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Exam Block #1 • Chapter 1 – Introduction to Geology • Chapter 2 – Plate Tectonics How to study for this class: 1. Read the chapter and answer the Review Questions. As you read, follow along with the Chapter Outline (download from our class site below) and these PowerPoint Notes. 2. Review each chapter using the GEODe: Earth CD-ROM in your book. 3. Go online and take the Concept Quizzes and Chapter Test for each chapter at: www.prenhall.com/tarbuck 4. Check your grades online at: http://pages.sbcglobal.net/solanogeo/index.htm BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 1 of 24 TAKE ADDITIONAL NOTES HERE: BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 2 of 24 Chapter 1 – Introduction to Geology 1 Geology – From the Greek geo “Earth” and logos “discourse”. Physical Geology – focuses on the materials and processes that operate on the Earth. Historical Geology – understanding of the origin of the Earth and life through time. What causes earthquakes? How do mountains form? What controls stream flow? How did the Ice Ages start? What are the basic minerals of the Earth? These questions and many other topics will be covered in this class! Charakusa Valley, Pakistan The Science of Geology 3 Many of the problems and issues addressed by geology are of practical value to people. Natural disasters are simply natural processes when people live where these processes occur. Earth is now gaining about 100 million people each year. Resources, such as water, soil, metals, non-metals, and energy (oil, gas, coal, geothermal) all come from the Earth. We will cover many aspects of our relationship with the physical environment. Historical Notes About Geology 2 The Science of Geology 5 Aristotle believed that motion was primarily determined by the nature of the substance that was moving. The four primary elements were considered to be: air, water, earth, and fire. So for example, smoke rose to be closer to a similar primary element: air. Objects fell towards their primary element: earth. Heavier objects would be even more like the earth and fall faster. BLOCK EXAM #1 - CHAPTERS 1 & 2 Historical Notes About Geology 4 Aristotle – (384-322 B.C.) Greek philosopher who made observations about the natural world that were influential throughout the Middle Ages (476-1450). Q: What was Aristotle explaining here?: “…a great many fishes live in the Earth motionless and are found when excavations are made.” A: His explanations were not based on observations or experiments, but were arbitrary pronouncements. He was regarded as the head and chief of all philosophers and his opinion on any subject was authoritative and final. Historical Notes About Geology 6 The father of experimental science is considered to be Galileo Galilei (1564-1642). The famous experiment at the Leaning Tower of Pisa in Italy is attributed to him, although there is no record in his journals. Aristotle believed that heavier objects fell at a faster rate. Is this true? Galileo correctly showed that objects fall at the same rate, regardless of their mass. Page 3 of 24 Historical Notes About Geology 7 The experiment consisted of dropping a heavy cannonball and a lighter wooden ball. The cannonball did strike the ground first, but the time difference was very slight. What would slow down the lighter ball? Galileo correctly attributed the slight time difference to air friction. However, since the heavier ball did reach the ground first (however slight), scholars viewing the experiment walked away still believing Aristotle’s ideas were correct. 9 Catastrophism Ussher’s work earned widespread acceptance among scientific and religious leaders. During the 1600 and 1700’s, catastrophism was popular. The idea was that the Earth’s landscape was shaped by great catastrophes. Mountains and canyons were formed by sudden disasters produced by unknowable forces that no longer operate (such as Noah’s Flood). The philosophy was to fit the rates of Earth processes to the then-current ideas on the age of the Earth. 11 Uniformitarianism 8 Idea of a Young Earth In 1650, James Ussher He also calculated: Adam and Eve were driven from Paradise on Monday November 10, 4004 BC, and that the ark touched down on Mt. Ararat on Wed. May 5, 2348 BC. (1580-1655) – Archbishop of Ireland, calculated that the Earth was created on Sunday October 23, 4004 BC. After counting the Patriarchal periods in the bible and elsewhere, Ussher came to this conclusion. This date was in fact printed in the margins of some editions of the King James Version of the Bible. 