![]() ![]() These primitive objects provide the best ages for the timeof formation of the Solar System. The oldest dated moon rocks,however, have ages between 4.4 and 4.5 billion years and provide a minimum agefor the formation of our nearest planetary neighbor.Thousands of meteorites, which are fragments of asteroids that fall to Earth,have been recovered. These rocks vary greatly in age, a reflection of their differentages of formation and their subsequent histories. The Moon is a more primitive planet than Earth because it has not been disturbedby plate tectonics thus, some of its more ancient rocks are more plentiful.Only a small number of rocks were returned to Earth by the six Apollo and threeLuna missions. In addition, mineral grains (zircon) with U-Pb agesof 4.4 Ga have recently been reported from sedimentary rocks in west-centralAustralia. The ages measured for Earth's oldest rocks and oldest crystals show that the Earth is at least 4.3 billion years in age but do not reveal the exact age of Earth's formation.The best age for the Earth (4.54 Ga) is based on old, presumed single-stage leadscoupled with the Pb ratios in troilite from iron meteorites, specifically theCanyon Diablo meteorite. ![]() The source rocks for these zircon crystals have not yet been found. In Western Australia, single zircon crystals found in younger sedimentary rocks have radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest materialsto be found on Earth so far. An interesting feature of these ancient rocks is that they are not from any sort of "primordial crust" but are lava flows and sediments deposited in shallow water, an indication that Earth history began well before these rocks were deposited. These ancient rocks have been dated by a number of radiometric dating methods and the consistency of the results give scientists confidence that the ages are correct to within a few percent. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years). These dating techniques, which are firmly grounded in physics and are known collectively as radiometric dating, are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to rehomogenize its radioactive elements.Ĭlickon the image to see a graphical representation of geologic timeĪncient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with half lives of 700 million to more than 100 billion years to stable isotopes of other elements. Nevertheless, scientists have been able to determine the probable age of the Solar System andto calculate an age for the Earth by assuming that the Earth and the rest of the solid bodies in theSolar System formed at the same time and are, therefore, of the same age. So far scientists have not found a way to determine the exact age of the Earth directly from Earthrocks because Earth's oldest rocks have been recycled and destroyed by the process of plate tectonics.If there are any of Earth's primordial rocks left in their original state, they have not yet beenfound.
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