Types of Igneous Rocks:
Basalt
Andesite
Dacite
Rhyolite
Typical characteristics:
Arcuate volcanic island chains occur along subduction zones.
Composition is more diverse and silicic than other volcanic provinces.
Basalt is generally subordinate.
More explosive eruptions.
Strato-volcanoes are the most common volcanic landform.
Aleutians
Marianas
Tongan
Japan
Typical cross-section shows complex tectonic settings.
High proportion of basaltic andesite and andesite found in subduction zones.
Common volcanic rock types classified by their relative proportions, e.g.:
Talasea, Papua: 9% B (Basalt), 23% B-A (Basaltic Andesite), 55% A (Andesite), 9% D (Dacite), 4% R (Rhyolite).
Types of Subduction Zones:
Ocean-ocean → Island Arc (IA)
Ocean-continent → Continental Arc (Continental Margin, ACM)
Alaska
Andes
Cascades
Mexican Volcanics
Characteristic igneous associations include:
Basalt, Andesite, Dacite, Rhyolite.
These products also display distinctive patterns of metamorphism and orogeny contributing to mountain belts.
Three primary magma series:
Tholeiitic (MORB, OIT, Subduction)
Alkaline (OIA, Subduction)
Calc-Alkaline
Melting related to subduction is confined to the mantle above the slab.
Involves andesite, dacite, and rhyolite through processes of:
Differentiation
Crystal Fractionation
Assimilation
Altered oceanic crust of the subducted slab
Mantle wedge between the slab and arc crust
Presence of water is significant to arc magmatism.
Temperature and depth influence melting and mineralogy in the mantle wedge.
Dehydration leads to the production of hydrous minerals.
Transition from Blueschist to Eclogite involves dehydration and changes in rock composition.
Blueschist: Glaucophane + Epidote.
Eclogite: Omphacite + Garnet.
Melting begins at ~100-120 km depth.
Produces basalts, which evolve to andesite or more silicic compositions through fractional crystallization and assimilation.
Arc magmatism involves complex interactions between slab fluids and mantle dynamics.
Subduction-induced melting produces hydrous basalts, followed by evolution to more silicic magma types (andesite, dacite, rhyolite).
Comparison of arc volcanics with mid-ocean ridge basalts (MORBs) shows significant differences in trace elements, influenced by slab-derived fluids.