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Plate tectonics theory online notes Plate tectonics theory online notes Document Transcript

  • Prof. C. ValentiPlate Tectonics Theory 1Plate Tectonics Theory. In order to examine the structural features of the planet,we need to be familiar with the forces operating to construct thesefeatures/landforms. Plate tectonics is not only responsible for orogenic events, butvolcanism and earthquakes as well. All forms of building and breaking the lithosphere.Collision processes take millions of years to complete, but in the geologist’s perspective,this is a smidgen in time. If plate tectonics were to cease, what would happen to theearth as we know it?In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comesfrom the Greek root "to build." Putting these two words together, we get the termplate tectonics, which refers to how the Earths surface is built of plates. The theoryof plate tectonics states that the Earths outermost layer is fragmented into ~20 largeand small plates that are moving relative to one another as they ride atop the hotter,more mobile asthenosphere.PLATE TECTONIC THEORY (developed in late 1960s)Plate tectonics theory is a model of the Earth in which the brittle lithosphere ‘floats’ onthe hot, plastic asthenosphere. The lithosphere is broken into about 20 plates (about12 large plates and several small ones). The plates move across the surface, each in adifferent direction from its neighbor at rates less than 1 to about 18 cm a year.plates = lithosphere = crust + rigid uppermost mantle up to 100 km thick beneath oceanic plates. up to 200 km thick beneath continents.asthenosphere = partially molten upper mantle beneath lithosphere (0 to ~350 kmthick).The rigid lithospheric plates move in response to flow in the asthenosphere below it.The basic energy source for tectonic movement is believed to be heat generated byradioactive decay of certain elements (mostly uranium, thorium, and potassium) andoriginal heat of the earth. This heat causes material in the asthenosphere to moveslowly by convection: the hot material rises to the base of the lithosphere, where itthen moves laterally, cools, and descends to become reheated.• Simple logic tells us that one plate can move relative to an adjacent plate in three ways. At a divergent plate boundary, two plates move apart, or separate. At a convergent plate boundary, two plates move toward each other and collide. At a transform boundary, two plates slide horizontally past each other.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 2Plate tectonics is a relatively new scientific concept, introduced some 30 years ago, butit has revolutionized our understanding of the dynamic planet upon which we live. Thetheory has unified the study of the Earth by drawing together many branches of theearth sciences. It has provided explanations to questions that scientists had speculatedupon for centuries -- such as why earthquakes and volcanic eruptions occur in veryspecific areas around the world, and how and why great mountain ranges like the Alpsand Himalayas formed.- Powerful, unifying theory that can account for most geologic processes.- Came about as a consequence of a large body of evidence in support of continental drift and sea-floor spreading.Before the advent of plate tectonics within the past 30 some-odd years some peoplealready believed that the present-day continents were the fragmented pieces ofPlate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 3preexisting larger landmass ("supercontinents" or Pangea meaning “all lands”)."According to the continental drift theory, the supercontinent Pangaea began to breakup about 225-200 million years ago, eventually fragmenting into the continents as weknow them today.The belief that continents have not always been fixed in their present positions wassuspected long before the 20th century; this notion was first suggested as early as1596 by the Dutch mapmaker Abraham Ortelius in his work Thesaurus Geographicus.Ortelius suggested that the Americas were "torn away from Europe and Africa . . . byearthquakes and floods" and went on to say: "The vestiges of the rupture revealthemselves, if someone brings forward a map of the world and considers carefully thecoasts of the three [continents]."However, it was not until 1912 that the idea of moving continents was seriouslyconsidered. This new idea - called Continental Drift – was introduced in two articlespublished by a 32-year-old German meteorologist named Alfred Wegener. He contendedthat around 200 million years ago the supercontinent Pangaea began to split apart. Heproposed that Pangaea first broke into two large continental landmasses, Laurasia in thenorthern hemisphere and Gondwanaland in the southern hemisphere, and Laurasia andGondwanaland then continued to break apart into the various smaller continents thatexist today. Since that time, the continents have been moving apart to their presentposition.Wegeners theory was based in part on what appeared to him to be the remarkable fitof the South American and African continents, first noted by Abraham Ortelius threecenturies earlier. Wegener was also intrigued by the occurrences of unusual geologicstructures and of plant and animal fossils found on the different