Abstract <p>The results of a comprehensive investigation of fluid-bearing magmatic systems of the island-arc type are presented, with Kamchatka serving as a case study. The research was conducted at the interface of petrology, geochemistry, and mineralogy, employing modern methods for analyzing melt and fluid inclusions (SIMS, LA-ICP-MS, Raman spectroscopy) and numerical modeling using the Petrolog software package. More than 50 volcanic objects were studied in detail; over 10 objects containing mineral parageneses reflecting the melting conditions of the mantle substrate were identified. It was established that, along with classic mantle sources (depleted mantle + aqueous fluid), melts formed during partial melting of lower crust amphibolites (8–10 kbar, 1130–1160°C, degree of melting &gt;25%) play a significant role in magma generation. A model of island-arc magmatism evolution is proposed, involving the sequential engagement of mantle, crustal, and restitic sources. Based on the analysis of products from nine catastrophic eruptions over the last 10 kyr, degassing volumes were estimated for the first time (≥8 Tt of gases in total, including 160 Mt of SO₂ and 250 Mt of Cl), along with their potential climate impact. Using the 2017 eruption of Bezymianny volcano as an example, a two-chamber model of the trigger mechanism for explosive eruptions was developed, associated with hornblende decomposition and fluid migration.</p>