CHAPTER 19 - TIME AND GEOLOGY
Overview
Relative time, establishing the sequence of geologic events, and isotopic
dating, establishing actual numerical age, are differentiated in the
introduction. The historical development of uniformitarianism is discussed and
compared to actualism, and the concept of absolute time is then developed.
Calibration of geologic time has been provided by absolute age dating using
radioactive isotopes, and relative time based on determining the sequence of
events through time.
A number of important principals are used to establish relative geologic time.
These include the principals of original horizontality, lateral continuity, crosscutting relationships, contact metamorphism and inclusions. These principals
are illustrated in the text by reference to a series of block diagrams (Figures
19.1 – 19.11) describing the geologic history of Minor Canyon (a fictitious
place).
Unconformities are defined as surfaces that represent gaps in the geologic
record, and most are buried surfaces of erosion. Three types of unconformity
are recognized: disconformities, angular unconformities, and nonconformities.
These can be differentiated on the basis of the physical relationships of beds,
including signs of erosion, and gaps in the record indicated by fossils. Possible
sequences of events that would produce each type of unconformity are
discussed and illustrated in Figures 19.13-19.15.
Correlation is the determination of age relationships between rock units or
geologic events in separate areas. Methods employed include: physical
continuity, similarity of rock types, and correlation by fossils (faunal
succession, index fossils, and fossil assemblages).
The standard geologic time scale (Table 19.2 - given without absolute dates;
illustrated later as Figure 19.26 with absolute dates) is based on establishing
the relative age of rock units based on stratigraphy and fossil assemblages.
Geologic time is subdivided into eras, periods, and epochs. The time scale
begins with the Precambrian, followed by the Paleozoic (old life), Mesozoic
(middle life), and Cenozoic (new life) Eras. The present day is part of the
Recent (Holocene) Epoch, within the Quaternary Period of the Cenozoic Era.
Absolute time is based on radiometric dating which determines age from the
radioactive decay of specific isotopes. Radioactive isotopes, their daughter
products, emission of alpha and beta particles, electron capture, and the halflife concept are described in the text, and supported by Figures 19.23 and
19.24 illustrating decay series and curves. Radiocarbon dating is described and
differentiated from the other parent-daughter systems. Calculation of an
absolute date is described in Box 19.4. Radiometric dating usually establishes
the time that a rock or mineral became a closed system, which may not be the
actual age of the rock itself. Dating is accurate within statistical expectations
for each isotope.
Radiometric dating provides numeric limits for relative ages, and has been
used to calibrate the geologic time scale (Figure 8.26). The chapter concludes
with a review of the history of attempts to establish the age of the earth –
currently though to be 4.5-4.6 billion years and inferred from the age of
meteorites. Comparison of the whole of geologic time is made to the height of
the CN Tower in Toronto (Figure 19.27), and a film shown at 32 frames per
second, with one frame equaling 100 years. Using these analogues, the
10,000 years since ice retreated from southern Ontario would be represented
by the uppermost millimeter of the Tower and less than 4 seconds on film.
Learning Objectives
1. Uniformitarianism (actualism) implies that geologic processes operating
today also operated in the past; "the present is the key to the past." Since
rates of deposition and other activities are slow, the expanse of geologic time
was necessarily broadened by application of uniformitarianism, but the term
doesn't imply rates were uniform.
2. Absolute time provides a date in years or some other time unit to a rock,
while relative time merely arranges events in a sequence.
3. Geologists think of the geology of an area in terms of the sequence of
events that form its history. Four basic principles are applied to recognize the
various steps in the geologic history of an area. Original horizontality implies
that the rocks were horizontal when first formed, and any change from
horizontal took place after deposition. Superposition implies that rock
sequences get younger toward their tops. Lateral continuity states that
sedimentary layers extend laterally until their edges pass into another
sediment type reflecting gradual changes in depositional environment. Crosscutting relationships imply that a disrupted rock unit is older than the cause of
its disruption. Figure 19.1-19.11 and Table 19.1 apply these principles to
understanding the sequence of events that developed Minor Canyon, a
fictitious location similar to Grand Canyon.
