hello grade 11 welcome in this video you will learn about the methods used to determine the age of rocks the relative method and the absolute dating method and you will also learn how these methods help geologists in determining the subdivisions of the geologic time [Music] our discussion will focus on the following learning competencies one describe the different methods used to determine the age of stratified rocks and two explain how relative and absolute dating were used to determine the subdivision of geologic time have you ever wondered how we were able to know what happened thousands or millions or even billions of years ago that was the time when technologies we have today are not yet available and even humans do not exist yet but how did geologists come up with the records of the past events determining the age of stratified drops using two methods the relative dating and the absolute painting help geologists know the sequence of age in geologic time now what is the difference between these two methods relative dating is a method of determining the age of rocks in each layer by identifying the relative order of previous events absolute dating however is a method of determining the numerical age of rocks using radioactive decay now let's discuss further how these two differ relative dating is determining which fossil or event came first second third etc while absolute dating is determining the actual age of rock using radioactive decay when we say the trilobite fossil is older than the dinosaur tooth fossil this is relative but when we say the trilobite fossil is 489 million years old that's absolute let's take a look at these two cars the red car and the grey car when we say the red car costs ninety thousand dollars and the silver car costs fifteen dollars this is what method yes absolute but when we say the red car is more expensive than the silver car this one is yes relative in determining the age of rocks these two methods are also used when the geologists say that this layer is 225 million years old this one is an absolute dating but when the geologists say this layer is older than this layer this one is relatively dating now you see the difference between the two absolute dating is determined through radioactive detail while relative dating is through geologic sequencing the determination of relative edges of rocks are based on the following principles now we have the law of superposition it states that each layer of sediments is piled on top of another layer making the one on top of the strata the youngest one while the layer at the bottom of the strata as the oldest as you can see in the illustration an example of stratified rocks following the law of superposition is found in the grand canyon [Music] next is the law of cross-cutting relationship which states that an igneous intrusion is always younger than the rock it cuts across a lava that cooled and hardened on its surface is called an extrusion the strata beneath the extrusion are always older while a magma that cooled and hardened beneath the surface is called an intrusion an igneous intrusion is always younger than the rock it has intruded this is also true to faults a fault a break on the earth's crust is always younger than the stratified rock that it comes next is the law of inclusion the rocks that mixed up in the formation of sediments are called inclusions these rocks are always older than the layer of sediments it is embedded the pebbles in this conglomerate are older than the conglomerate itself next we have the final succession fossils of plants and animals are grouped according to the period of geologic time they appeared strata containing those group of fossils are also formed on that particular period of time this becomes the basis of geologists in creating the geologic time scheme we also have the law of original horizontality according to this layers of sediments are originally deposited horizontally those strata that are found not in horizontal order may have been deformed by the movement of the earth's crust in absolute dating the most common way of determining the absolute age of rock is through radiometric dating where radioactive isotopes found in fossils or rocks are used carbon 14 is used to determine the age of fossils while uranium-238 for rocks for example uranium-238 will take 4.5 billion years for half the amount of it to decay into a daughter isotope led 206 [Music] radioactive decay refers to the process by which an atomic nucleus of an unstable atom loses energy half-life refers to the period of time it takes for a substance undergoing decay to decrease by half radioactive isotopes called parent isotope are unstable and gradually decays in a fixed rate into a daughter isotope which is more stable in the illustration is another isotope of uranium the uranium-235 which has a half-life of 700 million years and decays into a stable isotope lead to oa since the rates of decay of radioactive isotopes are known it can be used to determine the age of rocks by measuring the proportion of parent and daughter isotopes present in rocks for example if iraq is found to contain the same proportion of uranium-238 and lead 206 geologists can tell the age of rock is about 4.5 billion years old since some rocks on earth's surface were found in different timelines other isotopes with a different rate of decay are used like potassium 40 with a half-life of 1.251 billion years and uranium-235 mentioned earlier with a half-life of 700 million years carbon 14 with a half-life of 5730 years is also used in dating remains of plants and animals this is called carbon dating determining the subdivision of geologic time using relative and absolute dating to get more accurate age of rocks geologists both use the relative and absolute dating for comparison and verification determining the age of sedimentary rocks using uranium-238 and potassium-40 sometimes leads to the age of its component rocks the metamorphic and igneous rocks not the age of sediment accumulation that is why geologists use as other radioactive isotopes like carbon 14 to date the age of the fossils in the sediments and through the process called luminescence dating a process that measures the quantity of emitted light from energy stored in a rock the clues in rocks help scientists put together a picture of how places on earth have changed scientists noticed in the 1700s and 1800s that similar layers of sedimentary rocks all over the world contain similar fossils they used relative dating to order the rock players from oldest to youngest and they saw that the fossils in older rocks are different from the fossils in younger rocks for example older rock layers contain only reptile fossils but younger rock layers may also contain mammal fossils scientists divided earth's history into several chunks of time when the fossils showed similar things living on the earth they gave each chunk of time a name to help them keep track of how earth has changed for example one chunk of time when many dinosaurs lived is called the jurassic we find fossils of earth's first green plants from the chunk of time name or division ordering the rock players from oldest to youngest was a first step in creating the geologic time scale it showed the order in which life on earth changed it also showed us on how certain areas changed over time in regard to climate or type of environment however the early geologic time scale only showed the order of events it did not show the actual years that events happened with the discovery of radioactivity in the late 1800s scientists were able to measure the exact age in years of different rocks the jurassic period occurs after triassic period and before the cretaceous period and spans the time from about 205 million years ago to about 142 million years ago relative time is the physical subdivisions of the rocks found in the earth's geology and the time and order of events they represent and that ends our lesson congratulations i hope you learned something today thank you for watching [Music] you