What key discovery allowed scientists to begin measuring the absolute age of rock samples? We now know that this estimate is much, far too young*. But contrary to Ussher`s calculations, this estimate was of the order of millions of years and not of the order of 6,000. Geologists began to accept Hutton`s views that the Earth is incredibly old. [* In part, this estimate is due to the fact that these early geologists did not realize that conformities – representing missing units of time often caused by erosion – are prevalent in rock records, as is the fact that some metamorphic rocks were once sedimentary and therefore excluded from their calculations.] As mentioned above, a radiometric date tells us when a system was shut down, for example, when a mineral containing radioactive parent elements crystallized for the first time. A single mineral grain can have a long history after its formation. For example, it can erode from an igneous rock, then be transported over long distances and over long periods of time before finally being deposited and becoming one of billions in a layer of sedimentary rock (e.B sandstone). If a radiometric date were reached from this mineral grain, it would tell us when the mineral first formed, but not when the sedimentary rock formed (however, it would tell us the maximum possible age of the sedimentary rock layer). What key discovery then allowed geologists to assign absolute age data to rocks and finally discover the age of the earth? The answer is radioactivity. If you know the number of radioactive parent atoms remaining in a sample, as well as the original number, what additional key information is needed to calculate the age of the sample? Assumptions about the absolute age of rocks (and the events they represent) are determined from the rates of radioactive decay of certain isotopes of elements that occur naturally in rocks. True or false: Earth`s age was determined by dating a rock sample found at the bottom of the Grand Canyon. To calculate the age of a substance using isotopic dating, use the following equation: In the 1800s, practitioners of the young science of geology applied the uniformitarian views of Hutton and Lyell (see the introduction to this chapter) to try to determine the age of the Earth. For example, some geologists have observed how long it took for a certain amount of sediment to take (para.
B example, one centimetre of sand) accumulates in a modern habitat, and then applied this rate to the total known thickness of the sedimentary rocks. When they did, they estimated that the Earth is several million years old. If the oldest mineral grain is 4.4 Ga and the oldest rock is 4.0 Ga, how do we know the Earth is 4.54 Ga? The answer is radiometric dating of meteorite samples, which we believe formed around the same time as the Earth, the Sun, and other planetary bodies in our solar system. One of these dated meteorites comes from the Meteor crater in Arizona. Radioactive dating can also use other radioactive nuclides with longer half-lives to date of older events. For example, uranium-238 (which decays to lead-206 in a series of stages) can be used to determine the age of rocks (and the approximate age of Earth`s oldest rocks). Since U-238 has a half-life of 4.5 billion years, it takes so long for half of the original U-238 to decay into Pb-206. In a rock sample that does not contain significant amounts of Pb-208, the most abundant isotope of lead, we can assume that lead was not present when the rock was formed. Therefore, by measuring and analyzing the ratio of U-238:Pb-206, we can determine the age of the rock. This assumes that all existing lead-206 comes from the decay of uranium-238. If additional lead-206 is present, which is indicated by the presence of other lead isotopes in the sample, an adjustment should be made. In addition, heating mineral grains to high temperatures can cause them to flee from parent and daughter materials and reset their radiometric clocks.
This can be a problem when calculating radiometric data from samples of metamorphic rocks, which are sedimentary or igneous rocks that have been altered by large amounts of heat and/or pressure. Fusion, which is associated with metamorphic changes, can reset the radiometer clock. Suppose, for example, that an igneous rock formed 2.0 billion years ago. If it had undergone a metamorphosis 1.2 billion years ago, radiometric dating would tell us that a sample of the rock is 1.2 billion years old, not 2.0 billion years old. The field on the far left of the figure above represents an initial state in which the parent atoms are distributed in the molten rock (magma). As the magma cools, the grains of various minerals begin to crystallize. Some of these minerals (shown above as gray hexagons) incorporate the radioactive parent atoms (blue diamonds) into their crystal structures; This marks the beginning of the “half-life clock” (i.e. Start time or zero time). At the end of a half-life, half (50% or four) of the parent atoms of each mineral grain were converted into their daughter products (red squares).
After two half-lives, 75% (six) of the original parent atoms in each grain were converted to daughter products. How many parent atoms would remain if three half-lives passed? At the beginning of this chapter, you learned that the Earth is 4.54 billion years old. .