absolute ages of rock
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Absolute Ages of RockMr. Perez
Important vocabulary
• Isotopes• Radioactive decay• Half-life• Radiometric dating• Uniformitarianism
Absolute Ages• Absolute age is the age, in years, of a rock or other object.• Examples:
• How old are you?• How old is your house?• How old is your book bag?• How old is your pet?• How old is your sibling?
Radioactive Decay• Atoms are made up of:• Nucleus- central region
• Includes the protons (+) and neutrons (0)
• Electrons- cloud surrounding center (-)
• The number of protons(+) determines the identity of the element
• The number of neutrons (0) determines the form of the element (aka isotope)
Radioactive Decay• Some isotopes are unstable and break down into other
isotopes and particles• We call the above process radioactive decay
Radioactive Decay- Alpha and Beta Decay
Beta Decay• Neutron breaks
down into proton and electron
• Electron leaves atoms a beta particle
Alpha Decay• Two protons and
two neutrons are given off
• Leave as alpha particle
Both types of decay form a new isotope
Radiometric Decay- Half Life• Each parent isotope decays (breaks down) into its daughter
isotope at a certain rate• Based on this decay rate, it takes a certain period of time for
one half of the parent isotope to decay to its daughter product• The half-life of an isotope is the time it takes for half of the
atoms in the isotope to decay.• Example- It takes carbon-14 5,730 years for half of its atoms to
turn into nitrogen-14. Then, it takes another 5,730 years for half of the remaining carbon-14 atoms to turn into nitrogen-14, and so on…
THINK OF EXPONENTS! 23 = 8, 32 = 9, 44 = 256
Radioactive Decay- Half-life• Fill in the following table using this information:• Mr. Perez has 100g of carbon-14. After 5,730 years, how many
grams of carbon-14 will still be carbon-14? After 2? After 3?...
Year Grams of carbon-14
0 100g
1
2
3
4
Radiometric Dating
Radiometric Dating• By measuring the ratio of parent isotope to daughter product
in a mineral AND knowing the half-life of the parent, you can calculate the absolute age of a rock• This is called radiometric dating
• A scientist must decided which parent isotope to use when measuring the age of a rock.• If it’s old, the scientist will use an isotope with a long half-life
• Example: potassium-40 takes 1.25 billion years to decay to argon-40
• To avoid errors, conditions must be met for ratios to give a correct indication of age:1. The rock being studied must retain ALL of the argon-40 that
produced by the decay of potassium-402. It cannot contain any contamination of daughter product from
other sources
Radiometric Dating- Radiocarbon Dating• Carbon-14 is useful for dating bones, wood and charcoal up to
75,000 years old.• Living things take in carbon from the environment to build
their bodies• Most of this carbon is carbon-12, but some is carbon-14• The radio of these two isotopes is always the same
• After an organism dies, the carbon-14 decays slowly• By determining the amounts of isotopes in a sample, scientists
can evaluate how much the isotope ratio differs from that in the environment
Radiometric Dating- Radiocarbon Datin
Radiometric Dating-• Age Determinations• Rocks that can be radiometrically dated are mostly IGNEOUS and
METAMORPHIC rock• Sedimentary rocks cannot be dated by carbon-14 because they
are made up of particles eroded from older rocks• The Oldest Known Rocks• Using radiometric dating, the oldest rocks on Earth are about
3.96 billion years old• The Earth is about 4.5-4.6 billion years old
Uniformitarianism• James Hutton (Scottish scientist in the 1700s)• Estimated that the Earth is much older than we think• Uniformitarianism- principle that states that Earth’s processes
occurring today are similar to those that occurred in the past• “The present is the key to the past”
• Today, scientists recognize that Earth has been shaped by two types of change1. Slow, everyday processes that take place over millions of years2. Violent and unusual events
Videos• http://science.howstuffworks.com/environmental/earth/geolo
gy/carbon-14.htm
• http://teachertube.com/viewVideo.php?video_id=181054&title=Radioactive_decay
Resources• Florida Science Grade 7
Glencoe Science & McGraw Hill Publishing• Google Images
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