Oldest rock

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The oldest rock or rocks on Earth are from the Archean eon and are only partially exposed on the surface. There is some controversy about the oldest rocks based on the oldest dated mineral zircon. Some of the oldest surface rock can be found in the Canadian Shield, Australia, Africa and in other more specific places around the world. The age of these felsic rocks are generally between 2.5 and 3.8 billion years old. A claim of the oldest zircon mineral of about 4 billion years old was found in the Jack Hills of the Narryer Gneiss Terrane of Australia. The Canadian Shield in the Northwest Territories has Acasta Gneiss that is also mainly composed of the very old igneous and granite core of ancient mountain chains that have been exposed over millions of years from the scouring of moving glaciers during ice ages. The age of Acasta Gneiss is also claimed to be about 4 billions years old. The search for the oldest, and specifically a large enough size of a zircon mineral crystal found in Gneiss may take some time. Most scientists will need to agree upon a few important questions first. How large will the crystal need to be? What exactly is the smallest size of a rock? How many dated zircon crystals from around the world are needed to ensure that the truly oldest rock or at least a crystal has been found? What exactly does the oldest crystal of zircon tell us about the age of the surrounding minerals and rock? These questions might take a long time to debate and resolve.

Zircons from the Acasta gneiss have been dated at up to 4.03 Ga, making it the oldest rock ever found on Earth. Detrital zircons founds in sandstones in Australia have been dated at 4.3 Ga and even older but these were transported into the sandstones from an outside source, so they are the oldest mineral ever dated but the rock is not the oldest.