April 21 Monday

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1. Review sheet on absolute vs relative dating
2. Begin Absolute-Age Dating Notes pg.

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Review sheet absolute vs relative

• Determining the actual age of an event or object in years is called absolute dating.

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• Scientists often use radioactive isotopes to find the absolute age of rocks and other materials.

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• Isotopes- Atoms of the same element that have a different number of neutrons but the same number of protons

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Absolute-Age Dating

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Draw and label this element key for carbon.

Atomic number = number of protons and electrons

Atomic mass = number of protons + number neutrons

Why is carbon 14 unstable?

Absolute-Age Dating:

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• Radioactive isotopes- unstable isotopes that break down into other isotopes by a process called radioactive decay.

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• Parent isotope- unstable, radioactive isotope (Example: Carbon-14 has 6 protons but 8 neutrons)

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• Daughter isotope- stable isotope formed by the breakdown of parent isotope (Example: Carbon-12 and Nitrogen-14)

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• Half-life- time needed for half of a sample of a radioactive element to undergo radioactive decay and form daughter isotopes.
• After one half-life has passed, half of the parent isotope has changed into daughter isotopes.

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Absolute-Age Dating:

Parent isotope

Daughter isotope

• Scientists study the ratios of parent and daughter isotopes to date samples.

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• Radiometric dating- Finding the absolute age of a sample by determining the relative percentages of a radioactive parent isotope and a stable daughter isotope

How can the absolute age of a rock be determined?

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What is the best rock for radiometric dating?

• Best type of rock samples to use for radiometric dating are igneous rocks with all crystals roughly the same age.
• When igneous rocks form, minerals in them often contain only a parent isotope and none of the daughter isotope.
• This makes the isotope percentages easier to interpret and helps dating to be more accurate.

What is the best rock for radiometric dating?

• Not as useful for dating sedimentary and metamorphic rock.
• The individual grains within a sedimentary rock may contain radioactive isotopes and can be used to determine the age of the grain, but not when the grains were deposited to form the sedimentary layer.
• Process of metamorphism occurs over long intervals of time so not all crystals form at the same time.

Relative-Age vs. Absolute-Age(7:39)

Absolute-Age Dating:

What are some radiometric dating methods?

• Scientists use many different isotopes for radiometric dating.

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• The type of isotope used depends on the type of material being dated.

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• The half-life of the isotope used is also very important. It cannot be too short or too long compared to the age of the sample.

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1. Radiocarbon Dating

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• Radiocarbon dating is a method used for dating wood, bones, shells, and other organic remains.

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• All living things have a constant ratio of radioactive carbon-14(unstable) to carbon-12 (stable).

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• Once a plant or an animal dies, no more carbon is taken in. The ratio between the isotopes changes because carbon-14 undergoes radioactive decay.

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Absolute-Age Dating:

What are some radiometric dating methods?

The level of carbon-14 in living things is the same as the atmospheric level because plants and animals constantly absorb new carbon sources by photosynthesising or by eating. Dead animals and plants don’t absorb new carbon, so the longer they are dead, the less carbon-14 they contain. The ratio of carbon-14 to regular carbon-12 in dead material is the basis for carbon dating.

• Radiocarbon Dating (C14 Dating)

Absolute-Age Dating:

What are some radiometric dating methods?

All carbon-14 decays into nitrogen, but the level of carbon-14 on Earth is relatively constant because cosmic rays constantly produce new radioactive carbon from nitrogen in the upper atmosphere.

• The half-life of carbon-14 is 5,730 years. The number of half-lives of carbon-14 that have passed gives the absolute age.

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• Radiocarbon dating can be used to date organic matter only.

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• This method is used to date things that lived in the last 45,000-50,000 years.

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Absolute-Age Dating:

What are some radiometric dating methods?

Absolute-Age Dating:

What are some radiometric dating methods?

2. Potassium-Argon Dating

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• Potassium-argon dating is often used to date igneous volcanic rocks that are 100,000 years to billions of years old. (Potassium-40 to argon-40)

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Absolute-Age Dating:

What are some radiometric dating methods?

3. Uranium-Lead Dating

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• Uranium-lead dating is based on measuring the amount of the lead-207 daughter isotope in a sample. (Uranium-235 to lead-207)

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• Half life of U-235 is 704 million years, so it can be used to determine the age of igneous rocks that are between 100 million years and a few billion years old.

Absolute-Age Dating:

What are some radiometric dating methods?

Absolute-Age Dating:

How is radiometric dating used to determine the age of Earth?

• Radiometric dating can be used to find the age of Earth, but there are no Earth rocks which can be directly studied that are as old as our planet.

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• Meteorites are small, rocky bodies that have fallen from space to Earth’s surface. They are the same age as the solar system, including Earth.

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• The absolute age of meteorites and other rocks in the solar system is about 4.6 billion years.

Absolute-Age Dating:

How can fossils help to determine the age of sedimentary rock?

• Sedimentary rock layers and the fossils within them cannot be dated directly.

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• Igneous rock layers on either side of a fossil layer can be dated radiometrically.

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• Once the older and younger rock layers are dated, scientists can assign an absolute age range to the sedimentary rock layer containing the fossils.

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Absolute-Age Dating:

How can fossils help to determine the age of sedimentary rock?

• Index fossils are fossils used to estimate the absolute age of the rock layers in which they are found.

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• Once the absolute age of an index fossil is known, it can be used to determine the age of rock layers containing the same fossil anywhere on Earth.

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Absolute-Age Dating:

How can fossils help to determine the age of sedimentary rock?

• To be an index fossil, the organism from which it formed must:
• Have lived during a relatively short geologic time span.
• Be relatively common or abundant
• Be found over a large area
• Be unique

Absolute-Age Dating:

How are index fossils used?

• Index fossils act as markers for the time that the organisms were alive on Earth.

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• Index fossils can also be used to date rocks in separate areas.

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• The appearance of the same index fossil in rock of different areas shows that the rock layers formed at about the same time.

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Potential Issues with Absolute-Age Dating

When scientists date rocks, they don’t observe the atoms changing. They measure the products of the change which they assume took place in the past.

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Three Assumptions:

1. The original number of unstable atoms can be known
2. The rate of decay was constant
3. The daughter atoms were all produced by radioactive decay with no outside forces acting