Slowly and painstakingly, geologists have assembled this record into the generalized geologic time scale shown in Figure 1. This was done by observing the relative age sequence of rock units in a given area and determining, from stratigraphic relations, which rock units are younger, which are older, and what assemblages of fossils are contained in each unit.

Using fossils to correlate from area to area, geologists have been able radiometric dating of the oldest rocks on earth work out a relative worldwide order of rock formations and to divide the rock record and geologic time into the eras, periods, and epochs shown in Figure 1. The last modification to the geologic time scale of Figure 1 was in the s, before radiometric dating was fully developed, when the Oligocene Epoch was inserted between the Eocene and the Miocene.

Although early stratigraphers could determine the relative order of rock units and fossils, they could only estimate the lengths of time involved by observing the rates of present geologic processes and comparing the rocks produced by those processes with those preserved in the stratigraphic record. With the development of modern radiometric dating methods in the late s and s, it was possible for the first time not only to measure the lengths of the eras, periods, and epochs but also to check the relative order of these geologic time units.

Radiometric dating verified that the relative time scale determined by stratigraphers and paleontologists Figure 1 is absolutely correct, a result that could only have been obtained if both the relative time scale and radiometric dating methods were correct. Nonetheless, stratigraphy and radiometric dating of Precambrian rocks have clearly demonstrated that the history of the Earth extends billions of years into the past.

Radiometric dating has not been applied to just a few selected rocks from the geologic record. Literally many tens of thousands of radiometric age measurements are documented in the scientific literature. Since beginning operation in the early s, the Geochronology laboratories of the U. Geological Survey in Menlo Park, California, have alone produced more than 20, K-Ar, Rb-Sr, and 14 C ages. Add to this number the age measurements made by from 50 to other laboratories worldwide, and it is easy to see that the number of radiometric ages produced over the past two to three decades and published in the scientific literature must easily exceedThree basic approaches are used to determine the age of the Earth.

The first is to search for and israel online dating sites the oldest rocks exposed on the surface of the Earth. These oldest rocks are metamorphic rocks with earlier but now erased histories, so the ages obtained in this way free online dating sites for college students minimum ages for the Earth.

Because the Earth formed as part of the Solar System, a second approach is to date extraterrestrial objects, i. Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets. The third approach, and the one that scientists think gives the most accurate age for the Earth, the other planets, and the Solar System, is to determine model lead ages for the Earth, the Moon, and meteorites.

This method is thought to represent the time when lead isotopes were last homogeneously distributed throughout the Solar System and, thus, the time that the planetary bodies were segregated into discrete chemical systems. The results from these methods indicate that the Earth, meteorites, the Moon, and, by inference, the entire Solar System are 4. Before reviewing briefly the evidence for the age of the Earth, I emphasize that the formation of the Solar System and the Earth was not an instantaneous event but occurred over a finite period as a result of processes set in motion when the universe formed.

It is, radiometric dating of the oldest rocks on earth, more correct to talk about formational intervals rather than discrete ages for the Solar System and the Earth. Present evidence indicates, however, that these intervals were rather short million years in comparison with the length of time that has elapsed since the Solar System formed some 4 to 5 billion years ago. Thus, the ages of the Earth, the Moon, and meteorites as measured by different methods represent slightly different events, although the differences in these ages are generally slight, and so, for the purposes of this chapter they are here treated as a single event.

All the major continents contain a core of very old rocks fringed by younger rocks. 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 radiometric dating of the oldest rocks on earth of a rock. Most commonly, the event causes partial or total loss of the radiogenic daughter isotope, resulting in a reduced age.

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. The oldest rocks in North America, found in Minnesota, give a U-Pb discordia age of 3. 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 Table 6 ; within the analytical uncertainties, the ages are the same and indicate that these rocks are about 3. Whole-rock samples from the Sand River Gneisses in the Limpopo Valley, South Africa, have been dated by the Rb-Sr isochron method at 3. These samples are from rocks that contain inclusions of still older but as yet undatable rocks.

Recently, Basu and others 16 reported a nine-sample Sm-Nd isochron age of 3. Studies of the oldest rocks from the Precambrian shields show that the Earth is older than 3.

How Do We Know the Earth Is 4.6 Billion Years Old?

For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. In Darwin's theory of evolution , the process of random heritable variation with cumulative selection requires great durations of time. Science in Christian Perspective. On a larger scale, even between continents, fossil evidence can help in correlating rock layers. Protons and neutrons make up the center nucleus of the atom, and electrons form shells around the nucleus. Helens The new lava dome dacite from the at Mount St. Carbon is constantly being added to the atmosphere. Other scientists backed up Thomson's figures. Anomalies of radiometric dating. It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites.