When the Earth was born roughly 4.5 billion years ago, its iron nickel core formed first. These dense metals sank to the center while melted mantle turned into the lighter crust. When convection in Earth's interior caused deep portions of the mantle to rise upward, undergo additional melting and then separate once again by density, it altered the mantle's chemical composition.

Scientists had long believed that residues of crust formation mixed back into the mantle so thoroughly that evidence of the planet's oldest geochemical events was completely lost. New research out of UC Santa Barbara shows otherwise.

UCSB geologist Matthew Jackson and colleagues have identified a geochemical signature of material dating from early melting events that were part of the Earth's formation. Their findings appear in the journal Science.