Location via proxy:   [ UP ]  
[Report a bug]   [Manage cookies]                
SlideShare a Scribd company logo

Plate Tectonics

•Download as PPT, PDF•
•
Bhavik A Shah
Bhavik A Shah

The movement of earth's plate is responsible for the formation of continents.How it works ? what are the principle's?

1 of 42
GOVERNMENT ENGINEERING COLLEGE, BHAVNAGAR. Civil Engineering Department
Plate TectonicsPlate Tectonics GEOTECHNICS AND APPLIED GEOLOGY
Table of ContentsTable of Contents  Plate Tectonics: The Beginning  Layers of the Earth  What are Tectonic Plates- movement? Tectonic Plate boundaries 1. Convergent boundary 2. Divergent boundary 3. Transform Boundary (San Andreas Fault, J. Tuzo Wilson)  Faults and Folds  Plate movement over Geologic Time  Creation and change of Landforms Volcanic eruptions (Mt. St. Helens) Mountain building events (Appalachian vs. Himalayas)  Tectonics and the Ocean Floor Continental margins  Passive  Active
Plate Tectonics: The BeginningPlate Tectonics: The Beginning  At the beginning of the 20th Century, scientists realized that that they could not explain many of the Earth’s structures and processes with a single theory. Many scientific hypotheses were developed to try and support the conflicting observations. One hypotheses was continental drift, which was proposed by Alfred Wegener in a series of papers from 1910 to 1928.  The principal thought of continental drift theory is that the continents are situated on slabs of rock, or plates, and they have drifted across the surface of the Earth over time; however, originally, they were all joined together as a huge super-continent at one time.  In the 1960’s, the theory of continental drift was combined with the theory of sea-floor spreading to create the theory of plate tectonics. Alfred Lothar Wegener (1880-1930) Background
Additional evidence supporting the continental drift theory: 1. Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica and South America. 2. Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal could not have swum across the existing Atlantic Ocean! 3. Glacial deposits found in current warm climates and warm climate plant fossils found in what is now the Arctic. 4. Nearly identical rock formations found on the east coast of U.S. and the west coast of Europe and eastern South America and western Africa.  The idea for Wegener's theory was sparked by his observation of the nearly perfect “fit” of the South American and African continents. The “fit” of two continents. Plate Tectonics: ThePlate Tectonics: The BeginningBeginning
What are Tectonic Plates?What are Tectonic Plates? The Earth is made up of three main layers: 1. The Core is at the center of the Earth. It is divided into an inner and outer core. 2. The Mantle is the layer surrounding the core. The upper mantle is partially molten and called the asthenosphere. 3. The Crust, or lithosphere, is the rigid outer-most layer. Thick continental crust underlies continents, and thin, very dense oceanic crust underlies oceans. Inner CoreInner Core Outer CoreOuter Core MantleMantle CrustCrust ThicknessThickness 1,216 km 2,270 km 2,900 km Continental 35-90 km Oceanic 7-8 km PhysicalPhysical PropertiesProperties Solid Iron; extremely dense (17 g/cm3 ) Molten Iron, very dense (12 g/cm3 ) Made mostly of silicates of magnesium and iron; moderately dense. Behaves like melted plastic in upper-most section (5.5 g/cm3 ) Made of silicate rocks and oxides; slightly dense; rigid. (2.67-3.3 g/cm3 ) Percentage ofPercentage of Earths’ MassEarths’ Mass 30% 65% 5% MantleCore Crust The layers of the earth.
Earth’s Sublayers  Lithosphere: This layer combines the rigid crust plus the upper-most mantle. (Greek: Rock)  Asthenosphere: Partially molten part of upper mantle (Greek: weak). Tectonic plates are able to move about on top of the softer, partially molten asthenosphere. The outer- most layers of the earth. What are Tectonic Plates? (continued)What are Tectonic Plates? (continued)
What are Tectonic Plates? (continued)What are Tectonic Plates? (continued)  The Earth’s crust consists of about a dozen large slabs of rock, or PLATES, that the continents and oceans rest on. These tectonic plates can move centimeters per year— about as fast as your fingernails grow up to 15cm/yr in some places.  Tectonic plates are also called lithospheric plates because the crust and the upper-most mantle make up a sub-layer of the earth called the lithosphere. The plates can move about because the uppermost mantle, or the asthenosphere, is partially molten and possesses a physical property called plasticity, allowing the strong, rigid plates of the crust to move over the weaker, softer asthenosphere.  The word TECTONICS is of Greek origin and it means “to build.” The word “tectonism” refers to the deformation of the lithosphere. This deformation most notably includes mountain building. Plates and relative plate motion. South Carolina is located on the North American plate
 Tectonic plates, or lithospheric plates, are constantly moving, being created, and consumed simultaneously. The motion sometimes results in earthquakes, volcanoes, and mountain ranges at the plate boundaries.  Plate motion is driven by heat escaping from the mantle. The constant movement of heat in the mantle leads to circular convection currents. These hot convective cells are similar to the rolling boil that occurs when water is heated on a stovetop. The flowing mantle has also been compared to a “conveyor belt,” moving the rigid plates in different directions.  Fundamentally, convection occurs due to uneven heating and different densities within the liquid. What are Tectonic Plates? (continued)What are Tectonic Plates? (continued) = Convection currents within the mantle. CoreCore Upwelling Downwelling Spreading ridge Subduction zone
There are three basic ways that plates interact with one another. Each of these plate boundaries has the potential to create different geological features. Plate BoundariesPlate Boundaries 1. When plates collide with each other = Convergent boundary 2. When plates separate from each other = Divergent boundary 3. When plates slide along side each other = Transform boundary The tectonic plates and plate boundaries.
