The peculiarities of the junction between the Kuril–Kamchatka and Aleutian island arcs are studied using different techniques and experimental data. The models of medium structure that were obtained by using surface wave tomography and studies of S-wave anisotropy in the asthenosphere beneath the subduction plate detected an ascending asthenospheric flow of hot material and fluids along the northern boundary of the lithospheric plate. The presence of the ascending asthenospheric flow in the junction zone has been recently confirmed by detailed body-wave seismic tomography. Additional feeding of the Klyuchevskoi volcanic group from the asthenospheric flow explains the great difference in the eruptive activity of these volcanoes in comparison to other volcanoes located in the Eastern volcanic belt of Kamchatka. Comparative study of Quaternary volcanoes lying in the Eastern volcanic belt has revealed a strongly pronounced decreasing productivity from north to the south, i.e., with distance from the end of the subduction plate. This reduction is likely related to a depletion in the invasion of magmatic melts along the subduction plate from the asthenospheric flow in the junction of the Kuril–Kamchatka and Aleutian island arcs.