Transcript File - Ms. D. Science CGPA

Earth’s History - Course 2
4-3 Radioactive Dating
Essential Question:
How do Scientist study Earth’s past?
What is Radioactive Decay?
What is Radioactive Dating?
How do Scientist Determine Earth’s Age?
Marie Curie
My Planet Diary pg. 152
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 purposes—including finding the age of rocks!
1. What did Marie and Pierre Curie discover about radioactivity?
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2. What does the discovery of radioactivity tell you about how scientists work
together? _________________________________________________
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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.
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?
Assess Your Understanding pg. 153
I get it! Now I know that radioactive decay occurs when
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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.
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 halflives, 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.
1. Calculate: A rock from
The moon contains 12.5 % of
The potassium-40 it began
With. How old is the rock?
Hint 12.5%=1/8
2. Calculate: A fossil contains
1/16 of the carbon-14 it began
With. How old is the fossil?
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?
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Assess Your Understanding pg. 155
a. Identify Scientists use the method of (radioactive dating / relative
dating) to find the absolute age of a rock.
b. Apply Concepts The half-life of thorium-232 is 14 billion years. A
rock with 25% of its throrium-232 remaining is __________years
old.
c. 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?
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I get it! Now I know that radioactive dating is done by _____________
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How Do Scientists Determine Earth’s Age? Pg. 156
To determine Earth’s 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.
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?
Assess Your Understanding pg. 157
I get it! Now I know that scientists determine Earth’s age by
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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?____________________
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