10 Uniformitarianism James Hutton (1726-1797) – is considered the father of modern geology. He published: ‘Theory of the Earth’ in 1784 in which he showed that the earth in fact has had a long history - as he put it, "we find no vestige of a beginning, no prospect of an end." The idea was that the physical and chemical laws that operate today have also operated in the past is know as uniformitarianism, which is commonly stated as: “The present is the key to the past.” Uniformitarianism + Catastrophism 12 While uniformitarianism is the d = 3000 m Grand Tetons, Wyoming Q: Typical erosion rates are 3 cm/1000 yrs. How long would it take to erode a 3000 meter high mountain to sea level? [Hint: rate = distance / time] A: Rearrange: t = d / r = BLOCK EXAM #1 - CHAPTERS 1 & 2 general rule in geology, there is evidence for exceptional catastrophic events, such as the meteorite impact that killed the dinosaurs. Such events cause extinctions that are observed in the fossil record. Therefore, to completely explain Earth’s geologic history, you need both ideas: 99.9% uniformitarianism punctuated by 0.1% catastrophes = the Geologic Record! Page 4 of 24 ✓REVIEW QUESTIONS 13 1. Geology is traditionally divided into two broad areas – name and describe these two subdivisions. During the 1800’s, the geologic time scale was developed using the principles of relative dating. Relative dating means that events are placed in their proper sequence or order without knowing their ages in years. This was done using two simple principles: Law of superposition – each horizontal layer is younger than the one below. Principle of fossil succession – any time period can be recognized by its fossil content. In the early 1900’s, radiometric dating was developed to assign fairly accurate dates to events in Earth history. What is the age of the Earth? 2. Briefly describe Aristotle’s influence on the science of geology. 3. How did the proponents of catastrophism perceive the age of Earth? 4. Describe uniformitarianism. How did the advocates of this idea view the age of Earth? 15 Law of Superposition 14 Geologic Time Principle of Fossil Succession 16 Extinct amphibian from Permian age (248-290 million years) rocks in Texas (left). Younger Fossil fish of Eocene (33-54 million years) age from Wyoming (right). Older The Geologic Time Scale 17 The Geologic Time Scale – was put together using the principles of relative dating; dates were added later when radiometric dating was refined. We will study the geologic time scale in detail in Chapter 9. BLOCK EXAM #1 - CHAPTERS 1 & 2 Nature of Scientific Inquiry 18 How do you know that everything in your book is true?! All science is based on the assumption that the natural world behaves in a consistent and predictable manner. Hypothesis – a tentative, but testable explanation for a natural phenomenon. Theory – a well-tested and widely accepted view that scientist agree best explains the observed facts. Comprehensive theories are often called paradigms. Name some theories. Q: “The Sun rotates about the Earth.” Is this a hypothesis or a theory? Why or why not? Page 5 of 24 ✓REVIEW QUESTIONS 19 Nature of Scientific Inquiry 20 5. What is the age of the Earth? Scientific investigations involve the following steps: 6. The geologic time scale was established without the aid of radiometric dating. What principles were used to develop the time scale? The collection of scientific facts through observation and measurement. The development of one or more working hypotheses or models to explain these facts. Development of observations and experiments to test the hypotheses, and The acceptance, modification, or rejection of the model based on extensive testing. 7. How is a scientific hypothesis different from a scientific theory? Do Glaciers Move? Switzerland, 1874 21 A View of Earth 22 The four spheres of the Earth. Meteorology Biology Oceanography Geology GEOSPHERE Give examples of how the four sphere interact with one another. Hydrosphere & Atmosphere 23 Hydrosphere: The Earth is the blue planet. Water more than anything else makes Earth unique. The oceans cover 71% of the Earth’s surface and account for 97% of Earth’s water. 