Absolute vs. Relative Dating

What is Geology? Geology is the study of the Earth’s form and composition and the changes it has undergone.

Why do we care? Rock record provides evidence of geological events and life forms of the past. Processes observed on Earth in the present also acted in the past. Earth is very old and has changed over time.

Is the Earth changing now? Yes! Although many features of the Earth may seem to be unchanging over our lifetime, they are still changing, but on time scales of hundreds, thousands or even millions of years. This image shows Mount Hood in Oregon USA at the same time in late summer in 1985 and 2002.

How old is the Earth?

How old is the Earth? The universe is 13.7 billion years old The sun is 5 billion years old. The Earth is 4.6 billion years old. The moon is 4.5 billion years old.

How do we know? Relative Dating Absolute Dating

Relative Dating Relative Dating identifies which rock units formed first, second, third and so on. Relative dating determines the sequence in which events occurred, but not how long ago they occurred.

Relative Dating Law of Superposition – each rock layer is older than the one above it and younger than the one below it. Usually applies to sedimentary rock, but can also be applied to lava flows and layers of ash from volcanic rock.

Relative Dating Principle of Original Horizontality – layers of sediments are generally deposited in horizontal position.

Relative Dating Principle of Cross-Cutting – when a fault cuts through rock layers, or when magma intrudes other rocks and hardens, then the fault or intrusion is younger than the rocks around it.

Absolute Dating Absolute Dating identifies the exact age of a rock. Scientists use radioactive decay to determine the age of the rock.

Atom Review What are the subatomic particles of an atom? Proton Electron Neutron Which subatomic particles make up the mass of the atom? Protons Neutrons

Radioactive Decay During radioactive decay, unstable atomic nuclei spontaneously break apart, or decay, releasing energy.

Radioactive Decay What is an isotope? Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. Hydrogen Tritium Deuterium

Carbon Dating Carbon-14 dating, is based on the ratio of C-14 to C-12 in an organic sample less than 70,000 years old As long as the organism was alive, the ratio of carbon-12 to carbon-14 was in equilibrium with its environment. When the organism dies, the "clock" starts as the carbon-14 decays, changing the ratio

Radioactive Decay Radioactive isotopes decay until they are stable. Ex. Uranium-238 decays over time to form lead-206

Half Life Half life is the amount of time necessary for one half of the nuclei in a sample to decay to its stable isotope.

Radiometric Dating Radiometric dating, also called radioactive decay dating, is a way of calculating the absolute ages of rock that contain radioactive isotopes.

Practice Problems 1. The half life of nickel is 100 years. If you had 94 grams of nickel, how much would be left after 200 years? 1 half life = 100 years = 47 grams 2 half lives = 200 years = 23.5 grams

Practice Problems 2. Sodium-24 has a half life of 15 hours. How much of a 440 gram sample of sodum-24 remains after 60 hours? 1 half life = 15 hours = 220 grams 2 half lives = 30 hours = 110 grams 3 half lives = 45 hours = 55 grams 4 half lives = 60 hours = 27.5 grams

Practice Problems 3. Iron-59 has a half life of 45 days. A murder weapon was found in a lake and the original weapon was reported to have contained 1,200 grams of iron. After 6 months (180 days), the original sample decayed. How much iron-59 was left? 1 half life = 45 days = 600 grams 2 half lives = 90 days = 300 grams 3 half lives = 135 days = 150 grams 4 half lives = 180 days = 75 grams