地质学

nullnullChapter 5: Plate TectonicsPlate tectonics is a theory about how the surface of the Earth evolves due to strong internal forces. The surface of the Earth is composed of rigid plates that are mobile and move relative to one another. Plate tectonics was proposed and began to gain acceptance from the scientific community in the 1960’s. Prior to the development of the theory of plate tectonics, most geologists believed that the relative location of the continents was relatively unchanged through time. Plate tectonics is a unifying theory in Geology — different geologic phenomena such as mountain building, earthquakes, volcanoes, and the distribution of fossils and organisms can be explained through plate tectonics. nullContinental Drift Looking at a map of the Earth, it appears that the continents could fit together like a jigsaw puzzle. Wegener suggested that a single "supercontinent" called Pangaea once existed in the past. Alfred Wegener (1915) proposed the revolutionary idea called Continental Drift. nullWegener developed his idea based upon 4 different types of evidence: 1. Fit of the Continents 2. Fossil Evidence 3. Rock Type and Structural Similarities 4. Paleoclimatic EvidencenullEvidence for Continental Drift 1. Fit of the Continents It was the amazingly good fit of the continents that first suggested the idea of continental drift. In the 1960's, it was recognized that the fit of the continents could be even further improved by fitting the continents at the edge of the continental slope — the actual extent of the continental crust.nullEvidence for Continental Drift 2. Fossil Evidence Wegener found that identical fossils, such as mesosaurus, were located directly opposite on widely separated continents. This had been realized previously but the idea of "land bridges" was the most widely accepted solution. This not a generally reasonable solution for continents so distant from one another.Wegener found fossils to be convincing evidence that a supercontinent had existed in the past.nullEvidence for Continental Drift 2. Fossil Evidence (cont.) Wegener also noted that Glossopteris, a fossil fern, was widely dispersed in regions of Africa, South America, India and Australia. It has subsequently found in Antarctica as well. The current climatic zones for these regions are too diverse to all have supported this plant. In order to explain the distribution of Glossopteris during the same time of Earth history, Wegener concluded that these fossils were convincing evidence that the continents were once joined into a "supercontinent." nullEvidence for Continental Drift 3. Rock Type and Structural Similarities Wegener found similar rock types and geologic structures on continents on opposite sides of the Atlantic Ocean. The map shows that, if the continents are joined together, the mountains form a continuous belt. This mountain belt is ~300 million years old and represents the time when the continents collided to form the supercontinent Pangaea. The Appalachian Mountains trend along the eastern flank of North America. Mountains of similar age and structure also appear in Scotland and the Caledonian Mountains of Scandinavia. nullEvidence for Continental Drift 4. Paleoclimatic Evidence Glacial till of the same age is found in southern Africa, South America, India and Australia — areas that it would be very difficult to explain the occurrence of glaciation. In addition, the areas with extensive coal deposits from the same time period occur in regions that would have been equatorial.At the same time, large coal deposits were formed from tropical swamps in N. America and Europe. Pangaea with S. Africa centered over the South Pole could account for the conditions necessary to generate glacial ice in the southern continents. nullScience is based on more than mere empirical observation — we strive to understand the mechanisms. We develop scientific theories to explain our observations. Wegener’s work may more correctly be an hypothesis. It was not until the 1960's that further data led to the development of the theory of plate tectonics that could explain the movement of continents. Wegener's idea of continental drift was not generally accepted because no one could come up with a reasonable mechanism for the movement of the continents. Untold tragedies of continental drift.nullPlate Tectonics The ocean floor reveals valuable clues about plate tectonics. We are familiar with the wide diversity of landforms and geologic processes on the continents - but there is an equal diversity in the ocean basins. The oceans contain volcanoes, mountain chains, valleys, plateaus, etc.nullThere is a global mid-ocean ridge system that winds its way through the Earth’s oceans like the seams on a baseball. The mid-ocean ridge system is a nearly continuous volcanic ridge found in all oceans. null Wegener’s theory of Continental Drift was not accepted partly because there was not mechanism for how large portions of the Earth’s crust could move relative to one another on the surface. The recognition of seafloor spreading caused a scientific revolution where scientists began to reexamine Wegener’s theory of Continental Drift. Seafloor Spreading and Continental Drift were incorporated into the modern theory of Plate Tectonics. The story of Plate Tectonics is an excellent example of the cumulative nature of science and the scientific method. nullDivergent Boundaries Most divergent boundaries are located along mid-ocean ridges. Divergent plate boundaries are known as constructive margins because they are the site where new oceanic crust (lithosphere) is generated. A process known as seafloor spreading