At a depth of about 2900 kilometres is an abrupt transition from the mantle to the earth’s core. The concept 'earth’s core' may imply to you that this core is one single piece. This is not the case, however. The earth’s core consists of two parts: the outer core (2000 kilometres thick) and the inner core (1300 kilometres thick). The difference is that the outer core is liquid while the inner core is solid. We do not know exactly which stones the earth’s core consists. The core is so heavy it makes up more than 28.4 % of the earth's total mass.