Because of the earth surface and the interior of the earth decay into radioactive elements convective motion in the mantle
The main reason that the earth's interior is liquid today is decay of radioactive elements.
About 50% of the internal heat of the earth produced from radioactive decay. Four radioactive isotopes are mainly responsible for the most of the radiogenic heat.
These radioactive isotopes are uranium-238 (238U), uranium-235 (235U), thorium-232 (232Th), and potassium-40 (40K).
Therefore, the main reason that the earth's interior is liquid today is decay of radioactive elements.
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