Which area is associated with thermal metamorphism?

• A. Near magma intrusion
• B. Deep within the mantle
• C. At ocean floor midpoints
• D. In high-grade regional zones

Answer: (A)

Thermal metamorphism is driven by heat from a nearby heat source, typically an igneous intrusion. When magma forces its way into surrounding rocks, the intense heat radiates into the adjacent rocks, raising their temperatures enough to cause minerals to recrystallize and form new metamorphic assemblages. Because the heating is localized and pressure doesn’t increase much, this produces a narrow zone around the intrusion with textures that are often non-foliated (think hornfels or other contact-metamorphic rocks). That close-in zone is the classic setting for thermal metamorphism.

The other settings involve different processes. Deep in the mantle, metamorphism is mainly governed by high pressures and temperatures in a vastly different regime, not by heat from a local intrusion. At ocean-floor ridges, alteration is largely hydrothermal: hot fluids circulating through rocks, changing mineralogy rather than simply heating them. Regional, high-grade metamorphism occurs over large areas due to tectonic forces and elevated pressures, not just the heat from a single intrusion.