Chapter 2 Notes
Chapter 2 Notes GEOL 1301 - 001
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Date Created: 10/08/16
Chapter 2 Plate Tectonics: The unifying theory Geology1301-002 Professor John Wickham Purple- important people Blue-vocabulary Green-important dates/time periods • The lithosphere is made up of about a dozen plates which converge with or separate from other plates as they slide across the asthenosphere • The theory of plate tectonics describes the movement of plates and forces acting on them o explains volcanoes, earthquakes, and the distribution of mountain chains, rock assemblages, and structures on the seafloor • The discovery of plate tectonics o For almost 200 years, geologist have been developing theories about plate tectonics § Greek for builder § This term is used to describe mountain building, volcanism, earthquakes, and other processes that construct geologic features on Earth’s surface • It wasn’t until plate tectonics that a single theory could explain this range of geologic activity o Continental drift: the large scale movement of continents across earth’s surface, driven by the plate tectonic system § In the late 16 cen. and the 17 cen. geologists noted the jigsaw-puzzle resemblance of coasts on the Atlantic Ocean • It seemed as if you could put all the continents together like pieces of a puzzle § Edward Suess in the 19 cen. postulated that the present southern continents formed together to form a larger continent called Gondwana § Alfred Wegener, a German meteorologist wrote a book on the break up and drifting of continents in 1915 • Postulated the supercontinent known as Pangea for the breakup of all the continents that we know today o Greek for “all lands” • He was correct about the continents drifting apart but was wrong in his assumptions about how fast they were moving and what forces were pushing them across the earth o This reduced his credibility and many scientists concluded that earth’s crust was too rigid for continental drift and Wegener’s ideas were soon forgotten by the majority of scientists • Wegener’s ideas and drift hypothesis pointed to similarities in rock ages and trends in geologic structures on opposite sides of the Atlantic o Offered good arguments of continental drift based on fossil record and climate data § Ex. Mesosaurus has been found in South America and Africa but nowhere else, suggesting these two continents were connected § Plants and animals showed similar evolutionary history until their evolution differed as a result of being on separate continents § Rocks from glaciers were found across multiple continents as well • Seafloor spreading o Geologic evidence did not convince many scientists o No one could explain the driving force behind the split up of Pangea § Wegener thought that the continents moved like boats across the oceanic crust and that they were dragged by tidal forces from the sun and moon • This was quickly rejected and was shown that tidal forces were too weak to move the continents § Scientists realized that convection from the mantle could push and pull the continents • This allows for the creation of new oceanic crust through the process of seafloor spreading: The mechanism by which new oceanic crust is formed at a spreading center on the crest of a mid-ocean ridge. As two plates move apart, magma wells up into the rift between them to form new crust, which spreads laterally away from the rift and is replaced continually by newer crust o Arthur Holmes, a British geologist, proposed that convection currents “dragged the two halves of the original continent apart, with consequent mountain building in the front where the currents are descending, and the ocean floor development on the site of the gap, where the currents are ascending.” § Many still thought earth’s crust and mantle were still rigid and immobile § Maurice “Doc” Ewing, a marine biologist, discovered that the Atlantic seafloor was made of young basalt, rather old granite, like everyone thought § Bruce Heezen and Marie Tharp mapped the Mid- Atlantic Ridge and discovered a deep rift running down its crest • They soon discovered that almost all of the earthquakes that occurred in the Atlantic Ocean originated near this rift • Other oceans with similar rifts were found in the Pacific and Indian Oceans and they also had earthquake activity § Harry Hess of Princeton University and Robert Dietz of the Scripts Institution of Oceanography proposed that Earth’s crust separates along the rifts in mid-ocean ridges, and that new crust is formed by the upwelling of hot molten rock into these cracks in the 1960s • The new seafloor spreads laterally along these plates and continues to be replaced by newer crust • The Great Synthesis: 1963-1968 o Could seafloor and its underlying lithosphere by destroyed by recycling back into earth’s interior? § If not, earth’s surface would gradually become bigger over time § Heezen believed that the earth was getting bigger while other geologists believe in seafloor recycling • They believed this was occurring along the ring of fire, but the exact process remains unknown o Geologist J. Tuzo Wilson described plate tectonics as rigid plates moving across the surface of the earth in 1965 § Characterized three basic boundaries where plate collide, move apart, and move past each other • Other scientists soon showed that all contemporary plate deformation happened at these boundaries o Ex. Faulting, shearing, and compressing tectonic forces • Scientists measured rate and direction of plates and found them mathematically consistent with the idea of rigid plates moving across the surface of the earth § By the end of 1968, basic elements of the plate tectonic theory had been established • Plate tectonics: The theory that describes and explains the creation and destruction of Earth’s lithospheric plates and their movement over Earth’s surface • Greek for builder • By 1970 the evidence became so persuasive that textbooks were revised and Earth scientists embraced the theory • The Plates and their boundaries o According to plate tectonics, the lithosphere is not continuous, but rather a mosaic of rigid plates § Each plate travels as a distinct unit § The largest is the Pacific plate • This includes most of the Pacific Ocean Basin § Some plates are named after the continents they include § There are 13 major plates and a number of smaller plates • Ex. The Juan de Fuca travels between the North American and Pacific Plate o Divergent boundaries: plates move apart and new lithosphere is created (plate area increases) § In oceans these are narrow rifts that approximate with the idea of plate tectonics § mid-ocean ridge: the boundary between separating plates • as the seafloor spreads magma wells up between the rifts to create new crust o this occurs at a spreading center: A divergent boundary, marked by a rift at the crest of a mid-ocean ridge, where new oceanic crust is formed by seafloor spreading § Ex. The Mid-Atlantic ridge where the North American and Eurasian plates are separating o Created millions of square kilometers of oceanic crust § Continental rifting • Early stages of plate separation can be found on continents too o Characterized by rift valleys, volcanism, and earthquakes • Continents have separated enough for new oceanic crust to form a basin in which the ocean has flooded o Ex. The Arabian plate has moved north east relative to the African plate • Sometimes it slows or stops before the continents actually split apart o Ex. The Rhine Valley along Germany and France o Convergent Boundaries: Plates come together and one plate is recycled into the mantle (plate area decreases) § If plate separate in one place, they must converge somewhere else if the Earth’s surface area is to remain the same § Ocean-Ocean convergence • Subduction: when one plate descends beneath the other o The lithosphere sinks beneath the other plate into the asthenosphere and is eventually recycled by the mantle § Produces long, narrow deep sea trench • Ex. The Marianas Trench of the western pacific o Island arc: A chain of volcanic islands formed on the overriding plate at a convergent boundary by magma that rises from the mantle as water released from the subducting lithospheric slab causes fluid-induced melting § Ocean-Continent Convergence • Oceanic lithosphere is easily subducted because continental lithosphere is less dense and more buoyant o The submerges part of the continent is crumpled and deformed, forming mountains parallel to the trench § Over time materials of the submerging plate are scraped off and incorporated into the other plate • Leads to confusing record of the subduction process o Water is carried down by the subduction causing the mantle material to melt § Magma rises and forms volcanoes on the back side of the trench • Ex. The Andes mountains § Continent-Continent Convergence • The two continents come together to form mountain ranges o This creates a double thickness of the crust o Ex. The Himalayan mountains formed when the Indian and Eurasian plate collided (with the Eurasian plate overriding the Indian plate) o Transform Faults: plates slide horizontally past each other (plate area does not change) § These are fractures along which plates slide past each other § Rocks on either side of the fault are different types and ages § Creates large, extremely destructive earthquakes • Ex. San Andreas fault § typically found along mid-ocean ridges where the continuity of a spreading zone is broken § often a step-like pattern o A combination of boundaries § Sometimes there are “oblique” boundaries, which may combine all three types of boundaries • Ex. Nazca Plate is bounded on three sides by spreading centers, offset in a step-like pattern by transform faults o Continental and oceanic crust behave differently § Continental crust is lighter and weaker • Not as easily recycled into the mantle as oceanic crust • Plate boundaries tend to be more spread out and more complex because it is weaker § Oceanic crust is heavier • Rates and History of Plate Movements o The seafloor as a magnetic tape recorder § Magnetometers were used during WW2 to detect submarines • After the war, geologists used a modified version of this technology to measure the magnetism of rocks on the seafloor o They discovered magnetic anomalies: a pattern of long, narrow bands with a pattern of high or low magnetic intensity on the seafloor and are parallel and almost perfectly symmetrical to the crest of a mid-ocean ridge § Confirmed seafloor spreading hypothesis § Allowed geologists to trace plate movement in the distant past o The rock record of magnetic reversals on land § Magnetic anomalies show that earth’s magnetic field does not remain the same over time • Right now the magnetic north pole is closely aligned with the geographical north pole, but this wasn’t always the case • In the early 1960s geologists discovered thermoremanent magnetization occurred within rocks o This is when rocks are slightly, but permanently magnetized because the cold lava “remembers” the magnetization long after the magnetic field has changed o In layered lavas flows the younger rocks are at the top, the magnetization of each layer of rock shows the direction of the magnetic pole at the time § This has allowed geologists to work out a magnetic time scale: The detailed history of Earth’s magnetic field reversals as determined by measuring the thermoremanent magnetization of rock samples whose ages are known o The magnetic field has flipped frequently over time, so it is equally likely to have a magnetic pole facing the direction we have now, or a magnetic pole of the opposite direction § Magnetic chrons: major periods when the magnetic field is normal or reversed • Greek for time • They last