1. Convergent Boundary:1. Convergent Boundary: Ocean-Continent CollisionOcean-Continent Collision  Because the oceanic crust is more dense than continental crust, when these two collide, the continental crust rides up over the oceanic crust and the oceanic crust is bent down and subducted beneath the continental crust. This is called a subduction zone, where the old oceanic crust is dragged downward and “recycled.”  Deep-sea trenches are created at subduction zones. Trenches are narrow, deep troughs parallel to the edge of a continent or island arc. They typically have slopes of 4-5 degrees, and they are often 8-10 km deep. The deepest spots on earth are found in oceanic trenches. The Mariana Trench is the deepest ocean depth at 11 km (35,798 ft) below sea level. Figure depicting oceanic crust subducting beneath continental crust, creating volcanoes on the land surface above, and a deep- sea trench off of the coast.
 If two continental plates collide, mountain building usually takes place because they are both relatively low in density.  Earthquake activity at these boundaries is common; however, because igneous activity is different from ocean-continent collisions, volcanoes are rare. Convergent Boundary:Convergent Boundary: Continent-Continent CollisionContinent-Continent Collision Constructive mountain building during continent-continent collision. The Himalaya mountains are still forming today as the Ind-Australian Plate collides with the Eurasian Plate  Examples: The Himalayan and the Appalachian mountain chains.
CONVERGENT BOUNDARY:CONVERGENT BOUNDARY: OCEAN-OCEAN COLLISIONOCEAN-OCEAN COLLISION  If 2 oceanic plates collide, the older, denser one is subducted downward into the mantle and a chain of volcanic islands can form, called a volcanic arc.  Example: Mariana Islands (Mariana Trench). It is deeper than the earth’s tallest mountain is tall. Mariana Trench: 11,000 meters deep. Mt. Everest: 8850 meters high.  The interaction of the descending oceanic plate causes incredible amounts of stress between the plates. This usually causes frequent earthquakes along the top of the descending plate known as the “Benioff Zone.” The focii of Benioff earthquakes can be as deep as 700 km below sea level. Oceanic/oceanic collision resulting in a chain of island arcs. Benioff Zone
 Most volcanoes form above subduction zones because as one slab is subducted beneath the other, the interaction of fluids and geothermal heat form new magma. The new magma then rises upward through the overlying plate to create volcanoes at the surface.  The Andes Mountains are home to many volcanoes that were formed at the convergent boundary of the Nazca and South American Plates. Right: Red dots indicate general locations of volcanoes along western coast of South America. Convergent Boundary: VolcanismConvergent Boundary: Volcanism  Left: Image of the Nazca Plate subducting beneath the South American Plate.
 At a divergent boundary, two oceanic plates pull apart from each other through a process called sea-floor spreading.  Sea-floor spreading was proposed by Harry Hess in the early 1960’s. Hess proposed that hot magma rises from the asthenosphere and up into existing ocean crust through fractures. The crust spreads apart making room for new magma to flow up through it. The magma cools, forming new sea floor and resulting in a build-up of basaltic rock around the crack, which is called a mid-ocean ridge. 2. Divergent Boundary: Sea-floor Spreading2. Divergent Boundary: Sea-floor Spreading  New material is constantly being created. This is the opposite of a convergent boundary, where material is constantly being destroyed. Sea-floor spreading at an oceanic divergent boundary.
 The world’s longest mountain chain is underwater. It is 56,000 km long and is called the Mid-Atlantic Ridge. Divergent Boundary: Mid-Atlantic RidgeDivergent Boundary: Mid-Atlantic Ridge  The Mid-Atlantic Ridge is considered a slow-spreading ridge, spreading at about 1-2 centimeters per year. An example of a fast-spreading ridge is the East Pacific Rise, which spreads at about 6-8 centimeters per year. Mid-Atlantic Ridge EastPacific Rise Satellite bathymetry of the East Pacific Rise spreading ridge.