3% is fresh water found in streams, lakes, and glaciers. Running water and groundwater are responsible for erosion that sculpts the planet’s surface. Atmosphere: This blanket of air is very thin compared to Earth’s diameter. It provides life-giving gases and well as protection from the dangerous radiation from the Sun. The energy exchanges that occur between the atmosphere and the Earth’s surface produces weather and erosion. BLOCK EXAM #1 - CHAPTERS 1 & 2 Biosphere & Geosphere 24 Biosphere: The Biosphere includes all life on Earth. It extends from the ocean floor upward for several kilometers into the atmosphere. The Biosphere influences the other three spheres, which would be very different without life. Geosphere: This includes all of the solid Earth, from the center to the surface. We will not only discuss the surface features, but also the structure of the Earth’s interior. Page 6 of 24 25 Earth As A System A system is any size group of interacting parts that form a complex whole. Examples: a city’s bus system; your car’s cooling system; our solar system. A closed system is one that is self-contained – no matter goes in and out of the system – like your car’s cooling system. Energy (heat) moves in and out of the system, but the matter (water) does not leave it. By contrast, most natural systems are open systems - one in which matter and energy flow into and out of the system – like a hurricane. Water vapor (matter) moves into and out of the system. Global Warming Feedback Systems Mechanism 27 Most natural systems have mechanisms that tend to enhance change, or other mechanisms that tend to resist change and thus stabilize the system. Negative feedback mechanisms work to maintain the system as it is, or maintain the status quo. For example, when we get too hot, we perspire to cool down – this stabilizes our body temperature. Positive feedback mechanisms work to enhance or drive change. For example, the warmer the ocean water, the greater the intensity of a hurricane. Most of Earth’s systems, such as the climate system, contain a wide variety of negative and positive feedback mechanisms. Movie: The Day After Tomorrow Positive Negative 29 Ice Age! 9 - 18° C Warmer X 28 What caused the climate change?! Higher temperatures cause the highly reflective snow to melt - replaced by dark soils and plants that absorb sunlight. Higher temperatures cause greater evaporation and formation of clouds. Higher temperatures promote the growth of vegetation that remove carbon dioxide from the air. Man cutting and clearing the tropical rain forest. Man burning fossil fuels putting carbon into the atmosphere. Global Warming May Trigger An Ice Age! 26 Earth As A System The Earth System 30 Cycles in the Earth System: 1) Hydrologic Cycle – circulation of water, powered by the Sun. 2) Rock Cycle – changing of one rock type to another, powered by Earth’s internal heat. The two cycles are linked and interact with one another. BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 7 of 24 Early Evolution of Earth – Big Bang • • • 31 A. About 5 billion years ago, a huge rotating cloud of dust and gases (solar nebula) begins to contract. B. Most of material is swept to center, to form the Sun. Some material remains in the flattened disk. C. Solid particles begin to form. D. In time, most of the debris was collected into the nine planets or was swept into space by the solar wind. E. Final accretion of material into the eight planets. About 13.7 billion years ago, all mass was at one point and exploded to form the present day Universe (Big Bang Theory). Before our Sun formed, there was another larger Sun in this sector of space. Stars fuse lighter elements into heavier ones and in the process release energy. Hydrogen fuses to form Helium, and then Helium fuses into heavier elements up to Iron (Atomic No. 26). At this point, Iron cannot fuse to heavier elements and the star collapses and may explode in a supernova, ejecting gases and heavier elements into space. Some of this material then slowly collected to form our solar system about 5 billion years ago. The Nebular Hypothesis 32 The Nebular Hypothesis 33 Moon Formation – The Ultimate Impact! 34 The Moon should contain more iron for its size – if it formed in orbit around the Earth (Nebular Hypothesis). Where is Moon’s missing iron? In Earth’s core! ✓REVIEW QUESTIONS 35 9. How is an open system different from a closed system? 9. Contrast positive and negative feedback mechanism. 10. What are the two sources of energy for the Earth System? 11. Briefly describe the events that led to the formation of the solar system. BLOCK EXAM #1 - CHAPTERS 1 & 2 Earth’s Layered Structure 36 • Due to the heat of impacts and radioactive decay, the early Earth became hot enough for iron and nickel to melt and sink to the center of the planet. The lighter elements rose to create the crust and mantle. • This created the three compositional layers of the earth: crust, mantle and core. Page 8 of 24 Earth’s Compositional Layers 37 Two Types of Crust: 1. Oceanic crust – is about 7 km thick and composed of the dark igneous rock basalt. It is relatively young (less than 160 million years) and has density of 3.0 g/cm3. 