Relative and Absolute Dating

Hill Science 6

Relative Dating

• Fossils can be dated relative to one another by noting their positions in strata.

• Fossils found in lower strata were generally deposited earlier and are older.

Relative Dating

• Geologic processes can cut through older layers.

• Scientists have learned how to read these clues.

Relative & Radiometric Dating

Absolute Dating is another term for radiometric dating which is used to pinpoint the time period.

How is this done?

Atoms and Isotopes: Let’s review the basics

Atomic # - The number of protons in an atom determines which element it is.

• If you change the # of protons, the element changes & the mass changes.

Mass # = (protons + neutrons)• If you change the # of neutrons, the

element stays the same, but the mass changes

ISOTOPES - atoms of the same element that have different numbers of neutrons

http://www.saskschools.ca/curr_content/chem30/images/e_deuterium.jpg

Nucleus: protons and neutronsElectron cloud: electrons

Why Are Some Isotopes Radioactive?

Mass # = protons + neutrons

Carbon 12 and Carbon 14 are both isotopes of carbon, one with 6 neutrons and one with 8 neutrons (both with 6 protons).Which isotope is unstable?

Carbon – atomic # 6How many protons?

Isotopes that have the right amount of neutrons are called stable. They always stay the same.

Some isotopes have a few too many neutrons or not enough - This makes them unstable and radioactive.

The nuclei of these radioactive atoms change or decay by giving off radiation in the form of particles or electromagnetic waves until the atom reaches a stable state.

Radioactive Decay

Radioactive isotopes tend to break down into stable isotopes of the same or other elements in a process called

Chapter 6 Absolute Dating: A Measure of Time

Radioactive Particles

• Alpha particle– 2 protons and 2 neutrons

Radioactive Particles

• Beta Particle

Radioactive Particles

• Gamma Radiation

• Carbon-14 is produced in the atmosphere when neutrons from cosmic radiation react with nitrogen atoms.

Radiometric Dating

• A half-life is the time needed for half of a sample of a radioactive substance to undergo radioactive decay.

• After every half-life, the amount of parent material decrease by one-half.

How does radio activity allow scientists to date rock?

Radioactive elements decay or change to become stable.

“Parent atoms” decay into stable “daughter atoms.”

When molten rock cools, forming igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.

Textbook PowerPoint CD: Slide 47 visual conceptclick here

Carbon-14 Dating

Other Isotopes Used in Dating

• Half-life of carbon-14 is 5,700 years– Can only date objects up to about 60,000 years old.

• So use other isotopes• Potassium-40 is another radioactive element naturally found

in your body and has a half-life of 1.3 billion years. • Other useful radioisotopes for radioactive dating include

Uranium -235 (half-life = 704 million years)• Uranium -238 (half-life = 4.5 billion years)• Thorium-232 (half-life = 14 billion years)• Rubidium-87 (half-life = 49 billion years).• Brain, Marshall. "How Carbon-14 Dating Works" 03 October 2000. HowStuffWorks.com.

<http://science.howstuffworks.com/environmental/earth/geology/carbon-14.htm> 02 March 2014.

Bracketing• Fossils are generally

found in sedimentary rock—not igneous rock.

• Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.

• Scientists look for igneous rock above and below the fossils: bracketing

Bracketing• To date most older

fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.

• Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.

• By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be.

Volcanic Ash – Igneous Rock