Submarine volcanism

The volcanic complex at 85°E on Gakkel Ridge

In 1999, the global seismological network recorded an unusually long-lasting and strong earthquake swarm from the eastern part of Gakkel Ridge, where spreading rates are as low as 10 mm/y (Fig. 1). Over 7 months, more than 200 earthquakes with magnitudes up to 5.5 were registered at teleseismic distances. At the same time, the SCICEX submarine expedition surveyed the area twice and detected a fresh lava flow at a large volcanic complex near 85°E where the earthquake activity concentrated (Edwards et al., 2001).  The earthquake swarm attracted considerable attention (Müller & Jokat, 2000; Tolstoy et al., 2001) as submarine eruptions usually last less than a month and the associated earthquakes are too small to be recorded on land.

During the AMORE 2001 expedition, we recorded microseismicity on an ice floe approximately 35 km north of the volcanic complex at 85°E (Schlindwein et al., 2005). We detected a swarm of more than 200 explosive sound signals that originate at the seafloor near 85°E. We believe that these sounds are produced by the interaction of erupting lava with the sea water. This rare in situ observation of a submarine eruption stimulated many questions on the volcanic processes at this submarine volcanic complex.

• How do submarine volcanoes work at ultraslow spreading rates?
• Is there a magma chamber?
• What are the differences to volcanic episodes on other mid-ocean ridges?

We currently work on improved epicentre locations and focal mechanisms for the teleseismic earthquake swarm that will allow us to reconstruct the development of the seismic activity in space and time during the volcanic episode. Microearthquake data including the explosive volcanic signals from 2001 and the newly acquired data from AGAVE2007 will help to characterise the behaviour of this large submarine volcano.

Terceira rift

In direct comparison to Gakkel Ridge we are cooperating with the Centro de Vulcanologia e Avalicao de Riscos Geologicos Ponta Delgada / Azores on the investigation of seismic consequences of ultra-slow spreading along the Terceira Rift in the North Atlantic (Vogt and Jung, 2003). Part of the ultra-slow spreading is accommodated onshore on the islands of Terceira and Sao Miguel and due to the installation of a permanent network for hazard mitigation we are able to analyze the seismic structure of the complex using methods such as local earthquake tomography and double difference relocation of local events which will give us a deep insight into how ultra-slow spreading ridges work.

A third focus is the North of Iceland, where the NICE expedition in 2004 deployed 14 ocean-bottom short period seismic stations for a 3 month period and multibeam bathymetric soundings were used to analyze the crater region of the submarine Storagrunn volcano and the southernmost part of Kolbeinsey Ridge and its isolated small seamounts chains running oblique to the slow spreading. First results were presented in Riedel et al. (2006). This part of the Mid-Atlantic ridge is known as Tjörnes Fracture Zone and resembles in many way spreading scenarios as those on the Azores.