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Transcript
Unit 4 THE FORCES WITHIN EARTH Theory of Continental Drift A German scientist, Alfred Wegener, propose that continents had once been together. His publication was called, “The Origin of Continents and Oceans.” Wegener called his idea, “Continental Drift”. Alfred Wegener - 1912 • Proponent of the Continental Drift Theory/Hypothesis • German meteorologist Old Idea • There was a commonly held belief that continents simply ‘drifted’ through the ocean floor pushing up material in front, forming mountain belts. • THINK OF A BOAT MOVING THROUGH WATER PUSHING WATER UP IN FRONT OF IT! Theory of Continental Drift Hundreds of millions of years ago, a supercontinent called “Pangaea” existed. At approximately 200 million years ago, this landmass split into smaller pieces and drifted apart pushing up folded mountains until they reached their present positions. Continental Drift - EVIDENCE 1) Fit of the Continents: Atlantic Continental coastlines appear to fit together like a jigsaw puzzle. For example, South America and Africa. Continental Drift - EVIDENCE 2) Fossil Correlation: Fossils of similar organisms found in matching fossil beds on different continents. E.G. South America and Africa. E.g. Fossil fern plant (Glossopteris) and an aquatic reptile (Mesosaurus). Wegener’s Fossil Correlation Continental Drift - EVIDENCE 3) Rock Types and Structures: Type of rock on neighboring continents, shield areas, line up. E.G. Shield area of South America and shield area of Africa line up. On neighboring continents, structural similarities such as folded mountains line up . E.g. The Appalachian Mountains and Caledonian Mountains. Continuous Mountains Similar Rock Types Continental Drift - Evidence 4) Ancient Climates: Paleoclimates Glacial deposits found in South America, Africa, India, and Australia line up. Coal deposits that form in tropical swamps line up on different continents. Glacial striations in South America and Africa line up when continents are reunited. Glacial Striations SO WHAT WAS WRONG WITH THE THEORY/HYPOTHESIS OF CONTINENTAL DRIFT? Main criticism centered around how the continents move (i.e. the mechanism). Wegener suggested that Earth’s rotation and lunar gravitational forces (i.e. moon’s tidal influence) caused continents to drift (in a westward motion). He also suggested that large, sturdier continents broke through the oceanic crust (like an ice breaker). BUT IS THIS REALISTIC? BOO WEGENER! Early Contributions To Plate Tectonics Alexander DuToit: Mapped the fit of the continents more accurately by using the continental shelves instead of the actual coastlines. E.g. Africa and South America. Also proposed the formation of Laurasia and Gondwanaland. (Two pieces from the split of Pangaea). Early Contributions To Plate Tectonics The question Wegener failed to answer was explained by a scientist named Arthur Holmes. Holmes suggested that continents and the ocean floor move due to forces in the asthenosphere (i.e. upper mantle). NO SUCH THING AS DRIFITNG! Mantle material moves up at ridges and move away in opposite directions moving the ocean floors and continents. WHAT IS BEING TALKED ABOUT HERE? Convection Currents. Convection Current Convection Current Early Contributions To Plate Tectonics Harry Hess and Robert Deitz: Through observations of the ocean floor, these two guys proposed the “Sea Floor Spreading” hypothesis. Convection Current The sea-floor can be no greater than 200 million years old. Early Contributions To Plate Tectonics Convection and Seafloor Spreading Convection currents in the asthenosphere, proposed by Arthur Holmes, and the seafloor spreading idea, propose by Harry Hess and Robert Deitz, provides evidence for a mobile Earth. By combining the seafloor spreading theory with continental drift ( which can now be better explained by convection currents) and earthquake information, the new Theory of Plate Tectonics arose, which could better explain crustal movements. J. Tuzo Wilson – CANADIAN! Mapped earthquakes and volcanoes over several years. He put forth the idea that “Earth consisted of several fragments called plates. Not just one rigid, solid layer! There are 21 known plates! Huge contribution to the proposed Plate Tectonic Theory. Besides the idea of plates, Wilson also discovered stationary hotspots, which explained volcanism within plates. He also discovered transform faults along divergent plate boundaries. (WILL DEAL WITH THESE LATER). Plate Tectonic Theory Theory of Plate Tectonics States: “Earth’s crust is divided into approximately twenty (21) rigid slabs called tectonic plates.” These tectonic plates are in