The Earth Through Time
Chapter 3—Time and Geology
CHAPTER OUTLINE FOR TEACHING
I. Geochronology
A. Relative Geologic Dating
B. Actual (Absolute) Geologic Dating
II. Time and Geology
- Standard Geologic Time Scale
1.Divisions of time scale (chronologic units)
a.Eons (largest): Hadean, Archean, Proterozoic, and Phanerozoic
b.Eras: within Phanerozoic, Eons are Paleozoic, Mesozoic,and Cenozoic
c. Periods, Epochs, and Ages: decreasing order subdivisions ofgeologic time
2. Divisions of stratigraphy (chronostratigraphic units): correspond tochronologic units
a. System (rocks formed during a Period)
b. Series (rocks formed during an Epoch)
c. Stage (rocks formed during an Age)
3. Development of geochronologic nomenclature
a. Cambrian System: Cambria (Roman name for Wales)
b. Silurian and Ordovician Systems: Silures and Ordovices were ancient Celtic tribes
c. Devonian System: Devonshire, England
d. Carboniferous System: British coal measures
e. Permian System: Permia,an ancient Russian kingdom
f. Triassic System: set of three formations in Germany
g. Jurassic System: Jura Mountains, Franco-Swiss border
h. Cretaceous System: Latin for chalk (creta)
i. Cenozoic: means “recent life”
(i)Paleogene System
(ii) Neogene System
j. Quaternary:represents materials formed during and after the last ice age
(i) Pleistocene
(ii) Holocene
4. Geologists prominent in the early development of nomenclature
a. Adam Sedgwick (1785-1873): founder of the Cambrian System (1835);
co-founder with Murchison of the Devonian System (1839)
b. Roderick Impey Murchison (1785-1871): founder of the Silurian System (1835); co-founder of the Devonian System (1839); founder of the Permian System (1840)
c. Charles Lapworth: founder of the Ordovician System (1879; a compromise in the feud between Sedgwick and Murchison due to overlap of the Cambrian and Silurian Systems
d. Alexander von Humboldt (1769-1859): founder of the Jurassic System
B. Quantitative Geologic Time
1. Early estimates
a. Solar and lunar cycles versus events in the Old Testament: 6,000 years
b. Evolutionary rate calculation: beginning of Cenozoic was 80million years ago (Lyell, 1839)
c. Sediment deposition rate: computation age ranges from 1 million to 1 billion years (done in the 1850s)
d. Salinity of sea water calculations: 90 million years since seasdeveloped (Joly, 1899)
e. Rate of cooling calculation: 24 to 40 million years sinceformation (Kelvin, 1890s)
2. Modern radioactive-isotope methods and concepts
a. Radioactivity discovered in 1896 by Antoine Henri Becquerel (1852-1908);investigated by the Curies who found more radioactive elements
b. Reviewing atoms: element; proton; neutron; electron; atomic number; atomic mass; isotopes
c. Employs rate of natural, spontaneous breakdown of nuclearstructure of atoms: radioactivity
d. Parent nuclide = daughter product + particle expelled
e. Rate of nuclear decay is constant
f. Crystallization of minerals locks in an original quantity ofradioactive atoms
g. Radioactive isotopes: each has a unique rate and mode of decay
h. Three types of radioactive decay: alpha, beta, and gamma (gamma radiation)
i. Radiometric dating of a crystal: possible because daughterproducts are retained
j. Original quantity of parent determined by counting daughterproducts (P + D = O)
k. Mass spectrometer: device used to measure minute amounts ofisotopes
l. Half-life: span of time needed for one half of original atom todecay to daughter product
m. Minerals in igneous rocks give the most reliable age dates
3. Principal geologic timekeepers: useful decay processes
a. Uranium-lead method: using Uranium235 or U238
b. Potassium-40/Argon-40 method: studies trapped Argon gas
c. 87Rb/86Sr method: uses straight-line (isochron) plots
d. Carbon-14 method: limitation is the half-life of 5,730 years
e. Nuclear fission-track counting in crystals
4. Age of the Earth
a. Oldest known Earth material is 4.36 billion years (zirconcrystals from western Australia)
b. Other very old rocks (over 4 billion years) come from Canada and Greenland
c. Age of meteorites: U/Pb and Rb/Sr dating yields 4.6 billion years
d. Moon rocks: U/Pb and Rb/Sr dates range from 3.3 to 4.6 billion years
Answers to Discussion Questions
1.Geochronologic units bear the same name as the chronostratigraphic units to which they correspond, and are divisions of geologic time. Chronostratigraphic units are the full rock record of corresponding geochronologic units, and are material features.