2. Continental crust – is generally 35-70 km thick and composed of granite and may be 3.9 billion years old and has density of 2.7 g/cm3. Mantle & Core: • The mantle is solid and the upper mantle is composed of peridotite, with a density of 3.3 g/cm3. With increasing depth, peridotite compacts into more dense minerals. • The core is thought to be an iron-nickel alloy with minor amounts of other elements and has an average density of 11 g/cm3. 39 Earth’s Physical Layers • With depth in the Earth, there is a gradual increase in temperature, pressure, and density. • The increase in temperature and pressure with depth affects the physical properties and hence the mechanical behavior of the Earth materials. • How does a pressure cooker work? • • Without a lid, water boils at 100°C (212°F). Water expands greatly in volume as it goes from a liquid to a gas (steam). With a lid, the increased pressure keeps the water liquid at a higher temperature. At 15psi, water temp. is now 125°C (257°F). Earth’s Physical Layers 41 • Lithosphere: is the cool outer layer of the earth that is rigid and broken into tectonic plates. Thickness averages about 100 km, but may be up to 250 km beneath continents. • Asthenosphere: is beneath the lithosphere and to a depth of 660 km. It is soft (ductile), comparatively weak layer and may have some partial melting of rock near its top. The lithosphere ‘floats’ on the asthenosphere and is free to move about: Plate Tectonics! BLOCK EXAM #1 - CHAPTERS 1 & 2 38 Earth’s Compositional Layers Earth’s Physical Layers 3.0 2.7 g/cm3 3.3 11 40 • The pressure increase with depth tends to increase rock strength. This is because, when a solid rock melts, the volume must increase, and the pressure keeps this from occurring. • Even though the temperature is increasing with depth, the pressure affect wins – in all parts of the earth – except one: the outer core. • Thus, depending on the physical environment (temperature and pressure), the Earth material may behave like a brittle solid, deform in a puttylike manner, or even melt and become liquid. • Earth’s five physical layers: lithosphere, asthenosphere, mesosphere, outer and inner core. Earth’s Physical Layers 42 • Mesosphere: is the lower mantle (6602900 km depth) and rocks gradually strengthen with depth because pressure increases. • Outer core: is a liquid layer of ironnickel alloy which convects generating the Earth’s magnetic • Inner core: is a solid layer field. of iron-nickel alloy. Page 9 of 24 Earth’s Internal Layers 43 • The two divisions of Earth’s surface are: 1. Continents: are less dense, thicker and composed of granitic rocks. The thicker and less dense continental rocks float on top of the mantle to a higher level. 2. Ocean Basins: are more dense and composed of basaltic rocks. The Face of Earth - Continents 45 • The two divisions of the continents are: 1. Mountain Belts: are less dense and composed of granitic rocks (old and young belts). 2. Stable Interior: Also called a craton – composed of shields and stable platforms. 44 The Face of Earth Granite Basalt ✓REVIEW QUESTIONS 46 13. Describe Earth’s compositional divisions. 14. Contrast the asthenosphere and lithosphere. 15. Describe the general distribution of Earth’s youngest mountains. The Face of Earth – Ocean Floor 47 • The three divisions of the ocean floor are: 1. Continental Margins: are adjacent to continents and include the continental shelf, the continental slope (the boundary between the continents and the ocean basins), and continental rise. See Figure 13.7, Page 355. BLOCK EXAM #1 - CHAPTERS 1 & 2 The Face of Earth – Ocean Floor 48 • The three divisions of the ocean floor are: 2. Deep-Ocean Basins: contain abyssal plains (deep, flat regions), the deep-ocean trenches (deep, narrow features near mountains), and seamounts (submerged volcanic structures). See Figure 13.9, Page 357. Page 10 of 24 The Face of Earth – Ocean Floor 49 • The three divisions of the ocean floor are: 3. Oceanic Ridges: are the most prominent feature on the Earth with broad, elevated regions over 43,000 miles long; made of igneous rocks that has been uplifted and fractured. Igneous Rocks Igneous: form when molten rock, called magma, cools and solidifies. When magma cools at depth, it cools slowly and forms a course-grained (1) Intrusive igneous rock – a good example is granite – only exposed after uplift and erosion. Granite in Yosemite. 