Sea-floor ExplorationSea-floor Exploration  The Deep Sea Drilling Project (DSDP) began in 1968 aboard the research vessel Glomar Challenger. This ship was outfitted with a drill rig capable of drilling into the ocean floor beneath many kilometers of water.  Before this type of research was available, scientists had to rely on dredging or grabbing single rock samples from line weights on boats.  Scientists quickly determined that continental crust is thicker than oceanic crust, that continental crust is less dense than oceanic crust, and that the youngest seafloor is located at mid-ocean ridges and increases in age with distance from the ridge.  Before technology like this, most people thought the ocean floor was flat and smooth. This is understandable as 2/3 of the Earth’s surface lies under oceans. The Glomar Challenger (1968)
Sea-floor ExplorationSea-floor Exploration  In the mid-1960’s, magnetometer surveys at sea indicated that alternating magnetic anomalies existed within marine rock. These anomalies were aligned parallel to the Mid-Atlantic Ridge forming stripe-like patterns on the sea floor, and they were symmetrically distributed on either side of the Mid-Atlantic Ridge.  Geologists Fred Vine and Drummond Matthews first noticed these symmetric patterns of magnetic “stripes” and concluded that this pattern of magnetic anomalies at sea matched the pattern of magnetic reversals over time.  The Earth’s magnetic field has changed over the past 100 million years approximately once every 250,000 years. When the magnetic field “reverses” from today’s “normal” N-S direction it becomes a period of magnetic reversal. The normal magnetic field is considered a positive anomaly, and, when the magnetic field is reversed, it is considered a negative anomaly.  The Earth’s magnetic field flows from a southerly direction to northerly direction. This is what makes the arrow on our compasses point towards north.
Sea-floor Exploration: Age-datingSea-floor Exploration: Age-dating  The relative age of the sea floor can be determined by changes in magnetic polarities of the earth.  These periods of normal and reversed polarity are recorded in the magnetic minerals within the newly formed sea floor at mid-ocean ridges. Scientists can see a clear pattern of normal and reversed magnetization in the rock record that shows up as “magnetic stripes” on either side of mid-ocean ridges, which gives them a relative time of how old the sea floor is and lets them compare ocean basins to each other. Above: Figure depicting magma flowing out from a spreading ridge, cooling and spreading out symmetrically about the ridge to produce successively older rock as you travel in either direction from the ridge. The magnetized minerals within the rock “tell” how old it is. T1 T2 T3 Oldest Youngest
 The oldest oceanic crust found is ~ 180 million years old.  Because the age of the earth is ~4,600 million years, we know that oceanic crust is continually being formed at spreading ridges and being destroyed at subduction zones. The ocean floor is constantly changing shape and size through the processes of sea-floor spreading and subduction. The relative ages of the ocean floor.  Because no older ocean crust has been found, recycling of the ocean crust takes place about every 180 million years. Sea-floor Exploration: Age-datingSea-floor Exploration: Age-dating
3. Transform Boundary3. Transform Boundary  When two plates slide past each other moving in different directions or the same direction, it is termed a transform boundary and is characterized by a transform fault and earthquake activity.  An example of a transform fault is the San Andreas Fault in California. Here the North American Plate joins the Pacific Plate. The difference in plate motion along the contact (fault) leads to a buildup of strain energy that sometimes slips releasing a huge amount of energy and causing an earthquake. An aerial photo of the San Andreas fault line. Movement between the 2 plates at the San Andreas Transform Fault.
 J. Tuzo Wilson was a geophysicist who was fascinated by Wegener's theory of continental drifting. He was also inspired by Harry Hess, whom he studied under at Princeton University in the 1930’s.  Wilson is recognized today for advancing plate-tectonic theory by introducing three major concepts: 1. Wilson (1963) introduced the concept of a “stationary hotspot”, where the heat from the mantle could affect the thin crust, forming volcanic islands such as Hawaii. 2. Wilson (1965) proposed the “transform” boundary as the third type of plate boundary. They commonly offset ocean ridges and trenches, and transform the motion between the offset. Unlike ridges and trenches, transform faults offset the crust horizontally, without creating or destroying crust. 3. Wilson proposed what is known today as the Wilson Cycle. This concept explains the origin for the Appalachian Mountains. The “cycle” goes through the sequence: 1. The splitting of a supercontinent, 2. the opening of an ocean basin, 3. the closing of the ocean basin, 4. the collision of continents and formation of mountains. Transform Boundary (continued)Transform Boundary (continued)
 Plate tectonics cause many of the physical features that we see on earth today like volcanoes and earthquakes, but also many other geological features like faults. Faults are planar rock fractures along which movement has occurred. In the example above, it is obvious that the fence has been offset to the right, therefore it is called a right lateral strike-slip fault  Similar in movement, a strike-slip fault occurs through shearing when two blocks move in horizontal but opposite directions of each other. Depending on the direction of offset, it can be a “right-lateral offset” or a “left-lateral offset.” The photograph above displays a light-colored pegmatite vein offset to the right in a schistose matrix.  A transform fault occurs at a transform plate boundary like the San Andreas Fault in California. It connects two of the other plate boundaries. Mountain building processes, faults and foldsMountain building processes, faults and folds Right-lateral offset
 Faults caused by blocks of crust pulling apart under the forces of tension are called normal faults. Entire mountain ranges can form through these processes and are known as fault block mountains (examples: Basin and Range Province, Tetons).  In a normal fault, the hanging-wall block moves down relative to the foot-wall block.  The footwall is the underlying surface of an inclined fault plane.  The hanging wall is the overlying surface of an inclined fault plane. Faults: Normal FaultsFaults: Normal Faults Hanging Wall Footwall Relative movement of two blocks indicating a normal fault. Diagrammatic sketch of the two types of blocks used in identifying normal faults. Hanging Wall Foot Wall Footwall block Hanging wall block
 Faults caused by blocks of crust colliding under the forces of compression are called reverse faults.  Reverse faults form during continent-continent collision. Usually, there is also accompanying folding of rocks.  During reverse faulting, the hanging wall block moves upward (and over) relative to the footwall block. Faults: Reverse FaultsFaults: Reverse Faults Hanging Wall Foot Wall Footwall block Hanging wall block Relative movement of two blocks indicating a reverse fault. Diagrammatic sketch of the two types of blocks used in identifying reverse faults. Hanging Wall Footwall
FoldingFolding  During mountain building processes, rocks can undergo folding as well as faulting.  Sometimes rocks deform ductilely, particularly if they are subjected to heat and pressure. At elevated temperature and pressure within the crust, folds can form from compressional forces.  Entire mountain rages, like the Appalachians, have extensive fold systems. Z-fold in schist with white felsic dike (hammer for scale). Near Lake Murray, South Carolina. Large fold in outcrop (geologists for scale). Near Oakridge, Tennessee, Appalachian Mtns.
