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1Plate Tectonics: A Unifying Theory CHAPTER 2Plate Tectonics: A Unifying Theory
2Introduction Why should you know about plate tectonics? Plate tectonics affects all of us, whether from destruction caused by volcanic eruptions and earthquakes, or politically and economically due to the formation and distribution of valuable natural resources.Plate tectonics is the unifying theory of geology, tying together many seemingly unrelated geologic phenomena and illustrating why Earth is a dynamic planet of interacting subsystems and cycles.
3Early Ideas About Continental Drift Alfred Wegener and the Continental Drift Hypothesis.The idea that continents have moved in the past is not new.The concept of continental movement was first suggested when it was noticed that Africa and South America had coastlines which appeared to be counterparts of one another.This suggested they may once have been joined and drifted apart.Fig. 2.3, p. 30
4Early Ideas About Continental Drift Alfred Wegener and the Continental Drift Hypothesis.Alfred Wegener originally proposed the continental drift hypothesis in 1912.He postulated that all landmasses were originally united into a supercontinent named Pangaea.Fig. 2.2, p. 29
5Early Ideas About Continental Drift Alfred Wegener and the Continental Drift Hypothesis.Pangaea consisted of a northern landmass called Laurasia and a southern landmass called Gondwana.As Pangaea broke up, the various continents moved to their present-day locations.The Glossopteris fern, also known as the “Pangaea plant”Fig. 2.1, p. 28
6What is the Evidence for Continental Drift? Continental FitWegener and others amassed a large amount of evidence in support of continental drift.There are close fits between the continents at their margins off the coasts at depths of about 2000 m.Fig. 2.3, p. 30
7What is the Evidence for Continental Drift? Similarity of Rock Sequences and Mountain RangesMarine, nonmarine, and glacial rock sequences of Pennsylvanian to Jurassic age are nearly identical on all the Gondwana continentsFig. 2.4, p. 30
8What is the Evidence for Continental Drift? Similarity of Rock Sequences and Mountain RangesThe trend of several major mountain ranges produces a continuous mountain range when the continents are positioned next to each other as they were during the formation of Pangea.
9What is the Evidence for Continental Drift? Glacial EvidenceGlacial tills and striations on the bedrock beneath the till provide evidence of glaciation at the same time on all the Gondwana continents, with South Africa located at the South Pole.Fig. 2.5, p. 31
10What is the Evidence for Continental Drift? Fossil EvidenceSome of the most compelling evidence comes from fossils like the Glossopteris fern.One of the strongest examples is the Mesosaurus, a fresh water reptile.Fig. 2.6, p. 32
11What is the Evidence for Continental Drift? Wegener could not provide a convincing mechanism to demonstrate ‘how’ the continents could have moved.Continental drift was largely ignored.Fig. 2.2, p. 29
12Features of the Seafloor Topography of the seafloor often affected by plate tectonicsMajor features of the seafloor:Continental shelvesContinental slopesContinental risesAbyssal plainsOceanic ridgesSubmarine ventsOceanic trenchesContinental Margins
13Features of the Seafloor Continental margins lying below sea level separate the continents from ocean basins.The continental margins include the continental shelf, continental slope, and in some places a continental rise.Fig. 2.7, p. 33
14Features of the Seafloor Continental ShelfContinental shelves extend from the shoreline to the more steeply dipping continental slope. They vary in width from ten's of meters to more than 1,000 kilometers.Fig. 2.7, p. 33
15Features of the Seafloor Continental SlopeThe continental slope starts at an average depth of 135 m and is located where the slope of the sea floor increases from less than 1° to several degrees.Fig. 2.7, p. 33
16Features of the Seafloor Continental RiseA continental rise with less dip than the continental slope may be located at the base of the continental slope.Fig. 2.7, p. 33
17Features of the Seafloor Abyssal PlainsAbyssal plains are the flattest locations on Earth!Cover large areas of the seafloorDeep sea sediments accumulate on their surfacesFig. 2.7, p. 33
18Features of the Seafloor Oceanic ridgesOceanic ridges are long, continuous submarine mountain ranges composed of volcanic rock.Oceanic ridges nearly encircle the world but are offset at intervals by large fracture zones.Fig. 2.7, p. 33
19Features of the Seafloor Submarine Hydrothermal VentsSeafloor vents at or very near spreading ridges where circulating water is heated to over 400°C and discharged into seawater.As the hot water circulates through the oceanic crust it dissolves metals that discharge as plumes, called black smokers.Fig. 2.8, p. 34
21Features of the Seafloor Oceanic trenches are the seafloor expressions of subduction zones.They are long, narrow, features which reach the greatest oceanic depths, sometimes >1.1 km.Characterized by low heat flow and frequent seismic activity.Fig. 2.7, p. 33
