The main factor that causes the inner core to be a solid is the intense pressure and weight exerted on it by the outer layers of the Earth. The inner core is composed primarily of solid iron and nickel, and the immense pressure from the surrounding layers prevents it from melting into a liquid state, despite the extremely high temperatures it experiences. The pressures at the core can reach up to 3.6 million atmospheres, which is why the inner core remains solid.
