Instructor s Manual Chapter 10. Deformation, Mountain Building, and the Continents

Transcription

1 Chapter 10 Table of Contents Chapter Outline Learning Outcomes Chapter Summary Lecture Suggestions Enrichment Topics Common Misconceptions Consider This Key Terms Internet Sites, Videos, Software, and Demonstration Aids Chapter Outline Introduction LO1 Rock Deformation: How Does It Occur? LO2 Strike and Dip: The Orientation of Deformed Rock Layers LO3 Deformation and Geologic Structures LO4 Deformation and the Origin of Mountains LO5 Earth s Continental Crust Learning Outcomes After reading this unit, the students should be able to do the following: LO1 Explain how rock deformation occurs LO2 Understand strike and dip the orientation of deformed rock layers LO3 Identify the types of deformation and geologic structures LO4 Understand deformation and the origin of mountains LO5 Describe Earth s continental crust Chapter Summary Folded and fractured rocks have been deformed or strained by applied stresses. Stress is compression, tension, or shear. Elastic strain is not permanent, but plastic strain and fracture are, meaning that rocks do not return to their original shape or volume when the deforming forces are removed. Strike and dip are used to define the orientation of deformed rock layers. This same concept applies to other planar features, such as fault planes. 77

2 Anticlines and synclines are up- and down-arched folds, respectively. They are identified by strike and dip of the folded rocks and the relative ages of rocks in these folds. Domes and basins are the circular to oval equivalents of anticlines and synclines, but they are commonly much larger structures. The two structures that result from fracture are joints and faults. Joints may open up, but they show no movement parallel with the fracture surface, whereas faults do show movement parallel with the fracture surface. Joints are very common and form in response to compression, tension, and shear. On dip-slip faults, all movement is up or down the dip of the fault. If the hanging wall moves relatively down, it is a normal fault, but if the hanging wall moves up, it is a reverse fault. Normal faults result from tension; reverse faults result from compression. In strike-slip faults, all movement is along the strike of the fault. These faults are either right-lateral or left-lateral, depending on the apparent direction of offset of one block relative to the other. Oblique-slip faults show components of both dip-slip and strike-slip movement. A variety of processes account for the origin of mountains. Some involve little or no deformation, but the large mountain systems on the continents resulted from deformation at convergent plate boundaries. 78

3 A volcanic island arc, deformation, igneous activity, and metamorphism characterize orogenies at oceanic oceanic plate boundaries, whereas orogeny at an oceanic continental plate boundary is a result of subduction. Some mountain systems are within continents far from a present-day plate boundary. These mountains formed when two continental plates collided and became sutured. Geologists now realize that orogenies also involve collisions of terranes with continents. Continental crust is characterized as granitic, and it is much thicker and less dense than oceanic crust that is composed of basalt and gabbro. According to the principle of isostasy, Earth s crust floats in equilibrium in the denser mantle below. Continental crust stands higher than oceanic crust because it is thicker and less dense. Lecture Suggestions 1. A large sample of silly putty can be used to illustrate how a given material can respond differently to different stresses, or stresses applied at different rates. If a ball of the material is dropped from a short distance onto a table top, it will bounce, and although a small amount of the stress, resulting from the impact of the ball with the surface, is accommodated in a plastic manner (leaving a flat spot on the ball), most of the stress has been accommodated in an elastic fashion. The material can then be deformed plastically by squeezing it (compression) or stretching it out (tension), or simply by letting gravity pull on the ball as it sits on the table. Fracture can result, of course, if the material is pulled (by tension or shearing) too fast. Point out that even though silly putty is not a rock, rocks can respond in similar fashion under the right circumstances. 2. Strike and dip are difficult to visualize and need to be demonstrated in class. To illustrate strike, as well as apparent and true dip, stack some books and prop them at an angle. Let the binding s trace be the strike and the cover be the bed s dip surface. Take a pencil and orient it on the dip surface (cover), so it is parallel to strike (the binding). Now, slowly rotate the pencil, so its eraser end remains fixed on the dip surface and its point moves away from the dip surface in a horizontal plane, until it lies perpendicular to the strike (binding edge). Notice that the distance between the pencil point and the dip surface (book cover) increases from zero (when the pencil is parallel to strike) to a maximum value, when it is perpendicular to the strike. The angle between the eraser and the pencil s projection (or its shadow from an overhead light) on the cover will increase from zero to some maximum value. These are dip or inclination angles, and the maximum value is that of the true dip. 79

