What process leads to melting at a divergent boundary?

• A. Decrease in pressure causing the asthenosphere to melt
• B. Increase in pressure causing mineral stability
• C. Addition of water from subducting slab
• D. Cooling of magma at the surface

Answer: (A)

Decompression melting at divergent boundaries explains why melting occurs there. When mantle material rises toward the surface at a spreading ridge, the pressure drops even as temperatures stay high enough. This pressure decrease lowers the solidus temperature of the mantle rock, causing it to partially melt and form basaltic magma. That magma then rises and creates new oceanic crust as the plates move apart. The other options describe processes associated with different tectonic settings: adding water from a subducting slab triggers flux melting at subduction zones, increased pressure stabilizes minerals and prevents melting, and cooling magma at the surface leads to solidification rather than melting.