What causes volcanism at a subduction zone, and how does Bowen's ...

[geodog55]

What causes volcanism at a subduction zone, and how does Bowen's reaction series explain the
  composition of the magmas there? What will be an ideal response?

Question 2

How does mafic magma originate from ultramafic rock at a spreading ridge? What will be an ideal response?

[recede]

Answer to Question 1

At subduction zones, the magmas are intermediate or felsic. The suducting plate reaches a depth where
the temperature is high enough to begin partial melting, and the oceanic crust on top of the plate loses
its water, which rises into the overlying mantle and enhances melting. The melting of the oceanic crust
yields more silica-rich magmas than at the spreading ridges, and subducted sediments also contribute
silica to the magma. Finally, as the magma rises through the continental crust (if it's an ocean,
continent subduction zone), the magma picks up silica-rich minerals.

Answer to Question 2

As the ultramafic mantle melts to form magma, the first minerals to melt, according to Bowen's
reaction series, are the more felsic minerals in the mantle, quartz, potassium feldspar, and sodium-rich
plagioclase. When the ultramafic rock starts to melt, the felsic minerals have already become part of
the magma, so the magma that erupts at the spreading ridge is more felsic than its source

[geodog55]

TYSM

[kjohnson]

Wow, this really help