CONTINUED...FROM PREVIOUS POST... Once fluids like water are released from the minerals and are mixed in with surrounding rocks, their presence can lower the melting temperature to the point that partial melt can occur! This partial melt results in the material RISING, eventually into magma chambers or all the way to the surface as volcanic activity (Marshak, 2005)! Continued volcanic activity results in the formation of volcanic arcs, both in oceanic-oceanic subduction (island arcs) and oceanic-continental subduction (continental arcs). Real world examples of island arcs include the Philippine Islands, Mariana Islands, and the Tonga Islands. Examples of real world continental arcs include the Cascade Volcanic Arc, Andean Volcanic Belt, and the Central American Volcanic Arc. Google them for more info!
This process of release and partial melting occurs at relatively consistent depths of around 110km (Tatsumi, 1986). As the depth is consistent, the distance between the actual subduction trench and the volcanic arc can tell us information about the angle of the subducting plate. Essentially, it makes a triangular shape in which we know the length of two sides, and can use Pythagoras theorem to calculate the angle of the slab (the hypotenuse!).
Marshak, S. (2005). Earth: Portrait of a Planet. W.W. Norton & Company, New York.
Winter, J. D. (2001). An Introduction to Metamorphic and Igneous Petrology. Prentice Hall. New Jersey.
Tatsumi, Y., Hamilton, D. L., & Nesbitt, R. W. (1986). Chemical characteristics of fluid phases released from a subducted lithosphere and origin or arc magmas: Evidence from high-pressure experiments and natural rocks. J. Volcan. Geotherm. Res., 29, 293-309.
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