The larger number of fossilized species present and the relatively short period of time since their deposit allow this more precise dating. The largest divisions of the Phanerozoic eon are the Paleozoic, Mesozoic, and Cenozoic eras. Each lasted for millions of years and each is broadly characterized by the degree of development that the life within it has undergone.
The Paleozoic is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous which is sometimes divided into the Mississippian and Pennsylvanian eras and Permian periods. Each of these is further divided into several epochs, some named for places where their major characteristics were discovered, others simply divided into early, middle, and late epochs. During the Paleozoic era , insects, plants, the first vertebrate animals, amphibians, reptiles, fish, sharks, and corals all appeared. Often, it is the changes in the kinds of animals and plants that are used to decide boundaries between the different periods.
Despite the emphasis on life in describing the various ages of the Paleozoic, geologic processes were still. Supercontinents formed and broke apart, several ice ages advanced and retreated, temperatures fluctuated, and sea levels rose and fell. These diverse processes influenced the many changes in life that are recorded in the fossils of the era—coal deposits in Europe laid down during the Carboniferous period are one of its more famous features.
At the end of the Paleozoic era , a disastrous event known as the Permian-Triassic extinction led to the destruction of almost all Paleozoic species. Though there have been efforts to link this extinction to a meteorite impact, no convincing evidence of a large enough collision during this time period has been found. Dinosaurs appeared during the Mesozoic era. The names of the periods in the Mesozoic era may sound familiar: Triassic, Jurassic, and Cretaceous. During this million-year era, all the familiar dinosaurs such as triceratops, tyrannosaurus, stegosaurus, diplodocus, and apatosaurus flourished at different times.
Some modern animals have ancestors that first appeared during the Mesozoic era, including birds, crocodiles, and mammals. Plants continued to develop, and the first flowering plants appeared. The end of the Mesozoic era can be seen clearly in some rock layers. Known as the K-T Cretaceous-Tertiary boundary, this dark line of sediment is rich in the element iridium. Another massive extinction of species occurred at this time, possibly because of one or more meteorite impacts along with a period of intense volcanic activity.
This would have decreased the amount of sunlight reaching Earth's surface, killing plants and, eventually, animals. Not all geologists and paleontologists are convinced that the K-T extinction was a catastrophic event; some argue that it occurred over a few million years after slower climate changes.
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The Cenozoic era , the current era of geologic time, is divided into the Paleogene and Neogene periods, and further into the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene epochs. During the Cenozoic, the supercontinent of Gondwana broke apart, and the continents reached their current positions. Several ice ages occurred, and the poles became ice-covered. The first mammals began to flourish in the Paleocene; the first apes appeared in the Miocene; and the first human ancestors in the Pliocene.
Modern humans, along with large animals such as mammoths and wooly rhinoceroses, appeared in the Pleistocene. The Holocene epoch , currently ongoing, began with the end of the last ice age, less than 10, years ago. Though this vast span of time was largely understood by the end of the nineteenth century, geologists, paleontologists, and scientists of other disciplines were still curious about Earth's absolute age , using different approaches to tackle the problem. In the s William Thomson — , more commonly known as Lord Kelvin, applied his theories of thermodynamics to determine Earth's age.
He surmised that Earth was between 20 and 40 million years old by calculating the time it should take for it to cool from a liquid to a solid. Though his calculations and some of his assumptions were correct, he failed to account for heat added by radioactivity. Around the turn of the twentieth century, Irish geologist John Joly — estimated Earth's age by analyzing the salt content of the seas. He then assumed that the oceans had started off as freshwater, and that all the salt had washed into them from the land.
This relied on the assumption that the rate of salt coming into the oceans was constant and that no salt had ever been removed from the seas. By this calculation he arrived at an age of about million years. Scientists needed a method that relied on something measurable over Earth's entire lifespan.
