3.2 Absolute-Age Dating

What have geologists developed by measuring the absolute age of rocks?

Accurate historical records for many geologic formations.

How long of a life do the most useful radioactive isotopes for dating old rocks have?

All of them have long half-lives

What is an important radioactive isotope used for dating?

An isotope of carbon called radiocarbon.

What happens as time passes with radioactive decay?

As time passes, more and more unstable parent isotopes decay and form stable daughter isotopes.

When was radioactivity discovered?

At the beginning of the twentieth century when scientists were able to determine the absolute ages of rocks and other objects.

What are isotopes?

Atoms of the same element that have different numbers of neutrons.

Why is radiocarbon known as carbon-14 or C-14?

Becaue there are 14 particles in its nucleus - 6 protons and 8 neutrons

Why does it make sense to scientists that the Earth is 4.54 billion years old?

Because rocks and minerals on Earth are more than 4 billion years old, so scientists know Earth must be at least that old. Evidence suggests that Earth, the Moon, and meteorites formed at about the same time.

Why are radioactive isotopes not useful for dating sedimentary rocks?

Because sedimentary rocks are made from grains and the radioactive isotopes within these grains generally record the ages of the grains, NOT the time when the sediment was deposited.

What country were the oldest known rock formations found?

Canada

When is the rate of decay from parent isotopes into daughter isotopes different?

When there are different radioactive elements.

Where does C-14 mix wtih a stable isotope of carbon called C-12 or carbon-12?

in Earth's upper atmosphere

Where are electrons located?

outside the nucleus

What smaller particles are in an atom's nucleus?

protons and neutrons

What are the smaller particles that each atom contains?

protons, neutrons and electrons

What happens to an isotope after it reaches two half-lives?

One-half of hte remaining parent isotopes have decayed so that only one-quarter as much parent remains as at the start.

What kind of material is radiocarbon material useful for?

Only for dating organic material (which is material from once-living organisms)

Since when have scientists been able to determine the absolute ages of rocks and other objects?

Only since the beginning of the 20th century.

What do all atoms of a given element have the same number of?

Protons

What is located in the atom's nucleus?

Protons and neutrons

What does every atom contain?

Protons, neutrons and electrons.

What are unstable isotopes known as?

Radioactive isotopes

What is the half-life of carbon-14?

5,730 years

What two things must isotope atoms have?

(1) the same element, and (2) different numbers of neutrons

What is the approximate age of Earth?

4.54 billion years old

What kind of isotopes do geologists use to date rocks?

Different kinds of radioactive isotopes

What do geologists use different kinds of radioactive isotopes for?

For dating rocks

What happens to an isotope after it reaches three half-lives?

Half again of the remaining parent isotopes have decayed into daughter isotopes.

Which rock is easier to apply radiometric dating to? Sedimentary or Igneous rock?

Igneous rock

Where does radiocarbon form?

In the Earth's upper atmosphere.

What is organic material?

It is material from once-living organisms, such as bones, wood, parchment and charcoal.

What is radioactivity?

It is the release of energy from unstable atoms.

What is an atom?

It is the smallest part of an element that has all the properties of the element.

What is absolute age?

It means the numerical age, in years, of a rock or an object.

What must a rock contain in order to be dated by radiometric means?

It must have U-235 or other radioactive isotopes trapped inside it.

What happens to an unstable element in radioactive decay?

It naturally changes into another element that is stable.

What happens to the C-14 already present in an organism that is dying start to decay?

It starts to decay to nitrogen-14 (N-14)

What happens to the level of carbon-14 as an organism dies?

It stops taking in C-14

What does it mean when an isotope reaches one half-life?

It's means half the parent isotopes have decayed into daughter isotopes.

How is the rate of decay for a given isotope measured?

It's measured in time units called half-lives.

What is radiocarbon dating useful for?

Measuring the age of the remains of organisms that died up to about 60,000 years ago.

What provides the age of the rock that contains a mineral?

Measuring the ratio of U-235 to Pb-207 in a mineral

What is another way to say nitrogen-14?

N-14

Is radiocarbon dating useful for non-organic material?

No! It's only useful for dating organic material.

Are all isotopes stable?

No! Most of them are stable.

Can radioactive isotopes with short half-lives be used for dating old rocks?

No, because they don't contain enough parent isotope to measure.

What three things help scientists understand Earth's long history?

Radiometric dating, the relative order of rock layers, and fossils.

What are two rocks that scientists used for radiometric dating of the Earth?

Rocks from the Moon and meteorites

What is one of the oldest known rock formations?

Rocks near Hudson Bay in Canada

What does radiometric dating of rocks from the Moon and meteorites indicate about the age of Earth?

That Earth is 4.54 billion years old

What about the Earth, Moon, and meteorites make scientists think the Earth is 4.54 billion years old?

That they formed at about the same time.

What is radioactive decay?

The process by which an unstable element naturally changes into another element that is stable.

What is radiometric dating?

The process by which scientists measure the amount of parent isotope and daughter isotope in a sample of the material they want to date.

What happens to the ratio as time pasess with radioactive decay?

The ratio between the numbers of parent and daughter isotopes is always changing.

How were the rocks in Hudson Bay in Canada dated by geologists?

Using radiometric dating

What is an isotope's half-life?

The time required for half of the parent isotopes to decay into daughter isotopes.

How are x-ray images made?

They are made using radioactive energy.

What happens as a result of radioactive isotopes decaying at a constant rate?

They can be used like clocks to meaure the age of the material that contains them.

Where do the grains in many sedimentary rocks come from?

They come from a variety of weathered rocks from different locations.

How do scientists determine how much time has passed since a mineral was formed?

They measure the ratio U-235 to Pb-207

What happens as radioactive isotopes decay?

They release energy and form new, stable atoms.

What do electrons surround?

They surround the nucleus

Why do scientists use the term absolute age?

They use it to mean the numerical age, in years, of a rock or an object.

Why do scientists measure the ratio of U-235 to Pb-207 in a mineral?

To determine how much time has passed since the mineral formed.

Why do scientists measure the ratio of C-14 to C-12 in the remains of a dead organism?

To determine how much time has passed since the organism died.

How long does the rate of decay from a parent isotope into daughter isotopes continue?

Until nearly all parent isotopes have decayed into daughter isotopes.

What are the five most useful radioactive isotopes for dating old rocks?

Uranium-235, Potassium-40, Uranium-238, Thorium-232, and Rubidium-87

What helps make radiometric dating measurements more accurate?

Using a combination of radioactive isotopes to measure the age of a rock.

Are electrons part of every atom?

Yes

Are most isotopes stable?

Yes

Are neutrons part of every atom?

Yes

Are protons part of every atom?

Yes

Are some istopes unstable?

Yes

Can an element's atoms have different numbers of neutrons?

Yes

Do all atoms of a given element have the same number of protons?

Yes

Do radioactive isotopes decay at a constant rate?

Yes

Is the ratio of C-14 to C-12 in the atmosphere constant?

Yes

Can a rock be dated if it has other radioactive isotopes trapped inside it, but no U-235?

Yes!

Is the rate of decay constant for a given isotope?

Yes!

Do isotope atoms have to have the same number of protons?

Yes, because all atoms of a given element have the same number of protons.

Is the rate of decay from parent isotopes into daughter isotopes different?

Yes, for different radioactive elements.

What are examples of organic material?

bones, wood, parchment, and charcoal

What is radiocarbon also known as?

carbon-14 or C-14

What do radioactive isotopes do over time?

decay or change

What are one of the most common isotopes used in radiometric dating?

uranium-235 or U-235