4. Correlation establishes age relationships between rock units or events in
separate areas. Physical continuity implies that a rock unit can be traced from
one area to another. Similarity of rock types allows correlations to be made,
particularly if the character and sequence of rocks are distinctive. Correlation
by fossils utilizes the principal of faunal succession: fossil species occur in a
definite and recognizable order through time. Index fossils are short-lived
species whose age is well constrained and have widespread distribution. Most
geoscientists use groups of fossils, or fossil assemblages to make correlations.
5. The standard geologic time scale reflects relative time and is based on
stratigraphy and fossil assemblages. Eras are the largest divisions of the time
scale, followed by periods, and then epochs. Fossils become common at the
beginning of the Paleozoic Era, and rocks that precede that era were formed
during the Precambrian. The Mesozoic Era succeeds the Paleozoic Era, followed
by the Cenozoic Era. Those era boundaries are times of mass extinction. The
Cenozoic Era includes the Holocene Epoch in which we are now living. Most
geological investigations involve the use of relative time.
6. Unconformities are gaps in the geologic record, commonly represented by
buried erosion surfaces. A disconformity separates beds that parallel one
another but formed at very different times. Fossils may indicate a break in the
record. Angular unconformities separate tilted older strata from overlying
younger strata. Fossils and cross-cutting relationships may be used to
determine the relative age of the folding and tilting. Nonconformities separate
older plutonic or metamorphic rocks from younger sedimentary rocks.
7. Absolute time provides ages, usually in years, to geological features and
events. The Earth is estimated to be 4.5-4.6 billion years old. The oldest rocks
on Earth, 4.03 billion years old, are found in northwestern Canada. The oldest
dated mineral is a zircon from Australia, which is 4.4 billion years old.
8. Absolute dating is based on the decay of radioactive isotopes, particularly
uranium. Decay is expressed in half-lives, the time it takes for one-half of a
given amount of radioactive isotope to be reduced to its stable daughter
product. Radioactivity involves emission of alpha and beta particles and
electron capture that may change the atomic number or atomic mass number
of the atom.
9. Dating is based on a comparison of the amount of isotope originally present
compared to the amount present at the time of the analysis and the half-life of
the isotope involved. Usually the date provided by the analysis is the time that
the rock or mineral became a closed system. In igneous rocks, that would date
mineral crystallization, while in metamorphic rocks, it would be time of
metamorphism. Absolute dates have been assigned to the geologic time scale
by bracketing events whose relative time is known. It has been used to
subdivide the Precambrian, which comprises the bulk of geologic time.
10. Radiocarbon dating involves the formation and breakdown of C-14. Dates
are determined by comparing the amount of C-14 present to what would be
expected in a living organism. The half-life is only about 5,730 years and the
system only provides dates on organic materials for the last 40,000 years with
accuracy.
11. Determining the age of the Earth has been a controversial subject and
several attempts were made to establish its antiquity prior to the discovery of
radioactivity (biblical, rate of cooling). The age of the Earth has been
established as 4.5 - 4.6 billion years old based on isotopic dating of
meteorites.
12. Geologic time is vast, mostly represented by the Precambrian, and human
history represents an exceedingly small portion of that time.
Boxes
19.1 - IN GREATER DEPTH - HIGHLIGHTS OF THE EVOLUTION OF LIFE
THROUGH TIME - The oldest fossils are prokaryotic (no nucleus), singlecelled organisms that lived approximately 3.5 billion years ago. Nucleated,
single-celled organisms (eukaryotes) appeared 1.4 billion years ago.
Multicellular jellyfish and worms are found in rocks deposited 700-650 million
years ago. Fossils characterize rocks of the Paleozoic, Mesozoic, and Cenozoic
Eras. These organisms, such as trilobites, had hard parts that gave them much
more opportunity for preservation. The first vertebrates were fish that appear
in the late Cambrian Period. The Devonian Period is the ‘age of fish’,even
though amphibians, the first land dwelling vertebrates, appear in the late
Devonian. Land plants appear in the Ordovician Period and reptiles appear in
the Pennsylvanian Period. The Paleozoic ends with a mass extinction involving
more than 95% of species living at the time. Dinosaurs, mammals, birds,
marine reptiles, flying reptiles characterize the Mesozoic Era that ended with
another mass extinction event involving about 75% of living species at the end
of the Cretaceous Period. The Cenozoic Era is the ‘age of mammals’ and a
variety of mammals evolved during this time, some of which have already
become extinct. Hominids have a record spanning the last 4 million years.
