GENERAL GEOLOGY (GEOL 1113)
STUDY GUIDE FOR EXAM V
Copyright © 1999 Dr. Stephen K. Boss All Rights Reserved
The following study guide is provided as an aid to help you identify the major concepts you should have learned concerning GEOLOGIC TIME (Ch. 8), and lectures on HUMANS AS GEOLOGIC AGENTS, and THE GEOLOGY OF DOOMSDAY. Key vocabulary with which you should be familiar is highlighted in GREEN UPPER CASE throughout this document and on future documents of this type for the course. Additionally, some key words are highlighted in blue and underlined which means they contain hyperlinks to additional information of interest. Clicking on the blue words will transport you to various Internet locations or on-line images to enhance your studying. You should, at the very least, be able to define the highlighted terms in order to complete the exam. Ideally, however, I hope you will be able to do more than simply respond to definitions. I would like you to learn to be able to apply the definitions and the concepts they represent to a fuller understanding of the Principles of Geology and Earth as a planet by visiting the added hyperlinks.
GEOLOGIC TIME
RELATIVE DATING METHODS
RELATIVE DATING refers to those methods and principles which allow geologists to determine the general chronology of events. That is, which events occurred earliest (or a long time ago) and which events are more recent.
It turns out that determining the relative age of events is not that difficult - there are 4 fundamental principles which govern determining the relative age (i.e. oldest versus youngest) of geologic events.
THE PRINCIPLE OF ORIGINAL HORIZONTALITY: this principle simply states that layers of sedimentary rock are initially deposited as horizontal layers of sediment. Following their deposition, they may be disturbed by folding or faulting such that they are no longer horizontal, but in the beginning they are horizontal. This is a simple concept and one which is easily demonstrated.
THE PRINCIPLE OF SUPERPOSITION: this principle follows, in part, from the Principle of Original Horizontality. It states that in any succession of sedimentary rock layers, the layers near the bottom are the oldest and the layers near the top are the youngest.
THE PRINCIPLE OF CROSS-CUTTING RELATIONSHIPS: this principle is, perhaps, the most difficult to visualize, but is also quite obvious. It states that, in any geologic setting, the event or geologic feature which cuts across the most other features is the youngest. This derives from the fact that a geologic feature cannot be cross-cut unless it already exists prior to the cross-cutting event. Thus, the cross-cutting event is the youngest.
THE PRINCIPLE OF FAUNAL SUCCESSION: this principle states that fossil organisms preserved in rock layers become progressively less complex as rocks become older. It is interesting to note this principle was formulated in the early 1800’s, PRIOR to the time that CHARLES DARWIN postulated his hypothesis of EVOLUTION in his book, THE ORIGIN OF SPECIES (1859). (NOTE: the links in this section will connect you with the University of California Museum of Paleontology - one of the finest in the world. I encourage you to spend some time browsing their extensive web site - you will be amazed!)
It is also of interest to note that the Principle of Faunal Succession works as a means of determining the relative age of rocks. Regardless of one’s personal belief as to why organisms change through time, it is an established fact that they do and this fact is useful to geologists.
The SUCCESSION OF FOSSILS within sedimentary layers as well as grouping or ASSEMBLAGES OF FOSSILS in those rocks are used to bracket the relative ages of rocks.
HISTORICAL GEOLOGY is the branch of geology that deals with interpretation of the geologic history of Earth and the timing of major events in Earth history.
Since 1795, when JAMES HUTTON presented his THEORY OF THE EARTH, historical geologists have been able to reconstruct a reasonable chronology of events in our planets history. However, it is important to understand that the history of our planet is INCOMPLETE - just as there are geologic processes which PRESERVE the planet’s history, there are also geologic processes which DESTROY our planet’s history.
Gaps in the geologic record are referred to as UNCONFORMITIES. Unconformities represent intervals of time during which 1) no sediment was deposited in a region and therefore, no geologic record was generated or 2) erosion in a region removed some portion of the geologic record, creating a gap in the geologic record.
James Hutton was the first person to describe an angular unconformity at SICCAR POINT (SCOTLAND) and recognize that the boundary separating the tilted layers from the horizontal layers represented an extraordinary amount of time. Observations such as this led Hutton ultimately to propose that Earth might be "millions" of years old.