Plate Movement Over Geologic TimePlate Movement Over Geologic Time  Alfred Wegener proposed that all of the continents once formed a “supercontinent” called Pangaea.  From the Greek language, ‘pan’ meaning ALL and ‘gaea’ meaning EARTH. It was thought to have come together and formed approximately 200 million years ago.  Evidence for a supercontinent included: 1. Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica, and South America. 2. Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal could not have swum across the existing Atlantic Ocean! 3. Glacial deposits found in current warm climates and warm-climate plant fossils found in what is now the Arctic. 4. Nearly identical rock formations found on the east coast of U.S. and the west coast of Europe and on eastern South America and western Africa.
1. About 1,100 million years ago, a super- continent called Rodinia existed (pre- Cambrian). 2. Rodinia broke apart, and about 400 million years ago, the oceans began to close up to form a pre-Pangea (early Devonian). 3. Pangea formed around 250 million years ago and animals could migrate from the north to the south pole (Early Triassic).
5. Gondwananland split to form Africa, South America, Antarctica, Australia and India. Laurasia split to form North America, Eurasia (minus India) and Greenland. 4. Pangaea began to break apart into 2 halves approximately 200 million years ago (Early Jurassic). The northern half is called Laurasia and the southern half is called Gondwanaland. These two huge continents were separated by a body of water called the Tethys Sea. 6. Around 15 million years ago, the continents finally looked like they do today
In 50 million years, it is possible that the Mediterranean could close due to the collision of Africa with Europe. Australia may eventually join Asia. It is though that in another 250 million years, another Pangea will form. Continents in the future?Continents in the future? 30
Volcanoes  Most volcanoes form above subduction zones because as one slab is subducted beneath the other, it causes melting, forming new magma, which then rises upward. This is why most volcanoes are found near plate boundaries.  Volcanoes are constructive because they add new rock, form new islands, and create new land masses. However, they are also destructive when they erupt and change the landscape (possibly even the climate). Creation and Change of LandformsCreation and Change of Landforms Above: Diagrammatic sketch of a volcanic arc located above a subduction zone Left: Active volcanoes found along plate boundaries
Volcanic Eruptions:Volcanic Eruptions:  Being hot, magma is less dense than the solid rock surrounding it. This enables the liquid magma to rise up through cracks in the solid rock and collect in pockets within the earth, called magma chambers. Eventually some of the magma pushes through vents and fissures in Earth's surface, causing an eruption that may be violent or quiet. Once the magma reaches the earth’s surface, it is called lava.  The explosiveness of an eruption depends on the viscosity of the magma. Viscosity is the resistance of a liquid to flow. If the magma is very thick and viscous, gases can build up within the magma. Finally, when threshold is reached, there is a violent eruption from the built up pressure of the gases in the magma chamber.  Most volcanoes form above subduction zones because as one slab is subducted beneath the other, melting occurs, forming new magma, which then rises upward along the plate boundary. Volcano Cross-sectional view looking through the side of a volcano Core Crust Upper mantle Lower mantle Magma chamber Land surface Vent
VOLCANIC ERUPTIONS:  If magma is less viscous, or more fluid, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano and violent explosions are rare.  Sometimes, huge clouds of ash race down mountainsides destroying almost everything in their path. These are pyroclastic clouds. They can travel faster than a high-speed train. The ash produced from an eruption will fall back to the ground and suffocate humans, animals, and plants. 33
 A famous volcano that erupted in the U.S. nearly 30 years ago was Mt. St. Helens in the Cascade Range of Washington.  The volcanic activity of Mt. St. Helens is caused by the ongoing subduction of the Juan de Fuca plate under the North American plate. It is part of the Cascade Volcanic arc, which includes some 160 active volcanoes along the west coast.  On May 18, 1980 Mount St. Helens erupted in what was the most economically destructive volcanic event in U.S. history. Nearly 60 people lost their lives and 250 homes were destroyed.  Before the volcano exploded, it was 2,950 m tall. Afterwards, it was 2,550 m tall, and instead of a sharp peak at the summit, a mile-wide horseshoe shaped crater was left. The debris avalanche associated with the eruption was nearly 3 cubic kilometers in volume. Volcanoes: Mount St. Helens Before eruption… After eruption…  When Mount St. Helens exploded, it had not erupted for 123 years. Most people thought Mount St. Helens was a beautiful, peaceful mountain and not a dangerous volcano.
