It is now well established that the Earth is extremely old, but until the mid-nineteenth century our planet's age was largely ignored by scientists. In 1859, however, Charles Darwin published his Origin of Species, which stated that the Earth was several million years old, and this stimulated geologists to begin investigating the Earth's age.
Early attempts at estimating the absolute age of the Earth varied widely and most are now known to be grossly inaccurate; nevertheless, considerable progress was made in developing methods of determining the relative ages of rocks and fossils.
The discovery in the early twentieth century that certain radioactive isotopes can be used to determine absolute ages led to increasingly accurate estimates of the Earth's age, and today it is almost universally accepted that our planet is some 4,500 million years old, although continuing refinements of radioactive dating techniques are likely to produce minor amendments to this figure.
Absolute Dating
Radioactive (or radiometric) dating is the most commonly used method of absolute dating (that is, establishing how many years old a rock is, rather than merely whether it is older or younger than another rock).
It employs the natural phenomenon whereby radioactive isotopes (which are present in minute amounts in the Earth's crust) spontaneously decay into stable, non-radioactive elements at a fairly constant rate.
The central concept in radiometric dating is that of a half-life - the time it takes for half a given quantity of a radioactive isotope to decay.
For example, the half-life of uranium-238 is about 4,500 million years, so after this time has elapsed, only half the original amount of uranium-238 remains, the rest having been transformed into various other isotopes (the final decay product of uranium-238 is lead-206). Thus by measuring the proportion of parent material to decay products in a rock sample, scientists can calculate the specimen's absolute age.
The other main isotopes used for radiometric dating include potassium-40, and carbon-14. Potassium-40 has a half-life of about 1,300 million years and eventually decays into the gas argon-40. Carbon-14 has a half-life of only 5,570 years (eventually decaying to the gas nitrogen-14) and so is used to date recent geological events; it is probably more widely, used, however, to date archeological artifacts.
Although radiometric dating is very useful for estimating the absolute ages of rocks, the techniques involved require the utmost precision and are capable of yielding ages only within a certain margin of error (which increases with the age of the rocks tested). Furthermore, this dating method cannot tell us about the actual events that occurred in the Earth's past. For these reasons geologists still use more traditional methods -such as stratigraphy and paleontology - to study the Earth's history.
Absolute Dating of Moon Rocks and Metorites
Radiometric dating shows that the oldest materials that we know about on Earth - zircon crystals that were found in Australia - are about 4.4 billion years old.
However, that tells us that the Earth is at least that old; it doesn't tell us the Earth's exact age.
The Earth's surface is constantly changing as a result of plate tectonics. Rocks are constantly being moved around and being melted inside the Earth's core, where they are recycled and transformed into younger rocks. Radiometric dating only tells us the age of the new, recycled rock; it doesn't tell us the age of the rock before it was melted.
To get around this problem, scientists have used radiometric dating to determine the ages of the Earth's Moon and meteorites (pieces of Solar System debris that have landed on Earth.)
The Earth, the Moon, and the objects from which meteorites are made were all formed when the Solar System was formed. Therefore, they would all be the same age.
There are no plate tectonics on the Moon. The oldest rocks on the Moon are as old as the Moon itself. Astronauts on the Apollo missions brought back Moon rocks that were about 4.5 billion years old.
Radiometric testing of Meteorites on Earth shows that they are between 4.5 and 4.6 billion years old.
From this, we can conclude that the Earth is between 4.5 and 4.6 billion years old.
Stratigraphy
Stratigraphy is the study of the nature and origin of layered (stratified) rocks, their sequence in the Earth's crust and their correlation (that is, identifying rocks of a similar age but from different locations). Analysis of the nature and distribution of stratified rocks enables the geological history of the area from which the rocks were obtained to be reconstructed. In performing such a task, geologists make use of several basic principles, perhaps the most important of which is that of uniformitarianism.
Uniformitarianism
According to the principle of uniformitarianism, present-day geological processes, such as erosion, transportation and volcanic activity, have operated throughout the Earth's history and can therefore be used to explain the formation of ancient as well as contemporary geological features.
For example, sedimentary rocks containing structures that can be recognized in as yet unlithified sediments can be regarded as having formed under the same conditions as those modern sediments; such structures include ripple marks on beach sands and current bedding in river deposits.
But although the principle of uniformitarianism is fundamental to understanding the stratigraphic record, by itself it does not explain all the observed geological phenomena. For example, the African and Canadian banded ironstones (which are more than 2,200 million years old) could have formed only in an oxygen-poor atmosphere, which indicates that the atmosphere during the first half of the Earth's history was significantly different from that of the present day. By analyzing rocks, however, it is often possible to discover the conditions under which they formed and this information can be used to compile palaeogeographical maps, which depict the distribution of different environments during the Earth's past.
Stratigraphy
Discovering the relative ages of rocks in a stratigraphic sequence and correlating separate outcrops are essential in trying to elucidate the stratigraphic record.
Of the principles and techniques developed to help in this work, the simplest is probably the principle of superposition, which states that, in an undisturbed sequence of sedimentary rocks, the older rocks are overlain by younger rocks.