There is a large range in the amount of time represented by unconformities. Some unconformities may represent only a few years, while others exceed 1 billion years in duration.
Unconformities are very important features of the geologic history of Earth. Some estimates suggest that the preserved history of Earth represented by layers of sedimentary rocks might, in fact, be only 10% of Earth’s history. The remaining 90% of Earth history is lost in unconformities.
By piecing together the geologic record from many locations around the earth, geologists have been able to construct a STANDARD GEOLOGIC TIME SCALE which shows Earth’s history subdivided into EONS, ERAS, PERIODS, and EPOCHS.
EONS are subdivisions of geologic time with durations of BILLIONS OF YEARS.
ERAS are subdivisions of geologic time with durations of HUNDREDS OF MILLIONS OF YEARS.
PERIODS are subdivisions of geologic time with durations of TENS OF MILLIONS OF YEARS.
EPOCHS are subdivisions of geologic time with durations of MILLIONS OF YEARS.
This time scale was constructed solely from analyses of the relative age relations of rocks all over the world. Assignment of ABSOLUTE AGES to the subdivisions of this time scale has only been possible over the last 50 years or so.
ABSOLUTE DATING METHODS
Since World War II, methods have been developed for determining the ABSOLUTE AGE of geologic events and rocks. All of these methods rely on the physical properties matter referred to as RADIOACTIVITY and RADIOACTIVE DECAY.
As it happens, some of the elements from which matter is composed are NATURALLY UNSTABLE. Over time, these unstable elements can be observed to DECAY or transform from a PARENT ELEMENT to a DAUGHTER ELEMENT. Examples of this are the transformation (or decay) of Uranium to Lead or Carbon-14 to Nitrogen. Such spontaneous decay is what scientists recognize as RADIOACTIVITY.
The RATE OF RADIOACTIVE DECAY is described by the HALF-LIFE of a radioactive element. By definition, the half-life of a radioactive element is THE TIME REQUIRED FOR ONE-HALF OF A QUANTITY OF THAT ELEMENT TO DECAY TO ITS DAUGHTER ELEMENT.
It turns out that the half-life of a radioactive element is a statistical probability which is invariant - the half-life of a radioactive element does not change with temperature or pressure or any other known physical process. Thus, if we know the half-life of an element and we know how much of the element was originally present in a rock, we can determine its age in absolute terms (i.e. years).
Consider the following simple example: A "species" of Uranium, known as Uranium-235, will decay to a "species" of Lead, known as Lead-207, with a half-life of 713 million years. If I find a rock which contains 3.125 moles of Uranium-235 and 21.875 moles of Lead-207, how old is the rock?
To solve this problem, I need to know how much Uranium-235 was originally in the rock. Here, I am in luck because the daughter element (Lead-207) was originally Uranium-235. Thus, by just adding the amount of Uranium-235 presently in the rock with the amount of Lead-207 in rock, I will know how much Uranium-235 I started with. In this case, 3.125 + 21.875 = 25 moles of Uranium-235.
If I start with 25 moles of Uranium-235 and allow one half-life to elapse, then the rock will contain 12.5 moles Uranium-235 and 12.5 moles Lead-207. If I allow another half-life to elapse, the rock will contain 6.25 moles Uranium-235 and 18.75 moles Lead-207. If a third half-life elapses, the rock will contain 3.125 moles Uranium-235 and 21.875 moles Lead-207.
Since the last number is the one which matches the Parent-Daughter ratio in my rock, I know that the rock is 3 half-lives of Uranium-235 old. Since this half-life is known to be 713 million years, the age of the rock is 3 x 713 million years = 2.14 billion years old.
While the basic concept for determining rock ages using this method is simple, in practice it requires great care and precision because often we are attempting to measure very accurately small quantities of radioactive elements which occur in rocks.
It is through the use of radioactive decay that absolute ages have been assigned to numerous geologic events. The ages of some important geologic events in earth history are given on the STANDARD GEOLOGIC TIME SCALE.
TOP TEN EVENTS IN EARTH HISTORY (FALL 1998) # EVENT TIME (yrs ago) COMMENTS 1. 6 AUGUST 1945 53 Knock, knock.
Who's there?
KA
KA-who?
