Geology Quasi Textbook

Mizzou Online Geological Sciences 1100: Principles of Geology with Laboratory Goals

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Lesson 1: The Changing Planet
Purpose
This lesson discusses plate tectonics and the hydrologic cycle. Return to top of page

Objectives
1.1: describe plate tectonics. 1.2: describe the hydrologic cycle. 1.3: discuss the spatial and temporal scales that are commonly used by geologists. Return to top of page

Reading Assignment
Exploring Earth Chapter 1: pages 1–23 See the WWW sites page for additional information on topics in this lesson. Return to top of page

Laboratory Assignment
Laboratory Manual in Physical Geology

Laboratory One: Parts 1A and 1B, pages 1–13 Return to top of page

Commentary Index
The Changing Planet Introduction to Plate Tectonics An Introduction to the Hydrologic Cycle and Climate An Introduction to Scales

Commentary The Changing Planet
Earth is a dynamic planet. If we could go back in time a billion years or only a few million years, we would find that the continents had different shapes and were located in different positions compared to where they are today. The processes that alter Earth’s surface can be divided into two categories: those that wear away the land (hydrologic cycle) and those that construct new land (plate tectonics). Included among the destructional processes are weathering and erosion. Included among the constructional processes are volcanism and mountain building.

Volcanoes are powerful players in plate tectonics. Pictured above is Mount St. Helens in Washington state, the most recent volcano in the United States to have a major eruption. It erupted in 1980. Source: www.ClipArt.com

The energy cycle encompasses all the internal and external energy sources that drive Earth’s systems. The three sources of energy input are solar radiation (99.986% of the energy), heat from the interior of Earth’s interior (0.013%), and energy from tides (0.002%). The energy for the hydrologic cycle (destructional forces) comes from solar and tidal energy, and the energy for plate tectonics (constructional processes) comes from heat from Earth’s interior.
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Introduction to Plate Tectonics
Earth’s rigid outer shell, the lithosphere, is broken into several individual pieces called plates (see Figure 1.6 on page 7 in Exploring Earth). These rigid plates are slowly but continually moving due to the unequal distribution of heat within Earth. As hot material moves up from deep within Earth, it spreads laterally, and the plates are set in motion. This movement of Earth’s lithospheric plates generates earthquakes, volcanic activity, and the deformation of large masses of rock into mountains. Because each plate moves as a distinct unit, most interactions among them occur along their boundaries. The first approximations of plate boundaries were based on the locations of earthquake and volcanic activity. There are three distinct types of plate boundaries, which are differentiated by the relative movement among them (see Figure 1.7 on page 7 in Exploring Earth). These are as follows: 1. Divergent plate margins: zones in which plates move apart, leaving a gap between

them into which molten new crust is extruded 2. Convergent plate margins: zones in which plates collide, causing one to go beneath the other or both to buckle upward 3. Transform plate margins: zones in which plates slide past each other Divergence occurs at oceanic ridges. As the two plates move away from each other, a gap is created between them that is immediately filled with molten rock. This molten rock cools and produces a new sliver of sea floor. This process is called sea-floor spreading and has produced the entire floor of the Atlantic Ocean over the past 160 million years. A typical rate of sea-floor spreading is about 5 centimeters (2 inches) per year. This may seem very slow, but it is rapid enough to have generated all existing ocean basins within the past 5 percent of geologic time. New crust is continuously being generated at oceanic ridges; however, the total surface area of Earth remains constant. Therefore, crust must be destroyed somewhere at the same rate that it is being created at the ridges. Convergent boundaries are the site of this destruction. Typically, as two plates collide, the leading edge of one of the plates is bent downward, forcing it to slide beneath the other. These regions are called subduction zones. Other boundaries are located where plates slip past each other without producing or destroying crust. Although most transform plate margins are located within ocean basins, a few slice through the continents. The San Andreas fault of California is a well-known example (see Figure 1.10 on page 9 in Exploring Earth). In the past fifty or sixty years, geologists have realized that the interaction of plates along their margins is the cause of most of Earth’s volcanism, earthquakes, and mountain building. In addition, the locations of these plate margins do not remain constant through time. For example, a divergent plate margin that runs through eastern Africa has developed in the relatively recent geologic past. As long as the temperatures deep within Earth remain significantly higher than those near the surface, the material within Earth will move. This internal flow will keep the rigid outer shell of Earth in motion. Thus, while this internal heat engine is operating, the positions and shapes of the continents and ocean basins will continue to change, and Earth will remain a dynamic planet.
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An Introduction to the Hydrologic Cycle and Climate
The hydrologic cycle describes the movement of water among various reservoirs in Earth’s system (see Figure 1.12 on page 10 in Exploring Earth). Primarily, solar energy drives Earth’s hydrologic cycle. Water on the Earth’s surface exists in three forms or phases: solid (ice), liquid (water), and gas (steam or water vapor). Most of the water on Earth’s surface is

in the oceans (97.2 percent). Most of the freshwater exists as glaciers (75 percent of freshwater). Groundwater is the next largest source of freshwater. Lakes, soils, and the atmosphere contain very little water (less than 0.03 percent). Water evaporates from the ocean and land surfaces. This water remains in the atmosphere for about two weeks. After that, the water falls back to Earth’s surface as precipitation. Water that falls on the land surface can run off directly into rivers or streams, infiltrate the soil zone and become groundwater, be taken up by plant root systems, or evaporate back into the atmosphere. Water that enters rivers and streams flows down a gradient back into an ocean basin. Water that infiltrates the soil and groundwater environments eventually discharges into rivers or streams and also flows back to the oceans. (Some groundwater discharges directly to the oceans.) The water that is taken up by the plants is in large part released as water vapor into the atmosphere through the pores of the leaves (stomata). This is called transpiration. The time spent within any one of these reservoirs is called the residence time. In the atmosphere, water resides for approximately two weeks. On the other hand, water resides in the ocean for approximately 37,000 years. Click to see an animation of the hydrolic cycle. Everyone realizes that the equatorial areas are warmer than the polar areas. This difference, which has persisted throughout geologic time, is due to the angle of sunlight incidence, which is much lower at the poles (see Figure 1.14 on page 12 in Exploring Earth) than at the equator. Even though the equator is warmer than the poles, the overall climate of Earth has changed over geologic time. There have been periods during which there were no glaciers and periods of major glacial advances. The difference in temperatures between the equator and the poles sets up circulation cells by which heat is transferred from the equator to poles (see Figure 1.15 on page 13 in Exploring Earth). In all circulation cells, warm air flows upward and away from the surface of Earth, then laterally. As it flows laterally, air cools and sinks toward Earth. Once near the surface, flow is again lateral, completing the circulation cell.
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An Introduction to Scales
Earth is about 4.6 billion years old and is approximately 13,000 kilometers in diameter (see Appendix VI on page 518 in Exploring Earth). Comprehending the magnitude of geologic time and size of Earth can be difficult. The magnitudes of these numbers are not used in everyday situations. The textbook provides many useful analogies to help you build a framework for understanding the magnitudes of time and space that geologists use every day (see Figure 1.20 on page 17 and Figure 1 on page 19 in Exploring Earth). Geologists often say, “The present is the key to the past.” This means that the history of Earth can be understood by what we see happening today. This idea is referred to as uniformitarianism. These processes have shaped the Earth’s surface over a very long period

of time. Although it might seem that geologic processes are only slowly changing Earth’s surface, the cumulative effects over long periods of time can be tremendous—e.g., the growth of mountains followed by erosion to flat surfaces (shields).
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Study Questions
Exploring Earth Questions 1, 2, 3, and 5 on page 23 Study questions are for your benefit only; do not submit your answers for grading. You can check your responses to these study questions on the Answers to Study Questions page. Return to top of page

Laboratory Exercises
Laboratory Manual in Physical Geology Questions 1–4 on page 7 Questions 5–13 on page 13 Laboratory exercises are for your benefit only; do not submit your answers for grading. You can check your responses to these laboratory exercises on the Answers to Laboratory Exercises page. Return to top of page

Progress Evaluation
The material in Lesson 1 will be covered in the Lesson 2 Progress Evaluation. Utilities: Submit this Lesson/Check your feedback Preview a Progress Evaluation Review the Progress Evaluation Instructions Request an Exam

Check Your Grades Utilities: Submit this lesson/Check your feedback Review the Progress Evaluation Instructions Request an Exam Check Your Grades Return to top of page Printer-Friendly | Home
© 2005. 2011 University of Missouri Editor: Kari Bethel Web Design: Lane Barnholtz Multimedia Design: James Barnes and Julie Moriarity Questions/Comments? Contact Mizzou Online The University of Missouri is an Equal Opportunity/ADA Institution.