2.Earth’s formed about 4.6 billion years ago. Here are the ages of the beginning of the following geochronologic intervals:
Proterozoic Eon – 2.5 billion (2,500 million) years ago
Paleozoic Era – 542 million years ago
Mesozoic Era – 251 million years ago
Cenozoic Era – 65 million years ago
Pleistocene Epoch – 1.8 million years ago
3. The amount of parent isotope must be determined as well as the amount of daughter isotope so that the parent:daughter ratio can be established. This ratio is important in determining how much decay has occurred (e.g., how many half-lives have passed since the mineral formed and locked in the radioactive element being studied).
4.The concept of half-life is necessary because it represents a convenient ratio (50:50) of parent and daughter. Furthermore, half-life of most radiogenic elements studied is a convenient duration (several 10s of millions of years) regarding geologic processes. Carbon14 (half-life = 5,730 year) is an exception.
5.The pebble ages tell us that the sedimentary layer is younger than 300 million years.
6.Isotopes are atoms of the same chemical element that have variant mass numbers.All isotopes of the same element have an equal number of protons, but the number of neutrons varies, hence the mass variance.
7.Radioactive decay is a multi-step process with a predictable half-life. It behavesas a timekeeper (or hourglass) so long as the System is not "disturbed." Anyfracturing, repeating, or metamorphic process can "reset" the atomic clock.
8. In the potassium-argon (K-Ar) method, if Ar is lost, the Age will be erroneous and will be apparently younger than it really is.
9. C14 decays according to its half-life of 5,730 years. Having 12.5% of original C14 indicates passage of 3 half-lives or 17,190 years.
10.If there is ¼ parent and ¾ daughter isotopes, then two half lives have passed. Therefore, the age is 2,400 million years (or 2.4 billion years).
11. b
12.c
13.e
14.c
15.b
CHAPTER ACTIVITIES
Student activities for in-depth learning:
1. Make a geological time scale where one inch is equal to 100 million years. Plot this on a long piece of paper. Make the geological time scale wide enough so that you can write the names of the Eons, Eras, and Periods comfortably in the space you provide. Use the scale of one inch to 100 million years consistently throughout the diagram you are making. When finished, what do you notice about the distribution of lengths of time within the Eons, Eras, and Periods? Speculate on why this is. Take a look at a web page on chronostratigraphy (such as stratigraphy.org). Also, look at web pages on the distribution of fossils through geological time.
2. Using web pages research the steps in the process of radiometric decay from parent Uranium-238 to Lead-206(e.g., List all the steps in the decay process, indicate whether alpha, beta, or other decay was involved in each step, give the time required for each step to take place (on average), and list the parent and daughter products in each step. You should find a dozen or more steps in this decay process.
CHAPTER OVERVIEW
This chapter provides a detailed explanation of the methods used by geologists in constructing a geologic time scale. The concepts of relative and absolute dating clearly differentiate among and between eons, eras, epochs, and ages. Following this section is a discussion of all the recent eras: Paleozoic; Mesozoic; and Cenozoic. All of the factors that have been used to determine the age of the Earth including the evolution of fossils, sediment deposition rate, ocean salinity, cooling rate, and natural radioactivity are discussed.
LEARNING OBJECTIVES
By reading and completing information within this chapter, you should gain an understanding of the following concepts:
- Explain the difference between actual and relative geologic time.
- Describe the Divisions of the Geologic Time Scale, i.e., its eons, eras, periods, epochs, and ages; and how this scale was developed.
- Discuss the relationship between time units and time-rock units.
- Discuss the term half-life as it applies to radioactivity.
- Describe the various radioactive timekeepers, i.e., uranium-lead, potassium-argon, rubidium-strontium, carbon-14, and fission tracks.
- Speak with confidence regarding the 4.6 billion year-old age of the Earth.
