Explosive eruptions occur at mid-ocean ridges
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The volcanic eruptions at mid-ocean ridges have been thought only to be quietly effusive. Because we find glassy fragments of lava bubbles (limu o Pele) at mid-ocean ridges, there must be gas-rich mildly explosive eruptions at mid-ocean ridges as well. Benthic foraminifera often glue particles to their tests, perhaps for protection from predators. These particles may be sponge spicules, sand grains, or other detritus, depending on the materials available and the "specialty" of the foram. In sediment cores from the Gorda Ridge, we found forams that "specialized" in volcanic glass grains and others that "specialized" in limu o Pele. They effectively concentrated the glass samples for us! |
 Benthic foraminifera with agglutinated limu o Pele (foram ~2mm across) Image © MBARI 2003 |
For other examples in this website of explosive eruptions occurring at depth see also
Our research on explosive eruptions at mid-ocean ridges
The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.
Submarine strombolian eruptions
GORDA RIDGE - Compositionally variable limu o Pele (lava bubble-wall fragments) occurs in widely distributed sediments collected during ROV Tiburon dives along the Gorda Ridge axis. The fragments were formed deeper than the critical depth of seawater so are unlikely to have been generated by supercritical expansion of seawater upon heating in contact with hot lava. Discharge of CO2 through erupting lava is the most likely way to make such bubbles at >298 bars pressure. The distribution and composition of limu o Pele fragments indicate that low-energy strombolian activity is a common, although minor, component of eruptions along mid-ocean ridges.
Combined dissolved and exsolved volatile contents of N-MORB from the Gorda Ridge with 12.8-15.6% spherical vesicles are about 0.78% CO2 and 0.18 wt% H2O and exceed estimates of primary CO2 of only 0.07 to 0.095 wt% calculated from whole rock Nb concentrations. This discrepancy suggests that the magmas accumulated an exsolved volatile phase prior to eruption. The evidence that a separated volatile phase drives strombolian eruptions on the seafloor also implies that gas bubbles coalesce during storage or transport to the surface. The combination of large bubbles in otherwise dense magma suggests nearly complete coalescence of small bubbles and is most consistent with accumulation of the exsolved volatile phase, most likely near the tops of crustal magma chambers, prior to upward transport in shallow conduits to the eruptive vents on the seafloor. A portion of this CO2-rich separated fluid phase is released in brief bursts during eruptions where it becomes part of event plumes.
Reference: D.A. Clague, A.S. Davis, J. E. Dixon (2003) Submarine strombolian eruptions on the Gorda mid-ocean ridge, In: Explosive Subaqueous Volcanism, J.D.L. White, J.L. Smellie, and D.A. Clague (eds), Geophysical Monograph 140, American Geophysical Union, 111-128.
Glass grains offer clues
GORDA RIDGE - Volcanic glass fragments recovered from sediment collected in the Escanaba Trough and on traverses up the west walls of the Gorda Ridge greatly extend information on the petrologic diversity and volcanic evolution of the region, compared to that obtained from traditional samples from lava outcrops. Glass fragments were found loose in sediments and cemented on benthic foraminifera that constructed their tests from detrital grains. Many of the glass grains have compositions that closely match those of nearby lava outcrops, indicating that they formed by spalling off pillow rims and quench granulation. However, the glass compositions also provide information on compositions of unexposed flows that were covered by later eruptions and/or sediment. Bubble-wall glass shards (limu o Pele), found only in the sediment, give evidence of explosive eruptions that occurred despite great depth.
Some of the glass fragments from the walls of the axial valley have unusually high K2O (to >0.7%) that is not represented by any sampled outcrop; such enriched mid-oceanic-ridge basalt compositions have not previously been reported from the Gorda Ridge. Analyzing glass grains from the sediment in addition to samples from lava outcrops allows a more comprehensive characterization of the volcanic history of the region.
Reference: A.S. Davis, D.A. Clague (2003) Got glass? Glass from sediment and foraminifera tests contribute clues to volcanic history, Geology, 31:(2): 103-106. [Abstract] [Article]
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