Why is the center of the Earth so hot?

Question

BlackTom AI · Accepted Answer

To understand why the Earth's center is so hot, let's evaluate each claim in turn and see which one aligns with geophysical evidence.
Option 1: "Lava being subducted and sinking into the core". Subduction brings surface rocks into the mantle, but it does not transport lava into the outer core or core itself. The process is too slow and occurs at tectonic plate boundaries; it cannot account for the core’s internal heat source. This makes the statement scientifically inaccurate as a primary heat driver for the center.
Option 2: "Radioactive decay of heavy elements in the core". This is a key source of internal heat: long-lived radioactive isotopes (such as uranium, thorium, and potassium, especially when present in the mantle and core) decay and release heat, contributing to the overall thermal energy that keeps the interior hot. While not the only source (primordial heat from formation also matters), it directly explains sustained high temperatures inside the Earth. This option correctly identifies a major heat source.
Option 3: "Heat captured by the Sun". The Sun’s energy warms the Earth’s surface and atmosphere, but it does not significantly penetrate to the core. The heat at the center is internal and generated by radioactive decay and residual formation heat, not by solar heating. Therefore this option misattributes the source of core heat.
Option 4: "The incredibly strong gravity compressing rock into lava". Gravity does cause compression and associated heating during planetary formation, but over geological timescales the current core temperature is not maintained primarily by ongoing compression of rock into lava. The phrase also misrepresents the state of matter at the core (the inner core is solid, the outer core is liquid iron alloy), and compression heating alone cannot explain the sustained heat observed today. This makes the statement misleading as a description of the present heat source.
Overall, option 2 best matches the established understanding of Earth's internal heating, while the others either describe processes that are not the main heat contributors or misrepresent how core heat is generated and maintained.

ASTR-1-45046-F25 Lecture: Solar System Formation

Why is the center of the Earth so hot?

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