Unit 2 Lesson 3: Absolute Dating

Unit 2 Lesson 3: Absolute Dating

Poll

True or False: All rocks are made of matter and all matter is made of atoms.

• True

• False

Poll

True or False: We use calendars to measure the absolute age of people.

• True

• False

Poll

True or False: Someone tells you that he is older than you are. This tells you his absolute age.

• True

• False

Poll

True or False: If you cut a clay ball in two and then cut one of the halves in two, you will end up with four pieces of clay.

• True

• False

Match

Match the following term with its correct definition

• absolute dating

• half-life

• radioactive decay

• radiometric dating

• index fossils

determining the actual age of an event or object in years

the time needed for half of a sample of a radioactive substance to undergo radioactive decay.

The breakdown of a radioactive isotope into a stable isotope.

Determining the relative percentages of parent to daughter isotopes.

used to estimate the absolute age of rock layers in which they are found

• determining the actual age of an event or object in years

• the time needed for half of a sample of a radioactive substance to undergo radioactive decay.

• The breakdown of a radioactive isotope into a stable isotope.

• Determining the relative percentages of parent to daughter isotopes.

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

The absolute age of an event or object is the actual age in years.

• How can the absolute age of rock be determined?

Using Radioactive Isotopes

• Isotopes are atoms of one type of elements that have different numbers of neutrons.

• Many are stable, and stay the way they are.

• Some isotopes are not stable and break down over time. These isotopes are radioactive.

Unstable isotopes are called radioactive.

• They break down into different isotopes until a stable isotope (or different element) is formed.

• This is called radioactive decay.

The process...

• A neutron gets converted into a proton by releasing an electron.

• A stable daughter isotope is formed from a radioactive parent isotope.

• Energy ("radioactivity") is released in this process.

Each type of isotope decays at a specific, constant rate.

• This has been measured and determined by scientists through experiments.

Half-Life

• This is the time needed for half of a sample of a radioactive substance to undergo radioactive decay to form daughter isotopes.

• Half-life is always given in units of time.

Multiple Choice

How much of a radioactive parent isotope remains after one half-life has passed?

• 1/2

• 1/3

• 1/4

• 0

Radiometric Dating

• In a way, some radioactive isotopes in minerals act like chemical "clocks".

• The mineral crystals record the ages of the rocks in which they formed.

• Determining the relative percentages of radioactive parent isotopes to stable daughter isotopes helps determine age.

​An example of how radiometric dating can be used...

​You want to find the age of a sample that contains a radioactive isotope that has a half-life of 10 million years...

• You analyze the sample and find a 50% to 50% ratio of parent and daughter isotopes.

• That means that 50% of 1/2 of the parent isotope has decayed - one half life.

• The sample is 10 million years old.​

What is the best rock for radiometric dating?

• When igneous rock forms, elements are separated into different minerals in the rock.

• The minerals often contain all parent isotope and NO daughter isotopes.

• This helps to determine accurate percentages.

Methods of Radiometric Dating

• Since different isotopes have different half-lives, we need to have an idea of the age range we're dealing with.

• This is where relative age may come in useful.

• Then we can choose the right isotope to look for.

​Radiocarbon Dating

• The ages of wood, bones, shells, and other organic remains can be found using radio carbon dating.

• Measure decay of Carbon-14 to Carbon-12.

  • Half-Life of 5,730 years

• Only for organic matter

• Used to date things in the last 45,000 ye​ars.

Open Ended

You have found a bone in a layer of rock that you think is about 500,000 years old. Would you use radiocarbon dating to find the age of this bone? Explain.

​Potassium-Argon Dating

• Used to date igneous volcanic rocks

• Used to date rocks that are between 100,000 ye​ars and a few billion years old.

​Uranium-Lead Dating

• Used to measure the decay of uranium-328 to lead-206.

• The amount of daughter isotope - Lead-206 is measured.