KA-BOOM!2. RISE OF HUMANS 4,000,000 (4 million) The rest is history! 3. AGE OF MAMMALS 66,000,000 to present Humans are late arrivals! 4. DINOSAUR EXTINCTION 66,000,000 (66 million) Leading hypothesis for this event is the impact of an 10-km diameter asteroid. The supposed crater from this impact is 150 km across and occurs in the subsurface in the Yucatan Peninsula, Mexico. 5. PERMIAN EXTINCTION 245,000,000 (245 million) Of the known organisms on Earth at this time (i.e. those for which we have fossils), 95% became extinct at the end of the Permian Period. The Permian Extinction is the largest known extinction event in Earth History. 6. FIRST LAND ANIMALS 400,000,000 (400 million) Slithering, creeping, crawling into the virgin "forests" of the Devonian Period. 7. ORIGIN OF LAND PLANTS 440,000,000 (440 million) The earliest land plants are believed to have evolved during the Ordovician Period. 8. PHOTOSYNTHESIS approx. 2,000,000,000 (2.0 billion) Plants evolve photosynthetic metabolism and forever change the chemical composition of the atmosphere by introducing oxygen. 9. ORIGIN OF LIFE approx. 3,800,000,000 (3.8 billion) Earliest known fossils of microorganisms occur in rocks 3.8 billion years old in Australia. 10. ORIGIN OF EARTH 4,500,000,000 (4.5 billion) Let there be light! (or something like that) HUMANS AS GEOLOGIC AGENTS
Beginning with the dawn of the Industrial Age, humans have been able to transform the planet in ways that few other organisms have ever done. Many of the Earth-altering activities of people are related to industrialization and the development of modern human societies - but some activities are as old as life itself. As a result, human activities have become as important agents of change on our planet as the many geological processes we've discussed throughout the semester. Let's look at just a few of the activities of humans and their planetary impacts:
HUMAN POPULATIONS
We know from historical records that HUMAN POPULATION has increased dramatically over the last century. In the year 1900, the were just over 1 BILLION PEOPLE on Earth. However, Over the intervening 100 years, world population has grown by ever increasing amounts such that sometime during 1999, world population will reach 6 BILLION.
Most estimates suggest that 20 YEARS FROM NOW, world population will be 8 BILLION. The impact of these people on our planet has been impressive. As people seek to improve their quality of life, it is important to note that the only means they have to do this is to exploit the natural resources of the Earth.
NATURAL RESOURCES in this case can mean almost anything that is part of the Earth - SPACE on which people live, FOOD that people must eat, or other MATERIALS that must be used for our survival. It is important to recognize that EVERYTHING WE HAVE was derived from the Earth in some way - this is a staggering realization.
Even more staggering is to attempt to imagine how we will maintain the quality of our lives and the lives of other people using only those materials available to us on Earth under the increasing demand of ever-growing human populations.
NATURAL RESOURCES
Way back at the beginning of the semester, we dealt very briefly with the issue of NATURAL RESOURCES and RESERVES. Recall that a RESOURCE is defined as economically valuable materials of geologic origin that can be extracted from Earth.
RESOURCES are the total quantities of a particular geologic material of economic value. It includes UNDISCOVERED as well as DISCOVERED deposits.
RESERVES, on the other hand, are those deposits of economically valuable geologic materials which can be recovered with present technologies. Thus, reserves are a subset of resources - they represent only those resources which 1) HAVE BEEN DISCOVERED and 2) ARE RECOVERABLE WITH PRESENT TECHNOLOGY.
The primary cause for concern in today's world is not that resources are being depleted, but the RATE OF RESOURCE DEPLETION.
GLOBAL POLLUTION
Much has been made of mankind's exploitation of FOSSIL FUELS (COAL, PETROLEUM, NATURAL GAS) during the last century. The issues which are relevant in this debate are
- the RATE OF COMBUSTION of these resources;
- the KNOWN RESERVES and estimates of the TOTAL RESOURCE;
- ENVIRONMENTAL DEGRADATION (as a result of POLLUTION) caused by our reliance on this form of energy.
Aside from obvious effects of pollution, there is an issue of great concern regarding the combustion of fossil fuels in great abundance: THE GREENHOUSE EFFECT.
THE GREENHOUSE EFFECT is an ENHANCED WARMING OF EARTH due to the presence of increased concentrations of certain gases in the atmosphere which trap heat from the Sun.