22Features of the Seafloor Seamounts and GuyotsSeamounts are extinct oceanic volcanoes. Many rise more than a kilometer above the ocean floor. They formed as the oceanic crust moved over a hot mantle plume.Guyots have the same origin, but have flat tops created when the volcanoes sank to sea level. Once near sea level, erosion by waves flattened the tops.Fig. 2.9, p. 34
23Features of the Seafloor Aseismic RidgesAseismic ridges are long, narrow ridges and broad plateaus that rise off the seafloor.Commonly form near hot-spot volcanoes, consisting of seamounts and guyots extending away from spreading ridges.Fig. 2.9, p. 2.9
24Earth’s Magnetic Field Paleomagnetic studies during the 1950s revived interest in continental driftThey indicated that either the magnetic poles had wandered and each continent had its own pole (an impossibility), orThe continents had moved over time. If the continents are moved into different positions relative to each other, the separate poles could be resolved into one.Fig. 2.12, p. 36
25Earth’s Magnetic Field Paleomagnetism is the remnant magnetism in ancient rocks recording the direction and intensity of Earth’s magnetic field at the time of the rock’s formation.The Curie point is the temperature at which hot iron-bearing minerals cool enough to gain magnetism. As long as the rock is not subsequently heated above the Curie point, it will preserve its remnant magnetism.Fig. 2.11, p. 35
26Paleomagnetism and Polar Wandering How can the apparent wandering of the magnetic poles be best explained?The magnetic poles have remained near their present locations at the geologic north and south pole and the continents have moved.Evidence: When the continents are fitted together, the paleomagnetic data for all of them have the same magnetic poles.Fig. 2.12, p. 36
27Magnetic Reversals and Seafloor Spreading Earth’s present magnetic field is considered "normal."Normal - with the north and south magnetic poles located approximately at the north and south geographic poles, respectively.At various times in the geologic past, Earth’s magnetic field has completely reversed.Reversed: the magnetic south pole is near the geographic north pole and the magnetic north pole is near the geographic south pole.
28Magnetic Reversals and Seafloor Spreading Earth’s present magnetic field is normalThe existence of magnetic reversals was discovered in continental lava flows by:A) age datingB) determining the orientation of the remnant magnetism.Fig. 2.13, p. 37
29South magnetic pole (normal position) North magnetic pole (normal pole (reversed)South magneticpole (reversed)South magneticpole (normal)North magneticpole (normal)Figure 2.10: Magnetic Reversals.During the time period shown (a–d), volcanic eruptions produced a succession of overlapping lava flows. At the time of these volcanic eruptions, Earth’s magnetic field completely reversed—that is, the magnetic north pole moved to the geographic south pole, and the magnetic south pole moved to the geographic north pole. Thus, the end of the needle of a magnetic compass that today would point to the North Pole would point to the South Pole if the magnetic field should again reverse. We know that Earth’s magnetic field has reversed numerous times in the past because when lava flows cool below the Curie point, magnetic minerals within the flow orient themselves parallel to the magnetic field at the time. They thus record whether the magnetic field was normal or reversed at that time. The white arrows in this diagram show the direction of the north magnetic pole for each individual lava flow, thus confirming that Earth’s magnetic field has reversed in the past.North magneticpole (reversed)South magneticpole (reversed)Stepped ArtFig. 2-13, p. 37
30Magnetic Reversals and Seafloor Spreading Harry Hess proposed the theory of seafloor spreading in 1962.He suggested that the seafloor separates at oceanic ridges, where new crust is formed by upwelling magma.As the magma cools, the newly formed oceanic crust moves laterally away from the ridge.Fig. 2.15, p. 38
31Magnetic Reversals and Seafloor Spreading Deep-Sea Drilling and the Confirmation of Seafloor SpreadingSeafloor spreading was confirmed by magnetic anomalies in the ocean crust that were both parallel to and symmetric around the ocean ridges.This indicates that new oceanic crust forms along the spreading ridges.Fig. 2.14, p. 38
32Magnetic Reversals and Seafloor Spreading Deep-Sea Drilling and the Confirmation of Seafloor SpreadingSea floor spreading confirmed by:the ages of fossils in ocean sedimentsradiometric datingOceanic crust is youngest at the spreading ridges and oldest at the farthest points from the ridges.Fig. 2.15, p. 38
33Plate Tectonics: A Unifying Theory Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early 1970's.The theory is widely accepted because it explains so many geologic phenomena, including: volcanism, seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the distributions of natural resources.For these reasons, it is known as a unifying theory.
