Living Things

1
Earths History - Course 2
4-3 Radioactive Dating Essential Question How
do Scientist study Earths past? What is
Radioactive Decay? What is Radioactive
Dating? How do Scientist Determine Earths Age?
2
My Planet Diary pg. 152

• Marie Curie
• In 1896, French scientists named Marie and Pierre
  Curie heard about experiments that had been done
  by another scientist, Henri Becquerel.
• Becquerel had shown that by placing some uranium
  salt on a photographic plate, covered with black
  paper, the plate would be affected as if light
  had fallen on it. The effect is produced by
  special rays which are emitted by the uranium
  salt. My determinations showed that the emission
  of the rays is an atomic property is an atomic
  property of the uranium.
• Marie Curie later described what happened The
  property that Becquerel and the Curies discovered
  was called radioactivity. Today, radioactivity is
  used for many purposesincluding finding the age
  of rocks!
• What did Marie and Pierre Curie discover about
  radioactivity?
• __________________________________________________
  ____
• 2. What does the discovery of radioactivity tell
  you about how scientists work together?
  _________________________________________________
• __________________________________________________
  __________

3
What is Radioactive Decay? Pg. 153

• Some elements decay, or break down, over time,
  releasing particles and energy. This process is
  called radioactive decay, and the elements are
  considered to be radioactive.
• During radioactive decay, the atoms of one
  element break down to form atoms of another
  element.
• The half-life of a radioactive element
• is the time it takes for half of the
• radioactive atoms to decay.
• The rate of decay of each
• radioactive element never changes.

4
Radioactive Dating
Half-Life The half-life of a radioactive element
is the amount of time it takes for half of the
radioactive atoms to decay. What pattern do you
see in the graph?
5
Assess Your Understanding pg. 153

• I get it! Now I know that radioactive decay
  occurs when
• __________________________________________________
  __________________________________________________
  __

6
What is Radioactive Dating? Pg. 154

• Radioactive elements in igneous rocks decay over
  time into other elements. This slowly changes the
  composition of the rock.
• Geologists use radioactive dating to determine
  the absolute ages of rocks.
• In radioactive dating, scientists first
  determine the amount of a radioactive element in
  a rock. Then they compare that amount with the
  amount of the stable element into which the
  radioactive element decays.
• They calculate the age of the rock using this
  information and the half-life of the element.

7
Radioactive Dating pg. 154

• A rock contains 25 of the potassium-40 it
  started with. How old is the rock?
• Step 1 Determine how many half-lives have
  passed.
• -After one half-life, 50 of the potassium would
  remain. After two half- lives, 25 of the
  potassium would remain. So two half-lives would
  have passed.
• Step 2 Find the half-life of potassium-40.
• -The half-life of potassium-40 is 1.3 billion
  years.
• Step 3 Multiply the half-life by the number of
  half-lives that have passed.
• -1.3 billion years/half-life x 2 half lives
  2.6 billion years, so the rock is about 2.6
  billion years old.
• Calculate A rock from
• The moon contains 12.5 of
• The potassium-40 it began
• With. How old is the rock?
• Hint 12.51/8
• 2. Calculate A fossil contains
• 1/16 of the carbon-14 it began
• With. How old is the fossil?

8
Potassium-Argon Dating and Carbon-14 Dating

• Some elements used by scientists in radioactive
  dating include potassium-40 and carbon-14.
• Potassium-40 has a long half-life, which is
  useful in dating the most ancient rocks.
• All plants and animals contain some carbon-14,
  which decays after the organism dies.
• Scientists measure the amounts of carbon-14 to
  determine the age of a rock sample.
• Real World Inquiry using Carbon-14 Dating
• Scientists have dated these skeletons to
  5,000-6,000 years ago. But they do not use
  radioactive dating to find the age of stone
  artifacts made by people. Why not?
• __________________________________________________
  __________________________________________________
  ________

9
Assess Your Understanding pg. 155

• Identify Scientists use the method of
  (radioactive dating / relative dating) to find
  the absolute age of a rock.
• Apply Concepts The half-life of thorium-232 is
  14 billion years. A rock with 25 of its
  throrium-232 remaining is __________years old.
• Challenge A scientist finds stone tools in the
  ruins of an ancient house. The house also has
  ashes in a fireplace. How could the scientist
  estimate the age of the stone tools?
  __________________________________________________
  __________________________________________________
  ______
• I get it! Now I know that radioactive dating is
  done by _____________
• __________________________________________________
  _____

10
How Do Scientists Determine Earths Age? Pg. 156

• To determine Earths age, scientists use
  radioactive dating and evidence from rocks on
  Earth and the moon.
• Using radioactive dating, Earth rocks are
  estimated to be between 3.8 and 4.28 billion
  years old.
• However, Earth rocks have changed over time due
  to the processes of plate tectonics and erosion.
• Moon rocks, fortunately, have not changed over
  time.
• According to one theory, scientists infer that
  the moon formed when another object collided with
  Earth.
• Since the moon came from Earth, the moon is about
  the same age as Earth.
• Radioactive dating estimates moon rocks to be
  4.6 billion years old. Scientists estimate that
  Earth is about the same age.

11
Radioactive Dating pg. 157
Formation of the Moon According to one theory, a
planet-sized object struck Earth. Material from
the object and material from Earth were knocked
into orbit around Earth. Gravity pulled the
material together to form the moon. How do moon
rocks help scientists estimate the age of Earth?
12
Assess Your Understanding pg. 157

• I get it! Now I know that scientists determine
  Earths age by
• __________________________________________________
  _______
• Rock Dating
• This rock is from the Jack Hills in Australia.
• Tiny zircon crystals in this rock have survived
• From some of the earliest rocks that formed
• on Earth. The crystals preserved minerals that
• were trapped inside when the crystals formed.
• By examining these minerals, cientists can
• determine the conditions under which the
• crystal formed.
• Why would only tiny crystals from the earliest
• rocks have survived?____________________
• _____________________________________