• Uranium-238 has a half-life of 4.5 billion years.

• Can be used to determine the age of igneous rocks that are between 100 million and billions of years old. ​

Open Ended

Radiometric dating can be used to find the age of Earth's rocks, but why can't it be used to find the age of the Earth?

The first rocks that formed on Earth have weathered, eroded, melted, or too deeply buried.

• Rocks continuously change over time.

There are NO ROCKS on Earth that can be directly studied to answer this question.

• But we've gotten close...

• Zircon crystals have been dated to be about 4.4 billions years old!

• The age of the Earth is estimated to be about 4.6 billion years old.

Meteorites that have fallen to Earth are believed to have formed when the solar system did.

• So we can measure their absolute age to get a better idea of the age of Earth.

The oldest meteorite was found in May 2020 is the Sahara Desert.

Multiple Choice

Why are meteorites a better indicator of Earth's age than rocks found ON Earth?

• meteorites formed a long time before the solar system did

• meteorites change in a more predictable way in space

• meteorites don't undergo the changes that rocks on Earth do

• there are more meteorites to measure than other types of rock

Open Ended

Sedimentary rocks can not be directly dated. Why?

Sedimentary rocks are made up of bits and pieces of different kinds of materials.

• Igneous rock on either side of a sediment layer are dated to get an age of the sediment.

Index Fossils

• Certain types of fossils that only appear is certain layers of rock.

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

Requirements to be an index fossil.

• Organism must have lived during a relatively short geologic time period.

• Fossils must be very common and found over a large area.

• Must have characteristics that make them different from other fossils.

Tropites

• Marine mollusks; related to the squid.

• Coiled shells

• Lived between 230 million and 208 million years ago.

Trilobites

• Related to horseshoe crabs, spiders, and scorpions.

• Lived between 450 million and 360 million years ago.

Multiple Choice

Scientists use radioactive decay to measure...

• relative time
• absolute time
• half-lives
• time of day

Multiple Choice

The length of time required for half of the radioactive atoms in a sample to decay is its...

• era
• age
• half-life
• eon

Multiple Choice

How does absolute age differ from relative age?

• only absolute dating indicates which rock is older than another
• only absolute dating finds the age in years
• only relative dating finds the age in years

Multiple Choice

Radiometric dating is possible because the rates of decay of radioactive isotopes _____.

• change over time
• change from place to place
• are constant
• vary widely

Multiple Choice

If the half-life of a radioactive group of atoms is 100 years, then how old is the rock if there have been two half-lives?

• 25 years
• 200 years
• 400 years
• 800 years

Multiple Choice

Geologists use radioactive dating to determine THE ABSOLUTE AGE OF ROCKS.

• True
• False

Multiple Choice

An element that has broken down and released particles and energy is said to be _______________.

• radioactive
• subatomic

Multiple Choice

Scientists use radioactive decay to measure...

• relative time
• absolute time
• half-lives
• time of day

Multiple Choice

The original element that undergoes radioactive decay is known as the...

• daughter element
• half-life
• parent element
• carbon-14

Multiple Choice

The element that is the product of radioactive decay is known as the...

• daughter element

• half-life

• parent element

• carbon-14

Match

Match the following

• Proton

• Neutron

• Radiometric Dating

• Relative Age

• Unconformity

subatomic particle with a positive charge that determines the identity of an element

subatomic particle that has no charge who's # can vary in the nucleus of an atom to form Isotopes.

method of determining absolute age by comparing the % of parent & daughter isotopes

the age of an object in relation to the ages of other objects.

break in the geologic record created when rock layers are eroded.

• subatomic particle with a positive charge that determines the identity of an element

• subatomic particle that has no charge who's # can vary in the nucleus of an atom to form Isotopes.

• method of determining absolute age by comparing the % of parent & daughter isotopes

• the age of an object in relation to the ages of other objects.

• break in the geologic record created when rock layers are eroded.