Scientists can use more than one method to determine the age of the Earth. Recall that relative age simply states that an object is older or younger than another object. Another method used to learn more about Earth’s history is called absolute-age dating
. Absolute-age dating is numerical. It is specific regarding the number of years old an object is before the present. Methods used to determine absolute-age dating includes radiometric dating, tree rings, varves, and ice cores.
The circles inside a tree trunk are called growth rings. A growth ring is one new layer of alternating light and dark colored wood around the trunk of a tree each year. The light colored growth rings of wood are made during the spring and summer months. The dark colored growth rings are made during the autumn and winter months. When the tree is cut down, these layers of wood in its trunk look like concentric circles. The appearance of the annual rings inside the tree can provide scientists with information about the environment when they were formed. The width of the growth rings can indicate if the tree receives too much or too little water due to changes in climate. The absolute-age of a tree can be estimated by counting the number of growth rings inside its trunk.
The annual layer of sediment or sedimentary rock is called a varve, alternating light and dark bands of particles. Just like tree rings, varves are deposited each year. The varves form in glacial lakes. The light-colored band of coarse sediments forms in the bottom of the lake in the summer months. The dark-colored bands of fine sediments form a thin layer above the coarse sediments during the winter months. One varve consists of a coarse and fine layer of sediment. Counting the varves can help scientists estimate the absolute-age of the sediments. Scientists drill holes that are over a hundred meters deep inside glaciers to retrieve ice cores. They can study the ice cores to learn about how climate has changed over time and to also measure the concentration of atmospheric gases. Although ice cores, varves, and tree rings are useful for finding numerical-age; but they are not useful for establishing the absolute age for geologic events that occurred a really long time ago.
The rocks and soil that make up the Earth are naturally radioactive. Radioactivity is the release of energy or mass from an unstable atomic nucleus. Scientists use the method called radiometric dating to determine the absolute-age of some minerals and rocks. Radiometric dating uses radioactive decay to compare the current ratio of a radioactive isotope to a stable isotope of the same element. To get a better understanding of radioactivity; let’s review the basic building block of all matter, the atom.
An element consists of one type of atom. The periodic table lists all the known elements. The atomic number is the number of protons in an element. The number of neutrons is usually equal to the number of protons. Both the protons and the neutrons are found in the nucleus of an atom. The electrons of an atom orbit the nucleus. An isotope is an atom of an element that has the same number of protons as another atom of the element but a different number of neutrons. For example, carbon-12, the most common form of carbon, has six protons and six neutrons, whereas carbon-14 has six protons and eight neutrons.
Isotopes can be stable or unstable. Stable isotopes do not undergo radioactive decay. Unstable isotopes do undergo radioactive decay. Radioactive decay occurs when one element changes into a different element by losing protons as a result of radioactivity. The process of radioactive decay is irreversible, the atom is changed forever. For example, once a popcorn kernel pops into a fluffy white piece of popcorn, there is no going back!
Half-life is the amount of time it takes for one half of the mass of a radioactive isotope to decay to its stable isotope. The half-life of a radioactive isotope can range from hundreds of years to billions of years. The table below shows the half-life of some common radioactive isotopes used in radiometric dating. The ratio of parent radioisotope to daughter isotope can help scientists determine the age of rocks. An older rock sample would contain a greater amount of daughter isotopes compared to the parent radioisotope. Radiometric dating is used to date mostly igneous and metamorphic. The method is rarely used to find the age of sedimentary rocks because they undergo weathering and erosion. The sediments may be weathered particles of older rocks.
| Radioactive isotopes used in Radiometric Dating | ||
|---|---|---|
| Radioactive parent isotope | Stable Daughter isotope | Half-life |
| Potassium-40 | Argon-40 | 1.25 billion years |
| Uranium-238 | Lead-206 | 4.5 billion years |
| Rubidium-87 | Strontium-87 | 47 billion years |
Another method for dating rocks is called radiocarbon dating. It uses carbon-14 to date organic materials. This method is used to date the organic materials found in younger rocks. Carbon-14 is the radioisotope of carbon-12. Both are present in the atmosphere, plants, and animals. The ratio of carbon-12 to carbon-14 remains constant in all living organisms. When an organism dies, the amount of carbon-14 in its body steadily decreases as carbon-14 decays. The half-life of carbon-14 is about 5,730 years. Scientists can use the ratio of carbon-12 to carbon-14 to find the absolute age of organic materials that formed up to about 75,000 years ago.
Watch the video below. Use the first ten seconds to answer the questions below.
What do the red circles represent?
What do the blue circles represent?
What percent of the carbon and nitrogen was there after the first and second half life?
What can you assume from that? (answers: 50%, 25% and 75%, Radioactive Carbon-14 decays into nonradioactive nitrogen-14)