about half a million years on average • Magnetic subchrons: short-lived reversal of the magnetic field within major chrons o Usually last several thousand to 200,000 years § Magnetic Anomaly Patterns on the seafloor • The banded patterns of magnetism puzzled many scientists o In 1963 F.J. Vine and D.H. Matthews (Englishmen) and, independently L. Morley and A. Larochelle (Canadians) realized that the bands of high and low magnetism on the seafloor correspond with the magnetism of the rocks § Ships that were above rocks that were magnetized in a normal direction had a locally stronger field • This is known as a positive magnetic anomaly • A negative magnetic anomaly occurred when rocks were magnetized in a reversed direction and ships would read a locally weaker field o The seafloor acts like a magnetic tape recorder by forming two symmetrical bands as it moves away from a rift § Inferring seafloor ages and relative plate velocity • By using the already discovered ages of magnetic reversals, geologists could determine the age of the bands of magnetized rocks on the seafloor • Relative plate velocity: the velocity at which one lithospheric plate moves relative to another o On a divergent plate this is given by combining the speed rate and the spreading direction o An average spreading rate of mid-ocean ridges is about 50 mm per year • Mapping magnetic anomalies on the seafloor has been helpful in figuring out the history of oceanic basins o Deep Sea Drilling § In 1968, a deep sea drilling project was created in collaboration with many oceanographic institutions and the National Science Foundation § Small particles fall through ocean water and accumulate as sediment as soon as new crust begins to form • The age of the oldest sediment immediately on top of the crust tells geologists how old the crust is at that particular spot • The age of sediments can be calculated from single-cell planktonic organism fossils that sink to the bottom of the ocean when they die • The farther away from the oceanic ridge, the older the sample o Samples also almost perfectly agreed with the age of the seafloor determined by magnetic reversal data § This validated the magnetic time scale and provided strong evidence for seafloor spreading o Measurements of Plate Movements by Geodesy § Astronomical positioning • Geodesy: the science of measuring shapes on the earth and locating points on its surface o Ex. Astronomical positioning § Measuring positions of points on earth’s surface in relation to the fixed stars in the sky o Geodetic techniques did not play a significant role in the discovery of plate tectonics because it takes high accuracy to observe plate movements directly § Instead geologists had to rely on evidence of seafloor spreading • Beginning in the late 1970s an astronomical positioning method was developed that can measure intercontinental distances with an accuracy of 1mm o In 1986 a team of scientist used this method to find the distance between two telescopes had increased 19 mm per year which was very close to the rates predicted by plate tectonic models § The Global Positioning System • Geodesy is an expensive and impractical means of investigating plate movements in remote areas with no radio telescopes o Since the mid 1980s geologists have used GPS (global positioning system) to do the same job with the same accuracy § Set of 24 satellites § Used to measure plate movements • Changes in distances between land-based GPS receivers agree with magnetic anomalies on the seafloor § Serves an outside frame of reference § Emits high-frequency radio waves keyed to precise atomic clocks on board • These signals can be picked up by inexpensive, portable radios o These are similar to GPS receivers in car, but much more precise • The Grand Reconstruction o Seafloor Isochrons § Isochron: a contour that connects rocks of equal age • Boundaries between bands of magnetic anomaly data • Tells us the amount of time that has elapsed since rocks were injected as magma into a spreading zone o Widely-spaced isochrons indicate faster spreading rates § In 1990, geologists found the oldest oceanic rocks • These were about 200 million years old, which is only 4% of the earth’s age o The oldest known continental rocks are about 4 billion years old o The seafloor is relative young when compared to the continents o Reconstructing the History of Plate Movements § Earth’s plates behave as rigid bodies • The distance between to point on the same plate don’t change much o But the distance between two points on different plates can change • Transform-fault boundaries indicate the directions of relative plate movement o No overlap, buckling, or separation o Two plates slide past each other w/o creating or destroying material o Orientation of the fault measures the direction in which one plate is sliding past the other • Seafloor isochrons reveal the position of divergent boundaries in earlier times o Roughly parallel and symmetrical along the ridge axis where they were created o They are the same age on opposite sides of a ridge § Can be brought together to show past position of plates o The breakup of Pangea § Existed about 240 million years ago § Began to break apart when North America rifted away from Europe 200 million years ago • Opening of N. Atlantic accompanied by the separation of northern continents that were known as Laurasia at the time from the southern continents known as Gondwana o The breakup of Gondwana separated South America, Africa, India, and Antarctica § Created Southern Atlantic and southern oceans § Narrowed Tethys Ocean • Separation of Australia and Antarctica and the ramming of India into Eurasia closed the Tethys Ocean o The Assembly of Pangea by Continental Drift § Isochron maps tell us all the seafloor on the earth today has been created since the breakup of Pangea • Seafloor spreading and continental drift