ARD - Absolute Age Lesson

 

Absolute Age

By the end of the 19th century, geologists used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods and epochs of the geologic time scale. They did not know the exact timing of events, but they knew the order. By the late 1800's scientists were able to start assigning absolute age to rocks and fossils.   Absolute age is numerical and it tells us the number of years old an object is before the present time.  

Methods used to determine absolute age include tree rings, varves, ice cores, and radiometric dating.

Tree Ring Dating

Generally trees grow more rapidly in warm months than colder months.   This growth results in alternating bands of light and dark colored wood.   Dark bands of wood represent one winter.   If you were to count the number of dark bands, it is possible to find the age of the tree. The width of the rings can also tell scientists information about climate factors and disruptions such as a forest fire.   Droughts and other climatic events can cause trees to grow slower causing the bands to be thinner, or grow faster causing them to be wider. The ring variations will appear in all trees within a region. Scientists can match up growth rings to get a record of climate change and in finding the age of ancient structures.

 

 

 

 

 

Ice Cores

An ice core is a sample that is removed from a sheet of ice or a glacier.   Since ice forms from the annual layer of snow fall, lower layers are older than upper layers. During the winter a layer of ice will solidify, and in the summer there is a dusting of ice creating the layers in the ice.   Some ice cores have been drilled down to depths of two miles and have contained ice up to 800,000 years old. Scientists analyze these ice cores to determine how the climate has changed over time, as well as to measure concentrations of atmospheric gases.

 

 

 

 

Varves

Sediments found in lakes, especially those located near a glacier have annual ring patterns called varves. When the glaciers melt rapidly a thick deposit of sediment is left. These are alternated with layers of thick clay enriched layers, which are deposited in the winter. These layers help scientists determine information about past climate conditions. A thick layer might have resulted in a warm summer and lots of melting from the glacier. Scientists can count these varves to get an idea of age much like a tree ring.

Radioactive Decay

In the 20th century, radiometric methods of absolute age determination were developed. These methods allow the ages of certain types of rocks and minerals to be quantified in terms of years. By the 1960's absolute dating methods had been used to determine the ages of many rocks from all the continents and ocean floors. Repeatedly, the absolute age determinations confirmed what geologists already knew, for example that the Cambrian period occurred before (is older than) the Ordovician period. The absolute dating methods proved that the relative dating methods had been correct, and now geologists can say not only the sequence of geologic time, they can also estimate fairly accurately how many years ago each division in the sequence occurred.

What is radioactive decay?

An isotope is a particular type of atom of a chemical element, which differs from other isotopes of that element in the number of neutrons it has in its nucleus. By definition, all atoms of a given element have the same number of protons. However, they do not all have the same number of neutrons. The different numbers of neutrons possible in the atoms of a given element correspond to the different possible isotopes of that element.

For example, all carbon atoms have 6 protons. Carbon-12 is the isotope of carbon that has 6 neutrons. Carbon-13 is the isotope of carbon that has 7 neutrons. Carbon-14 has 8 neutrons in its nucleus, along with its 6 protons, which is not a stable combination. That is why carbon-14 is a radioactive isotope, it contains a combination of protons and neutrons in its nucleus that is not stable enough to hold together indefinitely. Eventually, it will undergo a spontaneous nuclear reaction and turn into a stable daughter product, a different isotope, which is not radioactive.

Each type of radioactive isotope has a half-life, a length of time that it will take for half of the atoms in a sample of that isotope to decay into the stable daughter product. Physicists have measured the half-lives of most radioactive isotopes to a high level of precision.

The properties of radioactive isotopes and the way they turn into their stable daughter products are not affected by variations in temperature, pressure, or chemistry. Therefore the half-lives and other properties of isotopes are unaffected by the changing conditions that a rock is subjected to as it moves through the rock cycle. If granite crystallizes with minerals containing radioactive isotopes, it is as though the rock crystallizes with a built-in batch of stopwatches that begin ticking away as soon as the granite has cooled.

Watch the video below to learn more about Radiometric Decay:

Radiometric Age Determination

Radiometric age determination is expensive and time-consuming. A geologist has to be sure that an age of a rock will help answer an important research question before he or she devotes time and money to making a radiometric age measurement. Before determining the age of the granite, it must be analyzed under a powerful microscope, and with an electron microprobe, to make sure that its original minerals have not been cracked and altered by metamorphism since the rock first formed. Separating the minerals from the granite is the next step in determining its age. High-precision laboratory analyses are then used to measure the amounts of radioactive parent isotope and stable daughter product in the minerals. Once these quantities have been measured, the half-life of the radioactive isotope is used to calculate the absolute age of the granite.

In the process of radiometric dating, several isotopes are used to date rocks and other materials. Using several different isotopes helps scientists check the accuracy of the ages that they calculate.

Carbon Dating

Earth's atmosphere contains three isotopes of carbon. Carbon-12 is stable and accounts for 98.9% of atmospheric carbon. Carbon-13 is also stable and accounts for 1.1% of atmospheric carbon. Carbon-14 is radioactive and is found in tiny amounts. Carbon-14 is produced naturally in the atmosphere when cosmic rays interact with nitrogen atoms. The amount of carbon-14 produced in the atmosphere at any particular time has been relatively stable through time.

Radioactive carbon-14 decays to stable nitrogen-14 by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon-14 decay can be estimated by measuring the proportion of radioactive carbon-14 to stable carbon-12. As a substance ages, the relative amount of carbon-14 decreases.

Carbon is removed from the atmosphere by plants during the process of photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. Therefore carbon-14 dating can be used to date plant and animal remains. Examples include timbers from an old building, bones, or ashes from a fire pit. Carbon dating can be effectively used to find the age of materials between 100 and 50,000 years old.

Watch the video below to learn more about Carbon Dating:

Potassium-Argon Dating

Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Because argon is a gas, it can escape from molten magma or lava. Therefore any argon that is found in a crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of potassium-40 to argon-40 will yield a good estimate of the age of the sample.

Potassium is a common element found in many minerals such as feldspar, mica, and amphibole. The technique can be used to date igneous rocks from 100,000 years to over a billion years old. Because it can be used to date geologically young materials, the technique has been useful in estimating the age of deposits containing the bones of human ancestors.

Uranium-Lead Dating

Two isotopes of uranium are used for radiometric dating. Uranium-238 decays to form lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life of 704 million years.

Uranium-lead dating is usually performed on crystals of the mineral zircon (Figure 11.26). When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. Therefore, if any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium.

Parent Isotopes, Daughter Isotopes, and Half-Lives

The dots in the cartoon below represent atoms of a parent isotope decaying to its stable daughter product through two half-lives. At time zero in the diagram, which could represent the crystallization of minerals in a rock, there are 32 red dots. After one half-life has passed, there are 16 red dots and 16 green dots. After two half-lives have passed, there are 8 red dots and 24 green dots.

The following graph illustrates radioactive decay of a fixed amount of an isotope. You can see how the proportions of the isotopes from the cartoon above are graphed as percentages at half-lives 0, 1, and 2 below.

 

IMAGES CREATED BY GAVS OR OPENSOURCE