Plate Tectonics PDF

6/13/2024 Plate Tectonics 1) The Discovery of Plate Tectonics 2) The Plates and Their Boundaries 3) Rates and History of Plate Movements 4) The Grand Reconstruction 5) Mantle Convection: The Engin...

6/13/2024 Plate Tectonics 1) The Discovery of Plate Tectonics 2) The Plates and Their Boundaries 3) Rates and History of Plate Movements 4) The Grand Reconstruction 5) Mantle Convection: The Engine of Plate Tectonics 6) The Theory of Plate Tectonics and the Scientific Method 7) The Eastern Mediterranean & Lebanon The Lithosphere is the EARTH’S STRONG, rigid outer shell of rock. It is broken into about a dozen plates, which: 1. slide past, 2. converge with, or 3. separate (diverge) from each other as they move over the weaker, ductile Asthenosphere. Plates are formed where they separate and recycled where they converge in a continuous process of creation and destruction. Continents, embedded in the lithosphere, drift along with the moving plates. The theory of plate tectonics describes the movements of plates and the forces acting on them. It also explains the occurrence of volcanoes and earthquakes, as well as the distribution of mountain chains, rock assemblages, and structures on the seafloor—all of which result from events at plate boundaries. 6/13/2024 Plate Tectonics 2 1  6/13/2024 1) The Discovery of Plate Tectonics Plate Tectonics: A single theory that satisfactorily explains the whole range of geologic processes. The scientific synthesis that led to the theory of plate tectonics began earlier in the 20th century with the recognition of evidence for continental drift [large-scale movements of the continents]. The concept of continental drift has been around for a long time. In the late 16th century and in the 17th century, European scientists noticed the jigsaw- puzzle fit of the coasts on both sides of the Atlantic Ocean. In 1915, Alfred Wegener wrote a book on the breakup and drift of continents, in which he laid out the remarkable similarity of geologic features on opposite sides of the Atlantic. Pangea (whole Earth) and Gondwanaland (the present-day southern continents). 6/13/2024 Plate Tectonics 3 Alfred Wegener CONTINENTS MOVE!! About 250 Ma ago, all continents were joined forming a supercontinent called PANGEA (Greek term = Entire Earth). Pangea split apart and fragments (continents) moved to their current positions. Alfred Wegener (meteorologist and geophysicist, Germany, 1880 – 1930) Gondwanaland (southern continents) In 1915, Alfred Wegener published the formal Theory of CONTINENTAL DRIFT. Plate Tectonics 4 2  6/13/2024 Wegener and advocates of the drift hypothesis pointed also to similarities in rock ages and trends in geologic structures on opposite sides of the Atlantic. They offered arguments, accepted now as good evidence of drift, based on fossil and climate data. 1. Identical 300-million-year-old fossils of the reptile Mesosaurus have been found in Africa and in South America, but nowhere else, suggesting that the two continents were joined when Mesosaurus lived. 2. Animals and plants on the different continents showed similarities in their evolution until the postulated breakup time. After that, they followed different evolutionary paths because of their isolation and changing environments on the separating continents. 3. Rocks deposited by glaciers that existed 300 million years ago were found distributed across South America, Africa, India, and Australia. 6/13/2024 Plate Tectonics 5 Gondwanaland Other reptile fossils. Cynognathus, Lystrosaurus 6/13/2024 Plate Tectonics 3  6/13/2024 Seafloor Spreading Convection in the Earth’s mantle could push and pull continents apart, creating new oceanic crust through the process of seafloor spreading. This map shows the North Atlantic seafloor, the cracklike rift valley running down the center of the Mid- Atlantic Ridge, and the locations of earthquakes (black dots). By the 1960s, many scientists were convinced with the creation of lithosphere at seafloor spreading and its recycling back into Earth’s interior in several regions of intense volcanic and earthquake activity around the margins of the Pacific Ocean basin, known collectively as the Ring of Fire. 6/13/2024 Plate Tectonics 7 Marie Tharp and Bruce Heezen (colleagues at Columbia University, USA) inspecting a map of the seafloor. Their discovery of tectonically active rifts on mid-ocean ridges provided important evidence for the seafloor spreading. The Great Synthesis: 1963–1968 The seafloor spreading hypothesis explained how the continents could move apart through the creation of new lithosphere at mid-ocean ridges. Geologists recognized that the seafloor is recycled in several regions of intense volcanic and earthquake activity around the margins of the Pacific Ocean basin, known collectively as the Ring of Fire. 6/13/2024 Plate Tectonics 8 4  6/13/2024 Distribution of Volcanoes and Earthquakes around the Pacific! 