Mizzou Online Geological Sciences 1100: Principles of Geology with Laboratory

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Lesson 2: Earth and the Solar System: Composition and Origin
Purpose
This lesson serves as an introduction to the origin and composition of Earth. Return to top of page

Objectives
After completing this lesson, you should be able to 2.1: define geology and describe its major branches. 2.2: explain current ideas regarding the origin of Earth, the Sun, and the planets. 2.3: discuss the relations between Earth and other bodies of our Solar System. 2.4: describe the basic composition and structure of Earth and discuss how it developed. 2.5: describe the scientific method. Return to top of page

Reading Assignment
Exploring Earth Chapter 2: pages 25–50 See the WWW sites page for additional information on topics in this lesson.

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Laboratory Assignment
Laboratory Manual in Physical Geology Laboratory Nine: Parts 9A and 9B, pages 144–162 Although this laboratory assignment is not directly related to the reading assignment, the skills you develop will be used throughout this course and, thus, you need to develop them early. Return to top of page

Commentary Index
Introduction to Geology Formation of Our Solar System The Scientific Method The Planet Earth

Commentary Introduction to Geology
The science of geology is defined as the study of Earth and, in recent years, the other planetary bodies. This study includes Earth’s composition and structure, history, life, and processes that have changed and that continue to change the planet. Will Durant (U.S. historian, 1885–1981) is quoted as saying, “Civilization exists by geological consent, subject to change without notice.” Geology overlaps with many of the other fields of science, including biology, chemistry, and physics. Some of the major branches of geology are listed below: Mineralogy—the study of the origin, chemistry, and structure of minerals

Petrology—the study of the origin, composition, and classification of rocks Stratigraphy—the study of the origin, distribution, and geological history of layered rocks Structural geology—the study of the origin, distribution, and history of geological structures Paleontology—the study of fossilized organisms, especially as they relate to the evolution of life and history of Earth Geochemistry—the study of the composition of Earth and the chemical processes that occur within and at the surface of Earth Geophysics—the study of the composition and structure of Earth using quantitative physical methods This course will touch on all of these subjects, although some will be covered in more depth than others.
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Formation of Our Solar System
The historical development of ideas concerning the origin of the Solar System provides a valuable lesson demonstrating how competing hypotheses are often developed to explain the same observation. As new evidence is obtained, hypotheses can be discarded, modified, or replaced with new hypotheses. One early hypothesis developed by Count Georges Leclerc de Buffon (French naturalist, 1707–1788) suggested that the gravitational attraction from a star passing close to the Sun pulled matter out of the Sun, leading to the formation of the planets. This idea is called the collision hypothesis. In 1755, the German philosopher Immanuel Kant envisioned the origin of the universe as a slowly rotating cloud of gas that condensed into the Sun and planets. This idea, called the nebular hypothesis, was placed into more formal mathematical terms in 1796 by the French mathematician and astronomer Marquis Pierre Simon de Laplace.

For centuries philosophers and and astonomers have speculated about the formation of the Solar System. Source: www.ClipArt.com

The nebular hypothesis held sway until the late 1800s, when the astronomer F. R. Moulton, at the University of Chicago, showed that the hypothesis failed to account for the angular momentum in the Solar System. In other words, the Sun, which contains most of the matter in the Solar System, does not rotate fast enough relative to the planets. Think of the Sun as a skater doing a spin: as the Sun’s gravity pulls in most of the matter from the surrounding cloud, it will rotate faster, just as when a skater pulls in his or her arms, he or she will spin faster. Another problem with the nebular hypothesis is that gas molecules evenly distributed within a nebula would not have enough gravitational attraction to form into planets. Moulton and T. C. Chamberlin, also of the Chicago University Geology Department, collaborated to develop a modified version of the collision hypothesis. They suggested that the material that pulled away from the Sun quickly condensed into millions of small, solid chunks called planetesimals. Gravitational attraction among planetesimals resulted in their coalescing into the planets. This idea, which could be mathematically shown to account for the angular momentum in the Solar System, became known as the planetesimal hypothesis. A number of problems were then discovered with the planetesimal hypothesis. Primarily, it did not account for the differentiation of matter among the planets. In other words, the inner planets contain mostly heavy elements and are rocky. The outer worlds are more gaseous and composed of lighter elements. If all of the planets are made of solar material, then they should be of similar composition. Also, it is unlikely that one star could pass close enough to another to pull matter away without being permanently captured by the mutual gravitational fields. A more recent hypothesis combines elements of both the nebular and planetesimal hypotheses in an attempt to account for all of the observations. It is now believed that the

Solar System might have begun as a rotating disk of gas (mainly hydrogen) and dust-sized solid particles, called the solar nebular, and was much the same as envisioned by Kant and Laplace. However, instead of matter being evenly distributed in the nebula, it formed distinct clumps. These clumps of matter had enough gravitational attraction to cause further accretion into larger, rotating, asteroid-sized bodies (or planetesimals). As envisioned by Chamberlin and Moulton, the planetesimals continued to fall into one another to form planets. Most of the matter drifted to the center of the disk and was compressed by gravity to a point where the temperature reached about 1 million degrees Celsius, and nuclear fusion began. Gravity in the center of the disk tended to hold heavier elements nearby, and lighter elements drifted to the outer portions of the disk, resulting in the differences in planetary composition. It probably took at least 100 million years for the formation of the Sun and planets to occur. The process of planetary accretion continues today as meteors and asteroid-sized bodies fall into planets, including Earth.
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The Scientific Method
The previous discussion of the origin of the Solar System is a lesson on the application of the scientific method. When scientists make an observation or a series of observations, they try to develop ideas that explain what they have seen. Such an idea is called a hypothesis. In order to determine whether the hypothesis is true, it is continuously examined in light of new observations or evidence. Sometimes multiple hypotheses are developed that explain an observation equally well. Ideas such as the age of Earth and organic evolution by natural selection have survived so many tests and challenges that they have been elevated to the status of theories. In other words, they are regarded as probably true and are likely to be modified only slightly by future evidence. But, in truth, we cannot be sure. Occasionally, a theory or related group of theories becomes so established by repeated observation that it is regarded as axiomatic to science and referred to as a scientific principle or scientific law. There are several of these in geology, including uniformitarianism, which was developed in the eighteenth and nineteenth centuries. It would be impractical to go into great length about the history of the development of every major geological idea presented in this course. Therefore, data will be presented as certain, even though they are really only hypotheses and theories. Many of these hypotheses and theories are still being developed and tested as we gather additional information.
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The Planet Earth
In its initial state, Earth was compositionally homogeneous. As planetesimals fell into Earth,

