We can't drill right down to the centre of the Earth to see what it is made of.
The waves from earthquakes are the key to mapping the inside of the Earth.
Seismic waves are bent (refracted) or reflected when they hit layers of different types.
Studying this information shows that there are several layers.
The core has an inner core of solid iron, an outer core of liquid iron.
The mantle is made of solid heavy rocks.
The thin outer layer, the crust, is the outline of the circle.
The crust does not sit on a 'molten sea'.
which is part of the upper mantle.
The rocks are kept nearly solid by the pressure of the rocks above.
They only melt if the pressure drops slightly.
The crust can move up and down slowly if the load on top changes.