1. The Pacific Ring of Fire. 2. The Circum- Pacific belt. Both mark convergent plate boundaries around the Pacific Plate where the oceanic lithosphere is being recycled! 6/13/2024 Plate Tectonics 9 Therefore, Wegener’s theory became accepted after 1960. Dr. Harry Hess, discovered the presence of paleomagnetic reversals in the seafloor crust. Proved that sea floor crust was growing from spreading centers. Sea floor crustal growth causes continents to move. Harry Hess (geologist, USA, 1906-1969) In 1965, the Canadian geologist John Tuzo Wilson introduced the words PLATE TECTONICS Plate (piece) + Tectonics (motion; Greek) The basic elements of the new theory of plate tectonics were established by the end of 1968. By 1970, the evidence for plate tectonics had become so persuasive that almost all Earth scientists embraced the theory. 6/13/2024 Plate Tectonics 10 5  6/13/2024 Textbooks were revised, and specialists began to consider the implications of the new concept for their own fields. https://education.nationalgeographic.org/resource/plate-tectonics-video 6/13/2024 Plate Tectonics 11 2) The Plates and Their Boundaries According to the theory of plate tectonics, the lithosphere is not a continuous shell, but is broken into a mosaic of rigid plates that move over Earth’s surface. 6/13/2024 Plate Tectonics 12 6  6/13/2024 Each plate moves as a distinct unit, riding on the asthenosphere, which is also in motion. The largest is the Pacific Plate, which comprises much of the Pacific Ocean basin. Some of the plates are named after the continents they include, but in no case is a plate identical with a continent. In addition to the 13 major plates, there are a number of smaller ones (e.g., the Arabian Plate, Cocos Plate, etc.) To see plate tectonics in action, go to a plate boundary. Depending on which boundary you visit, you may find earthquakes, volcanoes, rising mountains, long, narrow rifts, folding, or faulting. Many geologic features develop through the interactions of plates at their boundaries. 6/13/2024 Plate Tectonics 13 ▪ There are three basic types of plate boundaries, all defined by the direction of movement of the plates relative to each other: 1. Divergent boundaries: plates move apart and a new lithosphere is created (plate area increases). 6/13/2024 Plate Tectonics 14 7  6/13/2024 Subduction 2. Convergent boundaries: plates come together and one plate is recycled into the mantle (plate area decreases). Collision 6/13/2024 Plate Tectonics 15 3. Transform faults: plates slide horizontally past each other (plate area does not change). 6/13/2024 Plate Tectonics 16 8  6/13/2024 The continental crust is made of rocks that are both lighter and weaker than those of either the oceanic crust or the mantle beneath the crust. Consequently: 1. The continental crust is not as easily recycled back into the mantle as the oceanic crust. 2. Plate boundaries that involve continental crust tend to be more spread out and more complicated than those that involve oceanic crust. Divergent Boundaries Divergent boundaries are where the plates move apart. Divergent boundaries within ocean basins are narrow rifts that approximate the idealization of plate tectonics. The Mid-Atlantic Ridge, a divergent plate boundary, rises above sea level in Iceland. This cracklike rift valley, filled with newly formed volcanic rock, indicates that plates are being pulled apart. 6/13/2024 Plate Tectonics 17 CONTINENTAL RIFTING Early stages of plate separation, such as the divergence that forms the Great Rift Valley of east Africa can be found on some continents. These divergent boundaries are characterized by rift valleys, volcanism, and earthquakes distributed over a wider zone than is found at oceanic spreading centers. 6/13/2024 Plate Tectonics 18 9  6/13/2024 The Red Sea and the Gulf of California are rifts that are further along in the spreading process. a) The Arabian Plate is moving northeastward relative to the African Plate opening the Red Sea. b) Baja California, on the Pacific Plate, is moving northwestward relative to the North American Plate, opening the Gulf of California between Baja and the Mexican mainland. 6/13/2024 Plate Tectonics 19 Convergent Boundaries Where plates come together, they form convergent plate boundaries. There are three possibilities: 1. OCEAN-OCEAN CONVERGENCE If the lithosphere of both converging plates is oceanic, one plate (the older, colder, and denser) descends beneath the other in a process known as subduction. The fluids derived from the subducting plate causes the mantle material above it to melt. The resulting magma produces a chain of volcanoes, called an island arc, behind the trench. 