continents that hewent about compiling evidence for his new hypothesis.1. Continents fit like a puzzle (e.g., the Atlantic coastlines of Africa and South America). Best fit includes continental shelves.2. Similar fossils on all of continents matched. Similarity of amphibian and reptile fossils found on different continents (e.g., Mesasaurus, a freshwater reptile foundPlate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 4 only in South America and Africa). The distribution of the fossil fern Glossopteris found in South America, Africa, India, Australia, and Antarctica.3. The distribution of glacial deposits (1) not found in areas that are currently glacial environments, and the orientation of glacial scratches on different continents (2) lined up.4. The distribution of climate sensitive sedimentary rocks on the different continents (e.g., coal beds, need tropical plants). Coal beds found in Siberia, Pennsylvania.5. Various mountain ranges of the same age, rock type, and style of deformation on different continents line up (e.g., Appalachian Mountains have counterpart in Europe, other rock types in South America and Africa similar). Trends of faults and folds.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 5At the time Wegener introduced his theory, the scientific community firmly believedthe continents and oceans to be permanent features on the Earths surface. Notsurprisingly, his proposal was not well received, even though it seemed to agree with thescientific information available at the time. A fatal weakness in Wegeners theory wasthat it could not satisfactorily answer the most fundamental question raised by hiscritics: What kind of forces could be strong enough to move such large masses of solidrock over such great distances? He could not support his hypothesis with a mechanism.Wegener suggested that the continents may have moved by centripetal force of theearth rotating, and that when the continents moved, they plowed through the oceanfloor. But his contemporaries argued correctly that it was physically impossible for alarge mass of solid rock to plow through the ocean floor without breaking up.Undaunted by rejection, Wegener devoted the rest of his life to doggedly pursuingadditional evidence to defend his theory. He froze to death in 1930 during an expeditioncrossing the Greenland ice cap. With his death, so too went the controversysurrounding continental drift.Work in the 1940’s and 50’s set the stage for revival of Wegner’s work in the early1900’s in two ways.(1) New studies of the sea floor as a result of WWII technology and(2) Geophysical research in rock magnetism.Paleomagnetism: How it works. Rocks record the direction and strength of theearth’s magnetic field at the time the rocks form. Small magnetite crystals in a coolinglava flow act like tiny compass needles, preserving a record of the earth’s magnetic fieldwhen the lava solidifies. (When Rocks are heated above a temperature, the Curie Point,magnetic minerals lose their magnetism. When iron rich grains cool below their curiepoint, they become magnetized in the direction parallel to the existing magnetic lines offorce. Once the minerals solidify, the magnetism they possess will remain frozen in thisposition). Iron minerals ‘point’ toward the existing magnetic poles at the time of theircooling. If the rock is moved, or the magnetic poles change position, the rockmagnetism will retain its original alignment. The ‘magnetism’ of the old rocks thereforecan be measured to determine the direction of the earth’s magnetic field in the past,paleomagnetism.Paleomagnetism revived the interest in continental drift, and new work was being doneon reconstructing Pangaea.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 6Sea floor spreading hypothesis. Harry Hess. The Driving Mechanism: Sea Floor SpreadingHypothesis. (proposed in 1962 to account for continental drift)During the 1950s, intense oceanographic research lead to the discovery of a worldwide mid-ocean ridge and deep-sea trench system.♦ Harry Hess concluded in 1960 that new sea floor was being created at mid-ocean ridges by volcanic activity. But the earth is not getting larger. Therefore an equal amount of oceanic crust is probably being lost at trenches. The driving force is convection currents in the mantle caused by radioactive decay and gravity carrying the rigid crust away from the mid-ocean ridge like a conveyor belt and driving it into the mantle at trenches along continental margins. The youngest rocks of the sea floor should be found near the mid oceanic ridge axes while the oldest furthest away from the mor. New sea floor is accumulated at the mid ocean ridge while old sea floor is subducted into the mantle and recycled.♦ He suggested that the continents may be moving along with the sea floor, not plowing through it as Wegener suggested. If the continents are not plowing, the sea floor must be moving with it. The sea floor moves away from the mid ocean ridge as a result of mantle convection. According to this concept, the sea floor is moving like a conveyor belt away from the crest of the mid oceanic ridge, down the flanks and across the deep ocean basin, to disappear finally by plunging beneath a continent. The ridge crest or spreading axis is stationary. The driving force pushing the sea floor laterally away from the central axis is the result of convection currents or the circulation pattern driven by the rising of hot material/sinking of cold material in the mantle. Hot material is lower in density and rises (similar to circulation of water in a bowl. If the mantle flows plastically a slow circulation can be set up when rocks of different temperature sets up a current. Locations of spreading ridges (upwelling) and trenches (downwelling) are determined by the convection cells.