Which factors determine the magnitude of geological activity on a terrestrial planet?

• A. Its size and the amount of internal radioactive heating.
• B. Its distance from the Sun and atmospheric density.
• C. Its surface gravity.
• D. Its orbital eccentricity.

Answer: (A)

Geological activity on a terrestrial planet is driven by its internal heat budget. The heat mostly comes from radioactive decay inside the planet, and how long and how strongly this heat persists depends on the planet’s size: a larger planet has more rock and more radioactive material, so it can generate more heat and retain it longer because its heat loss per unit mass is lower. This sustained internal heat powers mantle convection, partial melting, and tectonic processes that drive volcanism and crustal deformation. The option that pairs a planet’s size with the amount of internal radioactive heating best captures what sets the magnitude of geological activity. Distance from the Sun and atmospheric density mainly affect surface conditions and weathering, not the planet’s internal energy source. While gravity and orbital eccentricity can influence how geology expresses itself, they don’t determine the overall heat reservoir that drives long-term geological activity.