In rocks older than about million years, it becomes impossible to use fossils to calculate their age because very few, if any, exist in these rocks. There are, however, a number of naturally radioactive elements that have been decaying since the formation of Earth. With the discovery of radiation and the calculation of half-lives in the twentieth century it finally became possible to determine the age of Earth's oldest rocks. Radioactive decay is the spontaneous change in the nucleus of an element by the escape of a proton or neutron.
Once a particle escapes the nucleus of an atom, it becomes a different isotope of the same element, or sometimes a different element altogether.
Age of the Earth: strengths and weaknesses of dating methods
The ratio of the original parent element to the daughter element produced by decay determines how long the element has been decaying. The half-life of an isotope is the amount of time it takes for half of the sample to decay. In , New Zealand -born British physicist Ernest Rutherford — discovered that uranium and thorium decayed into isotopes of lead. By Bertram Boltwood — , an American chemist studying radioactive materials, had calculated the age of certain rocks based on analysis of their radioactivity.
Radiometric dating, a well-regarded way to establish the age of rocks, is still based on the same principles laid out by Rutherford and Boltwood. It assumes that the half-lives of elements do not change over time, and that the sample has not been contaminated by the addition or removal of radioactive material. Zirconium crystals are usually analyzed because they trap uranium in their structure. Analyzing the decay of uranium to lead is useful because the half-life of uranium is million years. Even longer dates can be measured with potassium-to-argon decay, with a half-life of 1.
Carbon dating is useful for measuring very short ages on the geologic time scale. With a half-life of 5, years, carbon decay is useful for measuring dates up to about 70, years. This makes the method particularly useful for dating samples from the Holocene and late Pleistocene epochs. Radiometric dating is the key to developing and understanding an absolute time scale of Earth and its geologic ages.
When geological events, rock formations, and individual species can be placed accurately in time, it becomes possible to understand their relationships to each other and to events and circumstances present today. This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law which nearly all of us would accept and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.
That is the background to the intellectual drama being played out in this series of papers. It is a drama consisting of a prologue and three acts, complex characters, and no clear heroes or villains. We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds. Even less should we let that knowledge influence our judgment of the players, acting as they did in their own time, constrained by the concepts and data then available.
One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists. Most notable is William Thomson, ennobled to become Lord Kelvin in , whose theories make up an entire section of this collection.
He was one of the dominant physicists of his time, the Age of Steam. His achievements ran from helping formulate the laws of thermodynamics to advising on the first transatlantic telegraph cable. Harlow Shapley, who wrote an article in on the subject, was an astronomer, responsible for the detection of the redshift in distant nebulae and hence, indirectly, for our present concept of an expanding universe.
Russell, author of the article on radioactive dating, was familiar to me for his part in developing the Hetzsprung-Russell diagram for stars, but I was surprised to discover that he was also the Russell of Russell-Saunders coupling, important in atomic structure theory. The first act consists in a direct attack, led by Lord Kelvin, on the extreme uniformitarianism of those such as Charles Lyell, who regarded the earth as indefinitely old and who, with great foresight or great naivety, depending on your point of view: Sollas , assumed that physical processes would eventually be discovered to power the great engine of erosion and uplift.
The second act of the drama sees a prolonged attempt by a new generation of geologists to estimate the age of the earth from observational evidence, to come up with an answer that would satisfy the demands of newly dominant evolutionary thinking, and to reconcile this answer with the constraints imposed by thermodynamics.
The third act sees the entry of a newly discovered set of physical laws—those governing radioactivity. Lord Kelvin and his allies used three kinds of argument. The first of these referred to the rate of heat loss from the earth and the length of time it would have taken to form its solid crust. It is claimed that Homo sapiens appeared some , to , years ago. But doesn't it seem strange that after more than , years earth's population is still only 7 billion? After all, the population increased from 1 billion in to 7 billion in - a span of just years!
Of course, population growth is exponential, but even then the numbers don't add up.