Mountain-building forces  When two continental plates collide at a convergent boundary, the process produces a mountain range. Compressional forces drive the mountain building process.  The Appalachians, the Alps, and the Himalayas were formed through compression.  The Himalayan mountain chain was formed approximately 150 million years ago. When we think of the Himalayas, we think of very high, steep mountains, cliffs, and, of course, Mt. Everest.  In contrast, when we consider our own Appalachians, which formed about 400 million years ago, we see more subdued topography than in the Himalayas. This is because the process of wind and water erosion have eroded hundreds of vertical feet of land surface from the area and reduced high jagged mountains into the rolling hills present today. Continent/ continent convergence The Matterhorn, Alps The Appalachian Mountains.
 Even though approximately 70% of the earth’s surface is covered by water, it is the study of sea- floor rocks, sediment, and topography that provide most of the information used to validate the theory of plate tectonics. Ongoing research on the sea floor continues to provide clues to the Earth’s dynamic structural processes.  Research and technology have granted us access to study both the shallow continental shelves as well as the deep abyssal plains of the ocean basins. These include rock dredging, coring, drilling, geophysical studies (seismic and magnetic), and age dating.  One advancement in technology is the use echo sounders, which are used to draw profiles of submarine topography. Seismic profilers also use acoustic echoes but can penetrate the bottom of the seafloor. These profilers allow us to see a “picture” of the layered structure under the sea floor. Continental margins and deep-sea landformsContinental margins and deep-sea landforms Cross-sectional view of continents grading into deep ocean basins. Note change in topography. USA, east coast USA, west coast Africa, west coast Ocean Basin Continent
Continental Margins: Continental Shelves and Slopes  All continents are surrounded by a shallow, relatively flat platform called a continental shelf and a sloping surface called a continental slope that gently descends down to the deep ocean floor.  Continental shelves vary in width depending on the type of continental margin. On the U.S. Pacific coast, it is only a few km wide, but off of the Atlantic Coast it is up to 500 km wide.  The continental shelf area has thick accumulations of young sediment and has water depths less than 200 meters.  The continental slope has a relatively steep slope (4-5 degrees) and it joins the edge of the shelf to the deep ocean floor. Relatively little is known about the slopes, as it is difficult to drill on the steep surfaces.  Beyond the continental shelf is the continental break where flat shelf ends and the steep continental slope begins. Although typical continental slopes have an incline of around 4 degrees, some active margins, like the Gulf of California, slope at about 20 degrees.  The continental slope may be marked by channels called submarine canyons that transport sediment from the shelf to the sea floor, sometimes in violent events called turbidity currents. The continental rise has been built up by these thick deposits of sediment. California Continental Margin looking east towards Los Angeles. Shelf Slope
1. Passive Continental Margins  A passive continental margin includes a continental shelf, slope, and rise and these margins gently grade into a deep abyssal plain.  The ocean floor and continent usually belong to the same continental plate.  Passive margins form on “geologically quiet” coasts, also called trailing margins, where there is no tectonic or volcanic activity, such as the eastern seaboard of the U.S.  The continental rise is a wedge of sediment that extends from the lower part of the slope to the deep sea floor sloping at about 0.5 degrees. It grades into a flat abyssal plain at around 5km in depth.  Abyssal plains are found at the base of the continental rise and are the flattest features on earth. Abyssal plains form where turbidity currents carry amounts of sediment large enough to bury and obscure the rugged relief normal found on the sea floor. Continental Shelf Continental Slope Continental break Sketch of an abyssal fan forming on the sea floor. Sediment for the fan is carried from the shelf through the canyon in violent turbidity currents and are deposited, forming a fan .
Active continental margin with trench Passive continental margin 2. Active continental margin:  An active continental margin includes a continental shelf, slope, and an ocean trench. It is the result of an ocean plate colliding with a continental plate.  Active margins are tectonically active, characterized by earthquakes, volcanoes, and mountain belts. Tectonically active margins may have a deep trench instead of a continental rise where the oceanic plate is subducted beneath the continental plate.  The west coast of the U.S. and South America are examples of active margins.  Ocean trenches are elongate features parallel to the edge of the continent and are the deepest regions on earth at over 11 km below sea level.  Ocean trenches are associated with seismic Benioff Zones, which begin at the trench and dip underneath the continent as part of the subduction zone. Earthquakes are prevalent along the Benioff Zone, which is the site of one tectonic plate descending underneath another. Cross-sectional sketch showing the trench off the active continental margin and the rise off of the passive continental margin.
 The Mid-Atlantic Ridge is an example of a divergent boundary found extending around the world dividing the world’s ocean basins. It spreads at about 1-2 centimeters per year and is made of basalt. It is 56,000 km long and 2,500 km wide, and it rises 2 –3 km above the sea floor.  The spreading of the ridge creates a rift valley running down the crest of the ridge. The valley is about 1-2 km deep and several kilometers wide- similar to the dimensions of the Grand Canyon!  Shallow earthquakes are frequent along these ridges and long deep fractures run perpendicular to the ridge.  Seamounts, guyots, and black smokers are other geological features that can be found on the deep sea floor. Satellite bathymetry of the East Pacific Rise spreading ridge. Sea floor spreading at the mid-ocean ridge and rift valley.
