Rocks can tell a lot about the past. They can help offer insight into where rivers once flowed, when volcanoes erupted, and when the ground gave way to massive amounts of pressure and turned up a whole new section of the ground.

According to a report from CS Monitor, rocks can even tell us about how the Earth formed. A recent study has revealed that the Earth’s inner core is much younger than previously thought, suggesting that the planet went through some significant changes between its formation and reaching its current state.

The inner core of the Earth is made from a solid ball of iron encased by a liquid outer core. Researchers have long argued over the true age of the Earth’s core, with estimates ranging from 0.5 to 2 billion years ago.

A recent study from an international group of scientists takes away much of the margin of error, however. Researchers have analyzed the magnetic properties of igneous rocks, which form when lava is cooled. As they solidify, igneous rocks preserve the direction of the Earth’s polarity. Geologists can tell what the planet’s magnetic field was doing at the time of the rocks’ formation based on the orientation of certain magnetic metals inside the rock.

Researchers made the discovery by timing the switch in strength of the Earth’s magnetic fields to the first appearance of solid iron at the center of the planet. This is just around the time when some of the liquid iron in the Earth’s core began to solidify.

According to the study’s lead author, Andy Biggin, the results may very well change our understanding of the inside of the Earth and how it formed. The timing of the nucleation of the inner core is the subject of fierce scientific debate, but has remained a key provision for explaining the history of the Earth’s inside.

The age of the Earth’s solid iron core can determine much of what we know about the planet’s magnetic field, which serves as a shield from dangerous solar radiation as well as a useful navigational tool.

The liquid outer core generates currents that power the planet’s magnetic field. The biggest influencing factor in the patterns of the magnetic field is convection, or the circulating of hot molten rock rising and then falling.

The convection in the center of the Earth was jumpstarted when the iron at the very middle began to solidify. Lighter, non-metallic elements migrated to the outer core and changed the overall density of the liquid, which resulted in a much stronger convection pattern.

The study reveals that the Earth’s center is probably cooling down at a slower rate than previously thought. In addition to providing a more accurate age of the center of the planet, the study confirms that the molten core fizzling out is one less thing we will likely need to worry about.