Describe the source of the plutons of magma that rise into mountain ranges and feed volcanoes overlying subduction zones
The correct answer and explanation is :
The source of the plutons of magma that rise into mountain ranges and feed volcanoes overlying subduction zones is the melting of the mantle wedge above a subducting tectonic plate. This occurs due to the process of flux melting, which is triggered by the release of water and other volatiles from the subducting oceanic crust.
Explanation (300 words)
Subduction zones are tectonic boundaries where one lithospheric plate is forced beneath another into the mantle. Typically, an oceanic plate subducts beneath a continental or another oceanic plate due to differences in density. As the descending plate sinks into the asthenosphere, it experiences increasing pressure and temperature, leading to dehydration reactions in minerals such as amphibole and serpentine. This process releases water and other volatiles into the overlying mantle wedge.
Water significantly lowers the melting point of the mantle rock (mainly peridotite), causing partial melting to occur. This generates basaltic magma, which is less dense than the surrounding mantle and begins to rise toward the surface. The magma may either:
- Accumulate in magma chambers and slowly cool and crystallize, forming plutons of intrusive igneous rock (such as granite or diorite).
- Erupt explosively or effusively at volcanic arcs, forming stratovolcanoes, island arcs, and other volcanic features.
Over time, repeated intrusions and eruptions contribute to the growth of continental mountain ranges and volcanic arcs (e.g., the Andes, Cascades, and Japanese island arcs). Some of the magma differentiates, evolving into andesitic or rhyolitic compositions due to fractional crystallization, assimilation of continental crust, and magma mixing.
Thus, the interaction of subducting oceanic lithosphere, volatiles, and mantle melting is the fundamental source of magma that fuels the plutons and volcanoes found in subduction-related mountain belts.