Download Dating the Earth Power Point

Dating the
Earth
Grade 8 Science
The Principle of Uniformitarianism
• The past history of the Earth must be
explained by what we can see happening
today.
• Or: “the present is the key to the past”
Two Methods
• Relative Dating – list order of events, not ages
– Laws and Principles of Stratigraphy
• Absolute dating – actual age of events in
years.
– Radiometric dating
Relative Dating
Grade 8 Science
Relative Dating
• Is the science determining the relative order
of past events, without necessarily
determining their absolute age.
• Identifying which rocks formed first, second,
third and so on.
• Tells us the sequence in which events
occurred, not how long ago they occurred
Laws and Principles of
Stratigraphy
•
•
•
•
•
Law of Superposition
Law of Original Horizontality
Law of Cross Cutting Relationships
Law of Lateral Continuity
Law of Included Fragments
Law of Superposition
• In an unaltered sequence of sedimentary
rocks each bed or layer is older than the one
above it and younger than the one below it.
The Law of Superposition
Stratigraphic Colum Diagram
Law of Original Horizontality
• Sediment is generally
deposited in a horizontal
position
• Flat layers are undisturbed
• If rock layers are tilted or
bent, then they must have
been moved sometime after
they were first deposited.
•
Law of cross-cutting
Relationships
a geologic feature which cuts through another
is the younger of the two features.
• Applies to faults or igneous intrusions (dikes,
etc.)
• Faults or intrusions are younger than the rocks
affected
Law of Lateral Continuity
• Layers of sediment
initially extend
laterally in all
directions result in
rocks that are
otherwise similar, but
are now separated by
a valley or other
erosional feature,
• They can be assumed
to be originally
continuous.
The Law of Included Fragments
• Fragments of rock that are contained (or
included) within a host rock are older than the
host rock.
Absolute Dating by the Use
of
Radioactive Isotopes
absolute dating
the process of establishing the age of an object by
determining the number of years it has existed
Relative
Dating
http://evolution.berkeley.edu/evosite/lines/IIIAchronology.shtml
Absolute
Dating
Absolute Dating
• Does not mean it isn’t without error
• Radiometric dating is the most common type
of absolute dating.
Carbon element box
What can be determined?
Element
Carbon
6
Symbol
Atomic
Number
C
12.01
Atomic Mass
Atoms and Isotopes
• If you change the # of neutrons, the
element stays the same, but the mass
changes
• ISOTOPES - atoms of the same
element that have different
numbers of neutrons

Carbon-14 is also referred to as:


C-14
Carbon-14
• Types of carbon (isotopes)
Atomic mass
9
14
16
6
6
6
Atomic number
Atoms & Isotopes
• Some are stable and some are unstable
• When an isotope is unstable it will decay over
time and turn into another atom
• If an isotope is unstable it is considered to be
radioactive
• The process of breaking down is called
radioactive decay.
• Original isotope is the parent
• New atom is the daughter

