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why the inner zone is in solid state?
 The solidity of the inner core is a consequence of the melting temperature gradient being steeper than the geothermal gradient, as a result of the effects of pressure on the liquidus temperature. In smaller planets and satellites the entire core may be solid. As the Earth cools and solidification proceeds, the radius of the inner-core boundary increases with time.

The Earth's inner core is slowly growing as the liquid outer core at the boundary with the inner core cools and solidifies due to the gradual cooling of the Earth's interior (about 100 degrees Celsius per billion years. Many scientists had initially expected that, because the solid inner core was originally formed by a gradual cooling of molten material, and continues to grow as a result of that same process, the inner core would be found to be homogeneous. It was even suggested that Earth's inner core might be a single crystal of iron.With a radius of 1200 km the Earth’s inner core is only 30% smaller than its moon. Being the solid,central core of a planet, its dynamics are fascinatingand unique. The inner-core boundary is a solid–liquid phase boundary, where the geotherm crosses the liquidus temperature of the iron alloy that comprises the Earth’s core.


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