Uranium–lead dating - Wikipedia
Apr 24, Many rocks and organisms contain radioactive isotopes, such as U and C . These radioactive isotopes are unstable, decaying over time. Dating rocks. Radioactivity can be used to date rocks. Rocks often contain traces of uranium. This is unstable and eventually decays to lead, which is stable. Mar 17, Of all the isotopic dating methods in use today, the uranium-lead method is the In a million-year-old rock, U is at its half-life and there will be an for dating these rocks, which have no fossils to indicate their age.
The half-life of carbon is only 5, years, so carbon dating is only effective on samples that are less than 50, years old.
Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old. To determine the ages of these specimens, scientists need an isotope with a very long half-life.
Some of the isotopes used for this purpose are uranium, uranium and potassium, each of which has a half-life of more than a million years.
Unfortunately, these elements don't exist in dinosaur fossils themselves. Each of them typically exists in igneous rock, or rock made from cooled magma. Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.
But this sediment doesn't typically include the necessary isotopes in measurable amounts. Today, the magnetic field is directed downward in the northern hemisphere and upward in the southern hemisphere. Earth's magnetic field periodically reverses its polarity. During the time of reversed polarity, a compass needle would point south.
These reversals make excellent markers in the geologic record because they global in extend. The age of these reversals can be determined by radiometric dating. The age of a fossil can be determined by correlating the position of the strata of rock where it was found and where a reversal occurs. Amino-acid dating Amino-acid dating is based upon the principle that amino acids which make up proteins change when an organism dies.
The proteins produced by an orgamism when it is alive almost entirely consists of amino acids in a "left-handed" configuration. After death, amino acids begin to invert to their "right-handed" configuration.
Fossil Record ( Read ) | Biology | CK Foundation
This process is called racemization. In fossils, an equilibrium ratio is eventually reached. The time needed to reach this equilibrium depends mainly upon temperature and secondarily on the species of the organism. Once the absolute date for a region is determined using radiometric dating and the temperature history of a region is established, amino-acid dating can be used to determine the age of a fossil. In the example of marine mollusks, the ratio for the amino acid isoleucine increases from nearly zero in modern shells to an equilibrium value of 1.
At 10 degrees centigrade, it takes about 2 million years to reach equilibrium. At minus 10 degrees centigrade, it takes 20 million years to reach equilibrium. Hip Hop back to. Each of these unstable isotopes has its own characteristic half life. Some half lives are several billion years long, and others are as short as a ten-thousandth of a second. On a piece of notebook paper, each piece should be placed with the printed M facing down. This represents the parent isotope.
The candy should be poured into a container large enough for them to bounce around freely, it should be shaken thoroughly, then poured back onto the paper so that it is spread out instead of making a pile.
This first time of shaking represents one half life, and all those pieces of candy that have the printed M facing up represent a change to the daughter isotope. Then, count the number of pieces of candy left with the M facing down. These are the parent isotope that did not change during the first half life.
The teacher should have each team report how many pieces of parent isotope remain, and the first row of the decay table Figure 2 should be filled in and the average number calculated. The same procedure of shaking, counting the "survivors", and filling in the next row on the decay table should be done seven or eight more times. Each time represents a half life.
Each team should plot on a graph Figure 3 the number of pieces of candy remaining after each of their "shakes" and connect each successive point on the graph with a light line. AND, on the same graph, each group should plot points where, after each "shake" the starting number is divided by exactly two and connect these points by a differently colored line. After the graphs are plotted, the teacher should guide the class into thinking about: Is it the single group's results, or is it the line based on the class average?
U is found in most igneous rocks. Unless the rock is heated to a very high temperature, both the U and its daughter Pb remain in the rock. A geologist can compare the proportion of U atoms to Pb produced from it and determine the age of the rock. The next part of this exercise shows how this is done.
Each team is given a piece of paper marked TIME, on which is written either 2, 4, 6, 8, or 10 minutes. The team should place each marked piece so that "U" is showing. This represents Uranium, which emits a series of particles from the nucleus as it decays to Lead Pb- When each team is ready with the pieces all showing "U", a timed two-minute interval should start. During that time each team turns over half of the U pieces so that they now show Pb This represents one "half-life" of U, which is the time for half the nuclei to change from the parent U to the daughter Pb A new two-minute interval begins.