melting point of a substance depends on pressure too. at high pressure , the melting point also shifts to higher level. and at low pressure , the melting point shifts to lower level. the inner core of earth is mostly made of iron and temperature inside earth is much higher than the normal melting point of the iron. so ideally the iron inside the core must be in molten state but in reality it is in solid state there because the pressure at the core is very high. due to this high pressure , the melting point of the iron becomes higher than its normal value and hence it does not melt inside the core.