Chapter 3 Time and Geology

Transcription

1 Chapter 3 Time and Geology

2 Finding the age of rocks: Relative versus Actual Dating The science that deals with determining the ages of rocks is called geochronology.

3 Methods of Dating Rocks 1. 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). 2. Actual (Absolute) dating - Quantifying the date of the rock in years. This is done primarily by radiometric dating (or detailed analysis of the breakdown of radioactive elements within the rocks over time).

4 Geologic Time Scale The geologic time scale has been determined over many years of research through relative dating, correlation, examination of fossils, and radiometric dating. Boundaries on the time scale are placed 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 The modern geological time scale

7 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)

8 Precambrian The Archean and Proterozoic are together referred to as the Precambrian, meaning "before the Cambrian Period." The Precambrian encompasses 87% of geologic history.

9 Eras There are three eras within the eon called Phanerozoic. 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)

10 Periods Eras are divided into periods.

11 Paleozoic Era Permian Period Carboniferous Period (split into Mississippian and Pennsylvanian Periods in the United States) Devonian Period Silurian Period Ordovician Period Cambrian Period (oldest)

12 Mesozoic Era Cretaceous Period Jurassic Period Triassic Period (oldest)

13 Cenozoic Era Quaternary Period Neogene Period 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) The former Tertiary Period is now split into two.

14 Epochs Periods are subdivided into epochs. and Epochs are subdivided into ages.

15 Epochs of the Cenozoic Era Quaternary Period Holocene Epoch Pleistocene Epoch (oldest) Neogene Period Pliocene Epoch Miocene Epoch (oldest) Paleogene Period Oligocene Epoch Eocene Epoch Paleocene Epoch (oldest)

16 Chronostratigraphic units Chronostratigraphic units represent the actual rocks deposited or formed during a specific time interval. Chronostratigraphic units are sometimes called "time-rock units."

17 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 a given epoch) Stage (all rocks corresponding to a particular age)

18 Periods and Systems Geochronologic units (time units) have the same names as their chronostratigraphic units (time-rock units). For example, Cambrian Period is a time unit, and Cambrian System is a time-rock unit. The rocks of the Cambrian System were deposited during the period called Cambrian.

19 Principles of Radiometric Dating

20 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

21 Model of the atom

22 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.

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

24 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.

25 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 nonradioactive isotopes. Examples: carbon, potassium

26 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.

27 Radioactive Parent Isotopes and Their Stable Daughter Products Radioactive Parent Isotope Potassium-40 Rubidium-87 Thorium-232 Uranium-235 Uranium-238 Carbon-14 Stable Daughter Isotope Argon-40 Strontium-87 Lead-208 Lead-207 Lead-206 Nitrogen-14

28 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.

29 Radioactive Decay of Uranium

30 Subatomic Particles and Radiation Released by Radioactive Decay Alpha particles atomic weight = 4; atomic number = (The same as the nucleus of a helium atom. Has + charge of 2.) Beta particles an electron that is released when a neutron splits into a proton and an electron. (Like all electrons, the mass is negligible and there is a positive charge.) Gamma rays electromagnetic waves much like x-rays, but higher frequency (Like all electromagnetic waves, including light, there is no charge or mass associated this photon or "particle.")

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

32 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.

33 Mass Spectrometer The quantities and masses of atoms and isotopes are measured using an instrument called a mass spectrometer. The decay rates of the various radioactive isotopes are measured directly using a mass spectrometer.

34 Decay Rates are Uniform Radioactive decay occurs at a constant or uniform rate. The rate of decay is not affected by changes in pressure, temperature, or other chemicals. As time passes, the number of parent atoms decreases and the number of daughter atoms increases at a known rate.

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

36 Half-Lives for Radioactive Elements Radioactive Parent Stable Daughter Half-life Potassium-40 Argon billion yrs Rubidium-87 Strontium billion yrs Thorium-232 Lead billion years Uranium-235 Lead million years Uranium-238 Lead billion years Carbon-14 Nitrogen years

37 Rate of decay for Uranium-238

38 Rate of decay for Potassium-40

39 Rocks That Can Be Dated Igneous rocks are best for age dating. The dates from crystals in igneous rocks tell us when the magma cooled. When the magma cools and crystallizes, the newly formed crystals usually contain some radioactive elements, such as Potassium-40 or Uranium-238 that can be used for radiometric dating.

40 Minerals That Can Be Dated Potassium-40 decays and releases Argon-40 gas, which is trapped in the crystal lattice. Potassium-40 is found in these minerals: Potassium feldspar (orthoclase, microcline) Muscovite Amphibole Glauconite (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 Radioactive mineral grains in sedimentary rocks are derived from the weathering of igneous rocks. The radiometric dates of these grains give us the time of cooling of the magma, which formed the original igneous rock. These dates do not tell us anything about the age of the sedimentary rock.