2. OCEAN-CONTINENT CONVERGENCE If one plate has a continental edge, it overrides the oceanic lithosphere of the other plate because continental lithosphere is less dense and therefore less easily subducted. 3. CONTINENT-CONTINENT CONVERGENCE Where two continents converge. The collision of the Indian and Eurasian plates provides the best example. The collision creates a double thickness of the crust, forming the highest mountain range in the world, the Himalaya, as well as the vast high Tibetan Plateau. 6/13/2024 Plate Tectonics 20 10  6/13/2024 Transform Faults Plates slide past each other. The San Andreas Fault Lithosphere is neither created nor destroyed. The San Andreas fault in California, where the Pacific Plate slides past the North American Plate, is a prime example of a continental transform fault. Because the plates have been sliding past each other for millions of years, the rocks facing each other on the two sides of the fault are of different types and ages. Large destructive earthquakes such as the 1906 San Francisco earthquake can occur on transform faults. 6/13/2024 Plate Tectonics North American Plate Pacific Plate 6/13/2024 Plate Tectonics 22 11  6/13/2024 3) Rates and History of Plate Movements Geologists have developed ingenious methods to answer questions related to these issues and thereby to gain a better understanding of plate tectonics. The Seafloor as a Magnetic Tape Recorder Geologists towed sensitive magnetometers behind research ships to measure the local magnetic field created by magnetized rocks on the seafloor. Scientists discovered regular patterns in the strength of the local magnetic field. In many areas, the intensity of the magnetic field alternated between high and low values in long, narrow parallel bands, called magnetic anomalies, that were almost perfectly symmetrical with respect to the crest of a mid-ocean ridge. 6/13/2024 Plate Tectonics 23 These great discoveries confirmed the seafloor spreading hypothesis and led to the theory of plate tectonics. It also allowed geologists to trace plate movements far back in geologic time. Magnetic anomalies allow geologists to measure the rate of seafloor spreading. (a) An oceanographic survey over the Mid-Atlantic Ridge just southwest of Iceland revealed a banded pattern of magnetic field intensity. Magnetic anomalies are evidence that Earth’s magnetic field does not remain constant over time. 6/13/2024 Plate Tectonics 24 12  6/13/2024 Oceanic crust is composed mainly of volcanic igneous rocks Paleomagnetism (basalts). Basalts (solidified lava) contain minerals rich in iron (Fe); some of them are magnetic. While lava is liquid, the minerals tend to line up with the magnetic orientation pointing at the North Pole When the lava solidifies (forming a rock) the magnetic orientation is frozen By repeating these measurements at hundreds of places around the world, geologists have worked out the magnetic time scale of the past 200 million years. 6/13/2024 Plate Tectonics Sea floor spreading and Paleomagnetism The magma that escapes from the asthenosphere solidifies to form new basaltic rocks that become magnetized in the prevailing direction of the earth’s magnetic field at each time….. Time 1. Time 2. Basaltic rocks Basaltic rocks magnetized following a magnetized following a reversed magnetic field normal magnetic field Time 3. Time 4. Basaltic rocks Basaltic rocks magnetized following a magnetized following a reversed magnetic field normal magnetic field again again Symmetrical arrangement of bands…. The basalts of the seafloor have acted as magnetic tape recorders preserving a record of polarity reversals in the alternating bands of normally (like today) and reversely magnetized rocks. 6/13/2024 Plate Tectonics 26 13  6/13/2024 INFERRING SEAFLOOR AGES AND RELATIVE PLATE VELOCITY By using the ages of magnetic reversals revealed from magnetized lavas on land, geologists can assign ages to the bands of magnetized rocks on the seafloor. They could then calculate how fast the seafloor was spreading by using the formula: speed = distance ÷ time, where distance is measured from the mid-ocean ridge axis and time equals seafloor age. On a divergent plate boundary, the combination of the spreading rate and the spreading direction gives the relative plate velocity, the velocity at which one plate moves relative to the other. Mapping magnetic anomalies on the seafloor has been an amazingly effective and very expedient method for working out the history of ocean basins. 