Historical Geology
INTRODUCTION
WHAT IS GEOLOGY?
HISTORICAL GEOLOGY AND THE FORMULATION OF
THEORIES
ORIGIN OF THE UNIVERSE AND SOLAR
SYSTEM, AND EARTHÕS PLACE
IN THEM
Origin of the
Universe—Did It Begin With a Big Bang?
Our Solar System—Its Origin and Evolution
¬Perspective The Terrestrial and
Jovian Planets
Earth—Its Place in Our Solar System
WHY IS EARTH A DYNAMIC AND
EVOLVING PLANET?
ORGANIC
EVOLUTION AND THE HISTORY OF LIFE
GEOLOGIC TIME AND UNIFORMITARIANISM
HOW DOES THE STUDY OF HISTORICAL GEOLOGY
BENEFIT US?
SUMMARY
The following content objectives are presented in Chapter 1:
¬ Earth is a complex, dynamic planet that has continually evolved since its origin some 4.6 billion years ago.
¬ To help understand EarthÕs complexity and history, it can be viewed as an integrated system of interconnected components that interact and affect each other in various ways.
¬ Theories are based on the scientific method and can be tested by observation and/or experiment.
¬ The universe is thought to have originated approximately 14 billion years ago with a Big Bang, and the solar system and planets evolved from a turbulent, rotating cloud of material surrounding the embryonic Sun.
¬ Earth consists of three concentric layers—core, mantle, and crust—and this orderly division resulted during EarthÕs early history.
¬ Plate tectonics is the unifying theory of geology and this theory revolutionized the science.
¬ The theory of organic evolution provides the conceptual framework for understanding the evolution of EarthÕs fauna and flora.
¬ An appreciation of geologic time and the principle of uniformitarianism is central to understanding the evolution of Earth and its biota.
¬ Geology is an integral part of our lives.
To exhibit
mastery of this chapter, students should be able to demonstrate comprehension
of the following:
¬ the vastness of Earth history
¬ the interconnectedness of Earth systems
¬ the differences between physical geology and historical geology
¬ the basis of scientific theories, and the scientific method
¬ the Big Bang model of the origin of the universe
¬ the evolution of the universe and how its composition has been changing
¬ the general characteristics of our solar system
¬ the solar nebula theory for the formation of our solar system
¬ the formation of the Moon
¬ the characteristics that make Earth a dynamic planet
¬ the compositional and physical structure of Earth
¬ the impact of the theory of plate tectonics on the study of the Earth
¬ the basic concept of organic evolution, including the primary evidence and mechanisms
¬ the principle of uniformitarianism, and the derivation and significance of the geologic time scale
1. Earth can be viewed as a system of interconnected components that interact and affect each other. The principal subsystems of Earth are the atmosphere, hydrosphere, biosphere, lithosphere, mantle, and core. Earth is considered a dynamic planet that continually changes because of the interaction among its various subsystems and cycles.
Figure 1.1 Subsystems of Earth
Table 1.1 Interactions among EarthÕs Principal Subsystems
Enrichment Topic 1: The Gaian Hypothesis
Chemist James Lovelock and evolutionary biologist Lynn Margulis
proposed the Gaian Hypothesis, which maintains that the interactions of the
EarthÕs organisms control the Earth itself, including carbon dioxide levels in
the atmosphere, oceanic salinity, and temperatures. Although there are several
levels of interpretation of Gaia, most scientists now reject a Òstrong
hypothesis,Ó in which Earth is treated as an organism itself.
Access the thinkquest website on the Gaian Hypothesis (http://library.thinkquest.org/C003763/index.php?page=index, click on Planetary Bio,
then Gaian Hypothesis). Have your students investigate the Daisyworld animation
link, in which the organisms and feedback loop regulate the temperature of the
planet. This topic can be used to introduce discussions on climate change, and
whether the Earth can absorb the additional carbon dioxide introduced since the
Industrial Revolution. Have students discuss the pros and cons of governmental
regulations, and/or whether our planet can effectively regulate itself and its
systems.
