The interior of the Earth is hot for several reasons:
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Radioactive Decay: The decay of radioactive isotopes, such as uranium, thorium, and potassium, generates heat. This process produces heat as these isotopes undergo nuclear decay, contributing significantly to the overall temperature of the Earth's interior.
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Residual Heat from Formation: When the Earth was formed about 4.5 billion years ago, it accumulated heat from the gravitational energy released during the accretion of material. This primordial heat is still present in the Earth's interior and contributes to its overall temperature.
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Pressure Increases: As depth increases within the Earth, the pressure also increases, which can raise the temperature due to the compression of materials. This phenomenon, known as adiabatic heating, occurs as materials at greater depths are subjected to high pressures.
These factors combine to create a consistently high-temperature environment within the Earth's interior.