What process is responsible for or leads to magma that is intermediate and often variable in chemistry, when erupted in the cascades?

Intermediate magma is also known as andesitic magmas and these are created when an oceanic plate subducts beneath a continental plate. The magma is generated in the wedge of mantle rock beneath the crust. The Cascade Mountain Range is formed due to the subduction of the Juan de Fuca Plate beneath the North American Plate. 

4, two dominant and two recessive