The granite vanished, but the zircons ultimately came to rest 3 billion years ago in sandy Australian stream sediments. These sediments later hardened into rocks that subsequently were altered by heat and pressure. Both research teams used instruments called ion microprobes to date and analyze the zircon crystals, which often contain uranium, rare earth elements and other impurities. Uranium decays to lead at a known rate. Uranium-lead ratios in the zircons showed they formed as early as 4.4 billion to 4.3 billion years ago when they crystallized in molten granite.
Continental crust is different than crust that underlies the oceans.
Granite is a common rock in continents. And zircons commonly crystallize in granite. So the zircons indicate granite was present 4.3 billion to 4.4 billion years ago, while the granite means continents existed at that time. The ancient zircons are surviving vestiges of crustal granite from Earth's early years. Ion microprobe analysis of rare-earth elements within the zircon crystals also found levels typical of continental rocks, Peck said.