Why does the inner core of the earth acts as a solid layer?

The layer outside the inner core is the outer core, with a thickness of approximately 1,430 miles. The outer core consists primarily of iron and nickel, two valuable metals found in abundant amounts on the planet. read more

So, because the pressure effect overrides the temperature effect, the inner core is solidified. We also can speculate that the inner core began to freeze (solidify) about 1 or two billion years ago. the inner core makes up only 1% of the mass of the Earth; the outer core makes up about 32% of the mass of the Earth. read more