6/13/2024 Plate Tectonics 27 4) The Grand Reconstruction The supercontinent Pangaea was the only major landmass that existed 250 million years ago. Modern geology enabled the reconstruction of the events that led to the assembly of Pangaea and to its later fragmentation into the continents we know today. Seafloor Isochrons The boundaries between bands are contours of equal seafloor age, or isochrons. Isochrons tell us the time that has elapsed since the rocks were injected as magma into a spreading zone and, therefore, the amount of spreading that has occurred since they formed. The more widely spaced isochrons of the eastern Pacific signify faster spreading rates there than in the Atlantic. 6/13/2024 Plate Tectonics 28 14  6/13/2024 This colored map shows the ages of the rocks on the seafloor as determined from magnetic anomaly data and deep-sea drilling. The time scale at the bottom gives the age of the seafloor in millions of years since its creation at mid-ocean ridges. 6/13/2024 Plate Tectonics 29 Ages of rocks on both sides of MORs (Mid-Oceanic Ridges) increase away from the mid-ocean ridge, with the oldest basaltic rocks ~180 Ma. Younger ages at ridges indicate the creation of plates at mid-ocean ridges; hence an evidence of seafloor spreading. 6/13/2024 Plate Tectonics 30 15  6/13/2024 All existing seafloor is geologically young compared with the continents. Over a period of 100 million to 200 million years in some places, and only tens of millions of years in others, oceanic lithosphere forms by seafloor spreading, cools, and is recycled into the underlying mantle. Reconstructing the History of Plate Movements Earth’s plates behave as rigid bodies. That is, the distances between three points on the same rigid plate do not change very much, no matter how far the plate moves. But the distance between, say, New York and Lisbon increases over time because those two cities are on two different plates that are separating along the Mid-Atlantic Ridge. The direction of movement of one plate in relation to another depends on two geometric principles that govern the behavior of rigid plates on a sphere: 1. Transform-fault boundaries indicate the directions of relative plate movement. 2. Seafloor isochrons reveal the positions of divergent boundaries in earlier times. Using these principles, geologists have reconstructed the history of continental drift. 6/13/2024 Plate Tectonics 31 The Breakup of Pangaea 1. The supercontinent of Rodinia formed about 1.1 billion years ago and began to break up about 750 million years ago. 2. The supercontinent Pangaea was mostly assembled by 237 Ma, surrounded by a superocean called Panthalassa (Greek for “all seas”), the ancestral Pacific Ocean. The Tethys Ocean, between Africa and Eurasia, was the ancestor of the Mediterranean Sea. Continental rifting, drifting, and collisions assembled and then disassembled the 6/13/2024 supercontinent Pangaea. Plate Tectonics 16  6/13/2024 3. The breakup of Pangaea was signaled by the opening of rifts from which lava poured. Rock assemblages that are relics of this great event can be found today in 200-million-year-old volcanic rocks from Nova Scotia to North Carolina. 4. By about 150 million years ago, Pangaea was in the early stages of breakup. The Atlantic Ocean had partially opened, the Tethys Ocean had contracted, and the northern continents (Laurasia) had all but split away from the southern continents (Gondwana). India, Antarctica, and Australia began to split away from Africa. 5. By 66 million years ago, the South Atlantic had opened and widened. India was well on its way northward toward Asia, and the Tethys Ocean was closing to form the Mediterranean. 6. The modern world has been produced over the past 65 million years. India collided with Asia, ending its trip across the ocean, and is still pushing northward into Asia. Australia has separated from Antarctica. Continental rifting, drifting, and collisions assembled and then disassembled the supercontinent Pangaea. 6/13/2024 Plate Tectonics 33 5) Mantle Convection: The Engine of Plate Tectonics Why do plates move? According to the supporters of the continental drift, mantle convection is the “engine” that drives the large-scale tectonic processes operating on Earth’s surface. All Earth scientists now accept that the lithospheric plates somehow participate in the flow of this mantle convection system. Where Do the Plate-Driving Forces Originate? The gravitational pull exerted by the cold (and thus dense) slabs of subducting lithosphere pulls the plates downward into the mantle. Thus, the plates are not dragged along by convection currents rising from the mantle, but rather “fall back” into the mantle under their own weight. According to this hypothesis, seafloor spreading is the passive upwelling of mantle material where the plates have been pulled apart by subduction forces. 