2. Geology, the study of Earth, is generally divided into two broad areas: Physical geology is the study of Earth materials as well as the processes that operate within and upon EarthÕs surface; historical geology examines the origin and evolution of Earth, it continents, atmosphere, oceans, and life.
3. The scientific method is an orderly, logical approach that involves gathering and analyzing facts about a particular phenomenon, formulating hypotheses to explain the phenomenon, testing the hypotheses, and finally proposing a theory. A theory is a testable explanation for some natural phenomenon that has a large body of supporting evidence. Both the theory of organic evolution and plate tectonic theory are theories that revolutionized biology and geology, respectively.
4. The universe began with a Big Bang approximately 14 billion years ago. Astronomers have deduced this age by observing that celestial objects are moving away from each other in an ever-expanding universe. Furthermore, the universe has a pervasive background radiation of 2.7 K above absolute zero, which is thought to be the faint afterglow of the Big Bang.
Figure 1.2 The
Dopper Effect
Figure
1.3 The
Expanding Universe
Michael Lemonick (2003) noted
that while Òcosmology is sometimes pooh-poohed as more philosophy than
science,Ó scientists have made recent advances in quantifying the five
important numbers that are needed to explain the universe. These numbers are
13.7 billion years (the age of the universe); 200 million years (the interval
between the Big Bang and the first stars); 4% (the amount of the universe that
is ordinary matter); 23% (the proportion of the universe that is dark matter);
and 73% (the proportion of the universe that is dark energy). These numbers
were calculated using a satellite known as the Wilkinson Microwave Anisotropy
Probe. Discuss with your students what these numbers mean, and how they were
revealed. Time, Feb 24, 2003 v.61
n.8 p.45.
5. About 4.6 billion years ago, our solar system formed from a rotating cloud of interstellar matter. As this cloud condensed, it eventually collapsed under the influence of gravity and flattened into a counterclockwise rotating disk.
Within this rotating disk, the Sun, planets, and moons formed from the turbulent eddy of nebular gases and solids.
Figure 1.4 Diagrammatic
Representation of the Solar System
Figure 1.5 Solar Nebula Theory
6. Earth formed from a swirling eddy of nebular material
4.6 billion years ago, accreting as a solid body and soon thereafter
differentiated into a layered planet during a period of internal heating.
Figure 1.6 Homogeneous
Accretion Theory for the Formation of a Differentiated Earth (Active Figure)
In order to convey to students the size of our solar system, and the relative proximities of the planets to the Sun, use a football field analogy:
Put the Sun on the goal line.
Mercury is on the 1-foot line
Venus is on the 2-foot line
Earth is on the 1-yard line
Mars is on the 1 1/2 yard line
Jupiter is on the 5-yard line
Saturn is on the 10-yard line
Uranus is on the 20-yard line
Neptune is on the 30-yard line
(Although Pluto is not considered a planet, it can be placed on the 40-yard line).
On this same scale, the nearest star would be 500 miles away.
7. Earth
is differentiated into layers. The outermost layer is the crust, which is
divided into continental and oceanic portions. The crust and underlying solid
part of the upper mantle, also known as the lithosphere, overlie the
asthenosphere, a zone that behaves plastically and flows slowly. The
asthenosphere is underlain by the solid lower mantle. EarthÕs core consists of
an outer liquid region and an inner solid portion.
Figure 1.7 Cross Section of Earth Illustrating the Core, Mantle, and Crust
Enrichment Topic 4. Hypothesis for the Formation of
the Moon
Although the debate continues about the formation of the EarthÕs one
natural satellite, many scientists agree that the Earth was probably hit by a large
planetesimal in its early history. Material ejected from the Earth during
impact coalesced and cooled into the various lunar layers. What data support
this hypothesis? Have students investigate the mineralogical composition of the
Moon, and compare it with the EarthÕs core, mantle and crust. Can students
propose alternative hypotheses for the formation of the Moon? Students can
further investigate the various geographic features of the Moon, and discuss
their formations.
8. Plate tectonic theory provides a unifying explanation for many geological features and events. The interaction between plates is responsible for volcanic eruptions, earthquakes, the formation of mountain ranges and ocean basins, and the recycling of rock materials.