occurs where magma from the mantle wells up into the divergent boundary - forming new basaltic seafloor. Spreading rates average ~5 cm/year. nullThere is nearly continuous volcanic activity somewhere along the mid-ocean ridge system. The mid-ocean ridge has an elevated position on the seafloor because it is formed from relatively hot igneous rocks. As the seafloor moves away from the ridge, it cools and contracts — thus the seafloor generally is at a greater depth as you move away from the mid-ocean ridge. nullAlthough most divergent boundaries are found along mid-ocean ridge systems, some can develop on continents The figure shows the development of a continental rift that eventually evolves to form a new ocean basin. This is what happened when North and South American rifted from Europe and Africa to form the Atlantic ocean. nullThe East African Rift represents a modern example of a continental rift. If this rift is successful, eastern Africa may split off from the rest of the continent and a new ocean basin may form between the two “Africas.”This region is subjected to extensive volcanic activity (as you would expect) with large volcanoes such as Kilimanjaro and Mt. Kenya. The Red Sea and the Gulf of Aden formed when the Arabian Peninsula rifted from Africa ~20 million years ago. The Red Sea and Gulf of Aden will continue to widen if rifting and spreading continues. nullEarth’s Major Plates The Earth’s surface is composed of a strong, rigid layer known as the lithosphere. The lithosphere is broken into pieces known as tectonic plates. Lithospheric plates are thinnest in the oceans (<100 km thick) and may be more than 250 km thick on the continents. There are 7 major plates and over a dozen smaller plates. nullThe lithospheric plates overlie a weaker region of the mantle known as the asthenosphere. The rocks in the asthenosphere are near their melting point and are relatively weak and ductile. The asthenosphere allows the plates to move above it. Plates move slowly but continuously - generally on the order of a ~5 cm/year.nullNote that the plates generally include a continent or a portion of a continent AND a portion of the ocean floor. Each plate moves as a coherent unit relative to othersnullPlate Boundaries Interactions between plates occur along their boundaries — the locations of most earthquakes indicate the plate boundaries where they are moving relative to one another. nullPlate Boundaries There are 3 distinct types of plate boundaries that are defined based upon the relative movement between the plates 1. Divergent boundaries - where plates move apart (diverge) 2. Convergent boundaries - where plates move into one another (converge) 3. Transform fault boundaries - where plates grind past one anothernullThis figure illustrates the different major types of plate boundaries. We just considered two types of divergent boundaries: mid-ocean ridges and continental rifts. Now we will consider the different types of convergent boundaries. nullConvergent BoundariesWe discussed that new crust is continually be made at the mid-ocean ridges (constructive margins). Since the size of the Earth remains the same, portions of the crust must be consumed or destroyed at approximately the same rate that it is being made. Convergent margins are also known as destructive margins since oceanic crust is destroyed or consumed. Most convergent margins are associated with a subduction zone where one plate is descending into the asthenosphere beneath another plate. nullThe surface expression of a subduction zone is a deep-ocean trench - these trenches maybe thousands of km long, 50-100 km wide, and 8-12 km deep. nullThe map shows the world’s oceanic trenches. Note that the Pacific is Nearly encircled in deep-ocean trenches.nullConvergent Boundaries: Oceanic-Continental ConvergenceOceanic-continental convergence occurs when leading edge of one plate is composed of continental rocks (granitic) and the other is oceanic (basaltic). The denser oceanic plate dives beneath (subducts) the lower-density continental plate. Lower density granitic rocks tend to float in the asthenosphere. Dewatering of the subducted slab causes melting in the wedge of the asthenosphere above it. The magma that is produced is buoyant and rises through the mantle toward the Earth’s surface. nullThe magma that is produced in the asthenosphere is basaltic in composition. As the magma rises, it must penetrate through the thick continental (granitic) rocks. As it assimilates the continental rocks, the composition of the magma changes from mafic to intermediate. The magma results in volcanic activity along a line parallel to the subduction zone known as a continental volcanic arc. Examples of continental volcanic arcs include the Cascade volcanoes such as Mt. Ranier and Mt. St. Helens and the volcanoes of the Andes mountains along the west coast of South America. Mt. St. HelensnullnullConvergent Boundaries: Oceanic-Oceanic ConvergenceOceanic-oceanic convergence occurs when the leading edge of both plates consists of oceanic crust. These plate boundaries have many of the same features as in oceanic-continental convergence. In oceanic-oceanic convergence, the line of volcanoes forms a string of islands parallel to the subduction zone known as a volcanic island arc. Examples of island arc systems include the Aleutian Islands, Tonga, Indonesia, and Japan.nullConvergent Boundaries: Continental-Continental ConvergenceContinental-continental convergence defines a plate margin where the leading edge of both plates contains continental crust. This type of plate boundary is associated with mountain-building.nullContinent-continent