Some claim a world-wide catastrophe may have occurred around 70, years ago, reducing the human population to maybe just 1, breeding pairs. Let's do the maths on these 'catastrophe' figures. Of course, there are many factors that affect r, such as climate, disease, war, standard of living and so on. Typically, population growth rates are between 0. In words, earth's population should be some million, billion, billion, billion.
This an impossibly large number when compared to the earth's current 7 billion people. Either the population growth calculation is hopelessly wrong, or the theory of human evolution is suspect! This computation appears much more realistic. Earth dating via ocean sediments, magnetic field decay, atmospheric helium, short-period comets and other techniques point to a young earth. However, the scientifc accuracy of YE claims are frequently challenged e. In order to balance the discussion we should also challenge the currently accepted radiometric dating methods.
If these are suspect then the disputed methods take on more meaning. There are several causes for concern here. The K-Ar method dates rocks by measuring the accumulated Ar It is claimed the advantage of this method is that it circumvents the zero date problem i. In other words, all Ar in a rock is assumed to have been produced by in-situ radioactive decay of K within the rock since it formed and there was zero Ar in the rock when it solidified.
However, this primary assumption has been challenged e. This 'zero Ar' problem has also been identified by Snelling who comments for one research project:. Certainly it is known to diffuse easily from deeper rocks under pressure so surface rocks tend to have a higher Ar concentration than would be expected. This, coupled with the fact that potassium is easily washed out of minerals, suggests this technique can give an artificially high age for the earth and leads some to conclude that:.
If we question these techniques, there is an alternative method called isochron dating. The isochron dating method attempts to combat the zero date problem by using ratios of isotopes and samples of different minerals from the same rock. However, it still relies on certain basic assumptions, and in particular on the assumption that the specimen was entirely homogenous when it formed i. The method also assumes that all mineral samples will have the same initial Sr to Sr ratio, but this is not always the case.
Everything Worth Knowing About ... Scientific Dating Methods
So whilst isochron dating can give a straight line, the slope may have no significance [Vardiman et al]. What about the radiometric assumption of constant decay rate? Such an assumption rests on the old evolutionary concept of uniformitarianism. In broad terms this means the observed geological features are the result of slow geological forces of the same kind and intensity as those found today.
And for radiometric dating it means that the decay constant of the parent has not changed over earth's history. Scientific justification for this assumption is found for example in Radiogenic Isotope Geology, A.
How Science Figured Out the Age of Earth
The overall theme is that of a very old earth. In contrast, Humphreys has proposed an accelerated decay higher decay rate early in earth's history, leading to a younger earth. This idea has been rebutted by those who claim there is no known scientific mechanism to produce such a change, see for example Tim-Thompson: Others disagree and say that studies in theoretical physics suggest accelerated nuclear decay can occur e. Uniformitarianism is also challenged if we invoke the concept of a world-wide flood for which there is much evidence.
Vardiman et al claim that this would result in unreliable radioisotopic dating. They conclude from their research that:. Let's take a deeper look into the theory of accelerated nuclear decay. Classical OE dating radiometric dating is based upon the spontaneous breakdown or decay of atomic nuclei, where a radioactive parent atom decays to a stable daughter atom.
The clash between OE dating millions or billions of years and YE dating thousands of years centres on the decay constant K. As discussed, OE dating rests on the evolutionary concept of uniformitarianism and an assumed constant decay rate for all time. But this is not necessarily so. The Decreasing Speed of Light: In , Albrecht and Magueijo proposed a reduction in 'c' over time as a solution to cosmological puzzles. For example, theories in which light is traveling faster in the early periods of the existence of the Universe have been recognised as an alternative to the 'big bang' inflation scenario, see Pedram and Jalalzadeh.
So, rather than 'c' being constant with time, it has been proposed that the product 'hc' where here 'h' is Planks Constant and 'c' is the speed of light in a vacuum should be considered constant, see Setterfield.