6/13/2024 Plate Tectonics 34 17  6/13/2024 Convection in the mantle—the rising of hot matter in one place and the sinking of cold matter in another—is the driving force of plate tectonics. A schematic cross section through the outer part of Earth, illustrating two of the forces thought to be important in driving plate tectonics: 1. the pulling force of a sinking lithospheric slab and 2. the pushing force of plates sliding off a mid-ocean ridge. 6/13/2024 Plate Tectonics 35 How Deep Does Plate Recycling Occur? There are two competing hypotheses on the depth extent of the mantle convection system that recycles the lithosphere: a) Whole-mantle convection: The lower boundary of the mantle convection system could be as deep as 2890 km below Earth’s surface, where a sharp compositional boundary separates the mantle from the core. b) Stratified convection: The mantle might be divided into two layers: an upper mantle system, and a lower mantle system where convection is much more sluggish. The separation is maintained because the upper mantle is composed of lighter rocks floating on lower denser rocks. 6/13/2024 Plate Tectonics 36 18  6/13/2024 What Is the Nature of Rising Convection Currents? Scientists believe that the rising currents are slower and spread out over broader regions. This view is consistent with the idea that seafloor spreading is a rather passive process. There is one exception, however: a type of narrow jet-like upwelling called a mantle plume. The best evidence for mantle plumes comes from regions of intense, localized volcanism (called hot spots), such as Hawaii, where huge volcanoes form in the middle of a plate, far from any spreading center. 6/13/2024 Plate Tectonics 37 6) The Theory of Plate Tectonics and the Scientific Method In the context of the scientific method, plate tectonics is a confirmed theory whose strength lies in its simplicity, generality, and consistency with many types of observations. Like the theories of Earth’s age, the evolution of life, and genetics, the theory of plate tectonics explains so much so well, and has survived so many efforts to prove it false, that geologists treat it as a fact. But, why wasn’t plate tectonics discovered earlier? Continental drift and seafloor spreading were slow to be accepted largely because these audacious (bold) ideas came far ahead of any firm evidence. Scientists had to explore the oceans, develop new instruments, and drill the seafloor before the majority could be convinced. 6/13/2024 Plate Tectonics 38 19  6/13/2024 7) The Eastern Mediterranean and Lebanon Lebanon Distribution of lithospheric plates and active volcanoes over the Earth’s surface. 6/13/2024 Plate Tectonics 39 Lebanon is located at the boundary between two plates: What are they? The Arabian Plate is moving to the NE with a speed of ~ 15-20 mm/year! 6/13/2024 Plate Tectonics 40 20  6/13/2024 Transform faulting. Along the Gulf of Aqaba Active Nowadays (Lebanon divided by the extension of this fault) Divergence Opened the Gulf of Suez 5 Ma ago! Plate Tectonics 41 The Dead Sea Transform (east. Mediteran.) African Plate Arabian plate 6/13/2024 Plate Tectonics 42 21  6/13/2024 Some historical Lebanese earthquakes Yammouneh Fault. 13th century (1202)- 7.5 Roum Fault 1837- 7.1 Rachaya Fault Oct. 1759- 6.6 Serghaya Fault Nov. 1759- 7.4 Map showing the major faults (red lines) in Lebanon (Elias et al., 2007) 6/13/2024 Plate Tectonics Welcome to Google Earth Exercises 1. Enter “Mt. Everest” into your GE search engine and use the cursor to find its highest point. What is its approximate elevation above sea level (above mean sea level, or amsl)? a) 10,400 m amsl b) 7380 m amsl c) 8850 m amsl d) 9230 m amsl 2. Zoom out from Mt. Everest proper and take a look at the shape of the Himalaya as a whole (try an eye altitude of 4400 km). Which of the following descriptions best captures what you see? a) A triangular mountain range composed of a single high peak b) An east-west–oriented mountain range composed of dozens of high peaks along the southern rim of a high plateau c) A north-south–oriented mountain range composed of high peaks in the middle and lower peaks around the edges d) A circular mountain range closed around a central broad dome 6/13/2024 Plate Tectonics 44 22  6/13/2024 3. Using the answer to question 1 and using your cursor to note the maximum depth of Challenger Deep below mean sea level, calculate the approximate total difference in elevation of the two locations. Which of the following numbers is closest to that difference? a) 14,000 m b) 20,000 m c) 18,000 m d) 26,000 m 6/13/2024 Plate Tectonics 45 23