34Plate Tectonics: A Unifying Theory According to plate tectonic theory, the rigid lithosphere, is divided into different-sized plates.Fig. 2.16, p. 39
35Plate Tectonics: A Unifying Theory The lithosphere overlies the asthenosphere, and some type of heat-transfer system within the asthenosphere, moves the plates.As the plates move over the asthenosphere, they separate mostly at oceanic ridges, and collide and subduct into Earth’s interior at oceanic trenches.
36The Three Types of Plate Boundaries Plate tectonics has operated since at least the Proterozoic. It is important to understand how the plates move and interact with one another.Types of Plate BoundariesDivergentConvergentTransform
37The Three Types of Plate Boundaries Divergent Boundaries - Divergent boundaries form when two plates move away from each another.New oceanic lithosphere forms at the opening rift.Most divergent boundaries occur along the crests of oceanic ridgesThey are also present under continents during the early stages of continental breakup.Fig. 2.17, p. 41
38Narrow fault-bounded sea Volcanic activityMagmaContinental crustRift valleyCoastal mountainrangeNarrow fault-bounded seaContinental “seaboard”(coastal mountains gone)Wide oceanFigure 2.16: History of a Divergent Plate Boundary.Stepped ArtFig. 2-17, p. 41
39The Three Types of Plate Boundaries Divergent BoundariesCharacteristic features of ancient continental rifting include: faulting, dikes, sills, lava flows, and thick sedimentary sequences within rift valleys.Pillow lavas and associated deep-sea sediments are evidence of ancient spreading ridges.
40The Three Types of Plate Boundaries Divergent BoundariesModern example: East AfricaInitial stages of continental breakupRift valleys and volcanismRift valleys expand and create seas, such as Red SeaAs divergence continues, seas may expand into oceans and continents have passive marginsFig. 2.18, p.44
41The Three Types of Plate Boundaries Convergent BoundariesConvergent boundaries: where two plates collideThere are three types of convergent boundaries.An oceanic-oceanic boundary: two oceanic plates collide, one ocean plate will subduct beneath the margin of the other plate.An oceanic-continental boundary: an oceanic plate and a continental plate collide, the oceanic plate will subduct.A continental-continental boundary: two continents collide, no subduction of continents.
