How Old is that Rock?The age of the Earth is estimated to be 4. Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old. It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites. Because the time this accretion process took is not yet known, and predictions from different accretion models range from a few million up to about million years, the difference between the age of Earth and of the oldest rocks is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages. Studies of strata , the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence.
There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil? Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event. Second, it is possible to determine the numerical age for fossils or earth materials. Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.
Third, magnetism in rocks can be used to estimate the age of a fossil site. This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks. Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record.
For example, in the rocks exposed in the walls of the Grand Canyon Figure 1 there are many horizontal layers, which are called strata. The study of strata is called stratigraphyand using a few basic principles, it is possible to work out the relative ages of rocks. Just as when they were deposited, the strata are mostly horizontal principle of original horizontality.
The layers of rock at the base of the canyon were deposited first, and are thus older than the layers of rock exposed at the top principle of superposition.
All rights reserved. In the Grand Canyon, the layers of strata are nearly horizontal. Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers.
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer. This is the principle of original horizontality : layers of strata are deposited horizontally or nearly horizontally Figure 2. Thus, any deformations of strata Figures 2 and 3 must have occurred after the rock was deposited. Layers of rock are deposited horizontally at the bottom of a lake principle of original horizontality. Younger layers are deposited on top of older layers principle of superposition.
Layers that cut across other layers are younger than the layers they cut through principle of cross-cutting relationships. The principle of superposition builds on the principle of original horizontality.Relative Dating of Rock Layers
The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it Figures 1 and 2. Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.
Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited. This is the principle of cross-cutting relationships. The principle states that any geologic features that cut across strata must have formed after the rocks they cut through Figures 2 and 3.
According to the principle of original horizontality, these strata must have been deposited horizontally and then titled vertically after they were deposited. In addition to being tilted horizontally, the layers have been faulted dashed lines on figure. Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited.
The geological time scale and an age for the Earth of b.y. rely heavily on the uranium/thorium/lead radiometric dating methods.1,2,3. The result is that these dating methods only produce old ages for the Earth within the evolutionary theoretical system. Within the creation theoretical system. However, there are many methods that can be used to determine the age of the earth or other objects. The textbooks focus on relative dating.
The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at a single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks.
Each fossil species reflects a unique period of time in Earth's history.
The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks Figure 4.
Fossils occur for a distinct, limited interval of time. In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence — when it went extinct.
Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species i. For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed. If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed.
In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B. Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box.
Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box. Fossil species that are used to distinguish one layer from another are called index fossils.
Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area.
Because they are often rare, primate fossils are not usually good index fossils. Organisms like pigs and rodents are more typically used because they are more common, widely distributed, and evolve relatively rapidly. Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time Figure 4.
If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time. Thus, the principle of faunal succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas.
All elements contain protons and neutronslocated in the atomic nucleusand electrons that orbit around the nucleus Figure 5a. In each element, the number of protons is constant while the number of neutrons and electrons can vary.
Atoms of the same element but with different number of neutrons are called isotopes of that element. Each isotope is identified by its atomic masswhich is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: carbon 12 12 Ccarbon 13 13 Cand carbon 14 14 C Figure 5a.
C 12 and C 13 are stable. The atomic nucleus in C 14 is unstable making the isotope radioactive.
Because it is unstable, occasionally C 14 undergoes radioactive decay to become stable nitrogen N The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope.
Most isotopes found on Earth are generally stable and do not change. However some isotopes, like 14 C, have an unstable nucleus and are radioactive.
Methods of dating the age of the earth
This means that occasionally the unstable isotope will change its number of protons, neutrons, or both. Radioactivity was poorly understood. Different methods of measurement such as the decay of uranium to helium versus its decay to lead sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature.
The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics.
It was not untilwhen under the influence of Arthur Holmes, whose name recurs throughout this story the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved.
Critical to this resolution were improved methods of dating, which incorporated advances in mass spectrometry, sampling and laser heating. The resulting knowledge has led to the current understanding that the earth is 4. That takes us to the end of this series of papers but not to the end of the story.
As with so many good scientific puzzles, the question of the age of the earth resolves itself on more rigorous examination into distinct components. Such questions remain under active investigation, using as clues variations in isotopic distribution, or anomalies in mineral composition, that tell the story of the formation and decay of long-vanished short-lived isotopes.
For centuries scholars sought to determine Earth's age, but the answer had . Critical to this resolution were improved methods of dating, which. Using relative and radiometric dating methods, geologists are able to answer the Second, it is possible to determine the numerical age for fossils or earth. Before so-called radiometric dating, Earth's age was anybody's guess. the technique of dating rocks using the uranium-lead method.
Isotopic ratios between stable isotopes both on the earth and in meteorites are coming under increasingly close scrutiny, to see what they can tell us about the ultimate sources of the very atoms that make up our planet.
We can look forward to new answers—and new questions. You have free article s left. Already a subscriber? Sign in. See Subscription Options. Get smart. Sign up for our email newsletter. Sign Up. End of Summer Sale Subscribe. But the half-life for uranium is about 4. The carbon half-life is only years.
Cesium has a half-life of 30 years, and oxygen has a half-life of only The answer has to do with the exponential nature of radioactive decay. The rate at which a radioactive substance decays in terms of the number of atoms per second that decay is proportional to the amount of substance.
So after one half-life, half of the substance will remain. After another half-life, one fourth of the original substance will remain. Another half-life reduces the amount to one-eighth, then one-sixteenth and so on. The substance never quite vanishes completely, until we get down to one atom, which decays after a random time. Since the rate at which various radioactive substances decay has been measured and is well known for many substances, it is tempting to use the amounts of these substances as a proxy for the age of a volcanic rock.
