the isotope potassium-40 has a half-life of 1.3 billion years, and decays to argon and calcium. Argon is measured as daughter material and this method is to date rocks older than 100,000 years old.
uranium-238 is a radioactive isotope which decays in steps to turn into lead-206. The half-life of uranium-238 is 4.5 billion years and the older the rock is, the more daughter material, which is lead-206 will be in the rock. This method is used for rocks more than 10 million years old. Younger rocks can't be used for this method.
during the process of radioactive decay, the unstable parent isotope rubidium-87 creates a stable daughter isotope strontium-87. The half-life of rubidium-87 is 49 billion years and this method is to date rocks older than 10 million years.
the carbon isotopes of carbon-12, 13, and 14 mix with oxygen to create the gas carbon dioxide, which is absorbed from plants through photosynthesis. As a plant is living, new carbon dioxide with a steady carbon-14 to carbon-12 ratio is always being absorbed. Animals that eat plants get the same ratio. Once the plant or animal dies, the carbon-14 decreases as the living thing decays and the ratio lessens. The half-life of carbon-14 is 5,730 years so the Carbon-14 Method is used for dating things during the last 50,000 years.
remains or traces of organisms that lived a long time ago. They are usually preserved in sedimentary rock.
a fossil mark that is created in sediment by the movement of an organism
a mark or cavity in a sedimentary surface by a shell or other body
a fossil that is made when sediments fill in the cavity left by a decomposed animal
a fossil of organisms that lived during a short, well-defined geologic time span. They start the age of rock layers because they are found in one geologic age.
geologic time scale
the method to divide the Earth's natural history into parts
the largest division of geologic time
the unit of geologic time that has two or more periods
the unit of geologic time where eras are divided
the subdivision of a geologic period
the death of every member of a species