Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits

Abstract

Submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur. Consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows, however, that significant pyroclastic activity can occur even at depths in excess of 3000 m. In order to document and illustrate the full range of submarine eruption styles, we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions: (1) no gas exsolution; (2) gas exsolution but no magma disruption; (3) gas exsolution, magma disruption, and hawaiian-style fountaining; (4) volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse; (5) magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity; and (6) volatile content builds up in the top of a dike leading to vulcanian eruptions. We also examine the role of bulk-interaction steam explosivity and contact-surface steam explosivity as processes contributing to volcaniclastic formation in these environments. We concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities, the most likely circumstance is the quiet effusion of magma with minor gas exsolution, and the production of somewhat vesicular pillow lavas or sheet flows, depending on effusion rate. The amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption. The most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity, due to the localized concentration of volatiles in magma that has a low rise rate; magmatic gas collects by bubble coalescence, and ascends in large isolated bubbles which disrupt the magma surface in the vent, producing localized blocks, bombs, and pyroclastic deposits. Another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions; these deposits, being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater, should be easily distinguishable from strombolian and hawaiian eruptions. However, we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles. In this case, a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian-style activity; the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile-induced hawaiian eruption. Specifically, typical pyroclast sizes might be smaller; fountain heights may exceed those expe...

Did the 1999 earthquake swarm on Gakkel Ridge open a volcanic conduit? A detailed teleseismic data analysis

“…Sohn et al (2008) suggest deep gas accumulation and its rapid explosive discharge following models of Head and Wilson (2003) for these vents. Thus, it is likely that the seismic swarm with its high energy release could occur due to a rapid and contemporary magmatic intrusion that later delivers the magmatic ground mass for the discovered young Limu o Pele on the rim of these vents.…”

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas1This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology. Canadian Journal of Earth Sciences, 48: this issue.2MRNF Contribution BEGQ 8439-2010/2011-1. Natural Resources Canada, Earth Science Sector Contribution 20100253.

“…Explosive eruptions involving mafic to intermediate magmas are entirely feasible in the submarine environment, even at water depths greater than 1 km, as demonstrated by theoretical modeling (Head and Wilson 2003) and ocean floor drilling (Gill et al 1990). Submarine explosive eruptions can form density currents that are either gas-supported (true pyroclastic flows) or water-supported (aqueous currents).…”

Influence of volatile degassing on initial flow structure and entrainment during undersea volcanic fire fountaining eruptions

“…fragmentation in the submarine environment [2,16]. Thus submarine fire fountaining may be driven by volatile contents higher than those typically associated with the primary gas content of seafloor basalts.…”

“…Geological evidence suggests that submarine fire fountaining takes place in the marine environment based on the occurrence of fluidal breccias and highly vesicular basaltic clasts that show similarities to their subaerial counterparts [1]. Head and Wilson [2] have presented a comprehensive treatment of the theoretical considerations of submarine fire fountaining by exsolution of primary volatiles (CO 2 and H 2 O). In their model they assume that fragmentation occurs when a gas volume fraction of 0.75 is attained by gas exsolution either within the conduit or right at the vent.…”