Absolute Dating of Rocks

1. Absolute Dating of Rocks

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

3. 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. Neutron Proton C12 – Element: Stable form C14 – Isotope: Unstable form Atomic mass Atomic Number (Number of protons)

4. 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.

5. 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 N14 Half-life: 5,700 years C14 = isotope N14 = stable decay product The skull of a Smilodon was extracted from a tar pit in Southern California. The skull has been preserved well and C14 and N14 are both present in the skull. The ratio of C14 to N14 is 1:1 and therefore the skull is estimated to have an absolute age of 5,700 years old.

6. Radioactive Dating One quarter (1/4) of a Smilodon skull is composed of C14. The other three quarters (3/4) is composed of N14. How old is the Smilodon skull? C14 has a half life of 5,700 years. 5,700 years After one “half-life” period, ½ of the skull would contain C14 and ½ of the skull would contain N14. After two “half-life” periods, ¼ of the skull would contain C14 and ¾ of the skull would contain N14. The skull would be 11,400 years old. 5,700 years C14 = isotope N14 = stable decay product

7. Radioactive Dating A fossilized Smilodon Skull weighs 100 kg. If the skull is estimated to be 17,100 years old, How much of the original isotope (C14) remains in the skull? C14 = 100 kg 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 C14. 12.5 kg of the original isotope (C14) would remain after 17,100 years. C14 = 50 kg N14 = 50 kg 5,700 years C14 = 25 kg N14 = 75 kg 5,700 years C14 = 12.5 kg N14 = 87.5 kg C14 = isotope N14 = stable decay product

8. Isotope Half-life Each Radioactive isotope has its own unique half-life. C14 has a half life of 5.7 X 103 years U 238 has a half life of 4.5 X 109 years Certain isotopes are useful in dating rocks and/or geologic events of specific times in geologic history. Since U238 has a much longer half-life than c14, it is used to age very old rock and/or geologic events. C14 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