51 Igneous: When lava cools at the Earth’s surface, it cools quickly and forms a fine-grained (2) Extrusive igneous rock – a good example is basalt (like in Hawaii). Sedimentary Rocks 50 Igneous Rocks – 2 Types 53 Sedimentary: (2) Chemical sedimentary rocks form when material dissolved in water precipitates to form a solid rock – like limestone. Sedimentary Rocks - 2 Types Sedimentary: Sediments are derived from weathering of preexisting rocks and are lithified into (1) Detrital sedimentary rocks – like sandstone (made of medium size particles). Shale is made of fine clay-size particles). Metamorphic Rocks – 2 Types 54 Metamorphic: Formed by “changing” preexisting igneous, sedimentary or other metamorphic rocks; driving forces are increased heat and pressure; minerals align into a (1) Foliated texture – example is a gneiss – whose parent rock was a granite. Granite BLOCK EXAM #1 - CHAPTERS 1 & 2 52 Gneiss Page 11 of 24 55 Metamorphic Rocks Metamorphic: when crystals grow larger without alignment, a (2) Nonfoliated texture is produced – example is marble – whose parent rock was limestone. Limestone Marble The Rock Cycle 56 The Rock Cycle • The Rock Cycle: One of Earth’s subsystems. • It is the loop that involves the processes by which one rock changes to another. • The Basic Cycle: Magma cools over time to produce igneous rocks; these are uplifted and exposed and weather into sediments, which are transported by wind, water and ice and deposited; they are lithified into sedimentary rocks; mountain building processes can deeply bury the rock and heat and pressure produce metamorphic rocks; these can melt into magma – thus completing the rock cycle. ✓REVIEW QUESTIONS 57 58 17. List the three major topographic units of the ocean floor. 18. Name each of the rocks described below: ✓REVIEW QUESTIONS • Light-colored, coarse-grained intrusive rock = • Detrital rock rich in fine clay-size particles = • A fine-grained black rock that makes up the oceanic crust = • Nonfoliated rock, for which limestone is its parent rock = 59 19. Indicate whether it is associated with igneous, sedimentary, or metamorphic rocks: • May be intrusive or extrusive = • Lithified by compaction and cementation = • Sandstone is an example = • Some members of this group are foliated = • This group is divided into detrital and chemical categories = • Gneiss is a member of this group = 20. Explain: “One rock is the raw material for another.” BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 12 of 24 Chapter 2 – Plate Tectonics 1 2 Continental Drift This Chapter: Transformation of an old idea (continental drift) Æ modern (plate tectonics in 1968). Q: What year did man land on moon? Alfred Wegener (1915) – German geophysicist was first to present the continental drift idea with evidence. But there was much resistance to his ideas. His list of evidence included: 1. Fit of Continents – The shorelines of South America & Africa fit together like a jigsaw puzzle. CON: “…but the fit is not that good.” K7, Pakistan, Himalayas 3 1) Fit of Continents 2. Fossil Evidence – Existence of identical fossils on widely separated landmasses. CON: “…but there were land bridges.” Today We Know: The best fit is not along the shorelines, but along the continental slopes. 2) Fossil Evidence 4 2) Fossil Evidence Mesosaurus – aquatic reptile with a limited distribution. • Glossopteris – a fossil fern found on several continents. • 5 Today We Know: No land bridges have been found in the oceans. BLOCK EXAM #1 - CHAPTERS 1 & 2 3) Rock Type & Structural Similarities 6 3. Reassembling the continents connects mountain chains into a single linear belt [Appalachians in North America & Caledonians in Northern Europe]. Rocks should match in age and type. Today We Know: ages match from radiometric dating. Page 13 of 24 4) Paleoclimatic Evidence 7 Pangaea – 200 Million Years Ago 8 4. There is glacial debris on many continents located at present-day tropical latitudes, like Africa, India, and Australia. Glacial striations on bedrock indicate that the direction of ice movement was from sea to land – opposite of what you would expect. This could only be explained if the continents had been together during glaciation. Wegener also correctly positioned southern Pangaea near the south pole, since the rock record shows large tropical swaps must have existed in Northern Hemisphere that eventually became the major coal fields of the eastern USA, Europe, and Siberia. 4) Paleoclimatic Evidence 9 Evidence of glaciation of Pangaea. 4) Paleoclimatic Evidence 10 Location of swaps that eventually formed coal deposits. Striations: ice moved to southwest. Breakup of Pangaea 11 12 Plate Motions Through Time BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 14 of 24 Objections to Continental Drift 13 14 Alfred Wegener The main objection to continental drift was Wegener’s inability to identify a mechanism that was capable of moving continents. Wegener proposed that the tidal forces of the moon moved the continents. However, other scientists correctly countered that the magnitude of this force would halt the rotation of the Earth! Another problem was that Wegener incorrectly suggested that the continents broke through the oceanic crust, much like an ice breaker cuts through ice. Wegener died in 1930 with few believing his hypothesis. ✓REVIEW QUESTIONS 15 1. Who is credited with developing the continental drift hypothesis? 