Radiocarbon dating lab scientists and archaeologists should coordinate on carbon 14, called radiocarbon, is a naturally occurring isotope of the element. Potassium-argon dating, method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock.
The attraction of the method lies in the fact that one of the daughter elements is argon which is an inert gas. This means that the geologist can plausibly assume that all argon gas escapes from the molten magma while it is still liquid. He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes the initial argon content is zero. He imagines that his radioactive hour glass sealed when the rock solidified, and his radioactive clock started running.
And he hopes the rock has remained sealed until the time he collected his sample. With these assumptions the geologist only needs to measure the relative amounts of potassium and argon in the rock at the present time to be able to calculate an age for the rock. Although it is a simple calculation the big question is whether his assumptions about the rock were correct.
How can the geologist know? If the rock actually contained some argon when it solidified then the calculated age would be too old. On the other hand, if the rock was later disturbed by a geological upheaval and lost argon the age would be too young. What he does is check his calculated age with the ages produced by other dating methods. In other words, he checks to see if his calculated result falls into the range where he expects it to fall, given the geological situation of where he found his rock.
He always does this check because no dating method can be trusted on its own. What happens if the results conflict? By this he means that argon gas in his rock has come from the melting of some older rocks deep underground and contaminated his sample with a higher concentration of argon, which is why its age is too old.
This is a standard explanation and is essentially a new story about the past, different from the original story that explained how potassium-argon dating works. We could ask ourselves which of the details of this story have been observed. Photo Wikipedia It is a story about older rocks, melted rocks, solidified rocks and argon gas. It explains what each of these were doing deep inside the earth millions of years ago.
Too old compared with what? With the true age of the rock. The problem is that although radiogenic argon and excess argon have different names they are exactly the same isotope—argon It is impossible to distinguish between them experimentally. So, how do we work out how much excess argon we have? We can only calculate the amount of excess argon if we know the true age of the rock.
What happens when the age is too young? In this case the method is again salvaged by changing his assumptions about the past. Often a heating event is invoked to liberate the argon from the solid rock, although other assumptions are made as well.
What happens if the age falls into the range he expected? In this case the geologist assumes that everything went well, and he publishes his result as the crystallization age of the rock. The scores of dates that have been produced have had a life like hens in a chicken coop. So although the potassium-argon method has been used for dating rocks for decades, the results it has produced have tended to reinforce the geological framework that already existed.
At most it may have modified the framework a little. Whenever a new date is introduced it has to find its pecking order within the geological community. Some dates are accepted, some are rejected, some are overturned and some are modified until everything is in its place, and order reigns again. They usually make a small atmospheric correction for this. Return to text.
Examine the stratigraphic column diagram. What relationship seems to exist between the approximate age of the fossils and rocks layers and their depth in the earth? Following the law of superposition in geology, older fossils and rocks are found in lower strata than younger fossils and rock layers. How has tectonic activity influenced discoveries at Turkana? Answer Tectonic activity left some areas of land uplifted, and erosional forces from the lake, nearby rivers, and other forms of weathering exposed rock, even older rock layers, as outcroppings in the landscape.
This made the fossils easier for researchers to find. The volcanic material in tuff layers also makes it possible to get a more accurate date for the fossils. What is one technique that scientists use to date the fossils they find? Answer Potassium-argon dating is a form of isotopic dating commonly used in archaeology. Scientists use the known natural decay rates for isotopes of potassium and argon to find the date of the rocks.
The radioactive isotope converts to a more stable isotope over time, in this case decaying from potassium to argon. If scientists find the ratio of potassium to argon, it tells them how long the rocks have been around by how long the isotopes have been decaying. By understanding the dates of these rocks, scientists can deduce the age of the nearby fossils. What difficulties might paleontologists and archaeologists have when trying to find and date fossils? Answer There are many possible answers.
One answer based on a common problem encountered by scientists is that fossils are often encased in rocks or are similarly colored, so they blend in with their surroundings. Sometimes, only a small part of a fossil is showing. They might also be buried. Because of these characteristics, field crews have to carefully examine their surroundings to find possible fossils.
Fossils might also be fragile or found in small fragments. Archaeologists have to use their skill and patience to put small pieces back together, like a jigsaw puzzle. Although fossil dating is now more scientifically accurate, it still requires skill and experience as scientists have to make educated guesses based on any evidence and the dating available for the layers surrounding the fossils.
Why is a unique fossil name like this important? Answer Because each name is a unique identification, this helps scientists keep track of where and in what order fossils are found. Says Shea: It would be like having a watch that told you day and night. Also called single crystal argon or argon-argon Ar-Ar dating, this method is a refinement of an older approach known as potassium-argon K-Ar dating, which is still sometimes used.
Both methods date rock instead of organic material. As potassium decays, it turns into argon. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at least , years old. While K-Ar dating requires destroying large samples to measure potassium and argon levels separately, Ar-Ar dating can analyze both at once with a single, smaller sample.
Uranium series dating: The uranium-thorium method is often helpful for dating finds in the 40, to ,year-old range, too old for radiocarbon but too young for K-Ar or Ar-Ar.
Trapped Charge Dating Brosko Over time, certain kinds of rocks and organic material, such as coral and teeth, are very good at trapping electrons from sunlight and cosmic rays pummeling Earth. Researchers can measure the amount of these trapped electrons to establish an age. But to use any trapped charge method, experts first need to calculate the rate at which the electrons were trapped. This includes factoring in many variables, such as the amount of radiation the object was exposed to each year.
These techniques are accurate only for material ranging from a few thousand to , years old — some researchers argue the accuracy diminishes significantly after , years.
Silicate rocks, like quartz, are particularly good at trapping electrons. Researchers who work with prehistoric tools made from flint — a hardened form of quartz — often use thermoluminescence TL to tell them not the age of the rock, but of the tool. After shaping flint, toolmakers typically dropped the rocks into a fire.
Shea explains: Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium K ,decays to the gas Argon as Argon Ar By comparing the proportion of K to Ar in a sample of volcanic rock, and knowing the decay rate of K, the date that the rock formed can be determined.
How Does the Reaction Work? Potassium K is one of the most abundant elements in the Earth's crust 2. One out of every 10, Potassium atoms is radioactive Potassium K These each have 19 protons and 21 neutrons in their nucleus. If one of these protons is hit by a beta particle, it can be converted into a neutron. With 18 protons and 22 neutrons, the atom has become Argon Ar , an inert gas. For every K atoms that decay, 11 become Ar How is the Atomic Clock Set?