continuous slow motion relative to each other. As the plates move (thanks to the convection currents in the asthenosphere) the continents are carried along on top like a conveyor belt. (DRAW DIAGRAM ON WHITEBOARD) When plates collide at plate margins or when continents collide with other continents, mountain ranges are forced up. • Tectonic Plate is a massive, irregularly shaped slab of solid rock. (i.e. slabs of the lithosphere). • See Figure 19.17 on Pages 528-529. Shows the plates being moved by convection currents. The edges of these plates are called boundaries. • Plates are composed of oceanic crust only or oceanic and continental crust. • Oceanic crust is mostly basaltic in composition. Made from basalt (lava) and gabbro (magma). • Continental crust is largely granitic in composition. Made from rhyolite (lava) and granite (magma). • Plate boundaries can be mapped by tracing the ocean ridges, ocean trenches and fault zones. Plate Boundary Type • It is determined by the motion of the plates. Because oceanic lithosphere is created at ridges and destroyed at subduction zones (trenches), the oceanic basins are continuously being recycled and are relatively young.( ~200mya) Destroyed Created Sample Problem Explain why it is impossible for oceanic crust to be older than 200 million years. Answer: A geologic process called sea floor spreading causes the ocean floor to move.Ocean floor is created at oceanic ridges and is destroyed or consumed within Earth at subduction zones. This process of recycling the ocean floor occurs within a span of 180 - 200 million years. Ocean Depths During World War II, geologists carried out studies of the sea floor. Two important findings include: 1) Oceanic Ridges - long sinuous ridges that occupy the middle of the Atlantic Ocean. They also exist in the Indian Ocean and the eastern part of the Pacific Ocean. 2) Oceanic Trenches - deep trenches along the margins of continents, particularly surrounding the Pacific Ocean (i.e. the Pacific Ring Of Fire). Three Types of Plate Boundaries Include: 1) Divergent Boundary Two plates move apart. Upwelling molten material from the mantle creates new ocean floor. Features on the ocean floor are called ridges. This is where new ocean crust (ocean floor) is created. Tensional forces cause the two plates to move apart. * Look at directions on convection currents. Basaltic magma Ocean Moho Ocean Crust Magma Continental Crust Mid-Ocean Ridges: ALSO CALLED CONSTRUCTIVE MARGINS SINCE NEW OCEAN CRUST IS BEING CREATED. Ocean floor features: Trenches Sea mounts Mountain chains Fracture zones Sediments on the Ocean Floor: Three Types of Plate Boundaries Include: 2) Convergent Boundary Two plates move together. One slab of the lithosphere (i.e. one plate) is consumed into the mantle. It descends beneath the overriding plate. This is called subduction. Ocean Crust Ocean trenches are formed. Old crust is destroyed at these boundaries. Some can be really deep. Compressional forces cause the plates to move together. * Look at directions on convection currents. Trench Volcanic Island Arc Harry’s Idea Pacific Ocean Floor: Look at the Aleutian Trench WHY IS THE EARTH NOT GETTING ANY BIGGER? Ridge volcanism at divergent plate boundaries is responsible for the construction of new oceanic crust. BUT Subduction zones at convergent plate boundaries are responsible for the destruction of old oceanic crust. RECYCLING OF CRUST! Three Types of Plate Boundaries Include: 3) Transform Boundary (Discovered By J. Tuzo Wilson) Two plates move past each other (grind) in opposite directions. Lithosphere (or crust) is not created or destroyed at these boundaries. No vertical movement. For example, San Andreas Fault in California and Alpine Fault in New Zealand . Shearing forces cause the plates to move past one another. These forces are caused by the directions of the convection currents. Birds Eye View or Top View J. Tuzo Wilson’s transform faults! Ridges are not totally linear! Transform faults often link divergent and convergent plate boundaries! Divergant Boundaries and Rift Valleys Textbook – Figure 19.19 – Page 533 Upwelling of molten material from the mantle creates tensional forces. This tension pulls the continents apart causing the central region to drop down (normal faulting) forming a Rift Valley (top diagram). Continents split! Tensional forces cause the plates to continually move apart forming new ocean crust and a new ocean when the crust thins enough (middle diagram). A rift valley has evolved into a divergent plate boundary when ridge volcanism and seafloor spreading is occurring. Two segments move apart continually in opposite directions. E.g. East African