4 3. Students are likely to remember hanging wall and footwall if they are told that the hanging wall is characterized as that wall from which a geologist could only hang (as opposed to walk) and the footwall as that wall that would be beneath a geologist s feet. 4. Once they can recognize the hanging wall and footwall, it may help the students remember the types of dip-slip faults from the acronym FUN. This stands for Footwall (moved) Up, = Normal fault. Obviously, the other type is HUR ( Hanging Wall Up = Reverse ). 5. Ask students to compare and contrast the types of geologic features, structures, and activity that occur on continental continental, continental oceanic, and oceanic oceanic convergent plate boundaries. Also, compare and contrast the geologic features, structures, and activity along divergent, convergent, and transform plate boundaries. 6. The terms and structures of anticlines and synclines can be more readily understood if it is noted that cline means slope and anti means opposite or away from, while syn means together or toward. 7. A three-layered peanut butter and jelly sandwich, cut so as to illustrate a fault, can be used to demonstrate the types of forces and resulting structures that form along convergent, divergent, and transform boundaries. Enrichment Topics Topic 1. Devastating Indian Earthquakes. Plate tectonics is responsible for earthquakes in India in which tens of thousands of people die. In January 2001, more than 10,000 people were killed in an intraplate earthquake on the Indian subcontinent. The Indian subcontinent is driving ever northward into Asia, so even where the plates don t meet, weak spots are prone to earthquakes. India has suffered five such earthquakes since 1965; the January 2001 quake was on an ancient rift that originated when India separated from Gondwana 150 million years ago. At even more risk is the northeastern edge of the plate, where it is colliding with Asia and the strain is building very fast. ScienceNOW, January 29, Topic 2. Birth of a Mountain Range, But When? There are two camps in the debate on when the Sierra Nevada Mountains rose: One says that it was between 60 and 40 million years ago when a subducted ocean plate slid beneath the continent and raised it up. The other thinks that it occurred between 5 and 3 million years ago when a large chunk of crust broke off from beneath the continent, melted, became buoyant, and caused the range to rise. One way of trying to determine when the range rose is to look at hydrogen isotopes of ancient raindrops to distinguish the height of the cloud from which the drop fell. This technique favors the first explanation, that the Sierra Nevada Mountains rose about 50 million years ago, but the results are still controversial. ScienceNOW, July 6,

5 Common Misconceptions Misconception: Rocks are solid, permanent, unmovable, and undeformable, as shown in such expressions as solid/hard as a rock, like a rock, I am a rock, etc. Fact: Rocks respond to stresses just as other solid objects do, and given sufficient stress conditions, which may include a long enough interval of time, rocks will deform like a plastic, taking on new orientations, shapes, and dimensions. Consider This 1. What will happen to the western coast of California in 30 or 40 million years as the result of movements along the San Andreas Fault Zone? 2. Locate the shield areas of each of the continents. 3. If orogenesis is typically associated with the active margins of continents and the Rocky Mountains are relatively young and not the product of suturing of two continents, how can the mid-continent location of the Rocky Mountains be explained by plate tectonic theory? 4. What type of forces and geologic structures dominate the northern Rocky Mountains? Which dominate the middle Rocky Mountains? Key Terms anticline footwall block principle of isostasy basin fracture reverse fault compression geologic structure shear stress continental accretion gravity anomaly strain deformation hanging wall block stress dip isostatic rebound strike dip-slip fault joint strike-slip fault dome monocline syncline elastic strain normal fault tension fault oblique-slip fault terrane fault plane orogeny thrust fault fold plastic strain Internet Sites, Videos, Software, and Demonstration Aids Internet Sites 1. Himalayas: Where the Earth Meets Sky: library.thinkquest.org/10131/ The formation of the Himalaya Mountains. 81

6 Videos 1. Natural Landscapes of North America. Insight Media (1999, 21 min.) The eight major geologic regions of North America with descriptions of the natural forces that created and continue to shape them. 2. Mountain Building and Continents. Insight Media (1999, 18 min.) The evolution of the continents and major mountain belts. 3. Mountains and Mountain-Building Processes. Insight Media (2000, 23 min.) Mountain-building processes in several mountain ranges around the world depicted by graphics and live-action footage. 4. Earth Revealed. Annenberg Media: (1992, 30 min., free video): #7: Mountain Building. Animations reveal how mountains are built and how they erode with emphasis on plate tectonics, the rock cycle, and other processes. 5. Before the Mountains. AAPG Bookstore DVD (1987, 29 min.) The sedimentary rocks that came before the Rocky Mountains. 6. Birth of the Rockies. AAPG Bookstore DVD (1987, 28 min.) The thrust sheets that form the Rockies. Slides 1. GeoPhoto Publishing, 35 mm transparencies or digital images: Crustal Deformation 2. Educational Images Slide Sets: Sediments, Faults, Unconformities 82