Subduction Zones: Mechanics and Seismicity
📌 Earthquakes in subduction zones are distributed along the Wadati-Benioff zone, extending to several hundred kilometers deep.
🌋 The existence of deep earthquakes confirms that the lithosphere remains rigid and brittle as it sinks into the mantle, contrasting with the ductile behavior of the underlying asthenosphere.
📉 Subduction occurs when oceanic lithosphere becomes denser than the surrounding asthenosphere due to cooling as it moves away from the mid-ocean ridge.

Magmatism in Subduction Zones
🌋 Magmatism, resulting in surface volcanism (basalts, andesites, etc.) and plutonism (granitoids), is characteristic of subduction zones.
💧 The common trait of these magmatic rocks is their high water content.
🔥 Partial melting of peridotite cannot occur based on the subduction zone's geothermal gradient unless the rock is hydrated, which lowers the melting point.

Source of Hydration and Melting
💧 The melting originates in the overriding plate's peridotites because they become hydrated.
🧊 Oceanic lithosphere forms with hydrated minerals (containing hydroxyl groups, OH⁻) due to hydrothermal metamorphism near the spreading ridge (forming meta-gabbros in the green schist facies).
💧 As the subducting slab sinks, metamorphism leads to the formation of blue schist facies (containing glaucophane, still hydrated), and subsequently to the eclogite facies (containing jadeite and garnet), where water is expelled.

Key Points & Insights
➡️ Subduction initiation is driven by the increasing density of the oceanic lithosphere with age.
➡️ The water released from the subducting slab during high-pressure metamorphism (transitioning from blue schist to eclogite facies) migrates to the overriding plate.
➡️ Water lowers the solidus temperature of the mantle wedge peridotite, directly causing the necessary partial melting that fuels dangerous, explosive volcanism.

📸 Video summarized with SummaryTube.com on Jan 06, 2026, 18:43 UTC