The correct option is It comes from radioactive decay and residual heat.
Explanation
The Earth possesses a massive internal heat budget that drives endogenic geomorphic processes such as volcanism, plate tectonics, and earthquakes. This heat increases with depth, following a trend known as the geothermal gradient.Detailed Analysis:
- It comes primarily from solar radiation absorbed by the crust. is Incorrect: Solar radiation is the primary energy source for surface processes (exogenic forces) and atmospheric circulation. It penetrates only a negligible depth into the crust and does not contribute to the deep internal heat of the planet.
- It comes from radioactive decay and residual heat. is Correct: The Earth's internal heat is derived primarily from two main sources:
- Radioactive Decay (Radiogenic Heat): The spontaneous decay of radioactive isotopes, specifically Uranium (U-238, U-235), Thorium (Th-232), and Potassium (K-40), releases significant thermal energy. This process occurs predominantly in the mantle and the crust.
- Primordial (Residual) Heat: This is the heat trapped inside the Earth during its formation (accretion) and subsequent differentiation (separation into layers). The core retains a vast amount of this initial thermal energy.
- It is generated solely by the friction of moving plates. is Incorrect: While friction generated by the movement of tectonic plates and tidal forces does produce heat, it is a minor contributor compared to radiogenic and primordial heat. It is certainly not the sole source.
- It is uniform throughout the mantle. is Incorrect: The temperature and heat flow within the Earth are not uniform. The temperature increases with depth, and heat flow varies laterally based on tectonic settings (e.g., heat flow is higher at mid-ocean ridges than at stable continental cratons).
Key Takeaway:
The primary sources of Earth's internal heat are radiogenic heat generated by the decay of radioactive elements and primordial heat retained from the planet's formation.