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Chapter 3

Chapter 3. Time and Geology . Geochronology . The science that deals with determining the ages of rocks is called geochronology . Methods of Dating Rocks.

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Chapter 3

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  1. Chapter 3 Time and Geology

  2. Geochronology The science that deals with determining the ages of rocks is called geochronology.

  3. Methods of Dating Rocks • Relative dating - Using fundamental principles of geology (Steno's Laws, Fossil Succession, etc.) to determine the relative ages of rocks (which rocks are older and which are younger). • Absolute dating - Quantifying the date of the rock in years. This is done primarily by radiometric dating (or analysis of the breakdown of radioactive elements in the rocks over time).

  4. Geologic Time Scale • The geologic time scale has been determined bit-by-bit over the years through relative dating, correlation, examination of fossils, and radiometric dating. • Boundaries on the time scale are drawn where important changes occur in the fossil record, such as extinction events.

  5. Geochronologic Units The geologic time scale is divided into a number of types of units of differing size. From the largest units to the smaller units, they are: • Eons • Eras • Periods • Epochs These units are geochronologic units. Geochronologic units are time units.

  6. Eons Eons are the largest division of geologic time. In order from oldest to youngest, the three eons are: • Archean Eon - "ancient or archaic" - oldest rocks on Earth • Proterozoic Eon - "beginning life" (2.5 billion to 542 million years ago) • Phanerozoic Eon - "visible life" (542 million years ago to present)

  7. The Precambrian The Archean and Proterozoic are together referred to as the Precambrian, meaning “before the Cambrian Period”. The Precambrian covers 87% of geologic history.

  8. Eras There are three eras in the Phanerozoic Eon. Eras are divided into geologic periods. In order from oldest to youngest, the three eras are: • Paleozoic Era - "ancient life" (such as trilobites) • Mesozoic Era - "middle life" (such as dinosaurs) • Cenozoic Era - "recent life" (such as mammals)

  9. Periods Eras are divided into periods.

  10. Paleozoic Era • Permian Period • Carboniferous Period (Mississippian and Pennsylvanian Periods in North America) • Devonian Period • Silurian Period • Ordovician Period • Cambrian Period (oldest)

  11. Mesozoic Era • Cretaceous Period • Jurassic Period • Triassic Period (oldest)

  12. Cenozoic Era • Neogene Period (youngest – today) • Paleogene Period (oldest) On maps and in publications prior to 2003, you will see the two periods of the Cenozoic Era listed as: • Quaternary Period • Tertiary Period (oldest) These are not the same as the current two periods.

  13. Epochs Periods can be subdivided into epochs Epochs can be subdivided into ages

  14. Epochs of the Cenozoic Era • Neogene Period • Holocene Epoch (youngest - today) • Pleistocene Epoch • Pliocene Epoch • Miocene Epoch • Paleogene Period • Oligocene Epoch • Eocene Epoch • Paleocene Epoch (oldest)

  15. Chronostratigraphic units Chronostratigraphic units are the actual rocks formed or deposited during a specific time interval. (They are sometimes called time-rock units.)

  16. Chronostratigraphic units Chronostratigraphic units include: • Eonothem (all rocks corresponding to a given eon) • Erathem (all rocks corresponding to a given era) • System (all rocks corresponding to a given period) • Series (all rocks corresponding to an epoch) • Stage (all rocks corresponding to a particular age)

  17. Periods and Systems Geochronologic units (time units) have the same names as the chronostratigraphic units (time rock units) that they represent.For example, the Cambrian System is a rock unit, and the Cambrian Period is a time unit. The rocks of the Cambrian System were deposited during the Cambrian Period.

  18. Principles of Radiometric Dating

  19. Review of Atoms Atom = smallest particle of matter that can exist as a chemical element. The structure of the atom consists of: • Nucleus composed of protons (positive charge) and neutrons (neutral) • Electrons (negative charge) orbit the nucleus • Various subatomic particles

  20. Two models of atoms

  21. Ions Most atoms are neutral overall, with the number of protons equaling the number of electrons. If there is an unequal number of protons and electrons, the atom has a charge (positive or negative), and it is called an ion.

  22. Atomic Number Atomic number of an atom = number of protons in the nucleus of that atom. Example: The atomic number of uranium is 92. It has 92 protons.

  23. Mass number Mass number is the sum of the number of protons plus neutrons.Example: Uranium-235 has 92 protons and 143 neutrons. The mass number may vary for an element, because of a differing number of neutrons.

  24. Isotopes • Elements with various numbers of neutrons are called isotopes of that element. Example: uranium-235 and uranium-238 • Some isotopes are unstable. They undergo radioactive decay, releasing particles and energy. • Some elements have both radioactive and non-radioactive isotopes. Examples: carbon, potassium.

