April teaches high school science and holds a master's degree in education. Discover how Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another. Relative. Relative and absolute dating North Gwinnett Middle School Relative dating tells us the sequence in which events occurred, not how long. Earth's history is divided into different chunks of geologic time, going all . High School Era, but really short compared to the age of most rocks. Using radiometric dating methods, we can link absolute time to geologic time.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point.Laws of Relative Rock Dating
Geologists draw on it and other basic principles http: Relative age dating also means paying attention to crosscutting relationships. Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
Pretty obvious that the dike came after the rocks it cuts through, right? With absolute age dating, you get a real age in actual years.
Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times. The narrower a range of time that an animal lived, the better it is as an index of a specific time. No bones about it, fossils are important age markers. But the most accurate forms of absolute age dating are radiometric methods.
Geologic Age Dating Explained
This method works because some unstable radioactive isotopes of some elements decay at a known rate into daughter products. This rate of decay is called a half-life. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. The cards should be duplicated, laminated, and cut into sets and randomly mixed when given to the students.
It is recommended that students complete Procedure Set A and answer the associated Interpretation Questions correctly before proceeding to Set B. The cards in Set B represent rock layers containing various fossils. For Set Byou may want to color code each organism type i. Sequencing the rock layers will show the students how paleontologists use fossils to give relative dates to rock strata.
Return to top To enhance this activity, have students match the fossil sketches to real fossils. The following is a list of fossils in the John Hanley Fossil Teaching Set that may be useful in this activity. It may be useful to share with students after they have completed Set B and answered the Interpretation Questions.
High School Earth Science/Absolute Ages of Rocks - Wikibooks, open books for an open world
The first card in the sequence has "Card 1, Set A" in the lower left-hand corner and represents the bottom of the sequence. If the letters "T" and "C" represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers.
Since this card has a common letter with the first card, it must go on top of the "TC" card. The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer.
Sequence the remaining cards by using the same process. When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the "TC" card at the bottom of the stack representing the oldest fossils. Starting with the top card, the letters should be in order from youngest to oldest.
Please note that none of the letters in this sequence may be reversed and still be correct.
WHO'S ON FIRST? A RELATIVE DATING ACTIVITY
The sequence must be exactly in the order as written. It is not uncommon to have students reverse the M and D for example and begin the sequence with DM because that is the way they are printed on the card.
It is good at this time to remind them that these letters represent fossils in a rock layer and that one fossil next to another within a rock layer implies no particular sequencing; they both are approximately the same age as that particular rock layer.
The following question may help clarify this point. Scientists measure the rate of radioactive decay with a unit called half-life.
The half-life of a radioactive substance is the amount of time, on average, it takes for half of the atoms to decay. For example, imagine a radioactive substance with a half-life of one year. When a rock is formed, it contains a certain number of radioactive atoms. After the third year three half-lives After four years four half-lives6.
If you find a rock whose radioactive material has a half life of one year and measure 3. The decay of radioactive materials can be shown with a graph Figure Decay of an imaginary radioactive substance with a half-life of one year.
Radiometric Dating of Rocks[ edit ] In the process of radiometric dating, several isotopes are used to date rocks and other materials. Using several different isotopes helps scientists to check the accuracy of the ages that they calculate.
Carbon Dating[ edit ] Earth's atmosphere contains three isotopes of carbon. Carbon is stable and accounts for Carbon is also stable and accounts for 1.
Carbon is radioactive and is found in tiny amounts. Carbon is produced naturally in the atmosphere when cosmic rays interact with nitrogen atoms. The amount of carbon produced in the atmosphere at any particular time has been relatively stable through time. Radioactive carbon decays to stable nitrogen by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon decay can be estimated by measuring the proportion of radioactive carbon to stable carbon As a substance ages, the relative amount of carbon decreases.
High School Earth Science/Absolute Ages of Rocks
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 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 and 50, years old.
Potassium-Argon Dating[ edit ] Potassium decays to argon with a half-life of 1.