BY, • Bhavik Shah – 130210106049 • Dharmi Shah – 130210106050 • Ruchi Shah – 130210106051 • Yash Shah – 130210106052 • Sanket Shekadiya – 130210106053 • Anjali Solanki – 130210106054 • Digvijay Solanki – 130210106055 • Vivek Solanki – 130210106056 • Tejas Chaudari – 120210106052
THANK YOU FOR BEARING

More Related Content

Plate Tectonics

  • 3. Table of ContentsTable of Contents  Plate Tectonics: The Beginning  Layers of the Earth  What are Tectonic Plates- movement? Tectonic Plate boundaries 1. Convergent boundary 2. Divergent boundary 3. Transform Boundary (San Andreas Fault, J. Tuzo Wilson)  Faults and Folds  Plate movement over Geologic Time  Creation and change of Landforms Volcanic eruptions (Mt. St. Helens) Mountain building events (Appalachian vs. Himalayas)  Tectonics and the Ocean Floor Continental margins  Passive  Active
  • 4. Plate Tectonics: The BeginningPlate Tectonics: The Beginning  At the beginning of the 20th Century, scientists realized that that they could not explain many of the Earth’s structures and processes with a single theory. Many scientific hypotheses were developed to try and support the conflicting observations. One hypotheses was continental drift, which was proposed by Alfred Wegener in a series of papers from 1910 to 1928.  The principal thought of continental drift theory is that the continents are situated on slabs of rock, or plates, and they have drifted across the surface of the Earth over time; however, originally, they were all joined together as a huge super-continent at one time.  In the 1960’s, the theory of continental drift was combined with the theory of sea-floor spreading to create the theory of plate tectonics. Alfred Lothar Wegener (1880-1930) Background
  • 5. Additional evidence supporting the continental drift theory: 1. Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica and South America. 2. Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal could not have swum across the existing Atlantic Ocean! 3. Glacial deposits found in current warm climates and warm climate plant fossils found in what is now the Arctic. 4. Nearly identical rock formations found on the east coast of U.S. and the west coast of Europe and eastern South America and western Africa.  The idea for Wegener's theory was sparked by his observation of the nearly perfect “fit” of the South American and African continents. The “fit” of two continents. Plate Tectonics: ThePlate Tectonics: The BeginningBeginning
  • 6. What are Tectonic Plates?What are Tectonic Plates? The Earth is made up of three main layers: 1. The Core is at the center of the Earth. It is divided into an inner and outer core. 2. The Mantle is the layer surrounding the core. The upper mantle is partially molten and called the asthenosphere. 3. The Crust, or lithosphere, is the rigid outer-most layer. Thick continental crust underlies continents, and thin, very dense oceanic crust underlies oceans. Inner CoreInner Core Outer CoreOuter Core MantleMantle CrustCrust ThicknessThickness 1,216 km 2,270 km 2,900 km Continental 35-90 km Oceanic 7-8 km PhysicalPhysical PropertiesProperties Solid Iron; extremely dense (17 g/cm3 ) Molten Iron, very dense (12 g/cm3 ) Made mostly of silicates of magnesium and iron; moderately dense. Behaves like melted plastic in upper-most section (5.5 g/cm3 ) Made of silicate rocks and oxides; slightly dense; rigid. (2.67-3.3 g/cm3 ) Percentage ofPercentage of Earths’ MassEarths’ Mass 30% 65% 5% MantleCore Crust The layers of the earth.
  • 7. Earth’s Sublayers  Lithosphere: This layer combines the rigid crust plus the upper-most mantle. (Greek: Rock)  Asthenosphere: Partially molten part of upper mantle (Greek: weak). Tectonic plates are able to move about on top of the softer, partially molten asthenosphere. The outer- most layers of the earth. What are Tectonic Plates? (continued)What are Tectonic Plates? (continued)
  • 8. What are Tectonic Plates? (continued)What are Tectonic Plates? (continued)  The Earth’s crust consists of about a dozen large slabs of rock, or PLATES, that the continents and oceans rest on. These tectonic plates can move centimeters per year— about as fast as your fingernails grow up to 15cm/yr in some places.  Tectonic plates are also called lithospheric plates because the crust and the upper-most mantle make up a sub-layer of the earth called the lithosphere. The plates can move about because the uppermost mantle, or the asthenosphere, is partially molten and possesses a physical property called plasticity, allowing the strong, rigid plates of the crust to move over the weaker, softer asthenosphere.  The word TECTONICS is of Greek origin and it means “to build.” The word “tectonism” refers to the deformation of the lithosphere. This deformation most notably includes mountain building. Plates and relative plate motion. South Carolina is located on the North American plate
  • 9.  Tectonic plates, or lithospheric plates, are constantly moving, being created, and consumed simultaneously. The motion sometimes results in earthquakes, volcanoes, and mountain ranges at the plate boundaries.  Plate motion is driven by heat escaping from the mantle. The constant movement of heat in the mantle leads to circular convection currents. These hot convective cells are similar to the rolling boil that occurs when water is heated on a stovetop. The flowing mantle has also been compared to a “conveyor belt,” moving the rigid plates in different directions.  Fundamentally, convection occurs due to uneven heating and different densities within the liquid. What are Tectonic Plates? (continued)What are Tectonic Plates? (continued) = Convection currents within the mantle. CoreCore Upwelling Downwelling Spreading ridge Subduction zone
  • 10. There are three basic ways that plates interact with one another. Each of these plate boundaries has the potential to create different geological features. Plate BoundariesPlate Boundaries 1. When plates collide with each other = Convergent boundary 2. When plates separate from each other = Divergent boundary 3. When plates slide along side each other = Transform boundary The tectonic plates and plate boundaries.