2. What was probably the first evidence that led some to suspect that the continents were once connected? 16 6. Early in the 20th century, what was the prevailing view of how land animals migrated across vast expanses of ocean? 3. What was Pangaea? 4. List evidence Wegener gathered to support his hypothesis. Paleomagnetism ✓REVIEW QUESTIONS 5. Explain why the discovery of Mesosaurus in both South America and Africa, but nowhere else, supports the continental drift hypothesis. 17 • Little was done from 1930-1950’s on continental drift. • The breakthrough came by exploration of the seafloor using paleomagnetism or fossil magnetism. • When lava flows cool below about 585° C (Curie Point), the magnetic minerals align to the existing magnetic field. • Once cooled, the magnetism is “frozen” in place and point to the existing magnetic poles at the time of their cooling. This is know as paleomagnetism. • The Earth’s magnetic field not only has direction, but also dip or inclination. BLOCK EXAM #1 - CHAPTERS 1 & 2 7. How did Wegener account of glaciers in the southern landmasses, while areas in North America, Europe, and Siberia supported lush tropical swamps? Paleomagnetism 18 Earth’s magnetic field produces lines of force like a bar magnet (but it is not produced in this way.) Note: true north and magnetic north are not the same. Page 15 of 24 19 Paleomagnetism Å Above the Curie Point, the magnetic directions are randomly aligned. The Earth’s magnetic field not only has direction, but also dip or inclination. Below the Curie Point,Æ the magnetic directions are aligned with the Earth’s magnetic field at the time of cooling. Locked in are the direction and dip of the magnetic field. 21 Paleomagnetism The direction and dip determine the location of the Paleopole. Apparent Polar Wandering X 20 Paleomagnetism 23 Apparent Polar Wandering 22 • S. K. Runcorn discovered in the 1950’s that the apparent position of the paleopoles had wandered from the present position of the north magnetic pole. • The pattern of the “polar-wandering” paths were similar for lavas in North America and Europe, except they were separated by about 24º longitude – which is the width of the Atlantic Ocean. • If North America and Europe were brought back together, the paths matched. This rekindled interest in continental drift. • However, because of the spinning of the Earth, scientists believe that the magnetic pole does not wander very far from the geographic poles. So a better explanation is that the continents have moved. Apparent Polar Wandering 24 ✔ BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 16 of 24 A Scientific Revolution Begins 25 • During the 1950’s and 1960’s, extensive mapping of the ocean floor took place. This was the last missing piece to the puzzle of Plate Tectonics. • The oceanic ridge system was found to circle the globe, have high heat flow and volcanism, and indications of tensional stresses (divergence). • In other parts of the oceans, scientist were discovering and locating the many deep earthquakes associated with trenches. • Dredging of the oceanic crust found no rocks older than 160 million years. How could it be so young? Seafloor-Spreading Hypothesis 27 Seafloor-Spreading Hypothesis 26 • Harry Hess (early 1960’s) first proposed seafloor spreading by mantle convection. • His model is centered on the activity that happens in the ocean basins. • He proposed that the mantle upwells at ocean ridges and spreads laterally, carrying the seafloor in a conveyor-belt fashion away from the ridge crest. • Newly formed oceanic crust fills the gap. That explains why the oceanic crust is so young. • Trenches are the locations where oceanic crust is drawn back into the Earth’s mantle. • Hess also correctly proposed the continents passively moved by convection – unlike Wegener’s hypothesis of continents plowing through the seafloor. Geomagnetic Reversals 28 • The Earth’s magnetic field was found to have reversed polarity periodically in the past, based on lavas from around the world. • When reversed, the north magnetic pole becomes the south magnetic pole, and vice versa! Polarity reversals occur frequently (in a geologic sense) on average every 500,000 to 700,000 years. • About 171 reversals are known since the end of the Cretaceous period (about 65 million years ago.) • During a reversal, the intensity or strength of the Earth’s magnetic field decreases to almost zero, and then flips