After 1. So, if you happened to find a rock with 1 microgram of potassium and a small amount of argon, would you conclude that the rock is 1. If so, what assumptions have you made? In the previous hypothetical example, one assumption is that all the argon was produced from the radioactive decay of potassium But is this really known? How do you know for certain that the rock was not made last Thursday, already containing significant amounts of argon and with only 1 microgram of potassium?
In a laboratory, it is possible to make a rock with virtually any composition. Ultimately, we cannot know. But there is a seemingly good reason to think that virtually all the argon contained within a rock is indeed the product of radioactive decay.
Volcanic rocks are formed when the lava or magma cools and hardens. But argon is a gas. Since lava is a liquid, any argon gas should easily flow upward through it and escape. Thus, when the rock first forms, it should have virtually no argon gas within it. But as potassium decays, the argon content will increase, and presumably remain trapped inside the now-solid rock.
So, by comparing the argon to potassium ratio in a volcanic rock, we should be able to estimate the time since the rock formed. This is called a model-age method. In this type of method, we have good theoretical reasons to assume at least one of the initial conditions of the rock. The initial amount of argon when the rock has first hardened should be close to zero. Yet we know that this assumption is not always true.
We know this because we have tested the potassium-argon method on recent rocks whose age is historically known. That is, brand new rocks that formed from recent volcanic eruptions such as Mt. Helens have been age-dated using the potassium-argon method. Their estimated ages were reported as hundreds of thousands of years based on the argon content, even though the true age was less than 10 years.
Since the method has been shown to fail on rocks whose age is known, would it make sense to trust the method on rocks of unknown age? But many secular scientists continue to trust the potassium-argon model-age method on rocks of unknown age. If so, then their true ages are much less than their radiometric age estimates. The age estimate could be wrong by a factor of hundreds of thousands. But how would you know? We must also note that rocks are not completely solid, but porous.
And gas can indeed move through rocks, albeit rather slowly. So the assumption that all the produced argon will remain trapped in the rock is almost certainly wrong. And it is also possible for argon to diffuse into the rock of course, depending on the relative concentration. So the system is not as closed as secularists would like to think. There are some mathematical methods by which scientists attempt to estimate the initial quantity of elements in a rock, so that they can compensate for elements like argon that might have been present when the rock first formed.
Such techniques are called isochron methods. They are mathematically clever, and we may explore them in a future article. However, like the model-age method, they are known to give incorrect answers when applied to rocks of known age.
And neither the model-age method nor the isochron method are able to assess the assumption that the decay rate is uniform. As we will see below, this assumption is very dubious. Years ago, a group of creation scientists set out to explore the question of why radiometric dating methods give inflated age estimates. We know they do because of the aforementioned tests on rocks whose origins were observed.
But why? Which of the three main assumptions initial conditions are known, rate of decay is known, the system is close is false?
How Science Figured Out the Age of Earth
To answer this question, several creation geologists and physicists came together to form the RATE research initiative R adioisotopes and the A ge of T he E arth. This multi-year research project engaged in several different avenues of study, and found some fascinating results. As mentioned above, the isochron method uses some mathematical techniques in an attempt to estimate the initial conditions and assess the closed-ness of the system.
However, neither it nor the model-age method allow for the possibility that radioactive decay might have occurred at a different rate in the past. In other words, all radiometric dating methods assume that the half-life of any given radioactive element has always been the same as it is today.
The Age of the Earth, Dating. Methods, and Evolution. Roger Sigler, M.S.. Why is this Chapter Important? This chapter is important because an “ancient Earth” is.
If that assumption is false, then all radiometric age estimates will be unreliable. As it turns out, there is compelling evidence that the half-lives of certain slow-decaying radioactive elements were much smaller in the past.
While there are numerous experimental methods used to determine geologic ages, the most frequently employed technique is radiometric. “Science has proved that the earth is billion years old. Is radiometric dating a reliable method for estimating the age of something?. Boltwood gave up work on radiometric dating and Rutherford remained mildly curious about the issue of the age of Earth but did helium method until and then ceased.
This may be the main reason why radiometric dating often gives vastly inflated age estimates. First, a bit of background information is in order. Most physicists had assumed that radioactive half-lives have always been what they are today. Many experiments have confirmed that most forms of radioactive decay are independent of temperature, pressure, external environment, etc. In other words, the half-life of carbon is years, and there is nothing you can do to change it.
Given the impossibility of altering these half-lives in a laboratory, it made sense for scientists to assume that such half-lives have always been the same throughout earth history. But we now know that this is wrong. In fact, it is very wrong.
More recently, scientists have been able to change the half-lives of some forms of radioactive decay in a laboratory by drastic amounts. However, by ionizing the Rhenium removing all its electronsscientists were able to reduce the half-life to only 33 years!
In other words, the Rhenium decays over 1 billion times faster under such conditions. Thus, any age estimates based on Rhenium-Osmium decay may be vastly inflated. The RATE research initiative found compelling evidence that other radioactive elements also had much shorter half-lives in the past.
Several lines of evidence suggest this. But for brevity and clarity, I will mention only one. This involves the decay of uranium into lead Unlike the potassium-argon decay, the uranium-lead decay is not a one-step process. Rather, it is a step process. Uranium decays into thorium, which is also radioactive and decays into polonium, which decays into uranium, and so on, eventually resulting in lead, which is stable. Eight of these fourteen decays release an alpha-particle: the nucleus of a helium atom which consists of two protons and two neutrons.