• The heat driving the convection comes from two sources the original heat from when the earth formed 4.6 billion years ago and radioactive decay of isotopes in the mantle. Continents ‘ride’ with the sea floor so they don’t plow into anything but another continent.• Plate Boundaries are associated with active margins. The distribution of earthquakes and volcanic eruptions indicate that these phenomena are concentrated in belts or linear chains at the boundaries of the lithospheric plates. Overhead.Driving source for plate movements. In the mantle, which behaves plastically isflowing from heat released by radioactive decay of Uranium 238 to Uranium 235.Uranium is unstable with an atomic mass of 238. During the decay process, the elementloses mass and in the process releases heat keeping the rocks in the mantle close toPlate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 7their melting point. A slow convection current forms lifting lithosphere near thedivergent, spreading centers, and as rocks cool they become more dense and sink asthey spread away from the axis.• Explanations. If convection drives sea floor spreading, then hot mantle rock must be rising under the mid oceanic ridge systems. As rock continues to rise beneath the ridge crest, the circulation pattern splits and diverges near the surface. Mantle rock moves horizontally away from the ridge crest on each side of the ridge. This movement creates tension, cracking open the oceanic crust to form the rift valley. As the mantle rock moves horizontally away from the ridge crest it carries sea floor piggyback along with it. As the rock becomes cool and denser, it sinks deeper beneath the ocean surface. This downward plunge of cold rock accounts for the existence of oceanic trenches.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 8Plate Tectonics. Combines Wegner’s theory of continental drift and Hess’s theoryof sea floor spreading coupled with paleomagnetic data.1) Paleomagnetism. Vine and Matthews (1960’s) and Robert Dietz. With World War II in action, many people studied the sea floor for submarine navigation. So during the 1940’s and 1950’s much more data by surveying the sea floor and magnetism of the oceanic crust was collected. The study of paleomagnetism (ancient magnetic fields of the sea floor) was developed (geophysics) and it shows how igneous rocks (basalt) form at the mid-oceanic ridges along the sea floor. At MOR’s basalt is erupted volcanically and it rises and cools. New magma rises and cools and spreads the previous oceanic crust laterally away from the central axis of the MOR. Basalt has high levels of iron in the melt that act like tiny little compass needles trapped in molten rock. The earth has its own magnetic field, so when the rocks crystallize, the tiny little crystals align themselves with the earth’s magnetic field and when the melt cools, it traps the crystals aligned with the magnetic field of the earth. Every so often (thousands to tens of thousands of years) the earth’s magnetic field reverses and the little magnet crystals in the rocks forming during a reversal will be pointed in the other direction. Magnetometers measure the intensity of magnetism in a rock. So during normal polarity, signals are high, reversed polarity, signals are low. Normal polarity as present day magnets point to present day North Pole and reverse polarity compass will point to the South Pole. Vine and Matthews (1960’s) and Robert Dietz took a closer look at the patterns produced by magnetic reversals along the sea floor. They found that most magnetic anomalies at sea are arranged in bands that lie parallel to the rift valley of the mor. Alternating positive and negative anomalies form a stripe-like pattern parallel to the ridge crest.  The earth possesses a magnetic field (similar to what would be expected from a huge bar magnet buried at the earth’s center). Magnetic lines of force run around the earth from the south magnetic pole to the north. A compass needle aligns itself parallel to the lines of the magnetic field.  At MOR new sea floor is added and spreads out laterally from the axis. Magma of the asthenosphere emerging at MOR is high in Fe and Mg. As the magma cools and the iron bearing minerals crystallize they get trapped in an orientation. Like tiny compass needles they align themselves parallel to the lines of force of the earth’s magnetic field (varies with latitude).  Magnetic Reversals. Magnetic north has not always coincided with its present position. The earth’s magnetic field has flipped or reversed polarity throughout earth’s history with north and south magnetic poles switching places.