polarity, and rebuilds strength in the opposite direction. Geomagnetic Reversals 29 BLOCK EXAM #1 - CHAPTERS 1 & 2 Geomagnetic Reversals 30 Page 17 of 24 Seafloor Spreading 31 Vine & Matthews in 1963 linked reversals to simple patterns of magnetic variations on the ocean floor – thus proving the seafloor spreading hypothesis. J. Tuzo Wilson in 1965 put the last piece of the puzzle together by dividing the Earth’s outer shell in rigid plates – Plate Tectonics was born! Plate Tectonics 33 • Plate Tectonics – The uppermost mantle and crust (lithosphere) behave as a rigid layer (plate) and overlies a weaker region of the mantle (asthenosphere). The lithosphere is effectively detached from the layers below and can move horizontally great distances. • Plate boundaries are of three types: • Divergent – plates move apart: results in production of oceanic lithosphere. • Convergent – plates move together: results in the consumption of oceanic lithosphere. • Transform Fault – plates grind past each other: without gain/loss of lithosphere. Rates of Plate Motion 35 ✓REVIEW QUESTIONS 32 8. Explain how paleomagnetism can be used to establish the latitude of a specific place at some distant time. 9. What is seafloor spreading? Who formulated it? Where is it active today? 10. How did Vine & Matthews related the seafloor spreading hypothesis to magnetic reversals. 34 Plate Tectonics The lithospheric plates interact along their boundaries in three different ways. Divergent Boundaries 36 • Most divergent boundaries are located along crests of oceanic ridges and are constructive plate margins, since new oceanic lithosphere is generated here – also know as spreading centers. • The interconnected oceanic ridge system is the longest topographic feature on Earth, exceeding 43,000 miles in length. The central portion may have a down-faulted structure called a rift valley. • The primary reason the oceanic ridge is elevated is that the newly created oceanic crust is hot and the volume expands. As it moves away from the ridge, the new lithosphere cools and contracts and sinks, creating greater ocean depths away from ridges. BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 18 of 24 Oceanic Ridge 37 The oceanic lithosphere becomes thicker as it moves away from the ridge and cools – ocean depths also increase. 38 Continental Rifting When Pangaea split apart, continents were torn apart by divergence. The process starts as a continental rift and may progress to form a complete oceanic ridge system. Basin & Range Nevada East African Rift Red Sea What makes these a true Divergent Boundary? Atlantic Ocean Divergent Boundaries 39 40 Convergent Boundaries • Since Earth is not growing is size, older portions of the oceanic lithosphere must descend into the mantle along convergent boundaries – destructive plate margins. • The deep-ocean trench is the surface expression of the descending plate. These convergent boundaries are also called subduction zones because the cold oceanic lithosphere is more dense and sinks into the mantle. • There are three types of convergent boundaries: 1. Oceanic-Continental Convergence 2. Oceanic-Oceanic Convergence 3. Continental-Continental Convergence Oceanic-Continental Convergence 41 Oceanic-Continental Convergence 42 • Active Examples: Andes & Cascade Range • Inactive: Sierra Nevada BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 19 of 24 Oceanic-Oceanic Convergence 43 Oceanic-Oceanic Convergence 44 45 Continental-Continental Convergence 46 47 Transform Fault Boundaries 48 • Active Examples: Aleutian, Japan, Philippines, Mariana & Tonga Continental-Continental Convergence No active volcanoes! Why? • Active Examples: Himalayas & Alps • Inactive: Appalachians & Urals. Collision of India and Asia • Where plates slide by one another, there is no production or destruction of lithosphere – the lithosphere is conserved – this know as a transform fault boundary. • Most transform faults join segments of an oceanic ridge and produce prominent linear breaks in the oceanic crust known as fracture zones. • A few transform faults cut through continental crust and two examples are the San Andreas Fault of California and the Alpine Fault of New Zealand. Even in these cases, the transform fault connects oceanic ridges. BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 20 of 24 Transform Fault Boundaries 49 Notice that the transform fault is only active between offsets in the oceanic ridges. The fracture zones are linear scars in the topography of the ocean floor. Transform Fault Boundaries 51 Transform Fault Boundaries 50 North of Mendocino, the San Andreas Fault ends and subduction begins; this creates the active volcanoes of the Cascade Range. ✓REVIEW QUESTIONS 52 11. Where is lithosphere being formed? Consumed? Why must the amount of production and destruction of the lithosphere be equal? 