Rift Valley Evidence For Plate Tectonics Six Pieces of Evidence: 1) Earthquakes and Volcanoes 2) Paleomagnetism 3) 4) 5) 6) Polar wandering Magnetic Reversals and Seafloor Spreading Ocean Drilling Hot Spots Reference: Tarbuck and Lutgens 1) Earthquakes and Volcanoes Earthquakes and volcanoes occur in rather limited belts. These belts mark the locations of Plate Boundaries. The largest active belt of earthquakes and volcanoes in the world is “The Pacific Ring of Fire”. Mt. Fuji DOTS = LOCATIONS OF EARTHQUAKES AND VOLCANOES Hugo Benioff suggested that this Earthquake pattern shows a plate subducting (sinking) into the mantle. Wadati-Benioff Zone Intermediate -Focus Shallow-Focus Deep-Focus Draw Map View Of This On Whiteboard! 2) Paleomagnetism (Fossil Magnetism) Iron-rich minerals become magnetized in the direction of the magnetic field at the time when the rock solidified. If the rocks move or if the magnetic poles change, the rocks retain “remember” the location of the magnetic poles at the time they formed. When the shield rocks on the continents (e.g. North America and Europe) were tested their ancient magnetism no longer lines up with the modern magnetic poles. Therefore, the continents (and the plates they are on) must have moved. Draw Map View Of This On Whiteboard! Shields of N.A. and Europe on opposite sides of the Mid-Atlantic Ridge! 3) Polar Wandering A plot of this magnetism showed that the magnetic pole appeared to change position considerably over the past 500 million years. Either the magnetic pole had moved with time, known as polar wandering, or the rocks had moved, continental drift and/or plate tectonics. Plate Tectonic theory is believed to be the best explanation (Not Continental Drift). 4) Magnetic Reversals and Seafloor Spreading The earth’s magnetic field reverses polarity (south becomes north and north becomes south). Basaltic lavas solidifying during a time of reverse polarity would display opposite magnetism as rocks forming today. Rocks with magnetism the same as our present magnetic field is said to have normal polarity, while rocks with opposite polarity is said to have reverse polarity. Can be seen in: 1) successive lava flows on volcanoes; and 2) the basaltic rock making up the ocean floor (seafloor spreading). Magnetic Reversals and Seafloor Spreading At oceanic ridges, the plates move apart and new basaltic rock is added to each plate. The magnetism of these basaltic rocks appear to alternate to produce identical magnetic patterns on both sides of oceanic ridges. This is strong evidence to support seafloor spreading and therefore, plate tectonics. Normal Polarity Convection Currents Reverse polarity Asthenosphere Showing seafloor spreading at a ridge (divergent plate boundary). You can also see both normal and reverse polarity (same on both sides). 5) Ocean Drilling Deep Sea Drilling Project collected convincing evidence for the theory of Plate Tectonics. The age of the sediment increased with increasing distance from the ridge (on both sides). The age of the basaltic rocks increased with increasing distance from the ridge (on both sides). Ocean Drilling – Evidence of Seafloor Spreading (1) Seafloor Sediment and (2) Age Sediment thickness increases in both directions away from the ridge. The age of the oceanic crust (rocks) increases in both directions away from the ridge. Younger Older Map showing ages of oceanic crust. 6) Hot Spots (i.e. intraplate volcanism) In the Pacific Ocean, a chain of volcanoes and seamounts extend from the Hawaiian Islands to the Aleutian trench. A plume of magma exists beneath Hawaii, and the Pacific plate moves over this stationary magma chamber (i.e. hot spot). Each time the plate moves, the magma burns through to create a new volcanic island. This confirms that the tectonic plates do move. Dates of the seamounts and islands shows that the age increases the farther away you go from Hawaii (and the hot spot). First proposed by J. Tuzo Wilson Yellowstone National Park is another example of this! Older Asthenosphere Hot Spot Plate Boundaries - Review 1) Divergent Boundary Ridge volcanism Seafloor Spreading Tensional Forces Mid-Oceanic Ridges Shallow-focus Earthquakes 2) Convergent Boundary Subduction volcanism Compressional Forces Trenches Earthquakes Three types: O-O, O-C, and C-C. 3) Transform Boundary No volcanoes Shallow-focus Earthquakes Three Types of Convergent Plate Boundaries: 1) Ocean – Ocean Collisions Compressional forces cause plates to move together (both of equal density), one slab of lithosphere is subducted into the mantle initiating volcanic activity which creates volcanoes. (i.e. Island Arcs) Curved arc on the surface! Ocean trenches are formed at these boundaries. Subducting plate is basaltic. Subducting sediments are granitic. Mix the two and the result is andesitic magmas and lavas (diorite and andesite). SHIELD VOLCANOES Examples: Japan arc, Japan, Mariana Arc, Philippines, Aleutian Arc, Aleutian Islands. 