Two major reasons:
1. The earth was formed as a result of accretion. Small particles gradually attracted each other due to gravitational forces and formed bigger objects. This process accumulated a lot of heat; when two objects collide, heat is generated. And as a majority of the heat was retained and never lost, the earth’s core remained hot.
2. However, a majority of the heat (>90%) is produced through the **decaying of radioactive elements**. The nucleus of unstable elements like U238 (Uranium) gradually break down, releasing energy.
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