12. Why is oceanic lithosphere subducted while continental lithosphere is not? 13. How did the Himalaya Mountains form? 14. Differentiate between transform faults and the two other types of plate boundaries. Hot Spots and Mantle Plumes 53 Hawaiian Hot Spot 54 • Dating of some volcanic island chains show an age progression away from active volcanoes. The basalt erupted at hotspots is chemically different than that erupted at mid-ocean ridges, indicating two separate sources. • Mantle plumes rise from deep in the mantle (perhaps the core-mantle boundary) and melt beneath the lithosphere, creating a hot spot: an area of active volcanism. The hot spot stays stationary for long periods of time, so as the lithosphere moves, new younger volcanoes are created in a chain – or hot spot track – which record the direction of plate motion. BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 21 of 24 Hot Spots & Hot Spot Tracks 55 57 • Driving Forces: slab pull, ridge push, and slab suction. • Resistive Forces: mantle drag and plate resistance. Layered Mantle Convection 56 • Researchers generally agree on the following: 1. Convection in the mantle – warm, buoyant rock rises and cooler, dense material sinks – is the underlying driving force of plate movement. 2. Subducting oceanic plates drive the cold downwardmoving portion of convection, while the upwelling portion consists of the shallow oceanic ridges and the deeper mantle plumes. 3. The slow movements of Earth’s plates and mantle are ultimately driven by the unequal distribution of heat within Earth’s interior. Heat is transported away from the Earth’s core and up through the mantle. • Active Examples: Hawaii, Yellowstone, Iceland, Galapagos, Bermuda, Azores, and Easter Island. Forces That Drive Plate Motion What Drives Plate Motion? 59 • Problem: Seismic imaging of subducting slabs has shown they penetrate the 660-km boundary, thus upper and lower mantle would mix together. BLOCK EXAM #1 - CHAPTERS 1 & 2 Models of Plate-Mantle Convection 58 • Researches generally agree on the following: 1. In Harry Hess’s model, the convection currents came from deep within the mantle and spread laterally and pull the plates apart (the plates were viewed as being carried passively by the flow in the mantle). 2. However, the opposite appears to be true: the horizontal movement away from ridges is causing shallow upwelling and as the plates move, they drag the adjacent material in the mantle to induce flow. 3. Basaltic lavas erupted along oceanic ridges are fairly homogeneous in composition and depleted of certain trace elements. By contrast, hot-spot volcanism have concentrations of trace elements – so two distinct source regions must exist within the mantle. Whole Mantle Convection 60 • Problem: Whole mantle would mix together very quickly and eliminate the unique deep source of hot spot material – it would be the same as the ridge material. Page 22 of 24 Deep Layer Model 61 62 Deep Layer Model • Problem: The deep layer has not been observed in seismic images. But this is the most current model. From Louise Kellogg – UC Davis ✓REVIEW QUESTIONS 63 64 15. Some people predict California will sink into the ocean. Is this idea consistent with the theory of plate tectonics? Convection and Tectonics 16. What is the oldest ages for oceanic seafloor and continental rocks? 17. If hot spots are fixed, in which direction was the Pacific Plate moving when the Emperor Seamounts were produced, compared to the present Hawaiian Islands? ✓REVIEW QUESTIONS ✓REVIEW QUESTIONS 65 66 18. Which of 3 boundaries or hotspot are these located? 18. Which of 3 boundaries or hotspot are these located? • Himalayas: • Basin & Range: • Aleutian Islands • Alps: • Red Sea: • Philippines: • Andes Mountains: • Hawaii: • San Andreas Fault: • Urals: • Iceland: • East African Rift: • Japan: • Red Sea: • Mount St. Helens: • Tonga: • Sierra Nevada: • Yellowstone: • Appalachians: • Cascade Range: • Galapagos: • Alpine Fault, New Zealand: BLOCK EXAM #1 - CHAPTERS 1 & 2 Page 23 of 24 ✓REVIEW QUESTIONS 19. Describe the three proposed models for mantle-plate convection. What is lacking in each of these models? 67 ] If slab subducts into the lower mantle – the whole mantle would eventually mix – and basalts from MOR and hotspots would have the same composition. ] No evidence of a deeper layer in the mantle. BLOCK EXAM #1 - CHAPTERS 1 & 2 Distribution of Earthquakes (1980-1990) 68 Note the circum-Pacific belt (‘Ring of Fire’) of earthquakes. Most earthquakes occur at plate boundaries. Page 24 of 24