Characterized by a diverse and silicic composition
Generally more explosive volcanic activity
Strato-volcanoes are the most common landform
Continental Arcs:
Examples include Alaska, Andes, Cascades, and Mexican Volcanics
Structure of an Island Arc
Schematic Diagram:
Shows typical cross-section of an island arc
Volcanic Rock Composition:
High proportion of basaltic andesite and andesite
Most andesites occur in subduction zones
Example Localities and Composition Proportions
Talasea, Papua:
B: 9%, B-A: 23%, A: 55%, D: 9%, R: 4%
Little Sitkin, Aleutians:
B: 0%, B-A: 78%, A: 4%, D: 18%, R: 0%
Mt. Misery, Antilles:
B: 17%, B-A: 22%, A: 49%, D: 12%, R: 0%
Subduction and Volcanism
Two types of arcs related to subduction:
Ocean-Ocean → Island Arc (IA)
Ocean-Continent → Continental Arc (CA)
Magma Series
Key Types:
Tholeiitic (related to mid-ocean ridge and subduction)
Alkaline (common in ocean-island arcs)
Calc-Alkaline (typical in subduction zones)
Origin of Subduction Volcanics
Melting Mechanism:
Melting is largely confined to the mantle above the subducted slab
Results in the generation of abundant basalts, and andesites, dacites, and rhyolites
Factors Influencing Melting
Crystal Fractionation
Assimilation
Magma Source Components
Crustal Element:
Altered oceanic crust and forearc sediments
Mantle Wedge:
Interaction of fluids and subducted materials influences arc magmatism
Interaction of Slab and Mantle Wedge
Dehydration:
H2O released from the subducting slab plays a critical role in potentials for melting in the mantle wedge
Geological Indicators
Comparison of incompatible elements in mantle-derived basalts (MORBs) and subduction-derived volcanics indicates significant water enrichment in arc magmas
P-T-t Paths in Subduction Zones
Temperature and Pressure Paths:
Hotter conditions are above the slab, influencing mineralogical changes from blueschist to eclogite
Example Reactions
Transition from blueschist minerals (glaucophane + epidote) to eclogite minerals (omphacite + garnet)
Importance of Water in Mantle Wedge
Water released from the subducting slab hydrates the mantle, creating mafic magmas
Summary Points
Volcanic Activity Related to Subduction:
Calc-Alkaline volcanism dominated by high water content and interaction between fluids
Melting at depths of around 100 to 120 km significantly contributes to basaltic magma generation
Evolution of Magma:
Basalt evolves through fractional crystallization to produce andesites, dacites, and rhyolites
Cross-Contribution:
Subduction zones are a unique melting environment, differentiating them from plume melting typically observed at mid-ocean ridges