43 Dating Sedimentary Rocks If the sedimentary rock contains a mineral that formed at the same time as the rock formed, then it may be possible to use that mineral to obtain a radiometric age date. The sedimentary mineral glauconite contains potassium, and can be used for radiometric dating (employing the potassium-argon technique).

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

45 Dating Sedimentary Rocks For example, the age of the shale layers in both instances is between 110 and 180 million years.

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

47 Dating sedimentary rocks and fossils 1. Locate a sequence of sedimentary rocks that contains some igneous rocks (such as a lava flow, volcanic ash bed, intrusion, or underlying igneous rock). 2. Determine radiometric dates for the igneous rocks. 3. Use relative dating to determine the relative ages of the sedimentary rocks. Bracket the age of the sedimentary rocks using two igneous rocks with known ages.

48 Dating sedimentary rocks and fossils cont'd 4. Correlate the sedimentary rocks with sedimentary rocks in another area that 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. 5. 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 scale was compiled by using this method repeatedly at 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 including their fossils.

50 Carbon-14 dating 1. Cosmic rays from the sun strike Nitrogen-14 atoms in the atmosphere and cause them to turn into radioactive Carbon-14, which combines with oxygen to form radioactive carbon dioxide.

51 Carbon-14 dating 2. Living things are in equilibrium with the atmosphere, because radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and thus the carbon cycle. All living things contain a constant ratio of Carbon-14 to Carbon-12. (about 1 in a trillion).

52 Carbon-14 dating 3. At death, Carbon-14 exchange ceases and any Carbon-14 in the tissues of the organism begins to decay to Nitrogen-14, and is not replenished by new Carbon-14. The change in the Carbon-14 to Carbon-12 ratio in fossil material is the basis for this kind of radiometric dating.

53 Carbon-14 dating The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Assumes that the rate of Carbon-14 production (and hence the amount of cosmic rays striking the Earth) has been constant over the past 70,000 years.

54 Carbon-14 formed from Nitrogen-14 and its fate in the natural environment.

55 Rubidium-Strontium Method When Rubidium-87 expels a beta particle, it becomes Strontium-87. This method provides a useful check on the potassium-argon method of dating.

56 Rubidium-Strontium Method Strontium-86 is not a radioactive isotope, and it is employed in this method as well. Using a mass spectrometer, the ratio of Rubidium-87 to Strontium-86 and the ratio of Strontium-87 to Strontium-86 is determined for several samples. This is plotted on a graph and the line thus determined is called an isochron.

57 Rubidium-Strontium Method The slope of the line permits computation of the age of the mineral crystals being studied. In this instance, the slope angle of the isochron suggests an age of billion years for a granite from Sudbury, Ontario.

58 Fission Track Dating Charged particles from radioactive decay pass through a mineral's crystal lattice and leave trails of damage in the crystal called fission tracks. These trails are due to the spontaneous fission (or radioactive decay) of the uranium nucleus.

59 Procedure: Fission Track Dating Enlarge tracks by etching in acid (to view with light microscope) - or view them directly with electron microscope Count the etched tracks (or measure the density of such tracks in a given area of the crystal) The number of tracks per unit area is a function of age and uranium concentration.

60 Fission Track Dating Useful in dating: Micas (up to 50,000 tracks per cm 2 ) Other uranium-bearing minerals and natural glasses

61 The Oldest Rocks The oldest rocks that have been dated are meteorites. They date from the time of the origin of the solar system and the Earth, about 4.6 billion years old.

62 The Oldest Rocks Moon rocks have similar dates, ranging in age from 3.3 to about 4.6 billion years. The oldest Moon rocks are from the lunar highlands (lighter-colored areas on the Moon), and may represent the original lunar crust

63 The Oldest Rocks The oldest dates of Earth rocks are 4.36 billion-year-old detrital zircon grains in a sandstone in western Australia. These grains probably came from the weathering and erosion of 4.36 billion-yearold granite that must have been exposed at the time the sand grains were deposited.

64 Other Old Earth Rocks 1. Southwestern Greenland (granite; 4.0 b.y.) 2. Minnesota (metamorphic rocks; 4.0 b.y.) 3. Northwest Territories, Canada (gneiss; 4.04 b.y.) 4. Hudson Bay, northern Quebec (zircons; 4.28 b.y.) Still older rocks on Earth may remain to be found and dated using radiometric methods.

65 Why are Earth Rocks Younger than Meteorites and Moon Rocks? The Earth is geologically active. The older rocks may have been eroded away or destroyed by tectonic forces. Older rocks may remain deeply buried under sedimentary rocks, or under mountain ranges. Older rocks may have been heated, metamorphosed, or melted, and their isotopes "reset" to the time of the later events of heating, metamorphism, or melting.