2) Ocean – Continent Collisions Compressional forces cause an ocean plate and a continent plate to move together. The more dense ocean plate sinks into the asthenosphere. This region called a subduction zone. Lithosphere is destroyed. At depths, the oceanic plate melts producing magmas, which rise and melts its way up through the continental crust. The basaltic mantle and basaltic oceanic crust melts together with continental rocks (granitic) to produce granitic magma and lava (granite and rhyolite). Some magma may reach the surface and erupt through composite volcanoes as violent volcanic eruptions. A volcanic arc is created on the surface of the continent. Examples include Rocky Mountains and Andes Mountains. Ocean – Continent Convergent Boundary If the subduction occurs beneath continental crust, a continental volcanic arc is produced. Composite volcanoes that are granitic in composition (granite and rhyolite). 3) Continent – Continent Collisions Compressional forces cause two continental plates to move together. Because of the low density of continental crust neither plate will subduct and the two plates ram into one another forming mountains. Such a collision occurred when India collided with Asia forming the Himalayas. An ocean existed between two continents and both had continental shelves. The compressed shelves create the mountains once the oceanic crust has been subducted and the process of subduction ceases! The Appalachians in Newfoundland is another example. Mountain Building The processes that produce a mountain system is orogenesis. Mountain systems show evidence of great forces. The building up of mountains is directly related to plate tectonics and in particular, Convergent Plate Boundaries (two types of collisions relate to mountain building). Mountains and Convergent Plate Boundaries 1) Ocean – Continent Convergent Boundary Mountain Chain Over-riding Plate Mountains and Convergent Plate Boundaries 1) Ocean – Continent Convergent Boundary The Andes Mountains in South America and the Rocky Mountains in Western Canada are examples. Andes Mountains Pacific Plate South American Plate Mountains and Convergent Plate Boundaries 2) Continent – Continent Convergent Boundary Mountains and Convergent Plate Boundaries 2) Continent – Continent Convergent Boundary Himalayan Mountains Plate Tectonics and Newfoundland ~ 600 million years ago, North America was part of a much larger supercontinent (Rodina). This supercontinent broke apart. A mid-ocean ridge developed along the break and the Iapetus Ocean began to open. ~540 million years ago, the convection currents shifted and subduction began. As the continents “drifted” or pushed together, the ocean floor was squeezed and then pushed upward to form the Appalachian Mountains. Subduction and abduction occurred! Volcanoes were also occurring! Once the Iapetus Ocean had been subducted and abducted, a new supercontinent formed called Pangaea. Iapetus Ocean North American Plate African Plate Plate Tectonics and Newfoundland The Geological Formation of Newfoundland This mountain range now exists throughout central and western Newfoundland; however, it has been weathering and eroding for quite some time. ~225 million years ago, the forces in Earth’s mantle again reversed and slowly began to pull the continents apart again (divergent plate boundary again). The Atlantic Ocean began to open and is still opening (to the east of the island). New Divergent Boundary North Americam Plate African Plate During this process a small bit of Africa got left behind! The eastern part of Newfoundland was once a part of the African plate. North Americam Plate African Plate Left Behind New Divergent Boundary African Plate Plate Tectonics and Newfoundland The Geological Layout of Newfoundland Humber (Western) Zone Has been a part of the North American plate for at least the last billion years. Central (Zone) Mobile Belt Remnants of island arcs and the ancient Iapetus ocean crust. Avalon (Eastern) Zone Once part of the African plate, which remained attached as Pangaea split 200 million years ago. Humber Zone Central Mobile Belt Avalon Zone Sample Problem Use the diagram and your knowledge of the theory of Plate Tectonics to explain how the three geologic zones of the island portion of Newfoundland and Labrador were formed. Cape Ray/Baie Verte Line Fault HermitageDover Fault A B Answer: It is thought