  • 11. 1. Convergent Boundary:1. Convergent Boundary: Ocean-Continent CollisionOcean-Continent Collision  Because the oceanic crust is more dense than continental crust, when these two collide, the continental crust rides up over the oceanic crust and the oceanic crust is bent down and subducted beneath the continental crust. This is called a subduction zone, where the old oceanic crust is dragged downward and “recycled.”  Deep-sea trenches are created at subduction zones. Trenches are narrow, deep troughs parallel to the edge of a continent or island arc. They typically have slopes of 4-5 degrees, and they are often 8-10 km deep. The deepest spots on earth are found in oceanic trenches. The Mariana Trench is the deepest ocean depth at 11 km (35,798 ft) below sea level. Figure depicting oceanic crust subducting beneath continental crust, creating volcanoes on the land surface above, and a deep- sea trench off of the coast.
  • 12.  If two continental plates collide, mountain building usually takes place because they are both relatively low in density.  Earthquake activity at these boundaries is common; however, because igneous activity is different from ocean-continent collisions, volcanoes are rare. Convergent Boundary:Convergent Boundary: Continent-Continent CollisionContinent-Continent Collision Constructive mountain building during continent-continent collision. The Himalaya mountains are still forming today as the Ind-Australian Plate collides with the Eurasian Plate  Examples: The Himalayan and the Appalachian mountain chains.
  • 13. CONVERGENT BOUNDARY:CONVERGENT BOUNDARY: OCEAN-OCEAN COLLISIONOCEAN-OCEAN COLLISION  If 2 oceanic plates collide, the older, denser one is subducted downward into the mantle and a chain of volcanic islands can form, called a volcanic arc.  Example: Mariana Islands (Mariana Trench). It is deeper than the earth’s tallest mountain is tall. Mariana Trench: 11,000 meters deep. Mt. Everest: 8850 meters high.  The interaction of the descending oceanic plate causes incredible amounts of stress between the plates. This usually causes frequent earthquakes along the top of the descending plate known as the “Benioff Zone.” The focii of Benioff earthquakes can be as deep as 700 km below sea level. Oceanic/oceanic collision resulting in a chain of island arcs. Benioff Zone
  • 14.  Most volcanoes form above subduction zones because as one slab is subducted beneath the other, the interaction of fluids and geothermal heat form new magma. The new magma then rises upward through the overlying plate to create volcanoes at the surface.  The Andes Mountains are home to many volcanoes that were formed at the convergent boundary of the Nazca and South American Plates. Right: Red dots indicate general locations of volcanoes along western coast of South America. Convergent Boundary: VolcanismConvergent Boundary: Volcanism  Left: Image of the Nazca Plate subducting beneath the South American Plate.
  • 15.  At a divergent boundary, two oceanic plates pull apart from each other through a process called sea-floor spreading.  Sea-floor spreading was proposed by Harry Hess in the early 1960’s. Hess proposed that hot magma rises from the asthenosphere and up into existing ocean crust through fractures. The crust spreads apart making room for new magma to flow up through it. The magma cools, forming new sea floor and resulting in a build-up of basaltic rock around the crack, which is called a mid-ocean ridge. 2. Divergent Boundary: Sea-floor Spreading2. Divergent Boundary: Sea-floor Spreading  New material is constantly being created. This is the opposite of a convergent boundary, where material is constantly being destroyed. Sea-floor spreading at an oceanic divergent boundary.
  • 16.  The world’s longest mountain chain is underwater. It is 56,000 km long and is called the Mid-Atlantic Ridge. Divergent Boundary: Mid-Atlantic RidgeDivergent Boundary: Mid-Atlantic Ridge  The Mid-Atlantic Ridge is considered a slow-spreading ridge, spreading at about 1-2 centimeters per year. An example of a fast-spreading ridge is the East Pacific Rise, which spreads at about 6-8 centimeters per year. Mid-Atlantic Ridge EastPacific Rise Satellite bathymetry of the East Pacific Rise spreading ridge.