their kinetic energy converted to heat. In addition to heat from falling planetesimals, disintegration of radioactive elements produced heat. For instance, the radioactive heat emitted by 1 cubic centimeter of granite is equal to about 20 calories over 10 million years. Admittedly, this does not seem like a lot of heat, but given the insulating properties of rock, the total mass of rock, and the length of time, considerable radioactive heat could have built up. This radioactive heat is responsible for the internal heat of Earth today and is the driving force behind plate tectonics. (The heat conduction formulas that predict plate tectonics were developed about 170 years ago by the French mathematician Baron Jean Baptiste Fourier.) About the time the internal temperature of Earth reached approximately 1000° C, metallic iron began to melt at depths of 400 to 800 kilometers. The molten iron and other heavy elements then sank toward the center of the Earth as lighter elements moved outward. This movement resulted in the release of additional heat, and the average temperature of Earth rose to 2000°C. This resulted in the concentration of about one-third of Earth’s mass at the center and is responsible for the present differentiation of the planet, as well as the upper movement of lighter elements (silicates). Differentiation has resulted in strong chemical zonation of Earth. Earth, as a whole, is believed to be made up of 35 percent iron, 30 percent oxygen, 15 percent silicon, 13 percent magnesium, 2.4 percent nickel, 1.9 percent sulfur, and 2.7 percent everything else. The crust of Earth is composed of 46 percent oxygen, 28 percent silicon, 8 percent aluminum, 6 percent iron, 4 percent magnesium, 2.4 percent calcium, and 5.6 percent other elements. It is important to note that in the study of geology, we rarely have anything other than crustal rocks to work with. The only way for us to understand what the interior structure of Earth is like is to study seismic waves produced by earthquakes. Information about the chemistry of Earth’s interior is obtained by studying the composition of rare fragments of mantle that are occasionally brought up from volcanoes, as well as by investigating the composition of meteorites and moon rocks. Partial melting and differentiation turned Earth into a layered planet (see Figure 2.18 on page 38 in Exploring Earth and the illustration below). Earth contains three major compositional layers. At the center is the densest layer, the core, which is a spherical mass composed largely of metallic iron with lesser amounts of nickel and traces of other elements. The thick shell of intermediate-density rocky matter that surrounds the core is the mantle. Surrounding the mantle lies the thinnest and least dense layer, the crust.

Source: www.ClipArt.com

The core and the mantle have relatively uniform thicknesses. However, the thickness of the crust can differ by a factor of nine. There are two different types of crust. Oceanic crust has an average thickness of about 8 km, a density of 3.2 g/cm3, and a composition rich in calcium and magnesium. Continental crust has an average thickness of 30 to 70 kilometers, an average density 2.7 g/cm3, and a composition rich in silicon, aluminum, sodium, and potassium. Physical properties such as the strength of rock vary with depth in Earth. These changes are controlled mainly by the temperature and pressure within the Earth. The composition of a rock plays a much smaller, yet important, role than temperature and pressure in determining its strength. For instance, when a solid is heated, it loses strength (e.g., it is easier to bend a warm pipe than a cold pipe). The result of the changes in temperature and pressure within the Earth is that physical properties do not always coincide with compositional properties. Thus, there are two types of layering—compositional and mechanical—and the two do not always match (see Figure 2.26 on page 46 in Exploring Earth). The greatest change in physical properties is found deep within Earth’s core, where

pressures are so great that iron is solid despite its high temperature. The solid center of Earth is the inner core. Surrounding the solid inner core is a zone in which temperature and pressure allow iron to exist as a liquid. This is the outer core. The difference between the inner and outer cores is not one of composition but of physical state: one is a solid, the other a liquid. Differences in temperature and pressure divide the layers above the core into three distinct regions. In the lower part of the mantle, the rock is under such high pressure that it has very high strength yet is very hot. This solid region extends from the core-mantle boundary to a depth of about 350 km and is called the mesosphere. Above the mesosphere, from 350 to about 100 km below Earth’s surface, is the asthenosphere, where the balance between temperature and pressure is such that rocks have very little strength. Rock in the asthenosphere is weak and easily deformed, like butter or warm tar. Above the asthenosphere, corresponding approximately to the outermost 100 km of Earth, is a region where rocks are cooler, stronger, and more rigid than those in the asthenosphere. This hard outer region, which includes the uppermost mantle and all of the crust, is the lithosphere. Whereas the crust and mantle differ in composition, it is the strength of the rocks that differentiates the lithosphere from the asthenosphere. Rocks in the lithosphere are strong and brittle; rocks in the asthenosphere are weaker and ductile. The lithosphere is broken into a number of plates that move about on the hot, convecting asthenosphere and interact with each other along their boundaries. Thus, plate tectonics is one of the consequences of the differences in physical properties between the lithosphere and the asthenosphere.
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Study Questions
Exploring Earth Questions 7 and 8 on page 50 Study questions are for your benefit only; do not submit your answers for grading. You can check your responses to these study questions on the Answers to Study Questions page. Return to top of page

Laboratory Exercises
Laboratory Manual in Physical Geology Questions 1–4 and 9–10 on page 159

Laboratory exercises are for your benefit only; do not submit your answers for grading. You can check your responses to these laboratory exercises on the Answers to Laboratory Exercises page. Return to top of page

Progress Evaluation
You should now complete the Lesson 2 Progress Evaluation. It covers the material in Lesson 1 and Lesson 2. The progress evaluation consists of 15 multiple-choice questions worth 1 point each for a total possible of 15 points. Remember to mark your answers on a printed copy of the progress evaluation preview so you will have a record of them. (Select "Lesson 2" to preview this progress evaluation.) When you have finished filling in and checking your answers on a copy of the progress evaluation preview, click on Submit a Lesson for an actual lesson submission page. Note: Progress evaluations must be submitted in sequence, and you may submit no more than three progress evaluations in a seven-day period. You should keep the printed copy of each progress evaluation preview with your answers marked so you will have a record of them. Utilities: Submit this Lesson/Check your feedback Preview a Progress Evaluation Review the Progress Evaluation Instructions Request an Exam Check Your Grades Return to top of page Printer-Friendly | Home
© 2005. 2011 University of Missouri Editor: Kari Bethel Web Design: Lane Barnholtz Multimedia Design: James Barnes and Julie Moriarity Questions/Comments? Contact Mizzou Online The University of Missouri is an Equal Opportunity/ADA Institution.

Mizzou Online Geological Sciences 1100: Principles of Geology with Laboratory Goals

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Lesson 3: Minerals
Purpose
This lesson covers minerals and their chemical components. Return to top of page

Objectives
After completing this lesson, you should be able to 3.1: describe the structure of the atom and how atoms and molecules bond to form crystalline substances. 3.2: define the term mineral and describe mineral properties. 3.3: identify the common minerals. Return to top of page

Reading Assignment
Exploring Earth Chapter 3: pages 53–77 See the WWW sites page for additional information on topics in this lesson. Return to top of page

Laboratory Assignment

Laboratory Manual in Physical Geology Laboratory Three: pages 37–51 Laboratory Three is possibly the most important part of this lesson, if not the whole course. This is because you will need a strong understanding of minerals, including their chemical makeup, in order to do much of what follows in the course. Mineralogy is the very foundation of geology, because Earth is essentially a mass of minerals. While there is a discussion about using dilute hydrochloric acid (HCl) on page 46 to test for carbonate minerals, you do not have to use HCl to complete that section. You only need to read and understand that section. Required Materials for Laboratory Three 1. For this laboratory, you will need a small hand magnifying glass. 2. You also will need the mineral and rock sample set mentioned in the Materials/Textbooks section of the Overview. You must rent it from Mizzou Online. The mineral and rock sample set contains the following tools: a scratch plate a glass streak plate a steel nail a magnet The set also contains the following rock/mineral samples. There are many variations in any mineral or rock type. Since this sample set provides only one example of each major type, use this sample set as a general guide and keep in mind that variations of these rocks/minerals exist. Muscovite Calcite Orthoclase/Microline Plagioclase Quartz Hornblend Augite Pyrite in quartz Magnetite Biotite Andesite Porphyry Pumice Gabbro Rhyolite Scoria (Vesicular Basalt) Quartz Sandstone Breccia Conglomerate Shale Micrite (Limestone)

Talc Gypsum (Alabaster) Limonite Hematite (red) Fluorite Galena Halite Olivine (Peridotite) Diorite Granite Basalt Andesite Obsidian Return to top of page

Chalk Arkose Siltstone Fossiliferous Limestone Chert Oolitic Limestone Quartzite Slate Schist Gneiss Marble Phyllite Hornfels

Commentary Index
What Are Minerals? Crystal Structures Mineral Properties Mineral Groups

Commentary What Are Minerals?
Mineralogy is one of the basic branches of geology and is very closely related to a number of other sciences, such as chemistry, physics, metallurgy, and ceramics. These sciences have their historical roots in the practice of mineralogy from ancient times through the Renaissance. Today, mineralogists work closely with other geoscientists in gaining a better understanding of Earth’s processes and the origin and distribution of valuable resources. They also work closely with metallurgists and mining engineers to develop better ways of extracting and refining these resources. Increasingly, mineralogists are concerned with environmental problems such as how pollutants

interact with groundwater. A mineral is a naturally occurring inorganic solid that has a specific crystalline structure in which the chemical composition may vary within defined limits. A mineral has definite physical properties that result from its chemical composition and crystalline structure.

