At an oceanic - oceanic convergent plate boundary,

At an oceanic/oceanic plate boundary 2 oceanic plates will collide against each other. The older oceanic plate will sub-duct under the newer one. It will sub-duct into magma, and will melt into it. The plate and magma will create a reaction, and will build up pressure. With enough pressure, magma will erupt creating a volcano.

Ch4 is nonpolar and insoluble in water. since all of the hydrogen atoms are evenly spread around the carbon atom even through the c h bond is polar all the poles cancel each other out.