THE EARTH’S OLDEST ROCKS

All the major continents contain a core of very old rocks fringed by younger rocks. These cores, called Precambrian shields, are all that remain of the Earth’s oldest crust. The rocks in these shields are mostly metamorphic, meaning they have been changed from other rocks into their present form by great heat and pressure beneath the surface; most have been through more than one metamorphism and have had very complex histories. A metamorphic event may change the apparent radiometric age of a rock. Most commonly, the event causes partial or total loss of the radiogenic daughter isotope, resulting in a reduced age. Not all metamorphisms completely erase the radiometric record of a rock’s age, although many do. Thus, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock. Moreover, many of the oldest dated rocks intrude still older but undatable rocks. In all cases, the measured ages provide only a minimum age for the Earth.

So far, rocks older than 3.0 billion years have been found in North America, India, Russia, Greenland, Australia, and Africa. The oldest rocks in North America, found in Minnesota, give a U-Pb discordia age of 3.56 billion years. The oldest rocks yet found on the Earth are in Greenland, South Africa, and India. The Greenland samples have been especially well studied. The Amitsoq Gneisses in western Greenland, for example, have been dated by five different methods within the analytical uncertainties, the ages are the same and indicate that these rocks are about 3.7 billion years old.