Examples of common minerals include the following: Ice—as long as it forms in nature and not in your freezer Table Salt—which is the mineral halite Quartz—which is a common rock crystal and a major component of sand (see Figure 3.1 in Exploring Earth)
Quartz Source: Corel Corporation

Examples of naturally occurring substances that are not minerals include the following: Water—in its liquid form. If it is frozen into ice, it fits the definition of a mineral. Coal—although a solid, coal is the fossilized organic remains of plants. It lacks a crystalline structure, and has a highly variable chemical composition. Oil or Petroleum—oil is an organic liquid, whereas natural gas is an organic gas. Minerals are compounds, which are a combination of two or more elements. (If you need a refresher in basic chemistry, please read “Focus on 2.1” on page 27 and “Focus on 3.1” on pages 56–57 in Exploring Earth.) Chemical elements are defined as atoms with a particular number of protons. An atom may have a variable number of neutrons and still be the same element. An isotope occurs when two or more atoms of the same element have a different number of neutrons. Some isotopes of various elements are unstable and may undergo

radioactive decay. These are called radioactive isotopes. An atom with the same number of electrons as protons has no electrical charge. If an atom loses one or more of its electrons, it takes on a positive charge. If it gains one or more electrons, it takes on a negative charge. A charged atom is an ion. A positively charged ion is a cation, and a negatively charged ion is an anion. Minerals are compounds held together by chemical bonds. Chemical bonds between atoms are related to the way that the electrons orbiting the atoms interact with one another. Several kinds of chemical bonds are important in mineralogy. Ionic Bonds—formed by an electrostatic attraction between oppositely charged ions Covalent Bonds—formed when atoms share the outer layer of electrons with one another Metallic Bonds—formed when the electrons in the outer layer are dispersed (shared) among all of the atoms forming a solid Van der Waals Bonds—much weaker than the other types of bonds mentioned previously. Van der Waals bonds are formed by weak electrostatic attractions that occasionally form between molecules.
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Crystal Structures
A crystal structure is the three-dimensional arrangement of atoms and molecules in a symmetrical lattice. This arrangement results from the chemical composition and bonding of that mineral. The symmetry of crystals results as atoms and molecules are arranged in repeating patterns. The smallest division of atoms having the symmetry of a crystal is called the unit cell. Crystal symmetry was first described by Nicolas Steno in 1669 when he noticed that adjacent faces on quartz crystals always meet at the same angle. The laws of crystal symmetry were summarized by René Haüy in 1801. Crystals can be grouped into six crystal systems as follows: Isometric System—Isometric crystals have three axes. All axes are the same length and are perpendicular to one another. Hexagonal System—Hexagonal crystals have four axes, three of which are parallel. These axes are all the same length and have 120° intersections. The fourth axis is of any length and intersects the other three at a 90° angle. Tetragonal System—Tetragonal crystals have three mutually perpendicular axes. Two are of the same length, whereas the third is of any length.

Orthorhombic System—Orthorhombic crystals have three mutually perpendicular axes, each with a different length. Monoclinic System—These crystals have three axes of any length. Two of the axes are at oblique angles to each other, whereas the third axis is perpendicular to the plane of the other two. Triclinic system—Triclinic crystals have three axes of any length with oblique angles between them.
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Mineral Properties
Minerals can be described and identified on the basis of physical properties. These properties include the following: Cleavage—the tendency of a crystal to break along smooth planes of weakness. This is caused by differences in bonding strength along certain planes within the crystal lattice and is, therefore, a result of the mineral’s crystal structure and composition. Fracture—a broken surface that does not occur along a cleavage plane. Such a surface is usually not smooth and flat but typically rough and irregular. A conchoidal fracture is a distinctive type of fracture that results in a smooth, rounded, or scalloped surface. It is particularly well developed in the mineral quartz. Hardness—the measure of resistance of a mineral to abrasion. It is a function of the bonding strength between atoms. Frederick Mohs, an Austrian mineralogist, developed a relative scale of mineral hardnesses in 1822, which is as follows: 1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase 7. Quartz 8. Topaz

9. Corundum 10. Diamond The hardness of a mineral can be determined by scratching it against a solid of a known hardness. If the mineral leaves a mark, the mineral is harder than the solid. The solid can be one of the minerals from the Mohs scale or another substance with a known hardness. Some common substances with known hardnesses are a copper penny (2.5), glass (5.5), and a steel knife blade (6.5). Specific Gravity—the ratio of the weight of Calcite on fluorite Source: Corel Corporation a mineral’s volume to an equal volume of water. For instance, lead is 11 times heavier than water, so its specific gravity is 11. Specific gravity depends on the chemical composition and, to some extent, the crystal structure of a mineral. For most field identification purposes, minerals can be described as heavy, medium, or light. Color—imparted by the trace element content (small amounts of chemical impurities that may be present) and crystal structure. This is often not a very diagnostic property because trace elements impurities can cause many minerals to have a variety of colors. For instance, quartz, calcite, and fluorite can be almost any color and are, therefore, easily confused unless other diagnostic properties are used to differentiate them. Streak—the color of a mineral when it is a powder. This is usually more diagnostic than the color of the mineral as a whole. For instance, pyrite (fool’s gold) and gold are almost the same color, but pyrite’s streak is very dark green to almost black, whereas gold’s streak is yellow. Hematite can have an earthy-red or a metallic-gray color, but its streak is always reddish-brown. Streak is determined by rubbing the specimen across a piece of unglazed porcelain. Luster—describes the way in which the surface of a mineral reflects light. The two most important types of luster are metallic (looks like a metal) and nonmetallic (does not look like a metal). Nonmetallic luster may be subdivided into a number of types, such as vitreous (glassy), pearly, and earthy. Luster is imparted by the mineral’s composition and bonding. Minerals with metallic luster have crystal structures that are partly or completely held by metallic bonds.

Fluorite Source: Corel Corporation

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Mineral Groups
In the mid- to late 1800s, geologist James D. Dana developed a system for classifying minerals on the basis of their chemical compositions. This system is still used today. The main mineral groups are as follows: Silicate Minerals—among the most common rock-forming minerals. Their structure is based on an arrangement of four oxygen atoms with one silicon atom that forms a silica tetrahedron. The silicon and oxygen atoms are covalently bonded with one another to form a complex anion with a –4 charge. Silica tetrahedra can form covalent bonds with one another, or they can form ionic bonds with positively charged ions (see Figure 3.15 in Exploring Earth). Silicates may be subdivided on the basis of the kind of bonds and structures that silica tetrahedra form with one another. These subdivisions are illustrated in Table 3.2 on page 69 of Exploring Earth. Oxides—include minerals that are formed when oxygen combines with one or more metallic ions. These include such minerals as corundum (AlO2), hematite (Fe2O3), magnetite (Fe3O4), and uraninite (UO2). Sulfides—minerals that form when one or more metallic ion combines with sulfur. These include pyrite (FeS), galena (PbS), sphalerite (ZnS), and chalcopyrite (CuFeS2).

