Chapter 3—Time and Geology
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
I. Finding the Age of Rocks: Relative Versus Actual Time
II. A Scale of Geologic Time
A. Overview of the Time Scale
B. Divisions in the Geologic Time Scale
C. Evolution of the Geologic Time Scale
1. Cambrian System
2. Ordovician and Silurian Systems
3. Devonian System
4. Carboniferous System (Mississippian and Pennsylvanian)
5. Permian System
6. Triassic, Jurassic, and Cretaceous Systems
7. Subdivisions of the Cenozoic
III. Actual Geologic Time: Clocks in the Rocks
IV. Radioactivity Provides a Way to Date Rocks
V. What Occurs When Atoms Decay
A. The Alpha, Beta, and Gamma of Decay
B. Why Radioactivity Lets Us Date Ancient Rocks with Confidence
C. Why Igneous Rocks Give the Most Trustworthy Dates
D. Half-Life
VI. The Principal Radioactive Timekeepers
A. Uranium-Lead Methods
B. The Potassium-Argon Method
C. The Rubidium-Strontium Method
D. How Carbon-14 Enters the Environment
E. How Carbon-14 Dating Works
F. Fission Track Dating Method
VII. 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.
Multiple-Choice Questions
1. The geologic time scale was originally based on
a. the succession of fossil assemblages.
b. the theory of organic evolution.
c. the rock record.
d. the absolute age of rocks based on isotope age dates.
2. The dating method that places geologic events in a chronological order as determined from their position in the rock record.
a. relative dating c. isotopic dating
b. absolute dating d. neutron dating
3. All the rocks formed or deposited during a specific time interval are called __________ units.
a. rock interval. c. time-rock units.
b. formation. d. time units.
4. The correct order of time stratigraphic units from largest to smallest is
a. age, era, epoch, period. c. age, epoch, era, age.
b. period, epoch, age, era. d. era, period, epoch, age.
5. The Carboniferous Systems of Europe and Britain correspond to what systems of North America?
a. Triassic and Jurassic c. Cretaceous and Paleogene
b. Cambrian and Ordovician d. Mississippian and Pennsylvanian
6. The atomic number of an atom is the number of
a. protons and neutrons in the nucleus. c. protons in the nucleus.
b. neutrons in the nucleus. d. electrons in the outer shell.
7. An alpha particle is composed of
a. 2 electrons. c. 2 neutrons, 2 protons.
b. 2 neutrons. d. 2 protons, 2 electrons.
8. An element that has the same atomic number, but different atomic weights is an
a. ion. c. atom.
b. isotope. d. element.
9. The use of radioactive elements has proven successful for determining absolute (Quantitative) ages because
a. most have very long half-lives.
b. they are easily found in the field.
c. they are abundant in the Earth’s crust.
d. the rate of decay is constant for a particular element.
10. An isotope is an atom of an element with
a. the same atomic number, a different number of electrons, but the same number of neutrons.
b. the same atomic number, but different numbers of protons and electrons.
c. the same atomic number, the same number of electrons, but a different number of neutrons.
d. a different atomic number, but the same number of electrons and neutrons.
11. The type of radioactive decay that involves the emission of a high velocity electron from the nucleus is
a. alpha decay. c. gamma emission.
b. beta decay. d. electron reflection.
12. Three Eons span the time boundaries of the Earth’s history. The three eons from the oldest to youngest are:
a. Proterozoic, Archeon, Phanerozoic.
b. Archeon, Proterozoic, Phanerozoic.
c. Archeon, Phanerozoic, Proterozoic.
d. Proterozoic, Archeon, Phanerozoic.
13. The divisions of the Proterozoic Eon from the oldest to youngest given the Earth’s history are:
a. Meso-Archeon, Eo-Archeon, Paleo-Archeon, Neo-Archeon.
b. Paleo-Archeon, Meso-Archeon, Paleo-Archeon, Eo-Archeon.
c. Eo-Archeon, Paleo-Archeon, Meso-Archeon, Neo-Archeon.
d. Neo-Archeon, Meso-Archeon, Paleo-Archeon, Eo-Archeon.
14. The following represent time divisions in geology which are inclusive—from largest to smallest the correct order is:
a. Era, Eon, Period, Epoch, Age. c. Era, Period, Eon, Epoch, Age.
b. Eon, Era, Period, Epoch, Age. d. Eon, Period, Era, Epoch, Age.
15. The eon before the Precambrian
a. Phanerozoic. c. Neo-Archeon.
b. Archeon. d. Proterozoic.
16. This type of geologic dating tells which event preceded another or which rock mass is older or younger.
a. quantitative dating d. relative dating
b. uniformitarism e. absolute dating
c. Isotopic dating
17. An element having the same atomic number but different atomic weight.
a. ion d. proton
b. atom e. isotope
c. neutron
18. Associated with protons in a nucleus are particles having the same mass as protons; however, when they are electrically neutral they are called
a. Isotopes. c. alpha particles.
b. electrons. d. neutrons.
19. Which of the following rock source materials are used in the Potassium-Argon method?
a. zircon, uraninite c. muscovite, hornblende, glauconite
b. muscovite, limestone, zircon d. coralline limestone, muscovite, zircon
20. Which of the following reflects the correct parent isotope to daughter isotope?
a. potassium – 40, lead – 206 c. uranium – 238, lead – 206
b. thorium – 232, lead – 208 d. potassium – 40, strontium – 87
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