Figure 1.8 Movement of
EarthÕs Plates (Active
Figure)
Figure 1.9 EarthÕs
Plates (Active
Figure)
Figure 1.10 Relationship Between
Lithosphere, Asthenosphere, and Plate Boundaries (Active Figure)
Table 1.2 Plate Tectonics and Earth Systems
9. The central thesis of the theory of organic evolution is that all living organisms evolved (descended with modifications) from organisms that existed in the past.
10. Time sets geology apart from the other sciences except astronomy, and an appreciation of the immensity of geologic time is central to understanding EarthÕs evolution.
Figure 1.11 The Geologic Time Scale
In order to have students begin to examine the enormity of the 4.6 billion year old history of the Earth, use an analogy that one year equals one second. Choose a few student volunteers, and have each student clap the number of years that s/he has lived (Example: 18 years = 18 seconds = 18 claps). Ask students if any of them has existed 4.6 billion seconds. Calculate the amount of time needed for 4.6 billion seconds:
4.6 x 109 s x 1 min/60 s x 1 hr/60 min x 1 day/24 hr x 1 yr/365.25 days = 145.8 YRS!
11. The principle of uniformitarianism is basic to the interpretation of Earth history. This principle holds that the laws of nature have been constant through time and that the same processes operating today have operated in the past, although at different rates.
12. Geology is an integral part of our lives. Our standard of living depends directly on our consumption of natural resources, resources that formed millions and billions of years ago.
Students should begin thinking about Earth as a system. Lead them into an interactive discussion of these or related topics:
1. Where is the energy stored within the planet?
2. How is energy transferred from one subsystem to another? What are the effects of this transfer of energy?
3. How can the changes made in one subsystem affect another? For example, how might the warming of the atmosphere affect the hydrosphere and biosphere? How might human-induced changes—such as emissions from factories—affect the atmosphere and biosphere? Students may also come up with their own examples of interactions between subsystems.
4. Have your students investigate the ÒcyclicalÓ processes of the Earth. Most students are familiar with the hydrologic cycle, but they may be less familiar with the Ònitrogen cycleÓ and Òcarbon cycleÓ of our planet. The discussion of the carbon cycle can also be used to springboard to discussions on climate change, and proposed mechanisms of carbon sequestering.
Understanding the differences between theory, hypothesis, law, and principle is extremely important for students of science. It is even more important that they comprehend the scientific method to understand the basis for and significance of theories, especially those that may be controversial.
Have students read "Method of the Multiple Working Hypothesis" by T.C. Chamberlin and discuss the following questions. Even though Chamberlin wrote this in 1893, are his thoughts on the workings of science still valid? (This is available online at http://arti.vub.ac.be/cursus/2005-2006/mwo/chamberlin1890science.pdf#search=%22%22Method%20of%20Multiple%20Working%20Hypothesis%22%20%26%20Chamberlain%22)
1. What is the method of multiple working hypotheses? Can you construct an example, real or imagined, of this method?
2. What is the rationale for this approach to science? How might this method be considered good science? How might other approaches to understanding the world be considered bad science?
3. Have the students use their personal experiences with the Earth to describe how they have used this procedure. Alternately, they could explain how using this procedure might have gained better results.
1. Where in our solar system would we be most likely to find extraterrestrial life? What type of evidence should we seek in searching for extraterrestrial life? Students can investigate planets beyond our solar system on Planet Quest, http://planetquest.jpl.nasa.gov/
2. Students can investigate the latest planetary discoveries through research on the Internet. The NASA website provides a wealth of resources and new information (http://pds.jpl.nasa.gov/planets/). Have students compare and contrast compositions, atmospheres, surface temperatures, and tectonic activity among the planets of the solar system. What patterns are revealed among terrestrial planets? What patterns are revealed among Jovian planets?
1. Uniformitarianism: When discussing the principle of uniformitarianism, have the students give examples from their life experiences. (For example, students living in flood-prone areas may discuss various flooding events that are precipitated by record rainfall.) Discuss how difficult deciphering the history of the Earth would be if we did not accept uniformitarianism.