Halides—contain negatively charged halogen ions (Cl–, F–, Br–, and I–). Such minerals include halite (NaCl) and fluorite (CaF2). Carbonates—minerals that consist of one or more metal cations bonded with a carbonate (CO32–) anion. Common carbonates include calcite (CaCO3), aragonite (CaCO3), and dolomite (CaMg[CO3]2). Note that aragonite and calcite both have identical chemical compositions; they are, however, different minerals because they have different crystal structures. Sulfates—consist of metallic cations combined with the sulfate (SO42–) anions. Often one or more water molecules are held in the crystal structure by a weak bond. Sulfate minerals include gypsum (CaSO4 • 2H2O) and anhydrite (CaSO4). A rather useful sulfate mineral used for soaking tired feet after hard geological field work is epsomite (MgSO4 • 7H2O), or Epsom salt! Native Elements—include minerals composed of single elements. Some of these are gold (Au), silver (Ag), copper (Cu), and sulfur (S). Carbon is found in nature as graphite, one of the softest minerals, and diamond, the hardest mineral.

Copper Source: Corel Corporation

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Study Questions

Exploring Earth Questions 1–5, 7, and 9 on pages 76–77 Study questions are for your benefit only; do not submit your answers for grading. You can check your responses to these study questions on the Answers to Study Questions page. Return to top of page

Laboratory Exercises
Laboratory Manual in Physical Geology Questions 2, 4a, 4c, 5, 6, 8, and 15 on pages 49–51 Laboratory exercises are for your benefit only; do not submit your answers for grading. You can check your responses to these laboratory exercises on the Answers to Laboratory Exercises page. Return to top of page

Progress Evaluation
The material in Lesson 3 will be covered in the Lesson 4 Progress Evaluation. Utilities: Submit this Lesson/Check your feedback Preview a Progress Evaluation Review the Progress Evaluation Instructions Request an Exam Check Your Grades Return to top of page Printer-Friendly | Home
© 2005. 2011 University of Missouri Editor: Kari Bethel Web Design: Lane Barnholtz Multimedia Design: James Barnes and Julie Moriarity Questions/Comments? Contact Mizzou Online The University of Missouri is an Equal Opportunity/ADA Institution.

Mizzou Online Geological Sciences 1100: Principles of Geology with Laboratory Goals

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Lesson 4: The Rock Cycle and Igneous Rocks
Purpose
This lesson discusses the rock cycle and igneous rocks. Return to top of page

Objectives
After completing this lesson, you should be able to 4.1: describe the major rock types and their characteristics. 4.2: discuss the different types of volcanoes and possible results of eruptions. 4.3: describe the rock cycle. 4.4: describe Bowen’s reaction series. Return to top of page

Reading Assignment
Exploring Earth Chapter 4: pages 80–97 (“Igneous Rocks”) Chapter 4: pages 113–116 (“The Rock Cycle”) See the WWW sites page for additional information on topics in this lesson.

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Laboratory Assignment
Laboratory Manual in Physical Geology Laboratory Four: pages 66–75 Laboratory Five: pages 76–93 Return to top of page

Commentary Index
Rocks Igneous Rocks Types of Igneous Rocks Origin of Magmas Intrusive Structures and Plate Boundary Associations Extrusive Structures/Volcanoes

Commentary Rocks
Strictly speaking, a rock is any naturally formed aggregate of mineral matter. Rocks are the major component of Earth’s crust. An understanding of the origin and composition of rocks is integral to an understanding of geology. Earth’s crust is a dynamic system that was formed by the processes of plate tectonics and greatly modified at its surface by hydrologic processes. Rocks are constantly being formed, uplifted, eroded, and/or buried, where they are changed as a result of elevated heat and pressure. The rock cycle illustrates the rock processes of Earth’s crust (see Figure 4.30 in Exploring Earth). There are three major groups of rocks: igneous, formed when molten rock material (magma)

cools and solidifies; sedimentary, formed from the accumulation of mineral matter eroded from previously existing rocks; and metamorphic, formed when previously existing rocks are subjected to elevated heat and pressure but are not melted (usually after deep burial in Earth’s crust). Rocks are classified according to (1) their origin (i.e., igneous, sedimentary, metamorphic); (2) mineralogy and/or chemical composition; and (3) texture (size, shape, packing, and orientation of the component mineral crystals or grains).
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Igneous Rocks
Igneous rocks are classified on the basis of both texture (the size of the component crystals) and composition. Both texture and composition can tell us something about how the rock formed. Small crystals usually indicate that the liquid melt cooled rapidly. Such a texture is characteristic of a magma that has reached Earth’s surface (called lava). Such rocks are referred to as “volcanic” or “extrusive.” Coarse crystals indicate that cooling took a longer time. Such a texture usually indicates that the magma cooled slowly, deep within the Earth’s crust. These rocks are referred to as “intrusive” or “plutonic.” Geological structures formed by igneous rocks that cooled within the crust are called intrusions or plutons. Composition can tell us a great deal about the geological processes that affected the parent magma. The topic of igneous rock composition is discussed later in this lesson.

Cooled lava Source: www.ClipArt.com

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Types of Igneous Rocks

Textural terms and their interpretation include the following: Glass—amorphous (cryptocrystalline to noncrystalline). Such a texture occurs when the cooling of magma is so rapid that crystals either do not have time to form (therefore no true minerals are present) or are so small that an electron microscope is required to see them. Such a rock forms when magma reaches the surface as lava and cools very rapidly (within a matter of hours or minutes). Aphanitic Crystals—fine-grained and require a light microscope to be seen. Such a texture occurs when a magma reaches the surface as lava or intrudes into shallow, relatively cold rocks and cools rapidly (commonly within a matter of weeks). Phaneritic Crystals—are coarse-grained and can be seen with a 10X hand lens or by the naked eye. Such textures occur when magma cools slowly within the crust (taking thousands to millions of years to solidify completely). Porphyritic Crystals—relatively large crystals in a groundmass of smaller crystals (see Figure 4.5 on page 85 in Exploring Earth). Such a texture forms when magma begins to cool slowly, forming large crystals. Then it cools rapidly and forms the fine, crystalline groundmass. This might happen if the magma begins to cool within the crust and then completes cooling after extruding on the surface as a lava. Pyroclastic Rocks— consist of broken fragments of glassy material and crystals that are compacted into rock. Such a texture results from the violent extrusion of magma onto the surface as volcanic ash and cinder. Compositionally, there are two primary types of igneous rocks: those that are rich in silicon and aluminum, called felsic, and those containing abundant iron and magnesium, referred to as mafic. The term felsic is derived from the minerals feldspar and quartz (silica), which are two of the most common minerals in this type of rock. Mafic stands for magnesium and iron (also called ferrous and ferric). Sodium is another common element in felsic rocks, whereas calcium is more likely to be found in rocks of mafic composition. Felsic rocks, also called granitic, are found almost exclusively in continental crust. Mafic or basaltic rocks can be found either in continental or oceanic crust but are more indicative of an oceanic setting. Because of the high iron and magnesium content, mafic minerals tend to have a higher specific gravity than felsic mineral, making those rocks feel heavier.

The chart above shows how igneous rocks are classified according to their crystal size and mineral composition (see also Chart 4.4 on page 84 in Exploring Earth). Rocks may be aphanitic (fine-grained) and classified as rhyolite, andesite, or basalt on the basis of their mineral composition. Phaneritic rocks are classified as granite, diorite, gabbro, or peridotite according to their observed mineral content. Plagioclase feldspar is found mostly in igneous rocks and ranges from varieties that are high in sodium (Na) in felsic rocks to varieties that are rich in calcium (Ca) in mafic rocks. Because the minerals in aphanitic rocks are too small to be seen without the aid of a microscope, we often have to rely on color for identification. Felsic minerals tend to show lighter colors than mafic minerals. Therefore, if no identifiable minerals are present, these rocks will be classified as rhyolite when white or pink, andesite when grey, and basalt when black.