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 9  The floor of the ocean consists of alternating stripes or bands of normally and reversely magnetized rocks, symmetrically arranged around the MORs. • These patterns were symmetrical about the ridge crest and they match the magnetic reversals of the earth over geologic time. • These patterns measure rate of sea floor spreading, where larger bands indicate faster movement. • These patterns indicate the age of the rocks in millions of years.2) Hot spots. Hot spot mantle plumes (as in Hawaiian Islands) remain stationary as the sea floor moves on top of the asthenosphere. Indicates plate direction. A hot spot is a persistent volcanic center thought to be located directly above a rising plume of hot mantle rock. A mantle plume originates deep in the mantle, and its magma then rises through the lithosphere to erupt and form a volcano or volcanic island. Because a lithospheric plate continues to move over the relatively stationary asthenosphere, a mantle plume may generate magma continuously as a plate migrates over the plume. This process forms a chain of volcanic islands that becomes progressively younger toward one end (e.g., Hawaiian Islands). Hot Spots indicate the direction of plate movement.3) Sea Floor Sediment Ages/Lack of Ancient Sea Floor. Sediments (fossils) deposited on the sea floor and radiometric dating of basalt have ages no older than ~200 million years old (last Pangaea). Anything older has been recycled during subduction because the earth’s volume is finite. As spreading occurs at MOR, previously formed rocks are continually spread apart and moved farther from the ridge, while fresh magma rises from the asthenosphere to form new, younger lithosphere at the ridge. The rocks of the sea floor, and the bottommost sediments deposited on them, are youngest close to the MOR and become progressively older the farther away they are from the ridges on either side. The age pattern is symmetrical across each ridge. Plate tectonics states that many ocean basins open and close over time, therefore there are no sediments older than the last opening cycle-Pangea, ~200-250 million years ago.4) Supporting evidence as a result of sea floor spreading. (1) Earthquake studies conducted at deep sea trenches. (2) Distribution of Volcanoes.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 10PLATE BOUNDARIESTypes of plate boundaries. All plate boundaries are associated with eithervolcanism, earthquakes or both.1) Divergent. Also called spreading centers and rifts; occurs where two plates move apart horizontally and new lithosphere is created. Occur both in oceanic and continental crust: Mid oceanic ridges or continental rupture. Typical rock type is basalt, mafic. As the two plates separate, hot, plastic asthenosphere rock flows upward to cool and form new sea floor lithosphere in the gap left by the diverging plates (basalt). Along divergent boundaries where molten rock emerges, the ocean floor is elevated due to the upwelling forces and the lower density of hot material. As rocks cool and move laterally away from the ridge, the cooler and denser lithosphere sinks deeper into the asthenosphere. Average rate of spreading is ~5cm per year. Ocean-ocean Topography: Mid-oceanic ridge (e.g., Mid-Atlantic Ridge, East Pacific Rise). Geologic Events: Sea-Floor Spreading, shallow earthquakes, rising basalt magma, volcanoes. Located on the crests of mid oceanic ridges. Atlantic spreads at a rate of 1cm/yr and the Pacific is 6 cm/yr. Tensional cracks, normal faults. Continent-continent Topography: Rift Valley (e.g., East African Rift and the Red Sea, Pangaea broke apart, basalt deposits in New Jersey). Geologic Events: Continental rifting results in mantle plumes beneath a continent and thinning of crust. The continents are torn apart resulting in small to moderate magnitude shallow earthquakes and passive volcanic eruptions with rising basaltic magma. Rifts eventually form oceans. The crust is initially stretched and thinned. Numerous faults break the crust, and the surface subsides into a central graben. Shallow earthquakes and basalt eruptions occur in this rift valley. As divergence continues, the continental crust on the upper part of the plate separates, sea water floods into the linear basin between the two diverging continents. The upward rise of basaltic magma from the mantle rises to form new oceanic crust between the two diverging continents. The plates continue to diverge, widening the sea and layers of marine sediments blanket the new oceanic crust.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 112) Convergent. A convergent boundary develops where two plates are moving horizontally toward each other and therefore are colliding. Colliding plate boundaries, resulting in orogenic events (volcanism and mountain building) or deep oceanic trenches. Ocean-Continent During continental – oceanic collision, denser sea floor becomes subducted beneath lighter continental crust. When the leading edge of a plate capped with continental crust converge with a plate capped with oceanic crust, the plate with the less dense continental material remains floating, while the denser oceanic slab sinks into the asthenosphere. As the oceanic slab sinks into the asthenosphere, some of the sediments carried on the subducting plate, as well as pieces of oceanic crust are scraped off and