and collision took place the same way it does today o Since subduction has destroyed the seafloor, we must rely on evidence preserved on continents to identify and chart movements of ancient continents (paleocontinents) § Old mountain belts have helped us locate paleocontinental collisions • Ex. The Appalachian Mountains § Rock types and fossils also indicate the distribution of ancient seas, glaciers, lowlands, mountains, and climates • Knowledge of the climate helps geologists know the latitude at which paleocontinents formed, allowing them to put the pieces together • evidence from rock types, fossils, and magnetization has allowed scientists to reconstruct an earlier supercontinent, called Rodinia o formed 1.1 billion years ago o began to break apart 750 million years ago § scientists tracked fragments over 500 million years to see how they reassembled into Pangea • Implications of the Grand Reconstruction o Geologists have used the Grand Reconstruction to find minerals and oil based on the rock formations in which these resources exist on one continent with their pre-drift continuations on another continent o Paleontologists have rethought evolution as a result of continental drift o Geologists have broadened their focus from a particular region to a world- encompassing picture § The concept of plate tectonics provides a way to interpret, in global terms, such geologic processes as rock formation, mountain building, and climate change o Past Climate Changes § Movement of tectonic plates have affected the climate in profound ways • The waters of the Southern Ocean have isolated Antarctica warmer water and tropical air o This keeps the southern waters cooler and maintains massive ice sheets across the Antarctic continent § 66 million years ago, Australia was still connected to Antarctica allowing warmer water to circulate and heat the continent • 40 million years ago, the North and South American continents were separated and water could flow between the Atlantic and Pacific Ocean • 5 million years ago, subduction in the Eastern Pacific Ocean formed the isthmus of Panama connecting North and South America and separating the Atlantic and Pacific Oceans • these changes and the collision of India and Eurasia cooled the entire planet enough to create ice sheets of Antarctica and in Greenland • The resulting modification of the climate system is thought to have initiated oscillations of climate between very cold periods and somewhat warmer periods • Mantle Convection: the engine of plate tectonics o We can observe that faster moving plates are being subducted along a large fraction of their boundaries § Slower moving plates do not have a significant amount of descending slabs • These observations suggest that the gravitational pull exerted by the cold (and thus dense) slabs of subducting lithosphere pulls the plates downward into the mantle o Lithosphere is not dragged along by convection, but rather it “falls back” into the mantle under its own weight • How do plates pull apart? o One possibility is that the overriding plates, as well as the subducting plates, are pulled toward their convergent boundaries • Stratified Convection o However, some scientists think that the mantle might be divided into two layers: an upper mantle system above about 700 km, where the recycling of lithosphere takes place, and a lower mantle system from a depth of about 700 km to the core-mantle boundary, where convection is much more sluggish o the separation of the two systems is maintained because the upper system consists of lighter rock than the lower system and thus floats on top, in the same way the mantle floats on the core • Whole Mantle Convection o material from the plates circulates all the way through the mantle, down as far as the core-mantle boundary o the iron-rich liquid below this core-mantle boundary is much denser than the solid rock of the mantle, preventing any significant exchange of material between the two layers • From knowledge of past plate movements, we can estimate that, just since the breakup of Pangaea, lithosphere equivalent to the surface area of Earth has been recycled into the mantle o scientists have found regions of colder material in the deep mantle under North and South America, eastern Asia, and other sites adjacent to convergent boundaries o zones occur as extensions of descending lithospheric slabs, and some appear to go down as far as the core- mantle boundary o From this evidence, most scientists have concluded that plate recycling takes place through whole-mantle convection rather than stratified convection • what is the nature of rising convection currents? o scientists believe that the rising currents are slower and spread out over broader regions § consistent with the idea that seafloor spreading is a rather passive process: pull the plates apart almost anywhere, and you will generate a spreading center • there is one exception called a mantle plume: A narrow, cylindrical jet of hot, solid material rising from deep within the mantle thought to be responsible for intraplate volcanism o thought to be slender cylinders of fast-rising material, less than 100 km across, that come from the deep mantle o intense enough to literally burn holes in the plates and produce tremendous volumes of lava o may be responsible for outpourings of lava so massive that they may have changed Earth’s climate and caused mass extinctions o The mantle plume hypothesis was first put forward in 1970, soon after the theory of plate tectonics had been established, by one of its founders, W. Jason Morgan of Princeton University o best evidence for mantle plumes comes from regions of intense, localized volcanism (called hot spots) o remains very controversial § Ex. Hawaii, where huge volcanoes form in the middle of a plate, far from any spreading center
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