CHAPTER OUTLINE
- Finding the Age of Rocks: Relative Versus Actual Time
- A Scale of Geologic Time
- Overview of the Time Scale
- Divisions in the Geologic Time Scale
- Evolution of the Geologic Time Scale
- Cambrian System
- Ordovician and Silurian Systems
- Devonian System
- Carboniferous System (Mississippian and Pennsylvanian)
- Permian System
- Triassic, Jurassic, and Cretaceous Systems
- Subdivisions of the Cenozoic
- Actual Geologic Time: Clocks in the Rocks
- Radioactivity Provides a Way to Date Rocks
- What Occurs When Atoms Decay
- The Alpha, Beta, and Gamma of Decay
- Why Radioactivity Lets Us Date Ancient Rocks with Confidence
- Why Igneous Rocks Give the Most Trustworthy Dates
- Half-Life
- The Principal Radioactive Timekeepers
- Uranium-Lead Methods
- The Potassium-Argon Method
- The Rubidium-Strontium Method
- How Carbon-14 Enters the Environment
- How Carbon-14 Dating Works
- Fission Track Dating Method
- How Old is the Earth?
Key Terms (pages given in parentheses)
actual geologic dating (30): The actual age, expressed in years, of a geologic material or event.
alpha particle (37): A particle equivalent to the nucleus of a helium atom, emitted from an atomic nucleus during radioactive decay.
Archean Eon (30): Pertaining to the division of Precambrian beginning 3.8 billion years ago and ending 2.5 billion years ago.
atom (36): The smallest particle of matter that can exist as a chemical element.
atomic mass (36): A quantity essentially equivalent to the number of neutrons plus the number of protons in an atomic nucleus.
atomic number (36): The number of protons in the nuclei of atoms of a particular element. An element is thus a substance in which all of the atoms have the same atomic number.
bentonite (38): A layer of clay, presumably formed by the alteration of volcanic ash. It is composed essentially of montmorillonite and related minerals of the smectite group.
beta particle (38): A charged particle, essentially equivalent to an electron, emitted from an atomic nucleus during radioactive disintegration.
Cambrian System (32): Exposures of Strata in Wales provide a standard section with which rocks elsewhere in Europe and on other continents can be correlated and are known as Cambrian System by definition. All other sections deposited during the same time as the rocks in Wales are recognized as Cambrian by comparison to the standard section. The basal unit is 570 million years. The time interval covered is 570 to 505 million years.
Carboniferous System (34): A division of Paleozoic ranging in geologic time from 360 to 286 million years. It is further divided into Lower Carboniferous (Mississippian in the U.S., covering rocks formed 361-320 million years ago (and Upper Carboniferous (Pennsylvanian in the U.S., covering rocks formed 320-286 million years ago). This system designated strata that included beds of coal in north-central England.
Cenozoic (30): The era in which we are now living. It began at 65 million years (basal unit).
Cretaceous System (34): In geological time, the last period of the Mesozoic Era, preceded by Jurassic (Period) and followed by Paleogene (Period); it extended from 144 million years to 65 million years before present.
daughter element (37): An element formed by the radioactive decay of another element.
Devonian System (34): Included in Paleozoic and covers geologic time from 408 to 360 million years ago. It was determined to be a new system based on the fauna that was different from that of the underlying Silurian and overlying Carboniferous Systems.
electron (36): A negatively charged particle of very little mass that orbits the nucleus of an atom.
element (36): A unique combination of protons, neutrons, and electrons that cannot be broken down by ordinary chemical methods.
eon (30): A major division of the geologic time scale. Phanerozoic is an eon comprising all of the geologic periods from Cambrian to Holocene. The term is also sometimes used to denote a span of 1 billion years.
epoch (30): A chronological subdivision of a geologic period. Rocks deposited or emplaces during an epoch constitute the series for that epoch.
era (30): A major division of geologic time, divisible into geologic periods. There are three divisions: Paleozoic Era, Mesozoic Era, and Cenozoic Era.
fission tracks (44): Sub-microscopic “tunnels” in minerals produced when high-energy particles from the nucleus of uranium are forcible ejected during spontaneous fission.
gamma radiation (37): Emission in the radioactive decay process consisting of a form of invisible electromagnetic waves having even shorter wavelengths than X-rays.
geochronology (29): The study of time as applied to Earth and planetary history.
half-life (39): The time in which one-half of an original amount of a radioactive atoms decays to daughter products.
Holocene Series (34): A term sometimes used to designate the period of time since the last major episode of glaciation. The term is equivalent to Holocene and ranges from 10,000-12,000 years ago to present.
isotope (37): Variants of mass numbers of atoms of the same substance. Isotopes are two or more varieties of the same element that have the same atomic number and chemical properties but differ in mass numbers because they have a varying number of neutrons in the nucleus.
