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Absolute Dating of Rocks

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Absolute Age is the specific age of a rock, fossil, or geologic event from the past Radioactive Dating is the method by which to determine the absolute age of a rock, fossil, or geologic event from the past. Absolute Age and Radioactive Dating The Smilodon skull is 5,700 years old.

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Isotopes An element is a substance containing atoms that are chemically alike. Within the atoms of elements are subatomic particles – including protons and neutrons -- found within the nucleus of an atom. These subatomic particles determine the mass of the atom. Most elements have equal numbers of protons and neutrons. These elements are considered to be stable Isotopes have unequal numbers of protons and neutrons and usually a higher mass. Isotopes are unstable as a result. Atomic mass Atomic Number (Number of protons) C 14 – Isotope: Unstable form C 12 – Element: Stable form Proton Neutron

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Radioactive Decay The nucleus of many isotopes is unstable. The nucleus of an unstable isotope will release energy in its process to become more stable. The isotope (parent element) will slowly release energy from its nucleus and change into a stable form (Daughter element) over time. A half-life is the time required for half of the isotope (Parent material) of a given material to change to its stable form (Daughter Material). Half-life The half-life of “radioactive material X” is 5,000 years.

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Radioactive Dating the half-life of an isotope and the ratio of the amount of isotope vs. stable decay product can be used to determine the age of rock. This process is known as radioactive dating. If a scientist knows the half-life of a radioactive isotope and the ratio of parent material (Isotope) to daughter material (Decay product = stable form), he/she can determine the absolute age of a rock sample. RADIOACTIVE DECAY DATA PARENT MATERIAL DAUGHTER MATERIAL (ISOTOPE)(STABLE DECAY PRODUCT) C 14 N 14 Half-life: 5,700 years The skull of a Smilodon was extracted from a tar pit in Southern California. The skull has been preserved well and C 14 and N 14 are both present in the skull. The ratio of C 14 to N 14 is 1:1 and therefore the skull is estimated to have an absolute age of 5,700 years old. C 14 = isotopeN 14 = stable decay product

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Radioactive Dating One quarter (1/4) of a Smilodon skull is composed of C 14. The other three quarters (3/4) is composed of N 14. How old is the Smilodon skull? C 14 has a half life of 5,700 years. 5,700 years After one “half-life” period, ½ of the skull would contain C 14 and ½ of the skull would contain N 14. After two “half-life” periods, ¼ of the skull would contain C 14 and ¾ of the skull would contain N 14. The skull would be 11,400 years old. C 14 = isotope N 14 = stable decay product

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Radioactive Dating 5,700 years C 14 = isotope N 14 = stable decay product A fossilized Smilodon Skull weighs 100 kg. If the skull is estimated to be 17,100 years old, How much of the original isotope (C 14 ) remains in the skull? 5,700 years If the skull is estimated to have an absolute age of 17,100 years, the skull went through three “half-life” periods of C 14. 12.5 kg of the original isotope (C 14 ) would remain after 17,100 years. C 14 = 100 kg C 14 = 50 kg N 14 = 50 kg C 14 = 25 kg N 14 = 75 kg C 14 = 12.5 kg N 14 = 87.5 kg

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Isotope Half-life Each Radioactive isotope has its own unique half-life. C 14 has a half life of 5.7 X 10 3 years U 238 has a half life of 4.5 X 10 9 years Certain isotopes are useful in dating rocks and/or geologic events of specific times in geologic history. Since U 238 has a much longer half-life than c 14, it is used to age very old rock and/or geologic events. C 14 is useful in dating rocks and/or geologic events in earth’s recent past. The process of radioactive decay from isotope to stable decay product is NOT ALTERED by processes that change the Earth. INCREASING TEMPERATURES INCREASING PRESSURE CHEMICAL REACTIONS WITH MOLTEN ROCK WEATHERING/EROSION WILL NOT alter the half-life of an isotope

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