Have you ever wondered how scientists figure out how old dinosaur bones are or when a volcano last erupted? To establish when these events occurred scientists had to discover a way to determine the numerical ages of rocks. We introduce you to that process in this video. We will use some new vocabulary, explain the changes that occur in atoms as radioactive decay occurs, and describe how the relative proportions of different isotopes can be used to begin to establish the age of a rock. Today we calculate that earth this 4.55 billion years old. The oldest US rocks are examples of the metamorphic rock gneiss ("nice") that can be found in parts of Minnesota and Wisconsin. The rocks highlighted here are the oldest in the US and most formed more than a billion years ago One section of old rocks that is seen by millions of visitors each year is a 1.8 billion year old Vishnu schist, another metamorphic rock in the bottom of the Grand Canyon. So we have some old rocks, but how do we know how old they are? Scientists first used observations of radioactive decay to estimate the age of Earth about a century ago, and today we use information from the radioactive decay of elements preserved in minerals to establish the timing of geological events. Before we discuss radioactive decay, let's recap some information about elements. Every element has a unique atomic number represented by how many protons are present in it's nucleus. For example, potassium has 19 protons and uranium has 92 protons. Almost all elements also have neutrons in their nuclei. The total number of neutrons and protons is the mass number of an element. While the number protons stays constant, the number of neutrons may vary. Isotopes are different versions of an element with different numbers of neutrons and, hence, different mass numbers. For example, both potassium and uranium have three naturally occurring isotopes. Radioactive decay occurs when an isotope loses or gains protons and/or neutrons to convert to a different isotope or element. For example, potassium-40 can lose an electron through a process known as beta decay. This changes a neutron to a proton, converting potassium to calcium. The potassium is termed the parent isotope and the calcium is known as the daughter isotope or atom. The opposite occurs during electron capture. That is, an electron is absorbed by the nucleus to change a proton to a neutron. For example, this happens when an isotope of potassium is converted to argon. Finally, elements undergo alpha decay when an alpha particle, a combination of two neutrons and two protons, is booted of the nucleus of an atom. When this happens to an isotope of uranium, it is converted to thorium. This is just one step in a lengthy series of changes that will eventually form a stable lead isotope. So now we know that radioactive decay can occur as parent isotopes lose or gain protons and/or neutrons to become daughter atoms. The time it takes for half at the parent isotope to be converted to a daughter isotope is known as the half-life of the element. Let's see how this works with a simple analogy using something familiar. Imagine that this grid of green candies represents radioactive isotopes before decay began. As they decay, these parent isotopes will be converted to orange daughter atoms. One half-life will have occurred when half of the 64 green candies are replaced by their orange cousins. Another way to represent this information is using this graph. When the rock formed it only had parent isotopes, as time passes the amount of parent isotope decreases and the amount of daughter increases. Let's try that again. During a second half life, the number of remaining parent isotopes is again divided in half. Now only a quarter of the original parent isotopes remain. During a third half-life, this number, that quarter, is again reduced by half. Recognizing a pattern yet? So after two half lives the proportion of parent isotope is reduced to a quarter of the original number; and to an eighth of the original number after three half lives. As more time passes, there is a steady predictable change in the relative proportion of parent and daughter isotopes as represented by their ratio. The ratios would be the same regardless of how many radioactive isotopes we began with. It is these ratios that geologists are measuring when they determine the age of a rock. Eventually, after about a dozen half-lives, there will be so little parent isotope left, that it will become difficult to obtain an accurate date for the rocks. Once we know how many half-lives have occurred, we can multiply that number by the length of one half life to determine the age of the rock. For example, one half-life for the decay of potassium-40 to argon-40 takes 1.25 billion years. If we analyzed the relative proportions of potassium-40 and argon-40 and determined that two half-lives had occurred we would know the rock was 2.5 billion years old. So hopefully you leave this lesson with a stronger understanding of the processes geologists use to figure out the numerical ages of rocks and thus establish when different events occurred in Earth's long history. We had three learning objectives for this lesson. How confident are you that you can successfully complete each of these tasks?