Basalt Source: www.ClipArt.com

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Origin of Magmas
Rocks of the asthenosphere are composed primarily of the mafic minerals olivine and pyroxene. New seafloor, or oceanic crust, is created at mid-ocean ridges as mafic magma moves upward from the underlying asthenosphere. The seafloor moves away from the mid-ocean ridge like a conveyor belt until it is subducted beneath a continent or island arc (see Figures 4.3 and 4.16 in Exploring Earth). The driving force is convection in the mantle. The oceanic crust created at the mid-ocean ridge is mafic because that is the composition of the asthenosphere where the magma originates. Continental crust, on the other hand, is composed mainly of felsic rocks, such as granite. In fact, continents tend to “float” on the asthenosphere because felsic rocks are less dense than the underlying mafic rocks. As the seafloor moves away from the mid-ocean ridge, sediment accumulates on it. Eventually, the ocean crust is subducted beneath the continental crust, where it begins to melt. This melting is aided by the high water content in the sediments. The lower part of the continental crust also begins to melt, resulting in magmas that range in composition from mafic to felsic, with an average composition of the volcanic rock andesite. Magmas tend to be more mafic closer to the subduction zones. Magmas are more felsic landward of the subduction zones (see Figure 4.16 on page 97 in Exploring Earth).

Granite Source: www.ClipArt.com

When volcanic and/or plutonic rocks are examined closely, one can see that many different rock types can form from the same parent magma. This is because different minerals in igneous rocks melt and crystallize at different temperatures. In the early part of the last century, N. L. Bowen, a Canadian geologist, performed a series of laboratory experiments to determine the order in which minerals would crystallize when a melt with the composition of basalt was cooled. In this way, he hoped to show that a felsic rock could be derived from a mafic magma. The results of his experiments are summarized in the following chart.

Note that as the magma cools, more mafic minerals such as olivine, pyroxene, amphibole, and calcium-rich plagioclase crystallize first. These first-formed minerals are characteristic of rocks such as peridotite, gabbro, and basalt. As the mafic minerals form, the remaining magma becomes depleted in iron, magnesium, and calcium and relatively enriched in silicon, aluminum, sodium, and potassium. If the magma is separated from the earlierformed minerals and later cools, quartz, potassium feldspar (orthoclase), and sodium-rich plagioclase will form, creating felsic rocks, such as granite and rhyolite. This separation may occur as the magma moves upward through the crust. Thus, several distinctly different rock types can form from a single magma, each progressively more felsic. This process is called fractional crystallization and is discussed in detail on pages 86–88 in Exploring Earth.
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Intrusive Structures and Plate Boundary Associations
As mentioned earlier in the commentary, when magma invades the crust, structures are formed called intrusions or plutons. The rock that is intruded by the magma is called the country rock. A number of intrusive structures are described in Exploring Earth on pages 89–97. (Figure 4.9 on page 91 will help you with this section.) Batholith—large, irregular mass of igneous rock that may underlie an area of more than 100 square kilometers and does not have a distinct floor. Whole mountain ranges, such as the Sierra Nevada Mountains of the western United States, are exposed batholiths.

Sierra Nevada Mountains Source: www.ClipArt.com

Dike—an intrusion of magma into fractures that cut across strata Sill—an intrusion of magma between layered rocks Hydrothermal Vein—may be associated with intrusions but is not an intrusion itself. As magma cools, hot fluids from the cooling body may move into country rock along faults and fractures. As the fluids cool, minerals crystallize into the vein. Many valuable ore deposits are associated with hydrothermal veins, including gold and silver.
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Extrusive Structures/Volcanoes
Volcanic, or extrusive, rocks form when magma reaches and spreads across the surface as lava. Compositional magma changes occur as magma becomes lava and as water and gas are lost when as it reaches the surface. The chemistry related mostly to silicon content results in distinctively different structures being created by felsic and mafic lavas. As the silica content increases, the viscosity of the magma increases. A higher viscosity means the lava does not flow as easily as a lava with a lower viscosity. Mafic or basaltic lavas are low in silica; therefore, they tend to be thin and fluid, commonly flowing a great distance from the source before cooling into rock. Mafic volcanic eruptions tend to be less explosive than eruptions of felsic lavas because volatile gasses are more able

to escape the thin mafic lavas. Types of mafic lava flows include the following: AA—angular blocks of hardened lava Pahoehoe—smooth, ropy-textured lava flows Pillow Structures—ellipsoidal or “pillow-shaped” structures in a lava flow that result from the extrusion of lava under water Columnar Basalts—hexagonal columns of basalt that result from the shrinkage of the basalt during cooling

Lava flowing into water. Source: www.ClipArt.com

Types of volcanic cones include the following: Shield Volcanoes—composed of the build-up of many basaltic lava flows around a central vent. The shape of the volcano is a broad cone with gently sloping sides. The Hawaiian Islands are composed of large shield volcanoes. Composite Volcanoes—composed of the build-up of lava and pyroclastic deposits. These volcanoes erupt explosively. The shape is the classic steep-sided cone. Mount St. Helens is such a volcano. Cinder Cones—steep-sided cones that are formed mainly of volcanic cinder and ash blown from a central vent. Cinder cones are most commonly basaltic in composition and may be associated with flows. Cinder cones range from tens of meters to several thousands of meters in height, such as the mountains north of Flagstaff, Arizona.

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Study Questions
Exploring Earth Questions 1–4 on pages 118–119 Study questions are for your benefit only; do not submit your answers for grading. You can check your responses to these study questions on the Answers to Study Questions page. Return to top of page

Laboratory Exercises
Laboratory Manual in Physical Geology Questions 1a–1e on page 71 Question 1 on page 82 Question 8 on pages 90–91 Laboratory exercises are for your benefit only; do not submit your answers for grading. You can check your responses to these laboratory exercises on the Answers to Laboratory Exercises page. Return to top of page

Progress Evaluation
You should now complete the Lesson 4 Progress Evaluation. It covers the material in Lesson 3 and Lesson 4. The progress evaluation consists of 15 multiple-choice questions worth 1 point each for a total possible of 15 points. Remember to mark your answers on a printed copy of the progress evaluation preview so you will have a record of them. (Select "Lesson 4" to preview this progress evaluation.) When you have finished filling in and checking your answers on a copy of the progress evaluation preview, click on Submit a Lesson for an actual lesson submission page.

Note: Progress evaluations must be submitted in sequence, and you may submit no more than three progress evaluations in a seven-day period. You should keep the printed copy of each progress evaluation preview with your answers marked so you will have a record of them. Utilities: Submit this Lesson/Check your feedback Preview a Progress Evaluation Review the Progress Evaluation Instructions Request an Exam Check Your Grades Return to top of page Printer-Friendly | Home
© 2005. 2011 University of Missouri Editor: Kari Bethel Web Design: Lane Barnholtz Multimedia Design: James Barnes and Julie Moriarity Questions/Comments? Contact Mizzou Online The University of Missouri is an Equal Opportunity/ADA Institution.

Mizzou Online Geological Sciences 1100: Principles of Geology with Laboratory Goals

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Lesson 5: Sedimentary and Metamorphic Rocks
Purpose
This lesson describes sedimentary and metamorphic rocks, their compositions, and formation processes. Return to top of page

Objectives
After completing this lesson, you should be able to 5.1: describe types of sediments and how sedimentary rocks form. 5.2: describe how metamorphic rocks form. 5.3: discuss metamorphic grade and types of metamorphism. Return to top of page

Reading Assignment
Exploring Earth Chapter 4: pages 97–113 (“Sedimentary Rocks” and “Metamorphic Rocks”) See the WWW sites page for additional information on topics in this lesson. Return to top of page

Laboratory Assignment
Laboratory Manual in Physical Geology Laboratory Six: pages 94–113 Laboratory Seven: pages 114–127 Return to top of page

Commentary Index
Sedimentary Rocks Types of Sedimentary Rocks Sedimentary Basins Metamorphic Rocks Types of Metamorphism Metamorphic Grade Metamorphic Facies

Commentary Sedimentary Rocks
Even though they are the most commonly encountered rocks, sedimentary rocks cover only approximately 5 percent of Earth’s crust by volume. They are very important economically, providing much of our energy, metals, and aggregates. Important sources of energy, such as that provided by coal, oil, gas, and most uranium ores, are found in sedimentary rocks. Metals such as iron, lead, zinc, and a large number of copper and gold ores are found in sedimentary rocks. Lastly, aggregates, or construction and industrial minerals and materials that are extracted or manufactured from sedimentary rock, include limestone used in cement; sand and gravel used in building stone; clay used for bricks, refractory insulation, and ceramics; potash and nitrates used in fertilizers; and gypsum used in plaster and sheetrock.

