Absolute DatingAbsolute DatingChapter 7Chapter 7

Lesson 2Lesson 2

Absolute DatingAbsolute Dating

Main Idea Unstable parent isotopes change to

stable daughter isotopes at a constant rate. Geologists measure the amounts of these isotopes in minerals to determine how long ago the minerals formed which is called Absolute Dating.

IsotopesIsotopes

Atoms of the same element that have the same number of protons but have a different number of neutrons are called isotopes.

Most isotopes are stable , meaning they stay in their original form.

But some isotopes are unstable-we call these radioactive.

Radioactive DecayRadioactive Decay

The breakdown of a radioactive, unstable isotope (parent) of one element into a stable isotope (daughter) of another element is called radioactive decay.

Scientists compare the amount of parent isotope with the amount of daughter

isotope to determine the absolute age of a rock.

The more daughter isotope there is the older the rock is.

Half Life &Half Life &Radiometric DatingRadiometric Dating

Radiometric dating is finding the absolute date of a rock using

radioactive decay and the half life of an isotope.

A half life is the time it takes for one- half of a radioactive, unstable parent isotope to decay by ½ into daughter

isotopes.

2 Forms of 2 Forms of Radiometric DatingRadiometric Dating

Potassium-40Potassium 40 has a half life of 1.3 billion

years. It decays to 2 other stable elements called argon and calcium. This is

used to date rocks older than 100,000 years old.

Uranium-238Uranium-238 is a radioactive isotope that

decays to lead-206. Uranium-238 has a half life of 4.5 billion years. This is used to date rocks older than 10 million years

Half DomeHalf Dome

Using Uranium-238 to measure the ignous rock of half dome determined that Half Dome in Yosemite formed

about 85 million years ago.

Age of EarthAge of Earth

Even though there are no rocks on earth that we can measure the date that are from when the Earth was

formed, rocks from our solar system, the moon, and from meteorites have

been measured.The Earth and our Solar system are

about 4.6 billion years old