READ: Geologic Time
READ: Geologic Time
The earth is 4.6 billion years old. Arriving at this number wasn't easy but there are many lines of evidence that have allowed scientists to reach that conclusion.
During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. What methods can you think of for doing this? One example is that by measuring how much sediment a stream deposited in a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer. Not surprisingly, these methods resulted in wildly different estimates.
In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old, which he later lowered to 20 million years. He did this systematically assuming that the planet started off as a molten ball and calculating the time it would take for it to cool to its current temperature.
Kelvin’s calculations were soon shown to be flawed when radioactivity was discovered in 1896.
Radioactivity turned out to be useful for dating Earth materials and for coming up with a quantitative age for Earth. Scientists not only date ancient rocks from Earth's crust, they also date meteorites that formed at the same time Earth and the rest of the solar system were forming. Moon rocks also have been radiometrically dated.
Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossil ages have become increasingly accurate.
What Is Radioactive Dating?
Radioactive isotopes can be used to estimate the ages of not only of rocks, but also of fossils and artifacts made long ago by human beings. Even the age of Earth has been estimated on the basis of radioisotopes. The general method is called radioactive dating. To understand how radioactive dating works, you need to understand radioisotopes and radioactive decay.
Radioactive decay is the breakdown of unstable elements into stable elements. To understand this process, recall that the atoms of all elements contain the particles protons, neutrons, and electrons.
Isotopes
An element is defined by the number of protons it contains. All atoms of a given element contain the same number of protons. The number of neutrons in an element may vary. Atoms of an element with different numbers of neutrons are called isotopes.
Unstable isotopes decay by losing atomic particles. They form different, stable elements when they decay.
The decay of an unstable isotope to a stable element occurs at a constant rate. The decay rate is measured in a unit called the half-life. The half-life is the time it takes for half of a given amount of an isotope to decay.
Radioisotopes and Radioactive Decay
A radioisotope has atoms with unstable nuclei. Unstable nuclei naturally decay, or break down. They lose energy and particles and become more stable. As nuclei decay, they gain or lose protons, so the atoms become different elements. This is illustrated in the figure below. The original, unstable nucleus is called the parent nucleus. After it loses a particle (in this case a type of particle called an alpha particle), it forms a daughter nucleus, with a different number of protons.
The nucleus of a given radioisotope decays at a constant rate that is unaffected by temperature, pressure, or other conditions outside the nucleus. This rate of decay is called the half-life. The half-life is the length of time it takes for half of the original amount of the radioisotope to decay to another element.
How can the half-life of a radioisotope be used to date a rock?
After a rock forms, nuclei of a radioisotope inside the rock start to decay. As they decay, the amount of the original, or parent, isotope decreases, while the amount of its stable decay product, or daughter isotope, increases. By measuring the relative amounts of parent and daughter isotopes and knowing the rate of decay, scientists can determine how long the parent isotope has been decaying. This provides an estimate of the rock’s age.
Different Isotopes, Different Half-Lives
Different radioisotopes decay at different rates. You can see some examples in the table below. Radioisotopes with longer half-lives are used to date older rocks or other specimens, and those with shorter half-lives are used to date younger ones.
| Parent Isotope | Daughter Isotope | Half-Life |
|---|---|---|
| potassium-40 | argon-40 | 1.3 billion years |
| uranium-235 | lead-207 | 700 million years |
| uranium-234 | thorium-230 | 80,000 years |
| carbon-14 | nitrogen-14 | 5,700 years |
Limitations of Radiometric Dating
Radiometric dating is a very useful tool, but it does have limits:
- The material being dated must have measurable amounts of the parent and/or the daughter isotopes.
- Radiometric dating can only be done on some materials. It is not useful for determining the age of sedimentary rocks. For this, geologists date a nearby igneous rock. Then they use relative dating techniques to figure out the age of the sedimentary rock. They may not get it exactly, but there will be some idea.
Geologic Time Scale
To be able to discuss Earth history, scientists needed some way to refer to the time periods in which events happened and organisms lived. They created a listing of rock layers from oldest to youngest. Then they divided Earth’s history into blocks of time with each block separated by important events, such as the disappearance of a species of fossil from the rock record. Since many of the scientists who first assigned names to times in Earth’s history were from Europe, they named the blocks of time from towns or other local places where the rock layers that represented that time were found.
From these blocks of time the scientists created the geologic time scale (figure below). In the geologic time scale the youngest ages are on the top and the oldest on the bottom.
In what eon, era, period and epoch do we now live? We live in the Holocene (sometimes called Recent) epoch, Quaternary period, Cenozoic era, and Phanerozoic eon.
Summary
- Early geologists estimated Earth's age in a variety of inaccurate ways like the amount of time it might take for a sediment layer to be deposited.
- Radiometric dating of meteorites and Moon rocks indicate that Earth is 4.6 billion years old.
- The age of a rock or other specimen can be estimated from the remaining amount of a radioisotope it contains and the radioisotope’s known rate of decay, or half-life. This method of dating specimens is called radioactive dating.
- Radioisotopes with longer half-lives are used to date older specimens, and those with shorter half-lives are used to date younger ones.
- The geologic time scale divides earth history into named units that are separated by major events in earth or life history.
CC-BY-NC http://www.ck12.org/earth-science/Age-of-Earth/lesson/Age-of-Earth-HS-ES/?referrer=featured_content
CC-BY-NC http://www.ck12.org/physical-science/Half-life-and-Radioactive-Dating-in-Physical-Science/lesson/Half-life-and-Radioactive-Dating-MS-PS/
CC-BY-NC http://www.ck12.org/earth-science/Radioactive-Decay-as-a-Measure-of-Age/lesson/Radioactive-Decay-as-a-Measure-of-Age-MS-ES/
CC-BY-NC http://www.ck12.org/earth-science/Radiometric-Dating/lesson/Radiometric-Dating-MS-ES/
CC-BY-NC http://www.ck12.org/earth-science/Geologic-Time-Scale/lesson/Geologic-Time-Scale-HS-ES/?referrer=featured_content