convergence usually begins as oceanic-continental convergence (ex. Andes). As the oceanic crust is subducted, a continental block on the subducting plate may approach the continent. The Himalayan mountains were formed by the collision of the Indian subcontinent into the Asian mainland. nullThese figures show the convergence of India into Asia over the last 71 million years. nullTransform Fault BoundariesTransform plate boundaries are where plates slide past one another. Most transform boundaries are associated with mid-ocean ridges where they form linear breaks in the ridge system. The active transform boundary exists between the two offset ridge segmentsnullThis animation illustrates the offset portions of a mid-ocean ridge and movement along a transform fault. nullAlthough most transform boundaries occur in the ocean basins, some cut through continental crust. The San Andreas fault system connects a spreading center in the Sea of Cortez with a spreading center of the coast of northern California. nullStudy this figure and make sure that you understand the details.nullTesting and Evidence of Plate Tectonics Plate tectonics is a scientific theory that allow us to make predictions. Data and observations about the Earth are compared to predictions from the theory of plate tectonics to test its validity. As we’ve seen, the distribution of deep-ocean trenches, earthquakes and volcanoes supports the plate tectonics. There are numerous other lines of evidence that provide validity for the theory including: Data from ocean drilling Hot spots Magnetic reversalsnullEvidence: Ocean Drilling An active program of sampling and drilling in the seafloor has provided considerable evidence in support of plate tectonics. The figure shows the age of the seafloor. The pattern is as is predicted by the theory of plate tectonics. The seafloor is very young at the mid-ocean ridges and gets progressively older as a function of distance from the ridge. nullEvidence: Ocean Drilling The figure shows the thickness of sediment on the seafloor throughout the ocean basins. The seafloor at the mid-ocean ridges is young and has essentially no sedimentary cover. Generally, the sedimentary cover increases with distance from the mid-ocean ridge. nullEvidence: Hot Spots Mapping of the seafloor indicates that there are linear chains of volcanic islands structures known as seamounts. nullThe Hawaiian Islands are part of a chain of islands and seamounts extending to the Aleutian trench. The Big Island of Hawaii is the only island in the chain with active volcanism. Radiometric dating of the volcanic rocks of this chain indicate that they get progressively older as a function of distance from the Island of Hawaii. nullThe origin of the volcanic islands and seamounts of this and other chains is from an anomalously hot portion of the mantle known as a hot spot that remains relatively stationary. As the Pacific plate moves over the hot spot, volcanoes are formed from magmas generated by the hot spot. As the plate continues to move, the volcano moves off of the hot spot and becomes extinct but is replaced by a new one directly above the hot spot. These observations are consistent with the theory of plate tectonics and support it. nullEvidence: Magnetic Reversals The Earth is like a giant bar magnet. When volcanic rocks crystallize, some minerals (ex. magnetite) orient themselves to the Earth’s magnetic field - preserving a record of the orientation of the Earth’s magnetic field. The figure shows that the Earth’s magnetic field is currently oriented so that magnetic lines of force are entering the Earth near the north pole. nullCareful mapping of the remnant magnetic field of the seafloor indicates that it contains stripes of alternating magnetic polarity. In the figure, regions of normal polarity are indicated in white where magnetic north is coincident with the geographic north. The regions in red represent seafloor that crystallized when the Earth’s magnetic field appears to have reversed itself. It has since been recognized that the Earth’s magnetic field has reversed polarity many times in the past - known as magnetic reversals. nullThe figure shows several stages of development along a mid-ocean ridge during magnetic reversals. nullThe magnetic stripes in the seafloor are a record of the magnetic reversals that have occurred. Note that the stripes are generally parallel to the axis of the mid-ocean ridge. The more complex structure is due to movement and volcanism along transform faults. These magnetic data were very strong early evidence for seafloor spreading and plate tectonics. nullThe Breakup of Pangaea Now that we understand plate tectonics, we can use geologic data to reconstruct Pangaea and model the movement of the continents during the last 200 million years. By about 150 m.y. ago, the N.Atlantic began to open. ~130 m.y. ago, the S. Atlantic was opening and India began its journey north toward Asia. ~44 m.y. ago, India began to collide with Asia forming the Himalayan Mtns.nullnullWhat Drives Plate Motions? We have described plate motions but have not really defined the forces that drive the plates to move. This is an active area of research and there is a diversity of opinions. Most geologists agree on the following points about the driving forces for plate motion:1. The Earth’s mantle is convecting - hotter rocks rise buoyantly and cooler denser rocks sink. This motion helps drive plate motion. Mantle convection and plate tectonics are part of the same system. Density differences due to the unequal distribution of heat within the Earth’s mantle ultimately drive the mantle convection cells and plate motion.