  25. What happens when atoms decay? • Radioactive decay occurs by releasing subatomic particles and energy. • The radioactive parent element is unstable and undergoes radioactive decay to form a stable daughter element. • Example: Uranium, the parent element, undergoes radioactive decay, releases subatomic particles and energy, and ultimately decays to form the stable daughter element, lead.

  26. Radioactive Parent Isotopes and Their Stable Daughter Products

  27. Radioactive Decay of Uranium As uranium-238 decays to lead, there are 13 intermediate radioactive daughter products formed (including radon, polonium, and other isotopes of uranium), along with and 8 alpha particles and 6 beta particles released.

  28. Radioactive Decay of Uranium

  29. Subatomic Particles and Radiation Released by Radioactive Decay • Alpha particles - large, easily stopped by papercharge = +2 mass = 4 • Beta particles - penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.charge = -1 mass = negligible • Neutrons - highly penetrating; no charge mass = 1 • Gamma rays (high energy x-rays) - Highly penetrating electromagnetic radiation; can penetrate concrete. Lead shield can be used.Photons (light). No charge or mass.

  30. Radioactive Decay Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

  31. Radioactive Decay Rate • Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own constant rate. • Once this rate is measured, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.

  32. Mass Spectrometer • The quantities and masses of atoms and isotopes are measured using an instrument called a mass spectrometer. • The mass spectrometer came into use after WWI (1918). This led to the discovery of more than 200 isotopes.

  33. Measuring Decay Rates The decay rates of the various radioactive isotopes are measured directly using a mass spectrometer. Basically, the mass of a quantity of a radioactive element is measured. Then after a particular period of time, it is analyzed again. The change in the number of atoms over time gives the decay rate.

  34. Decay Rates are Uniform • Radioactive decay occurs at a constant exponential or geometric rate. • The rate of decay is not affected by changes in pressure, temperature, or other chemicals. • The rate of decay is proportional to the number of parent atoms present.

  35. Half-Life • Each radioactive isotope has its own unique half-life. • A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

  36. Half Lives for Radioactive Elements

  37. Decay Curve for Uranium-238

  38. Decay Curve for Potassium-40

  39. Rocks That Can Be Dated Igneous rocks are best for age dating. The dates they give tell when the magma cooled. When the magma cools and crystallized, the newly formed crystals may contain some radioactive elements, such as potassium-40 or uranium that can be dated.

  40. Minerals That Can Be Dated Potassium-40 decays and releases argon gas, which is trapped in the crystal lattice. Potassium-40 is found in these minerals: • Potassium feldspar (orthoclase, microcline) • Muscovite • Amphibole • Glauconite (greensand; found in some sedimentary rocks)

  41. Minerals That Can Be Dated Uranium may be found in: • Zircon • Urananite • Monazite • Apatite • Sphene

  42. Dating Sedimentary Rocks

  43. Dating Sedimentary Rocks Radioactive mineral grains in sedimentary rocks are derived from the weathering of igneous rocks. Their dates are the time of cooling of the magma that formed the igneous rock. The date does not tell anything about when the sedimentary rock was deposited.

  44. Dating Sedimentary Rocks If the sedimentary rock has a mineral that formed on the seafloor as the rock was cemented, then it may be possible to age date it. The greensand mineral, glauconite, contains potassium, and can be dated using the potassium-argon technique.

  45. Dating Sedimentary Rocks The ages of sedimentary rocks and fossils are determined using both relative and absolute dating.

  46. Dating Fossils The ages of fossils in a sequence of sedimentary rocks can be determined using both relative and absolute dating.

  47. Dating sedimentary rocks and fossils • Locate a sequence of sedimentary rocks that contains some igneous rocks (such as a lava flow, volcanic ash bed, intrusion, or underlying igneous rock). • Get a radiometric date for the igneous rocks. • Use relative dating to determine the relative ages of the sedimentary rocks.Bracket the sedimentary rocks between two igneous rocks of known age.

  48. Dating sedimentary rocks and fossils – cont’d • Correlate the sedimentary rocks with sedimentary rocks in another area which contain the same fossils. They are correlated (or "matched up") on the basis of the fossils they contain. They must contain the same species of fossils. • Using this method, the age of the rocks in other areas is determined indirectly, from the ages of the fossils they contain.

  49. The geologic time scalewas established by doing this repeatedly for many locations around the world. The geologic time scale is a composite vertical sequence representing all known rock units and their fossils, worldwide, in sequential order. Absolute ages of rocks have been determined through radiometric dating where possible. The geologic time scale provides a calibrated scale for determining the ages of rocks worldwide by examining their fossils.

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