ANSWER: 1-1.5 bilion
Scientists have found the inner core of the earth was formed 1-1.5 billion years back as it froze from the molten iron outer core. The inner core is Earth’s deepest layer and it is a ball of solid iron larger than Pluto surrounded by liquid outer core. Inner core is a recent addition to the planet and it was formed according to scientists who have debated its period of formation as ranging from 0.5 to 2 billion years. Researchers conducted an analysis of records from ancient igneous rocks and found sharp increase in magnetic field of the planet between 1 to 1.5 billion years ago. Increased magnetic field is a possible indication of the first occurrence of solid iron at the centre of the earth. The research also suggests average growth rate of the solid inner core is one mm per year. Earth’s magnetic field is generated by motion of liquid iron alloy in the outer core around 3000 km beneath the crust of the earth.