Which combination best describes the factors controlling geological activity?

• A. Distance from the Sun and atmospheric composition.
• B. The planet's age and magnetic field strength.
• C. Its size and the amount of internal radioactive heating.
• D. Its rotation rate and axial tilt.

Answer: (C)

Geological activity is driven by the planet’s internal heat powering mantle convection and tectonic processes. The amount and duration of that heat depend most on how big the world is and how much radioactive material it contains. A larger planet retains heat longer and produces more heat from radioactive decay, sustaining magma generation, mantle flow, and surface volcanism. Distance from the Sun and atmospheric composition affect surface climate, not the internal heat engine that drives geology. Age and magnetic field influence cooling history and core dynamics, but they don’t set the ongoing heat budget as directly as size plus internal radioactive heating. Rotation rate and axial tilt affect climate and surface dynamics, not the core energy source behind geology. So the combination of size and internal radioactive heating best explains geological activity.