Sedimentary rock is formed by the consolidation of sediment. Sediment consists of fragments of preexisting rocks or precipitates of chemicals dissolved from preexisting sediment. Sediment is deposited by water, wind, and ice. There are three major stages in the production of sediment, which are as follows: 1. Weathering and erosion of preexisting rocks: This process involves the breakdown and removal of preexisting rocks. Weathering is the breakdown of preexisting rocks. There are two types of weathering: physical and chemical. Erosion is the movement of material from its original location to another.

The results of weathering and erosion Source: www.ClipArt.com

2. Transportation of sediment: Sediment can be transport as bedload, suspended load, or dissolved load. These processes usually occur in running water but may involve wind and glacial ice. Clastic particles (so named because they are broken fragments of preexisting rocks) include gravel, sand, silt, and clay. Dissolved ions include Ca2+, Mg2+, Na+, Si2O62–(soluble silica anion), CO32–, and others. 3. Deposition of sediment: Sediment deposition takes place in various environments (see Figure 4.20 in Exploring Earth). Geologists place these environments into three major groups based on geographic setting. These are as follows: a. Continental (Terrestrial) Environments alluvial (rivers and streams and fan deposits formed at the base of mountain ranges) lakes (lacustrine) glaciers

wind-blown (eolean) b. Shoreline (Transitional) Environments deltas tidal flats estuaries barrier islands beaches c. Marine Environments organic reefs continental shelves continental slopes deep marine
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Types of Sedimentary Rocks
There are two basic kinds of sedimentary rocks: clastic and chemical or biochemical. Clastic rocks are those formed from fragments of preexisting rocks. Clastic rocks are named based on the size of the grains (see Figure 4.19 in Exploring Earth). These names include the following: Conglomerate—pebble to boulder sized (>2 mm) fragments usually in a matrix of sand or silt Sandstone—sand-sized particles (0.063 mm to 2 mm) Siltstone—cemented, silt-sized (0.004 mm to 0.063 mm) particles Shale/Mudstone—>67 percent clay-sized (<0.004 mm) particles

Sandstone and shale Source: www.ClipArt.com

Chemical or biochemical rocks are precipitates of chemicals dissolved from preexisting rocks. Chemical precipitation may occur by either inorganic or organic means. Some examples of each are as follows: Organically precipitated, such as Limestone—produced by the action of marine organisms. The dissolved ions are transported in the ocean, where marine organisms incorporate the ions into their shells. Dolomite—the chemical alteration of limestone or lime sediment by addition of Mg2+, which converts calcite and/or aragonite to the mineral dolomite. (Note that, in this case, both the rock and mineral have the same name. Some geologists prefer the term dolostone for the rock; I don’t.) Inorganically precipitated from a saline water, such as Halite—bedded NaCl (salt) Gypsum—bedded CaSO4 • 2H2O Anhydrite—bedded CaSO4, altered from gypsum Bedded Potash Deposits—These are composed of a single mineral that was deposited in an arid environment. The dissolved ions that were deposited were

originally components of preexisting rocks, which were weathered and eroded.
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Sedimentary Basins
The site of sediment deposition and sedimentary rock formation is called a sedimentary basin. A sedimentary basin is a low area on the surface of Earth (see Figure 4.25 in Exploring Earth). Such depressions include lakes, sea coasts, continental margins, rift valleys, and crustal downwarping. Examples of sedimentary basins include (1) a continental depression, such as the Great Rift Valley of Eastern Africa; (2) an inland sea, such as the Mediterranean Sea or Gulf of Mexico; and (3) the edge of an ocean where the continental shelf and slope plunges into the deep ocean.

The Mediterranean Sea off the coast of Greece Source: www.ClipArt.com

Gravity dictates that sediment eventually makes its way to a final resting site—a basin—where it is buried by more sediment and undergoes lithification and diagenesis. After sediment is deposited, it is buried by other sediment and undergoes compaction and cementation by minerals precipitated from groundwater (e.g., quartz and calcite). The result is a hardened sediment called sedimentary rock. Geological processes do not stop at that point. Further chemical and physical changes can occur in the rock after burial. These changes are called diagenesis. Diagenesis is defined as all processes that work on a sediment or sedimentary rock from the time of deposition until the rock undergoes metamorphism, melting, or weathering and erosion. This includes the processes of lithification. Classification and detailed descriptions of the many kinds of sedimentary basins are beyond the scope of this course. However, most sedimentary basins can be classified into two general categories on the basis of their relationship to plate tectonics:

1. Basins that form on the leading edge of continents: In these types of basins, subduction of an oceanic plate and mountain building has taken place. Such sedimentary basins are dominated by clastic sediment eroded from nearby highlands and may contain volcanic rocks. Examples of such settings are the West Coast of North and South America. 2. Basins that form on the trailing or passive edge of continents: Little tectonic activity takes place where these basins are found. Such basins contain clastic sediment eroded from distant mountain ranges and delivered by great river systems, such as the Mississippi or Amazon. In areas without large rivers, carbonate sediment (limestone) may be deposited. Volcanic rocks are rare in passive margin basins. Examples of passive margin basins include the Gulf of Mexico and the Coral Sea off the northeastern coast of Australia. Deep within sedimentary basins, temperatures and pressures may reach a point where plant matter is changed to coal, and microorganisms become oil and gas. Porosity can be decreased by compaction. Furthermore, mineralogical changes may occur in rocks associated with the modest increasing pressure and temperature. Such changes all fall within the realm of diagenesis.
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Metamorphic Rocks
Metamorphic rocks are formed when igneous and sedimentary rocks are altered by high heat and/or pressure, which can cause changes in texture and mineralogy to occur without melting. These changes may accompany the interaction between the rock and pore fluids (water containing dissolved ions), causing changes in the rock chemistry. The temperature and pressure at the time of metamorphism can be determined from the texture and mineralogy of the rocks. Mapping these characteristics can give an indication of the source and direction of heat and pressure that caused metamorphism.
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Types of Metamorphism
Figure 4.28 in Exploring Earth will help you understand the material in this section. Contact Metamorphism—occurs when rocks are altered by the heat and hydrothermal fluids associated with an igneous intrusion. The region of metamorphosed rock surrounding the intrusion is called a metamorphic aureole. Regional Metamorphism—occurs in areas of mountain building, especially where a subducting seafloor plate or the collision of continental plates results in elevated pressures and in the melting and upward movement of magma bodies that later form batholiths. In such regions, most if not all of the crustal rocks may be altered by

varying degrees of metamorphism. High-Pressure Metamorphism—occurs at higher pressures and lower temperatures than regional metamorphism. This is a special kind of regional metamorphism.
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Metamorphic Grade
Metamorphic grade gives the level or rank of metamorphism. Low-grade metamorphism is characterized by minor alterations of a rock, whereas high-grade metamorphism is characterized by major alterations. The grade of metamorphism is determined by the minerals present. Because certain minerals can be produced only under specific temperature and pressure conditions, the presence of these minerals is a good indicator of grade (see Figure 4.26 on page 108 in Exploring Earth).
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Metamorphic Facies
A metamorphic facies consists of a group of metamorphic rocks that contains a mineral assemblage that is characteristic of a particular grade of metamorphism. Because rocks of different original compositions can be metamorphosed at different rates, there may be considerable overlapping in the mineralogy of metamorphic facies. The following table shows a rough comparison among metamorphic grade, facies, and mineralogical composition (also see Figure 4.26 on page 108 in Exploring Earth). Metamorphic Facies Characteristic Minerals Chlorite Muscovite Talc Serpentine Actinolite Biotite Epidote Na Plagioclase Hornblende Andalusite Staurolite Na-Ca Plagioclase

