Decompression melting at a divergent boundary produces which type of magma and forms which rocks?

Decompression melting at a divergent boundary produces which type of magma and forms which rocks?

• A. Mafic magma; forms both extrusive and intrusive rocks
• B. Felsic magma; forms only intrusive rocks
• C. Ultramafic magma; forms only extrusive rocks
• D. Intermediate magma; forms mainly sedimentary rocks

Answer: (A)

At divergent boundaries, mantle material rises and pressure drops, causing decompression melting. This melting produces basaltic magma, which is mafic in composition (rich in Mg and Fe, relatively low in silica). As this magma moves upward, it can crystallize underground to form gabbro (an intrusive rock) or erupt at the surface to form basalt (an extrusive rock). So the magma is mafic, and it forms both extrusive and intrusive rocks.

At divergent boundaries, mantle material rises and pressure drops, causing decompression melting. This melting produces basaltic magma, which is mafic in composition (rich in Mg and Fe, relatively low in silica). As this magma moves upward, it can crystallize underground to form gabbro (an intrusive rock) or erupt at the surface to form basalt (an extrusive rock). So the magma is mafic, and it forms both extrusive and intrusive rocks.