Their are 2 layers of core this are inner core and outer core. outer core is made up of iron and nickel due to its high temperature iron and nickel MELT so outer core is liquid while the inner core is also made up of solid iron and nickel. it is also have high temperature but it is believed to have SOLIDIFIED as a result of pressure freezing w/c is common to liquids subjected under tremendous pressure
Outer core is liquid while inner core is solid hard.
Well it is kept in shape by the gravity of the Earth.