plastered against the edge of the over riding crust, forming an accretionary wedge. The descending plate will reach a depth of 100-150 km, and heat from friction and confining pressure will cause water and other volatile components from the subducting sediments into the overlying mantle. This induces partial melting of mantle rocks at reduced temperatures. Generates mafic magma, being less dense than the mantle rises up through the crust. Here, it may partially melt the continental felsic crust and subsequently produce intermediate lava. Topography: Mountains and Ocean Trenches (e.g., Andes and Nazca plate). Geologic Events: Subduction, deep earthquakes, rising magma, volcanoes, deformation of rocks. Ocean-Ocean Collision of two oceanic slabs will result in descent of one below the other initiating volcanic activity in a similar manner to oceanic-continental. The older (colder, denser) oceanic crust will subduct. Volcanic islands form on the ocean floor and eventually build up to protrude through the ocean surface. Topography: Island arcs and Ocean trenches (e.g., Western Aleutians, Caribbean Islands, Japan). Geologic Events: Subduction, deep earthquakes, rising magma, volcanoes, deformation of rocks. Continent-Continent When two plates carrying continental crust converge, neither plate will subduct beneath the other because of low densities and the buoyant nature of continental rocks. The result is collision and construction of large scale high pointy mountain chains. In addition to felsic rocks, deep sea sediments and oceanic crust may be found in the mountains. Prior to continental collision, an ocean separated the two continental masses. Subduction initiatesPlate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 12 partial melting in the overlying mantle rocks, which results in growth of a volcanic arc. Eventually as the intervening seafloor is consumed, these continental landmasses collide. This folds and deforms the accumulation of sediments along the continental margin. The new mountain ranges include deformed and metamorphosed sedimentary rocks, fragments of the volcanic arc and pieces of oceanic crust. Lithosphere is very thick in areas of continental continental collision. Topography: Mountains (e.g., Himalayas). Geologic Events: Deep earthquakes, deformation of rocks. Examples include the Appalachian Mountains (from collision of Gondwanaland and North America during the formation of Pangaea), India and Asia forming the Himalayan Mountain chain.3) Transform. Occurs when two lithospheric plates slide past one another, and neither is subducted. Juan de Fuca plate with North American plate. Volcanism is not generally associated with transform boundaries but earthquakes are. Crust is not created or destroyed. Ocean-Ocean Topography: Major offset of mid-oceanic ridge axis in the oceanic crust (e.g., offset of East Pacific Rise in South Pacific). Geologic Events: Shallow focus earthquakes. Continent-Continent Topography: Small deformed mountain ranges, deformations along faults (e.g., San Andreas Fault). Geologic Events: Earthquakes, deformation of rocks.Active versus passive continental margins. Eastern U.S. is situated in the middle of aplate (passive continental margin) where we are not influenced by active tectonics. Weare enjoying the ride but there are no earthquakes or volcanic activity here. Activecontinental margins such as Mediterranean, West coast of the U.S., Japan, all lie alongplate boundaries, and are affected by earthquakes and volcanoes. Ring o’ fire. Bothvolcanism (mountain building), continental – continental collision (mountain building)and earthquakes are associated with active continental margins.Plate Tectonics Theory
  • Prof. C. ValentiPlate Tectonics Theory 13DRIVING MECHANISM MODELS1. Convection Currents. Suggests that large convection currents in the mantle, where warm less dense rock rises and cooler, denser material sinks, drive plate motion. According to this proposal, the warm less dense material of the lower mantle rises very slowly in the regions of oceanic ridges. As material spreads laterally, it drags the lithosphere along like packages on a conveyor belt. Types of Mantle Convection 1) Whole-Mantle (entire mantle is convecting) 2) Two-Layer or Boundary Layer (mantle is split into two layers (upper mantle = above 670 km; lower mantle = 670 to 2900 km). Convection in the lower mantle drives convection in the upper mantle.2. Ridge Push/Slab Pull. "Ridge-push" - the ridge is topographically high, thus gravity pushes the oceanic lithosphere "downhill" towards the trench. The elevated position of an oceanic ridge could cause the lithosphere to slide under the influence of gravity, pushing the plate into the asthenosphere. "Slab-pull" - subducting cold slab of oceanic lithosphere is denser than the surrounding warmer asthenosphere, thus it pulls the rest of the plate with it as it subducts.3. Rising Plumes/Descending Slabs. Hot spots of thermal convection of isolated intraplate plumes also spread laterally and facilitate plate motion away from the zone of upwelling. A dozen or so ‘hot spots’ have been identified along ridge systems where they may contribute to plate divergence. Intraplate mantle plumes, generate strings of volcanic island chains.Plate Tectonics Theory