Transcript Relative and Absolute (Radiometric) Dating

Relative and Absolute Dating
Hill Science 6
Relative Dating
• Fossils can be dated
relative to one another
by noting their
positions in strata.
• Fossils found in lower
strata were generally
deposited earlier and
are older.
Relative Dating
• Geologic processes can
cut through older
layers.
• Scientists have learned
how to read these
clues.
Relative & Radiometric Dating
Absolute Dating is
another term for
radiometric dating
which is used to
pinpoint the time
period.
How is this done?
Atoms and Isotopes: Let’s review
the basics
Atomic # - The number of protons in an
atom determines which element it is.
Nucleus: protons and neutrons
Electron cloud: electrons
• If you change the # of protons, the
element changes & the mass changes.
Mass # = (protons + neutrons)
• If you change the # of neutrons, the
element stays the same, but the mass
changes
http://www.saskschools.ca/curr_content/c
hem30/images/e_deuterium.jpg
ISOTOPES - atoms of the same
element that have different
numbers of neutrons
Why Are Some Isotopes Radioactive?
Carbon – atomic # 6
How many protons?
Isotopes that have the right amount of
neutrons are called stable. They always stay
the same.
Some isotopes have a few too many neutrons
or not enough - This makes them unstable and
radioactive.
The nuclei of these radioactive atoms change
or decay by giving off radiation in the form of
particles or electromagnetic waves until the
atom reaches a stable state.
Mass # = protons + neutrons
Carbon 12 and Carbon 14 are both
isotopes of carbon, one with 6
neutrons and one with 8 neutrons
(both with 6 protons).
Which isotope is unstable?
Chapter 6
Absolute Dating: A Measure of Time
Radioactive Decay
Radioactive isotopes tend to break down into stable
isotopes of the same or other elements in a process called
Radioactive Particles
• Alpha particle
– 2 protons and 2 neutrons
Radioactive Particles
• Beta Particle
Radioactive Particles
• Gamma Radiation
•
Carbon-14
is produced
in the
atmosphere
when
neutrons
from cosmic
radiation
react with
nitrogen
atoms.
Radiometric Dating
• A half-life is the time needed for half of a sample of a
radioactive substance to undergo radioactive decay.
• After every half-life, the amount of parent material
decrease by one-half.
How does radio activity allow scientists to date rock?
Radioactive elements decay or change to become stable.
“Parent atoms” decay into stable “daughter atoms.”
When molten rock cools, forming igneous rocks, radioactive atoms are trapped inside.
Afterwards, they decay at a predictable rate. By measuring the quantity of unstable
atoms left in a rock and comparing it to the quantity of stable daughter atoms in the
rock, scientists can estimate the amount of time that has passed since that rock
formed.
Textbook PowerPoint CD: Slide 47 visual concept
click here
Carbon-14 Dating
Other Isotopes Used in Dating
• Half-life of carbon-14 is 5,700 years
– Can only date objects up to about 60,000 years old.
• So use other isotopes
• Potassium-40 is another radioactive element naturally found
in your body and has a half-life of 1.3 billion years.
• Other useful radioisotopes for radioactive dating include
Uranium -235 (half-life = 704 million years)
• Uranium -238 (half-life = 4.5 billion years)
• Thorium-232 (half-life = 14 billion years)
• Rubidium-87 (half-life = 49 billion years).
•
Brain, Marshall. "How Carbon-14 Dating Works" 03 October 2000. HowStuffWorks.com.
<http://science.howstuffworks.com/environmental/earth/geology/carbon-14.htm> 02 March 2014.
Bracketing
• Fossils are generally
found in sedimentary
rock—not igneous rock.
• Sedimentary rocks can be
dated using radioactive
carbon, but because
carbon decays relatively
quickly, this only works
for rocks younger than
about 50 thousand years.
• Scientists look for igneous
rock above and below the
fossils: bracketing
Bracketing
• To date most older fossils,
scientists look for layers
of igneous rock or
volcanic ash above and
below the fossil.
• Scientists date igneous
rock using elements that
are slow to decay, such as
uranium and potassium.
• By dating these
surrounding layers, they
can figure out the
youngest and oldest that
the fossil might be.
Volcanic Ash – Igneous Rock