The nucleus of an atom changes into a
new element
– The proton number (atomic number) must change
– A neutron changes into a proton
14
6
14
7
Atoms and Isotopes
• If you change the # of protons, the element changes & the mass
changes.
• Which element has 7 protons?
Nitrogen
• If I take away one proton, I would only have six! Which element has 6
protons?
Carbon
• If I add one proton to the original 7, I will have 8. Which element has 8
protons?
Oxygen
Radioactive Decay
• Alpha decay - Alpha decay is caused when there are too
many protons in a nucleus. In this case the element will
emit radiation in the form of positively charged particles
called alpha particles.
• Beta decay - Beta decay is caused when there are too many
neutrons in a nucleus. In this case the element will emit
radiation in the form of negatively charged particles called
beta particles.
• Gamma decay - Gamma decay occurs when there is too
much energy in the nucleus. In this case gamma particles
with no overall charge are emitted from the element.
Radioactive Decay
• When an atom (or isotope) undergoes
radioactive decay it can lost some atomic
particles (protons, neutrons or electrons) and
turn from one element into another.
• Radioactive decay – the process by which an
unstable element naturally changes into
another element that is stable.
Radioactive Elements Decay
• The universe is full of naturally occurring
radioactive elements. Radioactive atoms are
inherently unstable; over time, radioactive
“parent atoms” decay into stable “daughter
atoms.”
• Most commonly occurs in igneous rock.
• When molten rock cools radioactive atoms are
trapped inside.
• Afterward they decay.
How Long Does Radioactive Decay Take?
• Half-Life - the time it takes for half of the atoms
(radioactive isotopes) in a sample to decay.
– Measured in seconds, minutes, years, etc.
– Each isotope has its own unique half-life.
• From thousandths of a second to billions of years
For example:
The half life of carbon-14 is 5,730 years.
If you have a sample of carbon-14 with 1,000 atoms how many atoms
will be left after one half-life? 500
Common Isotope Pairs
Half-Life
• After two half-lives how much of the
original isotope will remain? One-fourth
• After three half-lives how much of the
original isotope will remain? One-eighth
.
If isotope X has a half life of 20 years and it decays over a period
of 100 years. How many half lives did it go through?
For example:
• An isotope of cesium (cesium -137) has a halflife of 30 years. If 1 gram of cesium – 137
disintegrates over a period of 90 years, how
many grams of cesium-137 would remain?
Time (yrs.)
Mass (g)
For example:
• An isotope of argon has a half-life of 500
years. If 100 grams of argon disintegrates over
a period of 2000 years, how many grams of
argon would remain?
Time (yrs.)
Mass (g)
Twizzler activity
Background
• Finding out how old something is helps scientists understand the
history of Earth and determine evolutionary pathways. Radioactive
dating is an important tool scientists use to do this. To find a
radioactive date, the object being dated must contain a radioactive
element such as uranium-235 or carbon 14. These elements are
decaying by emitting a small part of the atom. They also become
something new, uranium turns to lead, carbon 14 turns to carbon
12. By determining the ratio of uranium to lead or carbon 14 to
carbon 12, it can be determined how long that item has been
around. In this activity you will model the decay of three different
“radioactive” atoms, each with a different half-life. Half-life is a
common way to describe the length of time it takes for half the
atoms in a particular element to decay.
Purpose
• Purpose: The purpose of this lab is to use
licorice as a model of radioactive atomic
decay. You will cut several pieces of licorice in
half to model the decay of different
“elements.” This should help you understand
the concept of a half-life and how it is used in
dating fossils as evidence for evolution.
Procedure
1. Obtain a piece of each color licorice from your teacher. Different colors of
licorice represent different elements in nature. For example the red licorice
could represent an element like carbon 14.
2. The color of the licorice determines its half-life. Each time a half-life passes
1/2 of the element decays.
3. In each table record the initial amount of each element. Do this by massing
the piece of licorice.
4. Cut your licorice in half. Find the mass of the piece you cut. This is the
amount (in grams) of the element the licorice “decayed” into. Record in your
data table.
5. Measure the amount of licorice you have left. Record in your data table.
The time it took for this decay is the half-life.
6. Continue this process until you cannot cut your licorice any more (or you
cannot determine the mass using the electronic balance).
7. Label your graph x-axis “years” and y-axis “amount in grams to start”. Try to
fill up as much of the graph paper as you can.
8. Graph your data. (Use a different color for each element.)
9. Repeat steps 1-8 for each piece of licorice.
Color (atom)
Red-1
Black-1
Half-Life (years)
2000
500
Radiometric Ages
Atoms Don’t Age the Way We Do
1
2
3
4
Start with 16 baby
aliens
Have 70 year half-lives
4 half-lives = 280 years
Each atom has a 50%
chance of decaying
during a half-life.
http://www.colorado.edu/physics/2000/isotopes/ima
ges/age280_baby.jpg