Hornfels is formed by which metamorphic process, and typically involves which minerals?

• A. Contact metamorphism; biotite and andalusite
• B. Regional metamorphism; garnet and kyanite
• C. Contact metamorphism; quartz and feldspar
• D. Regional metamorphism; hornblende and plagioclase

Answer: (A)

Hornfels form when heat from a nearby igneous intrusion bakes surrounding rocks, causing metamorphism under relatively low pressure. This heat-driven, low-pressure environment leads to non-foliated, fine-grained textures and favors minerals stable at high temperature but low pressure. Andalusite forms under these low-pressure, high-temperature conditions, and biotite (a common metamorphic mica) also crystallizes readily in hornfels. Together, these minerals reflect the heat-centric, near-surface metamorphism characteristic of hornfels. In contrast, regional metamorphism involves higher pressures and directed stresses that produce foliated textures and minerals like garnet or kyanite, which is why the combination of biotite and andalusite best fits hornfels.