Jurassic System (34): A division of Mesozoic covering a geologic time span of 208 to 144 million years ago.
Mesozoic Era (30): Continued for about 179 million years during Phanerozoic covering the time period from 245 to 65 million years. Its basal unit is 245 million years ago.
Mississippian System (34): Division of Carboniferous System covering 360 to 320 million years ago. It is equivalent to the English division known as Lower Carboniferous.
Neogene Series (34): The middle of the three Cenozoic periods; encompasses Miocene and Pliocene Epochs.
neutron (34): An electrically neutral (uncharged) particle of matter existing along with protons in the atomic nucleus of all elements except the mass 1 isotope of hydrogen.
Ordovician System (34): The second period of Paleozoic, above Cambrian and below Silurian, from approximately 500 million to 440 million years ago.
Paleogene System (34): The older of the three Cenozoic systems; encompasses Paleocene, Eocene, and Oligocene Epochs.
Paleozoic Era (30): The era of geologic time from the end of Precambrian until the beginning of Mesozoic.
parent element (37): An unstable element that changes by radioactive decay into a stable daughter element.
Pennsylvanian System (34): A division of Late Paleozoic, extending from 320 to 280 million years ago, varyingly considered to rank as a part of Carboniferous; named for outcrops of coal-bearing rock formation in Pennsylvania.
period (30): A subdivision of an era.
Permian System (34): Division of Paleozoic covering geologic time ranging from 286 to 245 million years ago. Named for a Russian province. Fossils were determined to be intermediate between those of Carboniferous below and Triassic above.
Phanerozoic Eon (30): The eon of geologic time during which the Earth has been populated by abundant and diverse life. It is subdivided into Paleozoic, Mesozoic, and Cenozoic. The basal unit is 570 (or 544) million years and it continues through the present geologic time.
Pleistocene Series (34): The older of the two epochs comprising the last geological period, Quaternary. Spans about 1.8 million to 10,000 years ago. It represents the interval of geological time (and rocks accumulated during that time) extending from the end of Pliocene to the start of Holocene. It is commonly characterized as an epoch when the Earth entered its most recent phase of widespread glaciation. Also known as the Ice Age.
Precambrian Period (30): Pertaining to all of geologic time and its corresponding rocks before the beginning of Paleozoic.
Proterozoic Eon (30): Refers to the time interval from 2500 to 544 million years ago.
proton (36): An elemental particle found in the nuclei of all atoms that has a positive electric charge and a mass similar to that of a neutron.
Quaternary System (37): The youngest of the three Cenozoic systems; encompasses Pleistocene and Holocene Epochs.
radioactivity (37): The spontaneous emission of a particle from the atomic nucleus, thereby transforming the atom from one element to another.
relative geologic dating (29): Dating which involves placing geologic events and the rocks representing those events in the order in which they occurred without reference to actual time or dates measured in years. This method tells which event preceded or followed another event, or which rock mass was older or younger relative to others.
series (30): The time-rock term representing the rocks deposited or emplaced during a geologic epoch. A series is a subdivision of a system.
Silurian System (34): System within Paleozoic rocks determined by the studies of fossiliferous strata outcropping in the hills of southern Wales. The geologic time interval is 438 to 408 million years ago.
stage (32): The time-rock unit equivalent to an age. A stage is a subdivision of a series.
system (32): Refers to the actual rock record of a period.
time-rock unit (= chronostratigraphic unit) (30): The rocks formed during a particular unit of geologic time.
time-unit (=geochronologic unit) (30): Represents increments of time.
Triassic System (34): A division of Mesozoic rocks with a basal age of 245 million years. Geologic time covered is 245 to 208 million years ago. The term Triassic refers to a threefold division of rocks of this age in Germany.
CHAPTER 3
Time and Geology
FINDING THE AGE OF ROCKS: RELATIVE VERSUS ACTUAL DATING
The science that deals with determining the ages of rocks is called geochronology.
Dating rocks started some 400 years ago with Nicholas Steno.
METHODS OF DATING ROCKS
1.Relative dating - Using fundamental principles of geology (Steno's Laws, Fossil Succession, etc.) to determine the which rocks are older and which are younger. In other words, determine the sequences of events without regard to a specific date.
2.Actual (Absolute) dating - Quantifying the date of the rock in years before the present. This is done primarily by radiometric dating (or detailed analysis of the breakdown of radioactive elements within the rocks over time).
GEOLOGIC TIME SCALE