Kinds of Convergence Zones

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When a denser oceanic plate collides with a continental plate, the ocean plate goes under the continental plate. A great deal of energy is dissipated at the collisional interface which results in the melting of the continental crust. The surface manifestation of a subduction zone is active vulcanism. The kinds of volcanoes which can occur depends on the chemical composition of the magma which determines its viscosity. There are basically three types of flows:

• Basaltic - low viscosity --> makes lava plains (e.g. Central Oregon) or large Shield Volcanoes
• Andesite --> fairly viscous and which makes large Strato-Volcanoes that you can ski on later. The Andes mountains are made mostly of Andesite
• Dacite or rhyolite --> real cool stuff which is so viscous that it solidifies upon extrusion and plugs of the vent or fissure. Pressure builds and the dacite dome explodes. Good examples of this are Crater Lake and Mt. Saint Helens

Detailed diagrams of the subduction process are shown below. In the case of oceanic-oceanic plate subduction still occurs and volcanism rsults. For continental-continental plate collisions there is no subduction and hence no volcanism but just general uplift. The Himalayan mountain range formed in this manner.

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