2. Geologic Time: Understanding the depth of geologic time requires an understanding of "Big Numbers" as well as the complexity of the 3rd and 4th dimensions. Students can use a variety of media to represent geologic time (adding machine tape, yarn, a football field, or the face of a clock). Describing large numbers in terms of familiar items may help students understand that the time of our human time scale—or even the existence of Homo sapiens on Earth—is miniscule.
3. The Geologic Time Scale: To help students familiarize themselves with the time scale, ask them to ÒbrainstormÓ a mnemonic for the time divisions you are requiring them to learn. Investigate with them the origin of the names of the eons and Phanerozoic eras (and smaller divisions as appropriate to your class). Students should learn these divisions from the oldest to the youngest in order to emphasize the order of events. The simple task of starting each class by writing the basic time divisions on a chalkboard or overhead slide (with the aid of your students) will familiarize them with the geologic time scale—without forcing them to initially memorize the eons, eras, and periods.
4. Emphasize that the geologic time scale is a human-made scale that helps scientists organize the EarthÕs history and events. Students may create their own portions of the geologic time scale throughout this course by using the TimeScale Creator at http://www.tscreator.com
1. What is the difference between a theory and a fact? Why do geologists consider Òplate tectonicsÓ a theory? Can you list any facts that support the theory of plate tectonics?
2. Why is plate tectonics called the unifying theory of geology? Can the distribution of volcanoes, earthquakes, mountain ranges, and mineral deposits be explained without this theory?
3. Why is organic evolution via the mechanism of natural selection called a theory? Does the use of the word ÒtheoryÓ indicate a lack of confidence in this important construct by scientists?
4. How do scientists differ in their use of the word ÒtheoryÓ from the general population, including social scientists? How can geology students emphasize to the general population that a ÒtheoryÓ in the sciences is the most highly respected level of knowing?
|
asthenosphere |
hypothesis |
principle of uniformitarianism |
|
Big Bang |
Jovian planets |
scientific method |
|
core |
lithosphere |
solar nebula theory |
|
crust |
mantle |
system |
|
fossil |
organic evolution |
terrestrial planets |
|
geologic time scale |
plate |
theory |
|
geology |
plate tectonic theory |
|
|
1. c |
5. a |
9. d |
|
2. c |
6. e |
10. d |
|
3. b |
7. e |
|
|
4. c |
8. b |
|
11. Earth is made of interconnected components that interact and affect each other in many ways. For example, it is impossible to study sedimentary rocks without knowing something about the atmospheric processes that weather, erode, transport and deposit rock materials or about the biological processes that lead to life and fossils. To truly understand Earth, we must learn about it as a system. Humans are an integral part of Earth system as part of the biosphere. Although humans are relative newcomers in EarthÕs history, Homo sapiens have arguably affected the Earth more than any other species. Examples of human effects on the Earth system are visible within the lithosphere (mining, farming activities), atmosphere (acid rain, greenhouse gas emission), biosphere (monoculture, human-extinction of species),and hydrosphere (levees on river systems, pumping of groundwater).
12. Earth is dynamic because it has continuously changed during its 4.6 billion year existence—it is not static. The three concentric layers are the core, mantle, and crust. EarthÕs core is metal. It is solid in the inner core, and liquid in the outer core. The mantle is solid, very dense, dark rock, which in some places can flow. The crust is solid, rigid rock. The density decreases from the core to the mantle and crust.
13. Uniformitarianism states that the natural processes operating in todayÕs world are the same processes, with the same underlying physical principles, that have operated throughout EarthÕs history. Catastrophic events, such as mass extinctions, have occurred many times in Earth history, but they are not very frequent.
14. Plate tectonic theory is important
to geology because it is a unifying theory, and the theory has provided a
framework for interpreting the composition, structure, and internal processes
of the Earth on a global scale. It has also led scientists to the realization
that the continents and ocean basins are part of a
lithosphere-atmosphere-hydrosphere that evolved together with the EarthÕs
interior. Therefore, the movement of the plates has affected not only the lithosphere,
but other subsystems of the Earth.
