The outer core of Earth is molten, but the inner core is solid. The reason for this is that the:

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The actual temperature at the center of the Earth is estimated to be around 6,000 degrees Celsius, which is much higher than the melting point of even the most heat-resistant materials. However, the pressure at the center of the Earth is estimated to be over 3.5 million times greater than atmospheric pressure, which can have a dramatic effect on the melting point of minerals. In the case of the Earth's core, this extreme pressure actually raises the melting point of the metals present in the outer core, causing them to remain in a liquid state. However, the pressure in the inner core is even greater than that of the outer core, which raises the melting point of the materials even further, to the point where they are solid despite the high temperatures.

The Outer Core of Earth Is Molten, but the Inner