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Geology

First evidence of deep explosive volcanic activity at the Marsili Seamount

Paper: Iezzi, G., Caso, C., Ventura, G., Vallefuoco, M., Cavallo, A., Behrens, H., Mollo, S., Paltrinieri, D., Signanini, P., Vetere, F., 2014. First documented deep submarine explosive eruptions at the Marsili Seamount (Tyrrhenian Sea, Italy): A case of historical volcanism in the Mediterranean Sea. Gondwana Research 25: 764-774.

Introduction

In this study, Iezzi et al. document for the first time the occurrence of two tephras found in a gravity core collected on the Marsili Seamount, located between mainland Italy and Sicily. As defined by the USGS, tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions, or carried upward by hot gases in eruption columns or lava fountains. The significance of the study is that it puts new, unexpected constraints on the age and type of activity of submarine volcanoes in the Mediterranean area and the hazard implications.

Findings

The Marsili Seamount is the about 3.2km tall with a 70km by 30km width, making it the largest volcanic complex in the Mediterranean area and Europe. It has long been thought the seamount formed from effusive lava flows (a non-explosive eruption with lava acting like a thick, sticky liquid) between 1 and 0.1 Ma. By analyzing the tephra deposits found in a core taken from the seamount, Iezzi et al. have concluded that the effusive flow model and age constraints are incorrect. Instead, the tephras are a result of explosive submarine volcanic activity that occurred only 3 thousand years before present time. This is the first evidence of explosive volcanic activity at significant depth (500-800 meters below sea level) found in the Mediterranean.

Location of gravity core on the Marsili Seamount, between Sicily and mainland Italy.

Location of gravity core on the Marsili Seamount, between Sicily and mainland Italy.

While some of the initial evidence suggests a provenance of the ashes from nearby subaerial Mt. Etna or Pantelleria, these ashes are ultimately excluded from those sources because they lack the Na-alkaline affinity.  Meaning, when chemical composition of the glass in the ash is analyzed, those from Marsili have less elemental sodium than others nearby.  The determined chemical composition can be used like a finger print for glass origin. Other regional volcanoes are similarly excluded as the tephra source for inadequate timing or the wrong rock type. In addition, morphological and stratigraphic evidence further supports submarine origin.

Analysis of the tephra supports the explosive submarine volcanic model, however it would help to bolster Iezzi’s case if the data set included more than one gravity core.  The team does acknowledge the lack of deposits in the proximal bathyal plains, but suggests this implies that the two eruptive events must have been low energy and did not produce enough material to travel far. Further expeditions in the region should consider adding to the data set.

Significance

Because of known seismicity and active hydrothermal degassing in the region, this study means that the still active Marsili Seamount could potentially erupt explosively in our time. The seamount should continue to be monitored closely and the related hazards should be evaluated for nearby communities.

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