15. Big Bang is a model for the evolution of the universe, in which a hot, dense state was followed by cooling and expansion. Evidence in support of the Big Bang includes the expanding universe, and the presence of background radiation of 2.7 K. The background radiation is thought to be the fading afterglow of the Big Bang event.
Astronomers
determine the age of the universe by measuring the rate of expansion of
galaxies as they move away from each other and the distance they are from the
point at which they were once all together. Because the amount of time the
galaxies have been traveling, multiplied by their rate of travel, is equal to
the distance theyÕve traversed, we can plug in the numbers to determine the
amount of time galaxies have been traveling. This is the age of the universe.
16. The solar nebula theory describes the origin of our solar system through the collapse and condensation of interstellar material in a spiral arm of the Milky Way Galaxy. The terrestrial planets formed nearer the Sun by accretion of rocky material and metallic elements, while the Jovian planets formed farther from the Sun from the condensation of gases. The solar nebula theory accounts for most of the characteristics of the planets and their moons, the differences in composition between the terrestrial and Jovian planets, and the presence of the asteroid belt, which was prevented from accreting into a planet by JupiterÕs tremendous gravitational field.
17. Plate tectonic theory is the concept that plates of lithosphere are in motion relative to one another, driven by convection cells in the mantle. The theory accounts for the distribution of mountain chains, major fault systems, volcanoes, and earthquake epicenters. It is the underlying set of processes for the rock cycle, and explains the biogeography of fossil and living organisms.
18. Historical geology is an important field of study because as we investigate past events in EarthÕs history, we may also draw implications for the todayÕs global ecosystem and the Earth system. Many of the principles involved in the study of historical geology have practical applications as well, including application in mineral and oil exploration, and the interpretation of the planets and moons of our solar system.
1. To formulate such a hypothesis, we should first collect data on rock types and their ages and geologic structures to identify what about them is similar or different. Then, we should use the data to formulate a hypothesis describing how such geographically separate mountain ranges could be so similar. One reasonable hypothesis would be that the continents on which the mountain ranges currently lie were once connected and the mountains formed adjacent to each other at the same time. To test this, we could try to piece the continents together and see if rock types and structures do indeed match up. We could use the hypothesis to predict where other continents might match up and at what time, and look for evidence to see if this is true. We might call this hypothesis continental drift!
2. Scientists who examine global temperature changes during the past need to accurately record the temperature differences within a standard time scale. Then, these scientists can examine the time intervals between global temperature fluctuations of the past, and perhaps search for clues within EarthÕs history to determine if there were specific causes for these fluctuations. The geologic time scale is extremely important in the current debate on global warming. We know that the EarthÕs climate has gone through cycles of long and short duration, and that global warming and cooling are completely natural processes in Earth history. However, if humans are affecting climate on an extremely short timescale, over a period of decades or centuries, this may not be part of one of the EarthÕs natural cycles. It is imperative that we understand the geological time scale in order to detect the potential disastrous consequences of human-induced global warming, such as the loss of habitat and species.
3. We can approach global warming from a global systems perspective by investigating the effects of one system upon another. If carbon dioxide levels rise in the atmosphere, a resulting rise in temperature will affect the atmosphere. Rising atmospheric temperatures will affect the hydrosphere with increased melting of polar and glacial ice. This, in turn, affects sea levels, coastal erosion, and the biosphere. Rising sea levels and increasing temperatures will affect the types of organisms living in each ecosystem. Undoubtedly, anthropogenic carbon dioxide emissions have increased, and have some effect on the Earth. Debate continues over whether the changes in atmospheric carbon dioxide are responsible for warming trends, and to what extent the changes in temperatures may be attributed to trends or anomalous events. There are several lines of discussion for mitigating global warming, including reduced greenhouse gas emissions, alternative energy sources, and carbon sequestering. We can investigate whether warming has occurred in the geologic past through an investigation of our planetÕs data. Some organisms are good indicators of past temperatures, in that the organisms only live within certain temperature ranges. (For example, certain species of microfossils indicate ocean temperatures.) Investigation of these organisms from EarthÕs past can inform us whether temperatures were warmer, and whether global warming has occurred in the past. Additionally, oxygen isotope data can also be used to determine past temperatures.
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