There are, however, complications; although all strata are originally deposited sequentially in horizontal layers, subsequent deformation may tilt or even overturn the strata - as happened, for example, when the primordial continents of Africa and Eurasia collided. Hence to establish the correct order of deposition it may be necessary to use other criteria, such as grading of the rock beds (in the formation of a sedimentary rock mass the coarsest particles are usually laid down first and are subsequently overlain by progressively finer - and therefore younger - sediments).
Rocks folded and deformed during a period of intense tectonic activity may be raised above sea level and severely eroded before further deposition occurs. The younger strata then rest discordantly on the upturned, worn-down edges of the older strata, with eroded fragments of the underlying deformed rocks marking the unconformity surface - as occurs between the Upper Cambrian (younger) and upturned Precambrian (older) metamorphic rocks in the Grand Canyon in the United States.
The other main principle used by geologists in determining an area's geological history is the principle of cross-cutting relations, according to which any feature that cuts across rock strata - a fault or an igneous intrusion such as a basalt dyke, for example - is younger than the strata themselves.
Radiometric dating can be used to ascertain maximum and minimum ages of the strata and, if these strata contain fossils, rocks that occur elsewhere with the same suite of fossils can also then be dated.
However, radiometric dating is most useful in assigning ages to rocks containing very few fossils, such as those from the Precambrian Era.
Stratigraphic Records
The stratigraphic record is very incomplete; unconformities are common and evidence of erosion is widespread. Moreover, although there are many examples of slow, apparently continuous sedimentation - the deep-sea oozes in the Indian Ocean and the estuarine sediments of the Netherlands, for instance - it is probable that catastrophic events such as landslides and volcanic eruptions have had the greater effect on the stratigraphic record.
Most near-shore sediments are destroyed almost as soon as they are created, although in some circumstances thick piles of sediment can accumulate rapidly; in the Italian Apennines, single beds of sediment up to about 20m deep are thought to have been deposited by a single rush of turbid water. Much better known, however, are the effects of volcanic eruptions, many of which produce vast amounts of ash and other debris; when it settles, the debris tends to form thick, uniformly-graded beds. In the famous eruption of Vesuvius in Ad79, for instance, the city of Pompeii was covered with debris to a depth of some 7m.
Fossils
Apart from stratigraphic investigations of the rocks themselves, the other major source of information about the Earth's past has come from studying fossils (paleontology).
Probably only a small fraction of the countless millions of living organisms that once existed have been preserved as fossils; living plants and animals may be attacked by predators, and dead organisms may be eaten by scavengers or their tissues may decay.
Even hard skeletal parts such as bones and shells last only a few years when exposed to the elements. For an organism to be preserved it must be rapidly buried in sediment (or another protecting medium) and then not subsequently destroyed by heat, pressure or weathering.
Most sediment accumulates in the sea, and so the fossil record is heavily biased towards marine organisms. Nevertheless some of the best preserved fossils are of terrestrial life-forms - insects trapped in the amber resin of trees and woolly mammoths frozen in the Siberian soil, for example.
Fossil Environments
Most organisms flourish only in certain conditions, and the abundance of different types of fossils is therefore valuable in reconstructing past environments. For example, some present-day species of ghost shrimps live only in the intertidal zone, so the presence of similar fossil ghost shrimps identifies a previous shoreline. Fossils that define the environment in which the rocks were laid down are called fades fossils.
Evolution and Fossils
In the early nineteenth century William Smith (a British canal engineer) and Georges Cuvier (a French zoologist) discovered that rocks of the same age contain the same suite of fossils, which always appear in the same order.
This principle of faunal succession was explained by Charles Darwin's theory of evolution (1859): the fossils of any specific past period are unique because that mixture of life forms existed only at that particular time. At some later time the less successful life forms become extinct and new life forms evolve, producing a new, distinctive suite of fossils.
As a result of more than 150 years of collecting and classifying fossils, geologists have determined the times during which the major groups of animals and plants evolved and have compiled a standard stratigraphic column showing this information.
By correlating a fossiliferous rock sample with the stratigraphic column a geologist can date the sample without even needing to know the correct scientific names of the fossils in the rock. For example, if a stratum contains both fossil leaves of a flowering plant and dinosaur bones then the rock must date from the Cretaceous Period as flowering plants did not appear until then and dinosaurs were extinct by the end of the Period.
On a more precise scale, palaeontologists have subdivided the stratigraphic column into a series of zones, each characterized by a particular group of fossils. These zone fossils must fulfill three important criteria: they must have evolved rapidly, they must be easily identifiable and they must be widely distributed. Ammonites (marine animals with coiled shells) meet all of these requirements and have enabled paleontologists to subdivide the Jurassic Period into 63 zones; the presence of a particular species of ammonite can date a stratum to within one million years. Various other fossils are used as zone fossils, including some corals and shellfish; graptolites (small, marine colonial organisms), which are used to subdivide the Paleozoic Era; and microscopic foraminiferans, which are valuable in Cenozoic biostratigraphy, and also widely used to date rocks in geological surveys for oil deposits.