapagos Spreading Center were uncorrelated with abrupt variations in the depth of the along-axis melt lens, so these variations are apparently not controlled by hydrothermal cooling differences. We also found no statistical difference (for a significance level of 0.05) in plume incidence between where the seismically imaged melt lens is shallow (2 ± 0.56 km, ph = 0.108 ± 0.045) and where it is deep (3.4 ± 0.7 km, ph = 0.121 ± 0.015). The Galapagos Spreading Center thus joins mid-ocean ridges near the Iceland (Reykjanes Ridge), St. Paul-Amsterdam (South East Indian Ridge), and Ascension (Mid-Atlantic Ridge) hot spots as locations of anomalously scarce high-temperature venting. This scarcity implies that convective cooling along hot spot–affected ridge sections occurs primarily by undetected diffuse flow or is permanently or episodically reduced compared to normal mid-ocean ridges.

Recent crustal deformation and the earthquake cycle along the Ecuador–Colombia subduction zone

Recent results from Global Positioning System (GPS) measurements show deformation along the coast of Ecuador and Colombia that can be linked to the rupture zone of the earthquake in 1979. A 3D elastic boundary element model is used to simulate crustal deformation observed by GPS campaigns in 1991, 1994, 1996, and 1998. Deformation in Ecuador can be explained best by 50% apparent locking on the subduction interface. Although there have not been any historic large earthquakes (Mw>7) south of the 1906 earthquake rupture zone, 50% apparent elastic locking is necessary to model the deformation observed there. In Colombia, only 30% apparent elastic locking is occurring along the subduction interface in the 1979 earthquake rupture zone (Mw 8.2), and no elastic locking is necessary to explain the crustal deformation observed at two GPS sites north of there. There is no evidence from seismicity or plate geometry that plate coupling on the subduction zone is reduced in Colombia. However, simple viscoelastic models suggest that the apparent reduction in elastic locking can be explained entirely by the response of a viscous upper mantle to the 1979 earthquake. These results suggest that elastic strain accumulation is occurring evenly throughout the study area, but postseismic relaxation masks the true total strain rate.

Distribution of isolated volcanoes on the flanks of the East Pacific Rise, 15.3°S–20°S

Volcanic constructions, not associated with seamount (or volcano) chains, are abundant on the flanks of the East Pacific Rise (EPR) but are rare along the axial high. The distribution of isolated volcanoes, based on multibeam bathymetric maps, is approximately symmetric about the EPR axis. This symmetry contrasts with the asymmetries in the distribution of volcano chains (more abundant on the west flank), the seafloor subsidence rates (slower on the west flank), and the distribution of plate-motion-parallel gravity lineaments (more prominent on the west flank). Most of the isolated volcanoes complete their growth within ∼14 km of the axis on crust younger than 0.2 Ma, while seamount chain volcanoes continue to be active on older crust. Volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities, suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities. The volume of isolated near-axis volcanoes correlates with ridge axis cross-sectional area, suggesting a link between the magma budget of the ridge and the eruption of near-axis volcanoes. Within the study area, off-axis volcanic edifices cover at least 6% of the seafloor and contribute more than 0.2% to the volume of the crust. The inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the Mid-Atlantic Ridge (<4 km), northern EPR (<20 km), and southern EPR (<28 km), so that isolated volcanoes form primarily on lithosphere younger than 0.2 Ma (< 4-6 km brittle thickness), independent of spreading rate. This suggests some form of lithospheric control on the eruption of isolated off-axis volcanoes due to brittle thickness, increased normal stresses across cracks impeding dike injection, or thermal stresses within the newly forming lithosphere.

High-Resolution Surveys Along the Hot Spot–Affected Galapagos Spreading Center: 2. Influence of Magma Supply on Volcanic Morphology

The Galapagos Spreading Center (GSC) at 89°–95°W exhibits large gradients in magma supply at a relatively constant intermediate spreading rate, making this area an ideal natural laboratory to study the effects of magma supply on volcanism at seafloor spreading ridges. Prior work shows that the GSC develops from axial valley to shallow axial rise and a shallow magma sill, much like a typical fast spreading ridge, as the contribution of the hot spot increases. The volcanic morphology varies with magma supply in a predictable manner that we divide into three terrains based on the characteristic style of volcanic emplacement and edifice construction within each terrain. The volcanic cone terrain comprises most of the GSC and is characterized by prominent volcanic cones within a >1 km wide and >100 m deep axial graben. Approaching the area of maximum mantle plume influence at 91°W, the GSC axis lies along an elevated axial rise split by a <1 km wide and <100 m deep axial graben, and the style of volcanism shifts to axial volcanic ridge terrain characterized by axis-elongate, low-relief ridges of pillow lava. The lava channel terrain comprises only one segment on either side of the maximum magma supply at 91°W, where sheet lava flows and lava channels are relatively widespread. A general lengthening of seafloor fissures with increasing magma supply suggests a greater tendency toward linear source eruptions, in agreement with the volcanic observations. These results suggest that magma supply rather than magma chamber depth or rate of tectonic extension is the primary influence on lava morphology, hence eruptive processes, at seafloor spreading ridges in general. In both the axial volcanic ridge and lava channel terrains, a single prominent volcanic cone exists within each volcanic segment, suggesting a segment-centered magma focusing.

A modified basal outlining algorithm for identifying topographic highs from gridded elevation data, Part 1: Motivation and methods

A new approach is developed to improve the automated identification and characterization of topographic highs having quasi-elliptical basal shapes. It is designed for the study of volcanic edifices in subaerial and submarine environments, but may be applied to identify any enclosed topography feature within a digital elevation model. The procedure utilizes the results of a standard closed-contouring approach, but then adjusts the elevation of the volcanic base by evaluating the shape of the edifice along a series of topographic profiles. The algorithm overcomes the principal limitations of a stand-alone closed-contouring approach and provides improved estimates of edifice size that are less sensitive to topographic gradients and the choice of contour search interval.

Transition from seamount chain to intraplate volcanic ridge at the East Pacific Rise

A number of large submarine intraplate volcanic ridges have been discovered throughout the South Pacific Basin, but their origins are enigmatic. Recent shipboard geophysical surveys reveal that the Sojourn Ridge, one of these large intraplate ridges, becomes a chain of discrete seamount volcanoes that intersects the ridge axis. This seamount chain exhibits several features that suggest that it is directly related to the Sojourn Ridge. The Sojourn Seamount Chain grows continuously in both volume and number of seamounts with distance from the spreading axis; several loci of recent volcanic activity along the chain are evident in the side-scan imagery, and a mantle Bouguer anomaly low underlies the entire length of the chain. This evidence provides new constraints on the origin of intraplate volcanic ridges. The continuation of the Sojourn Ridge system as a volcano chain that extends to within 5 km of the spreading axis implies active generation of magma and a focusing mechanism, such as flexural stresses induced by the mass of the volcanic pile, as the probable mechanism for developing volcanic ridges and long seamount chains.

Eruptive timing and 200 year episodicity at 92°W on the hot spot‐influenced Galapagos Spreading Center derived from geomagnetic paleointensity

Eruptive timing in mid-ocean ridge systems is relatively poorly constrained, despite being an important variable in our understanding of many mid-ocean ridge processes, including volcanic construction; magma recharge, flux, and storage; and the stability of hydrothermal systems and biological communities. Only a handful of absolute eruption chronologies exist, yet they are essential in understanding how eruptive timing varies with important controlling variables. To construct an eruptive history at one location on the Galapagos Spreading Center, we present age determinations derived from geomagnetic paleointensity. To aid interpretation of the paleointensity data, we also present results from on-bottom magnetic anomaly measurements and forward modeling of topographic-induced magnetic anomalies. Anomalies may lead to a 1–2 µT bias in flow-mean paleointensities, which does not significantly affect the overall interpretation. Paleointensity results for the three youngest sampled units are indistinguishable, consistent with the flows being emplaced in relatively rapid succession. Comparisons with models of geomagnetic field behavior suggest these flows were erupted sometime in the past 100–200 years. The fourth sampled unit has a significantly higher paleointensity, consistent with an age of roughly 400 years. The possible bias in paleointensity data allows for ages as young as ∼50 years for the youngest three flows and 200–400 years for the oldest flow. This age distribution demonstrates an episodicity in the emplacement of the largest flows at this location, with a 200–300 year period of relative quiescence between emplacement of the oldest unit and the three youngest units.