Absolute Dating of Rocks

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.

Isotopes Atomic mass Atomic Number (Number of protons) 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 12C – Element: Stable form 14C – Isotope: Unstable form Atomic mass Atomic Number (Number of protons)

The half-life of “radioactive material X” is 5,000 years. 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.

RADIOACTIVE DECAY DATA 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) 14 C  14N 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 14C and 14N are both present in the skull. The ratio of 14C to 14N is 1:1 and therefore the skull is estimated to have an absolute age of 5,700 years old.

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

12.5 kg of the original isotope (14C) would remain after 17,100 years. 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 (14C) remains in the skull? 14C = 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 14C. 12.5 kg of the original isotope (14C) would remain after 17,100 years. 14C = 50 kg 14N = 50 kg 5,700 years 14C = 25 kg 14N = 75 kg 5,700 years 14C = 12.5 kg 14N = 87.5 kg 14C = isotope 14N = stable decay product

INCREASING TEMPERATURES INCREASING PRESSURE Isotope Half-life Each Radioactive isotope has its own unique half-life. 14C has a half life of 5.7 X 103 years 238U 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 238U has a much longer half-life than 14c, it is used to age very old rock and/or geologic events. 14C 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