The primary gas associated with the Greenhouse Effect is CARBON DIOXIDE.
Carbon dioxide in Earth's atmosphere has increased dramatically (about 30%) since 1900). An inevitable effect of increasing CO2 in the atmosphere is that the WORLD WILL GET WARMER.
Since 1985, the 10 WARMEST YEARS OF THE HISTORICAL RECORD HAVE OCCURED.
AVERAGE GLOBAL TEMPERATURE HAS INCREASED about 0.5oC since 1900.
GLOBAL CLIMATE WILL CONTINUE TO CHANGE for the foreseeable future.
DEFORESTATION
Forested land worldwide is being rapidly DEFORESTED for a variety of reasons:
DEBT PAYMENT: Developing nations use income from logging to help repay national debts.
SETTLEMENT: As a result of growing populations, people migrate into forested lands and clear the land for development.
CONVERSION TO AGRICULTURE: Forested land is converted to agriculture to feed growing populations.
FUELWOOD: Forested land is being stripped of timber by populations in need of cheap fuel.
THE GEOLOGY OF DOOMSDAY
For centuries, many pundits have speculated about the END OF EARTH. "Doomsday prophecies" typically include descriptions of great catastrophes, fire, and brimstone. How will the Earth end? Will it be a cataclysm of cosmic proportions? While no one can say how the world will end, consider the following scenario:
Whether or not mankind manages to avoid blowing itself up is not a factor in how the Earth will "die" in the planetary sense. All around us in the solar system are other planets which are best described as "dead". But what makes them so? Obviously, there is not life on them - this is one factor. But what else makes the other planets of our own Solar System different from Earth?
A new area of endeavor among Geologists and other Physical Scientists is known as EARTH SYSTEM SCIENCE. Earth System Science views the Earth as an in somewhat non-traditional terms, combining integrated studies of all sciences across disciplines. As a result of this new view, we now have a better understanding of why Earth is different from the other planets.
A principal factor is the existence on Earth of a dynamic geological process, PLATE TECTONICS. Most of the topics we've covered in this semester have been related to the PLATE TECTONIC PARADIGM in Geology. The circulation of the ASTHENOSPHERE and the effects of that circulation on plate motions and the ROCK CYCLE creates a mechanism on Earth for continual renewal and replenishment of vital materials. Without these cycles, life of any kind on Earth IS NOT SUSTAINABLE.
The source of energy for CONVECTION in the ASTHENOSPHERE is RADIOACTIVE DECAY of various elements within the Earth (primarily URANIUM, and POTASSIUM). We now know that there is a finite quantity of time which can elapse before these elements are depleted through the natural process of RADIOACTIVE DECAY.
One day in our collective and very distant future, the HEAT PRODUCTION from radioactive decay will not be sufficient to keep the great engine of ASTHENOSPHERIC CONVECTION running. As a result, the plates will slow down; VOLCANIC ACTIVITY at MID-OCEAN RIDGES and SUBDUCTION ZONES will cease. EARTHQUAKES will not longer plague the inhabitants of PLATE BOUNDARIES.
The great mountain ranges of the planet will ERODE, and be levelled by WEATHERING PROCESSES. The weathered debris from the mountains will be DEPOSITED as broad sheets of SEDIMENT along the continental margins. CHEMICAL WEATHERING of rocks exposed to the atmosphere will consume many of the gases in the atmosphere, forever changing its composition; in particular, OXYGEN and CARBON DIOXIDE will be scrubbed from the atmosphere by ROCK WEATHERING PROCESSES. However, without VOLCANOES to replenish the supply of carbon dioxide in the atmosphere, plants soon run short of this necessary resource for photosynthesis - and without photosynthesis, the oxygen in our atmosphere cannot be replensihed. Thus, life itself becomes impossible.
And, in the end, Planet Earth...simply......grinds.......inexorably............to......................a.................................halt.
This concludes the study guide for Exam V. Comprehending geologic time is not easy - but it can be done with lots of practice. I hope you will practice and learn the basic methods of determining the age of the Earth. As always, if you can master the information on this study guide, I know you will do well on the exam. HAPPY STUDYING and GOOD LUCK! - Dr. Boss :-)
Copyright © 1999 Dr. Stephen K. Boss All Rights Reserved