Low Grade

Greenschist

Intermediate Grade

Epidote-AlbiteAmphibolite

High Grade

Amphibolite

Partial Melting Grade

Granulite

Augite Sillimanite Pyrope Garnet Ca Plagioclase

Note the changes in metamorphic facies as the grade increases. The epidote-albiteamphibolite facies is characterized by the presence of actinolite (a low-temperature amphibole), biotite, and sodium plagioclase (called albite). As the temperature increases, a higher temperature amphibole (hornblende) appears, along with a more calcic plagioclase. At the highest grade, pyroxene (augite) replaces the hornblende, while plagioclase becomes calcium-rich. The similarities to Bowen’s reaction series should be obvious. As the temperature increases, minerals stable at higher temperatures form. As pressure increases, minerals stable at higher pressures form. Figures 4.26 and 4.27 show the relation among metamorphic facies, temperature, and pressure. Temperature is measured in degrees Celsius, and pressure is measured in kilobars (kb). One kilobar is approximately 1000 times greater than atmospheric pressure.
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Study Questions
Exploring Earth Questions 5, 6, and 9 on pages 118–119 Study questions are for your benefit only; do not submit your answers for grading. You can check your responses to these study questions on the Answers to Study Questions page. Return to top of page

Laboratory Exercises
Laboratory Manual in Physical Geology Question 6 on page 106 Question 5 on page 125 Laboratory exercises are for your benefit only; do not submit your answers for grading. You can check your responses to these laboratory exercises on the Answers to Laboratory

Exercises page. Return to top of page

Progress Evaluation
The material in Lesson 5 will be covered in the Lesson 6 Progress Evaluation. Utilities: Submit this Lesson/Check your feedback Preview a Progress Evaluation Review the Progress Evaluation Instructions Request an Exam Check Your Grades Return to top of page Printer-Friendly | Home
© 2005. 2011 University of Missouri Editor: Kari Bethel Web Design: Lane Barnholtz Multimedia Design: James Barnes and Julie Moriarity Questions/Comments? Contact Mizzou Online The University of Missouri is an Equal Opportunity/ADA Institution.

Mizzou Online Geological Sciences 1100: Principles of Geology with Laboratory Goals

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Lesson 6: Geological Time
Purpose
This lesson will help you understand how geological time is measured. Return to top of page

Objectives
After completing this lesson, you should be able to 6.1: describe the use of stratigraphy, uncomformities, and fossil correlations as methods of dating Earth. 6.2: describe the geologic time scale. 6.3: discuss radioactive dating as a means of dating rocks. Return to top of page

Reading Assignment
Exploring Earth Chapter 6: pages 151–175 See the WWW sites page for additional information on topics in this lesson. Return to top of page

Laboratory Assignment

Laboratory Manual in Physical Geology Laboratory Eight: pages 128–139 Return to top of page

Commentary Index
Geological Time Unconformities Absolute Time

Commentary Geological Time
The modern framework of geological time has been developed over the past 200 years, as discussed in Chapter 6 of Exploring Earth. Four general principles or laws are used to determine geological time. These principles, first developed by Nicolaus Steno (a Danish geologist), were published in 1669. Law of Superposition—states that beds of sedimentary and extrusive igneous rocks are found in order of their age, with older beds on the bottom and younger beds on top. The only time that this law does not hold is when rocks have been overturned by tectonic processes. Law of Original Horizontality—states that sedimentary beds are originally deposited in a nearly horizontal position. Any tilt in sedimentary bedding is the result of later structural deformation. Law of Included Fragments—states that if fragments from one rock unit are included in another rock unit, the latter rock unit is younger (see Figure 6.4 in Exploring Earth) Law of Cross-Cutting Relations—states that if a rock unit (for instance a dike) cuts across another rock unit, the latter rock unit is older (see Figure 6.4 in Exploring Earth)

By using these four simple laws, the order of events as documented by many (but not all) rock records can be told.
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Unconformities
Using the above principles, it was possible for geologists of the eighteenth and nineteenth centuries to determine the order in which geological events occurred. Another leap in knowledge came in the late 1700s when James Hutton, a Scottish geologist, determined that there were gaps in the geological record, which he called unconformities. Geologists now classify unconformities into three general categories: disconformities, angular unconformities, and nonconformities. Disconformities—develop when flat-lying sedimentary beds are eroded and another series of flat-lying sedimentary beds are deposited on top of the erosional surface or following periods of non-deposition. The erosional surface represents the time that elapsed between deposition of the underlying and overlying beds. This can be millions of years. Angular unconformities—develop when beds undergo tilting or folding prior to erosion and subsequent deposition of flat-lying beds (see Figure 6.5 on page 157 in Exploring Earth) Nonconformities—develop when crystalline metamorphic or intrusive igneous rocks undergo erosion and are then overlain by sedimentary rocks. It is important to understand that the igneous rocks are older than the overlying sedimentary rocks. Where igneous rocks have intruded sedimentary rocks, the overlying sedimentary rocks would be older, and a metamorphic aureole rather than an erosional surface would be at the contact.

The Grand Canyon is a spectacular example of layered beds of rock. Source: www.ClipArt.com

Steno also determined that sedimentary rocks of the same age had the same fossils. But the first person to really use fossils to determine the relative ages of strata was William “Strata” Smith, an English civil engineer, who published the first geological map of England and Wales in 1815. Smith developed the principle of faunal succession (fossil correlation), which states that fossil organisms underwent progressive development throughout geological time. These changes in fossils can be used to determine the relative ages of the rocks in which they occur.
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Absolute Time
All of the principles mentioned thus far give only the relative ages of rocks. In other words, we can use the principles discussed to determine which rocks are older than others and the order in which they were deposited, intruded, structurally deformed, and/or metamorphosed. None of these methods is used to determine how old the rocks are in number of years. This led to a great controversy about the age of Earth in the nineteenth century. Using the principles discussed in this lesson, geologists have constructed the time scale (see Figure 6.15 on page 169 in Exploring Earth). Dates were later determined using radiometric methods as discussed on pages 168–174 in Exploring Earth. The precise age of rocks and geological events is referred to as absolute time. A more detailed study of geological time, including the evolution of life on Earth, is included in introductory courses in historical geology. You can enroll in a historical geology

course at almost any university or college.
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Study Questions
Exploring Earth Questions 1, 2, and 4–6 on page 175 Study questions are for your benefit only; do not submit your answers for grading. You can check your responses to these study questions on the Answers to Study Questions page. Return to top of page

Laboratory Exercises
Laboratory Manual in Physical Geology Question 1 on page 130 Questions 4 and 6 on page 135 Question 8 on page 139 Laboratory exercises are for your benefit only; do not submit your answers for grading. You can check your responses to these laboratory exercises on the Answers to Laboratory Exercises page. Return to top of page

Progress Evaluation
You should now complete the Lesson 6 Progress Evaluation. It covers the material in Lesson 5 and Lesson 6. The progress evaluation consists of 15 multiple-choice questions worth 1 point each for a total possible of 15 points. Remember to mark your answers on a printed copy of the progress evaluation preview so you will have a record of them. (Select "Lesson 6" to preview this progress evaluation.) When you have finished filling in and checking your answers on a copy of the progress

evaluation preview, click on Submit a Lesson for an actual lesson submission page. Note: Progress evaluations must be submitted in sequence, and you may submit no more than three progress evaluations in a seven-day period. You should keep the printed copy of each progress evaluation preview with your answers marked so you will have a record of them. Remember: After you have completed the Lesson 6 Progress Evaluation and received your score, you should prepare to take the midterm examination. Utilities: Submit this Lesson/Check your feedback Preview a Progress Evaluation Review the Progress Evaluation Instructions Request an Exam Check Your Grades Return to top of page Printer-Friendly | Home
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