My daughter and I have been getting into volcanoes. We both had many questions about them. I couldn’t explain subduction-related volcanoes but thankfully this book could, with cool pictures. I ended up reading the whole thing because plate tectonics is so fascinating. (But the detailed descriptions of orogenies were very sleep inducing.)
From the book:
“At first glance, it would seem paradoxical that subduction zones, some of the coldest tectonic zones, at given depths produce prolific amounts of magmatism. How can locations where cold lithosphere is rapidly subducted, in turn, cause magma to rise and feed intense plutonism and volcanism?”
“Melting above the subduction zone is a complicated process but can be described in a simple way: Sediments and ocean crust … release water upwards. The water-bearing fluids rise into the hot asthenosphere wedge between the lower and upper plate and cause melting. Melting is initiated because the solidus temperature … is exceeded at this location below the volcanic belt. When the solidus temperature is exceeded, the melting process begins … water depresses the melting point of rocks.”
“The critical role of water in the evolution of subduction-related magmatic rocks is mirrored in their water content, generally comprising several percent by weight.”