that the geology of the Island of Newfoundland resulted long ago when the Iapetus Ocean was closing. The North American plate collided with the African plate and as a result a portion of the Iapetus Ocean floor was sandwiched in between. Zone “A” is referred to as the Western Zone and was part of the North American plate. Zone “C” is referred to as the Eastern Zone and is thought to be part of the African plate. Zone “B” is referred to as the Central Mobile Belt (or Central Zone) and is thought to be once part of the ancient Iapetus Ocean. C Crustal Deformation Deformation- a general term that refers to all changes in the original form and /or size of a rock body. It may also produce changes in the location and orientation of rocks. MOST CRUSTAL DEFORMATION OCCURS ALONG PLATE BOUNDARIES! Force - that which tends to put a stationary object in motion or change the motion of moving bodies. Stress- the amount of force applied to a given area. Three types of forces/stresses that produce crustal deformation: 1.) Compressional - A force or pressure that attempts to flatten or squeeze a material. ( reduces volume). Stresses or forces that shorten a rock body. 2.) Tensional - A stretching force on an object. Stresses or forces that elongate a rock body. 3.) Shear- Any external force acting perpendicular to the material. Stresses that cause two separate rock bodies to slide past one another. Crustal Deformation Three types of deformation include: 1.) Elastic- A temporary shape change that is self-reversing after the force is removed. The object returns to its original shape. 2.) Brittle- Deformation that results in fracturing or breaking and is permanent. 3.) Ductile- When rocks bend or flow, like clay, it is permanent. (solid state flow) Crustal Deformation Factors affecting deformation: 1.) Temperature The colder the Earth materials, the more brittle the deformation will be. The warmer the Earth materials, the more elastic or ductile the deformation will be. 2.) Confining Pressure Will only result in elastic or ductile deformation. Brittle deformation typically occurs on the surface with no pressure from above. 3.) Rock Type All rock types can be deformed, but sedimentary rocks could be more easily deformed since they are softer. 4.) Time Elastic or ductile deformation occurs over a long period of time. Rapid deformation (short time) tends to cause brittle deformation. Faults Fault A break or crack in Earth’s crust (rock layers) along which subsequent motion occurs. Rapid, continual tensional (or compressional) forces, usually at shallower depths, result in brittle deformation (faulting). Three parts of a Fault include: 1) Hanging Wall - The top part of the rock above the fault plane/line. 2) Foot Wall – The bottom part of the rock below the fault plane/line. 3) Fault Plane - The surface that separates the two moving pieces. Fault Plane HW FW You could draw a vertical line down through the rock blocks. You will “hit” the HW first and you will “hit” the FW second. Two Major Types of Faults: 1) Dip-Slip - Normal (i.e. tensional forces) Horst and graben (i.e. tensional forces) Reverse (i.e. compressional forces) Thrust (i.e. compressional forces) 2) Strike-Slip (i.e. transform) - Left-lateral (shear forces) - Right-lateral (shear forces) Dip-Sip Faults 1) Normal Fault (dip-slip) Caused by tensional forces. Hanging wall drops in relation to the foot wall. Hanging Wall 2) Reverse Fault (dip-slip) Caused by compressional forces. Foot Wall Hanging wall moves upward in relation to the foot wall. Dip-Slip Faults Horst (dip-slip) An uplifted block of crust Graben bounded by two normal faults. Caused by tensional forces. Graben (dip-slip) A valley formed by the downward displacement of a block of crust bounded by two faults. Caused by tensional forces. Horst Dip-Slip Faults Hanging Wall 3) Thrust Fault (dip-slip) Caused by Compressional forces. Hanging wall moves upwards over foot wall. Low angle reverse fault. Less than 45 degrees Foot Wall Strike-Slip Faults Transform Fault (strike-slip) Caused by shearing forces. Two plates slide side by side. No vertical movement. Place yourself on a fault block and ask yourself which way the opposite block is moving. Left or right? Folds The bending of rock layers into a wave like pattern due to slow, continual, compressional forces, usually at depth. (i.e. ductile deformation). Folds Parts of a fold include: 1) Anticline Caused by compressional forces. Crust moves upward forming a hill. Referred to as an up-fold. Anticline 2) Syncline Caused by compressional forces. Crust moves downward forming a valley. Referred to as a down-fold. Syncline Draw a horizontal line. Is an A formed?