19.2 - ASTROGEOLOGY - DEMISE OF THE DINOSAURS - WAS IT
EXTRATERRESTRIAL?- The Cretaceous-Tertiary (K-T) boundary is marked
by the extinction of the dinosaurs and many other species of animals and
plants. It has been proposed that this extinction was caused by the collision of
a 10 km asteroid with the Earth that produced a dust cloud causing a drop in
temperature and disruption of the food supply. Evidence cited in favor of this
collision are K-T boundary layers with high iridium content, shocked quartz,
weathered glass spheres, and a presumed impact crater of the proper size and
age buried at Chicxulub, Mexico. Opponents of the asteroid impact hypothesis
explain the K-T climatic changes and extinction by volcanism. While
‘unfortunate’ for the dinosaurs, the K-T boundary extinctions provided the
opportunity for population of the Earth by mammals.
19.3 - ENVIRONMENTAL GEOLOGY - RADON, A RADIOACTIVE HEALTH
HAZARD Radon is an intermediate daughter product produced by the decay of
U-238 to Pb-206. Concentrations of radon may occur in areas where the
bedrock contains uranium, and exposure to concentrations of the gas is
thought to cause cancer. The greatest risk appears to be seepage through a
building's foundation and entrapment in houses sealed for air conditioning and
heating. Controversy between the EPA and other concerned groups exists as to
the number of homes at risk, but the problem can be eliminated by ensuring
good air circulation.
19.4 - IN GREATER DEPTH - CALCULATING THE AGE OF A ROCK - The age
of a mineral is calculated by multiplying the half-life divided by the decay
constant times the natural log of the number of atoms present in the parent
divided by the number of atoms present in the original present. An example
using the system U-235 to Pb-207 is given that provides an age of 1.032
billion years.
Short Discussion/Essay
1. Discuss the reliability of isotopic dates obtained from igneous, sedimentary
and metamorphic rock samples.
2. Discuss the difference between a physical correlation and time correlation
and the methods for determining both.
3. Why aren't the major divisions of geologic time (eras, periods, epochs) the
same duration, if all are expressed in years?
4. What assumptions are involved with determining the age of the Earth?
5. Why is there no complete record of geologic time preserved anywhere on
earth?
Longer Discussion/Essay
1. Why isn't the age of the Earth established by dating rocks on the Earth?
2. Why is most of geologic history based on relative time?
3. How are absolute dates assigned to the geologic time scale?
4. Why do extinctions play such a significant role in the geologic time scale?
5. Why are very young igneous and metamorphic rocks difficult to date
isotopically?
Selected Readings
Berry, W.B.N. 1968. Growth of a Prehistoric Time Scale. San Francisco: W.H.
Freeman and Co.
Ausich, W.I. and Lane, N.G. 1999. Life of the Past. Upper Saddle River, NJ:
Prentice Hall, Fourth Edition.
Dalrymple, G.B. 1991. Age of the Earth. Stanford, CA: Stanford University
Press.
Isachsen, C.E., Bowring, S.A., Landing, Ed and Samson, S.D. 1994."New
Constraint on the Division of Cambrian Time." Geology 22: 496-498. (The
absolute date for the Precambrian-Cambrian boundary has been moved from
600 m.y. (Kulp, 1960) to 570 m.y. (Palmer, 1984) to 543.9 m.y. in this paper
- it is assigned an age of 551 m.y. in the textbook).
Palmer, A.R. 1983. Decade of North American Geology, Geologic Time Scale.
Geology 11:503-504 (available as Geological Society of America, Map and
Chart Series MC-50). This time scale was revised in 1999 by Palmer and
Geissman and is available on-line from GSA.
Zen, E. 2001."What is Deep Time and Why Should Anyone Care?" Journal of
Geoscience Education 49(1):5-9.
The January, 1982, issue of the Journal of Geological Education is devoted to
the creation-evolution controversy and contains several articles on dating the
age of the earth.