In a magnetism-related thought experiment, which rock type would be moved more easily?

• A. Felsic
• B. Mafic
• C. Intermediate
• D. Ultramafic

Answer: (B)

The main idea is how strongly a rock responds to a magnetic field, which depends on magnetic susceptibility—the presence of iron-rich minerals that can align with the field. Mafic rocks contain more iron and often minerals like magnetite, giving them a higher magnetic susceptibility. That stronger magnetic response means, in a magnetic field gradient, the force acting on a mafic rock is larger, so it would move more easily than rocks with less iron. Felsic rocks are silica-rich and iron-poor, so their magnetic response is weak and they’d move less. Intermediate rocks have a moderate iron content and respond somewhere in between. Among the options given, the rock with the greatest tendency to be moved by a magnetic field is the mafic rock.