Explosive mud volcano eruptions and rafting of mud breccia blocks

Abstract

<p>Azerbaijan hosts the highest concentration of mud volcanoes (MVs) on Earth with structures that may reach several kilometres in diameter and the height up to 600 m. Many of these structures alternate between periods of dormancy with vigorous eruptions. The frequency of the eruptive activity varies between MVs, and is typically related to the time required to build sufficient overpressure able to drive the extrusion of fluids and mud breccia at the surface.</p><p>Lokbatan is possibly the most active MV on Earth exhibiting powerful eruptions occurring every 3-5 years. These phenomena manifest with spectacular gas flares that reach several tens of meters in height and the bursting of thousands of m<sup>3</sup> of mud breccia resulting in spectacular mud flows that extend for more than 1.5 kilometres. Unlike other active MVs, Lokbatan does not show any visual evidence of diffuse degassing (e.g. active pools of gryphons) in the crater zone. Gas flux measurements completed with closed-chamber technique sensitive to ppmv, reveal extremely low values throughout the structure around the crater with average CH<sub>4</sub>=0.13 g/m<sup>2</sup>day and CO<sub>2=</sub>4.53 g/m<sup>2</sup>day. We suggest that after eruptive events, the mud breccia is able to seal the structure preventing the gas release and therefore promoting the overpressure build-up in the subsurface. This self-sealing mechanism allows a fast recharge of the MV resulting in more frequent and powerful eruptions. These spectacular phenomena are able to release in short time intervals massive amounts of gas, erupted mud breccia and energy due to the sudden overpressure release. Our field observations reveal the presence of large (up to ~50,000 m<sup>3</sup>) stratified blocks that were originally part of a large crater cone. These blocks were rafted > 1 km from the vent on top of mud breccia flows. We use a lubrication theory model to show that it is reasonable to transport blocks this large and this far provided the underlying mud flow was thick enough. The presence of large rafted blocks is not a unique phenomena observed at Lokbatan MV and is documented also at other large-scale structures.</p>