Absolute Ages of Rocks

Different Types of Isotopes

Radioactive isotopes with short half-lives can't be used for dating old rocks because they don't contain enough parent isotope to measure. Geologists often use a combination of radioactive isotopes to measure the age of a rock, which helps make the measurements more accurate.

Dating Sedimentary Rocks

A rock must have U-235 or other radioactive isotopes trapped inside it in order to be dated by radiometric means. The grains in many sedimentary rocks come from a variety of weathered rocks from different locations. The radioactive isotopes within these grains generally record the age of the grains, not the time when the sediment was deposited.

Isotopes

All atoms of a given element have the same number of protons. All forms of hydrogen contains only one proton regardless of the number of neutrons.

Atoms

An atom is the smallest part of an element that has all the properties of the element. Protons and neutrons are in an atom's nucleus, and electrons surround the nucleus.

Half-life

An isotope's half-life is the time required for half of the parent isotopes to decay into daughter isotopes.

Isotopes

Isotopes are atoms of the same element that have different numbers of neutrons.

Radioactive Decay

Radioactive decay is the process by which an unstable element naturally changes into another element that is stable.

Radioactive decay

Radioactive isotopes decay, or change, over time. As they decay, they release energy and form new, stable atoms. The unstable isotope that decays is called the parent isotope, and the unstable isotope that forms is called the daughter isotope.

The Age of Earth

Evidence suggests that Earth, the Moon, and meteorites all formed at about the same time. Understanding Earth's history can help scientists understand changes occurring on Earth today- as well as changes that are likely to occur in the future.

Absolute ages of rocks

Geologists have developed accurate historical records for many geologic features. Radioactivity is the release of energy from unstable atoms.

Radiometric Ages

In the process of radiometric dating, scientists measure the amount of parent isotope and daughter isotope in a sample of the material they want to date. From this ratio, they can determine the material's age by making precise measurements in laboratories.

Radiocarbon Dating

Radiocarbon (carbon-14) has six protons and eight neutrons and forms in Earth's upper atmosphere. It then mixes in with a stable isotope of carbon-12, and the ratio of C-14 and C-12 is constant. All living things use carbon as they build and repair tissues, but an organism stops taking in C-14 when it dies. Scientists measure the ratio of C-14 to C-12 in the remains of the dead organism to determine how much time has passed since the organism died.

Dating Rocks

Radiocarbon dating is useful only for dating organic material- material from once-living organisms, such as bones, wood, parchment, and charcoal. Most rocks and fossils don't contain organic material.

Absolute age

Scientists use the term absolute age to mean the numerical age, in years, of a rock or object.

Half-life

The rate of decay from parent isotopes into daughter isotopes is different for different radioactive elements, but the rate of decay is constant for a given isotope. 1.object contains 100%. 2.one half-life: 50% parent and 50% daughter. 3.two half-lives-25% parent and 75% daughter. 4.three half-lives: 12.5% parent and 87.5% daughter.

Dating Igneous Rocks

Uranium-235 is often trapped in the minerals of igneous rocks that crystallize from hot, molten magma. As soon as U-235 is trapped in a mineral, it begins to decay to lead-207 , or Pb-207. Scientists measure the ratio of U-235 to to Pb-207 in a mineral to determine how much time has passed since the mineral formed, and this provides the age of the rock that contains the mineral.