42The Three Types of Plate Boundaries Convergent BoundariesOceanic-Oceanic BoundariesOne oceanic plate subducts beneath the other and a volcanic island arc forms on the non-subducted plateAn oceanic trench forms parallel to the volcanic island arc where the subduction occurs.Volcanoes result from rising magma produced by the partial melting of the subducting plate.Fig. 2.19a, p. 45
43The Three Types of Plate Boundaries Convergent BoundariesOceanic-Continental BoundariesAn oceanic plate and a continental plate converge, with the denser oceanic plate subducting under the continental plate.Like an oceanic-oceanic boundary, a chain of volcanoes forms on the nonsubducted plate.Fig. 2.19b, p. 45
44The Three Types of Plate Boundaries Convergent BoundariesContinental - Continental BoundariesTwo continents converge, the ocean floor separating them subducts, the two continents collide. Neither plate will subduct.When the two continents collide, they are welded together to form an interior mountain chain.Fig. 2.19c, p. 45
45The Three Types of Plate Boundaries Convergent BoundariesRecognizing Ancient Convergent Plate BoundariesIntensely deformed rocks, andesite lavas, and ophiolites are all evidence of ancient subduction zones, marking former convergent plate boundaries.Fig. 2.20, p. 46
47The Three Types of Plate Boundaries Transform BoundariesPlates slide laterally past each other along transform faultsThese boundaries change one type of motion between plates into another type of motion.Fig. 2.21a, p. 49
48The Three Types of Plate Boundaries Transform BoundariesThese are boundaries along which plates slide laterally past each other along transform faultsGenerally, no diagnostic features are left by transform faults.Example: San Andreas faultFig. 2.22, p. 48
49Hot Spots and Mantle Plumes What are hot spots?A hot spot is the location on Earth’s surface where a stationary column of magma, originating deep within the earth (possible near the mantle-outer core boundary) has slowly risen to the surface and causes volcanism.
50Hot Spots and Mantle Plumes Hot spot plumes apparently remain stationary within the mantle while plates move over themThe resulting hot spot leaves a trail of extinct and progressively older volcanoes that record the movement of the plate.Fig. 2.23, p. 49
51Hot Spots and Mantle Plumes Determining rate and direction of plate movementHot spots may be used to determine the absolute motion of plates. They provide an apparently fixed reference point from which the rate and direction of plate movement can be measured.
52Plate Movement and Motion Determining rate and direction of plate movementThe average rate of plate movement is most commonly determined by dividing the distance from an oceanic ridge axis to any magnetic anomaly in the crust of the seafloor by the age of that anomaly.Satellite-laser ranging techniques are also used to determine the current rate of movement and relative motion of one plate with respect to another.Fig. 2.24a, p. 49
53The Driving Mechanism of Plate Tectonics What moves the plates?Most geologists agree that some type of convective heat system is the basic process responsible for plate motion.Radioactive decay in the Earth's interior provides heat for convection.Additional heat comes from the core.Insert Fig. 2.25, p. 50Fig. 2.25, p. 50
54The Driving Mechanism of Plate Tectonics Mantle Convection Cell ModelSpreading ridges: Hot ascending limbs of cells.Trenches: Cooled part of convention cells descend.Insert Fig. 2.25, p. 50Fig. 2.25, p. 50
55The Driving Mechanism of Plate Tectonics Gravity driven plate motionBesides convection, gravity-driven mechanisms may have a major role.Fig. 2.27, p. 51“Slab-pull” involves pulling the plate behind a subducting cold slab of lithosphere.“Ridge-push” involves gravity pushing the oceanic lithosphere away from the higher spreading ridges and toward the subduction trenches.
56Plate Tectonics and the Distribution of Natural Resources PetroleumPlate movements are partly responsible for abundant petroleum in the Middle East.During the Mesozoic, much of the Middle East was a broad marine shelf near the equator.Countless microorganisms lived in the warm surface waters, died and accumulated in sediments.Burial of the organic-rich sediments from subduction created the right amount of heat to produce petroleum.Plate collisions between Iran and the Arabian Plate folded rocks and created traps for the petroleum to accumulate.
57Plate Tectonics and the Distribution of Natural Resources Mineral DepositsMany metallic mineral deposits are related to igneous and associated hydrothermal activity in convergent and divergent plate boundaries.Copper, iron, lead , zinc, gold and silver ore deposits are associated with plate boundaries.Fig. 2.28, p. 53
58Plate Tectonics and the Distribution of Life Plate tectonics affects organic evolution.Organisms occupy biotic provinces controlled mostly by:ClimateGeographic barriersPlate movements create mountains and continental barriers that influence climate and organic evolution.Insert Fig. 2.29, p. 54Fig. 2.29, p. 54
59Plate Tectonics and the Distribution of Life Plate tectonics and organic evolution in the AmericasFormation of Panama by plate movements about 5 million years ago isolated Caribbean and Pacific marine organisms promoting separate evolution.North American land animals migrated to South America, causing extinctions of South American species.Insert Fig. 2.29, p. 54Fig. 2.29, p. 54