Radiometric Dating

  Radiometric Dating


The aging process in human beings is easy to see. As we age, our hair turns gray, our skin wrinkles etc

However, rocks and other objects in nature do not give off such obvious clues about how long they have been around. So we rely on radiometric dating to calculate their ages.

Radiometric dating is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes. Different methods of radiometric dating can be used to estimate the age of a variety of natural and even man-made materials.

The whole process is only possible because of radiactive decay.

  Radioactive Decay


Radioactive elements are unstable and they are always trying to move to a more stable state. So they do this by giving off radiation. This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay.

The thing that makes this decay process so valuable for determining the age of an object is that each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life. So if you know the radioactive isotope found in a substance and the isotope's half-life, you can calculate the age of the substance.

Half Life

It is the time required for a quantity to fall to half of its starting value. So you might say that the 'full-life' of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive.

When the isotope is halfway to that point it has reached its half-life.

 

  How radioactive dating works?


Example - C 14 Dating

There are two basic forms of carbon: one that occurs naturally, called carbon-12 (12C), and one that forms from processes acting on nitrogen in the atmosphere, called carbon-14 (14C).

When a cow eats grass, its body absorbs the carbon (both12C and 14C) in the plant. When the cow dies, it stops taking in carbon (for obvious reasons). The amount of 12C in the cow’s body stays the same after death, but the amount of 14C changes because it returns to nitrogen.

As time goes on, the amount of 14C continues to decrease until nothing is left

When a paleontologist finds a bone (or a piece of wood), measure the amount of 14C and 12C it contains. Based on how much 14C is left, can supposedly calculate when the animal (or plant) died.

Elements Used in dating

Geologists regularly use five parent isotopes to date rocks:

These parent radioisotopes change into daughter lead-206, lead-207, argon-40, strontium-87, and neodymium-143 isotopes, respectively. Thus geologists refer to

  • uranium-lead (two versions),
  • potassium-argon,
  • rubidium-strontium,
  • or samarium-neodymium dates for rocks.

Note that the carbon-14 (or radiocarbon) method is not used to date rocks because most rocks do not contain carbon.

 

Radioactive  dating  is a way of determining the age of sample of material  using  the decay rates of radioactive  elements to provide  a clock.

Measuring the ratio of parent to daughter isotopes, determines how many half-lives have passed

——Number of half-lives X length of half-life = age of sample

  Atoms and Isotopes


—The number of protons in an atom determines which element it is. Change in number of protons, changes the element & its mass changes. However, change in the number of neutrons, the element stays the same, but the mass changes.

——Isotopes - atoms of the same element that have different numbers of neutrons

—

Why Are Some Isotopes Radioactive?

Isotopes that have the right amount of neutrons are called stable.  They always stay the same.

Some isotopes have a few too many neutrons or not enough - This makes them unstable and radioactive.

The nuclei of these radioactive atoms change or decay by giving off radiation in the form of particles or electromagnetic waves until the atom reaches a stable state.

 

  Types of Radioactive decay


Alpha decay - Alpha decay is caused when there are too many protons in a nucleus. In this case the element will emit radiation in the form of positively charged particles called alpha particles.

Beta decay - Beta decay is caused when there are too many neutrons in a nucleus. In this case the element will emit radiation in the form of negatively charged particles called beta particles.

Gamma decay - Gamma decay occurs when there is too much energy in the nucleus. In this case gamma particles with no overall charge are emitted from the element.

  Types of Dating


Carbon-14 dating

Carbon-14 method : The  element carbon  occurs naturally  in three  isotopes - C-12, C-13, C-14. C-12 and C-13 are stable.

C-14 is radioactive  with half life 5730 years. Plants get C-14 from the environment, animal get C-14 from plants, when an organisms dies; it stops taking in C-14. The C-14 already in the organisms doesn't stop decaying so as time goes on that is less and less C-14 remains, if we measure how much C-14 there currently is, we can tell how much  C-14 there was when organisms died and therefore how much  decayed, on this basis we can know how old the sample.

Uranium-Lead Dating

Used to find the age of a uranium-containing mineral. It works because we know the fixed radioactive decay rates of uranium-238, which decays to lead-206, and for uranium-235, which decays to lead-207.

So we start out with two isotopes of uranium that are unstable and radioactive. They release radiation until they eventually become stable isotopes of lead.

These two uranium isotopes decay at different rates. In other words, they have different half-lives. The half-life of the uranium-238 to lead-206 is 4.47 billion years. The uranium-235 to lead-207 decay series is marked by a half-life of 704 million years.

These differing rates of decay help make uranium-lead dating one of the most reliable methods of radiometric dating because they provide two different decay clocks. This provides a built-in cross-check to more accurately determine the age of the sample.

Potassium-Argon and Rubidium-Strontium Dating

Potassium-argon dating we can tell the age of materials that contain potassium because we know that potassium-40 decays into argon-40 with a half-life of 1.3 billion years. With rubidium-strontium dating we see that rubidium-87 decays into strontium-87 with a half-life of 50 billion years. By anyone's standards, 50 billion years is a long time. In fact, this form of dating has been used to date the age of rocks brought back to Earth from the moon.

Radioactive Dating Application

Addition to the ages of earth,  moon  and meteorites radiomatric dating has been used to determine ages of fossils,including early man, timing of glaciation,  ages of  mineral deposits,  recurrence  rates of earthquake and volcanic eruption, the reversal  of earth's magnetic field and the age and duration of a wide variety of other geological events.

 

 

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