How do convergent and divergent plate boundaries cause magma formation?
Plate boundaries create cracks in the earth's crust that allow liquidized rock from the mantle to penetrate the crust and become magma.
Divergent plates like the mid Atlantic ridge are places where two ocean plates separate and pull apart. The crack created by the separation allows liquid rocks to press out. The heavy rock usually forms basalt from the magma. In places like Iceland new Islands come to the surface from the magma pressing up from the mantle.
Subduction Zones a form of converging plants not only provide a place for liquid rock to reach the crust but also melt rocks. In a subduction zone the ocean plate is forced under the lighter (less dense) land plate. The friction as the huge plates of earth are pushed past each other creates heat that melt the plates ( distroying the sedimentary layers of the ocean plate) The magma that reaches the surface forms volcanos and becomes lava. The magma that fails to reach the surface becomes granite.
The so called ring of fire around the Pacific Ocean are volcanos formed by subduction zones.