The world's strongest known spreading-related seismicity swarm occurred in 1999 in a segment of the Gakkel Ridge located at 85 degrees E as a consequence of an effusive-explosive submarine volcanic eruption. The data of a seismic network deployed on ice floes were used to locate hundreds of local earthquakes down to similar to 25 km depth and to build a seismic tomography model under the volcanic area. Here we show the seismicity and the distribution of seismic velocities together with the 3D magmatic-thermomechanical numerical model, which demonstrate how a magma reservoir under the Gakkel Ridge may form, rise and trigger volcanic eruptions in the rift valley. The ultraslow spreading rates with low mantle potential temperatures appear to be a critical factor in the production of volatile-rich, low-degree mantle melts that are focused toward the magma reservoirs within narrow magmatic sections. The degassing of these melts is the main cause of the explosive submarine eruptions. Joint implementation of seismic tomography and numerical thermo-mechanical modeling sheds light to the causes of explosive submarine volcanic eruptions on the Gakkel Ridge and deeply penetrating seismicity down to 25 km depth.