  • 17. Sea-floor ExplorationSea-floor Exploration  The Deep Sea Drilling Project (DSDP) began in 1968 aboard the research vessel Glomar Challenger. This ship was outfitted with a drill rig capable of drilling into the ocean floor beneath many kilometers of water.  Before this type of research was available, scientists had to rely on dredging or grabbing single rock samples from line weights on boats.  Scientists quickly determined that continental crust is thicker than oceanic crust, that continental crust is less dense than oceanic crust, and that the youngest seafloor is located at mid-ocean ridges and increases in age with distance from the ridge.  Before technology like this, most people thought the ocean floor was flat and smooth. This is understandable as 2/3 of the Earth’s surface lies under oceans. The Glomar Challenger (1968)
  • 18. Sea-floor ExplorationSea-floor Exploration  In the mid-1960’s, magnetometer surveys at sea indicated that alternating magnetic anomalies existed within marine rock. These anomalies were aligned parallel to the Mid-Atlantic Ridge forming stripe-like patterns on the sea floor, and they were symmetrically distributed on either side of the Mid-Atlantic Ridge.  Geologists Fred Vine and Drummond Matthews first noticed these symmetric patterns of magnetic “stripes” and concluded that this pattern of magnetic anomalies at sea matched the pattern of magnetic reversals over time.  The Earth’s magnetic field has changed over the past 100 million years approximately once every 250,000 years. When the magnetic field “reverses” from today’s “normal” N-S direction it becomes a period of magnetic reversal. The normal magnetic field is considered a positive anomaly, and, when the magnetic field is reversed, it is considered a negative anomaly.  The Earth’s magnetic field flows from a southerly direction to northerly direction. This is what makes the arrow on our compasses point towards north.
  • 19. Sea-floor Exploration: Age-datingSea-floor Exploration: Age-dating  The relative age of the sea floor can be determined by changes in magnetic polarities of the earth.  These periods of normal and reversed polarity are recorded in the magnetic minerals within the newly formed sea floor at mid-ocean ridges. Scientists can see a clear pattern of normal and reversed magnetization in the rock record that shows up as “magnetic stripes” on either side of mid-ocean ridges, which gives them a relative time of how old the sea floor is and lets them compare ocean basins to each other. Above: Figure depicting magma flowing out from a spreading ridge, cooling and spreading out symmetrically about the ridge to produce successively older rock as you travel in either direction from the ridge. The magnetized minerals within the rock “tell” how old it is. T1 T2 T3 Oldest Youngest
  • 20.  The oldest oceanic crust found is ~ 180 million years old.  Because the age of the earth is ~4,600 million years, we know that oceanic crust is continually being formed at spreading ridges and being destroyed at subduction zones. The ocean floor is constantly changing shape and size through the processes of sea-floor spreading and subduction. The relative ages of the ocean floor.  Because no older ocean crust has been found, recycling of the ocean crust takes place about every 180 million years. Sea-floor Exploration: Age-datingSea-floor Exploration: Age-dating
  • 21. 3. Transform Boundary3. Transform Boundary  When two plates slide past each other moving in different directions or the same direction, it is termed a transform boundary and is characterized by a transform fault and earthquake activity.  An example of a transform fault is the San Andreas Fault in California. Here the North American Plate joins the Pacific Plate. The difference in plate motion along the contact (fault) leads to a buildup of strain energy that sometimes slips releasing a huge amount of energy and causing an earthquake. An aerial photo of the San Andreas fault line. Movement between the 2 plates at the San Andreas Transform Fault.
  • 22.  J. Tuzo Wilson was a geophysicist who was fascinated by Wegener's theory of continental drifting. He was also inspired by Harry Hess, whom he studied under at Princeton University in the 1930’s.  Wilson is recognized today for advancing plate-tectonic theory by introducing three major concepts: 1. Wilson (1963) introduced the concept of a “stationary hotspot”, where the heat from the mantle could affect the thin crust, forming volcanic islands such as Hawaii. 2. Wilson (1965) proposed the “transform” boundary as the third type of plate boundary. They commonly offset ocean ridges and trenches, and transform the motion between the offset. Unlike ridges and trenches, transform faults offset the crust horizontally, without creating or destroying crust. 3. Wilson proposed what is known today as the Wilson Cycle. This concept explains the origin for the Appalachian Mountains. The “cycle” goes through the sequence: 1. The splitting of a supercontinent, 2. the opening of an ocean basin, 3. the closing of the ocean basin, 4. the collision of continents and formation of mountains. Transform Boundary (continued)Transform Boundary (continued)
  • 23.  Plate tectonics cause many of the physical features that we see on earth today like volcanoes and earthquakes, but also many other geological features like faults. Faults are planar rock fractures along which movement has occurred. In the example above, it is obvious that the fence has been offset to the right, therefore it is called a right lateral strike-slip fault  Similar in movement, a strike-slip fault occurs through shearing when two blocks move in horizontal but opposite directions of each other. Depending on the direction of offset, it can be a “right-lateral offset” or a “left-lateral offset.” The photograph above displays a light-colored pegmatite vein offset to the right in a schistose matrix.  A transform fault occurs at a transform plate boundary like the San Andreas Fault in California. It connects two of the other plate boundaries. Mountain building processes, faults and foldsMountain building processes, faults and folds Right-lateral offset
  • 24.  Faults caused by blocks of crust pulling apart under the forces of tension are called normal faults. Entire mountain ranges can form through these processes and are known as fault block mountains (examples: Basin and Range Province, Tetons).  In a normal fault, the hanging-wall block moves down relative to the foot-wall block.  The footwall is the underlying surface of an inclined fault plane.  The hanging wall is the overlying surface of an inclined fault plane. Faults: Normal FaultsFaults: Normal Faults Hanging Wall Footwall Relative movement of two blocks indicating a normal fault. Diagrammatic sketch of the two types of blocks used in identifying normal faults. Hanging Wall Foot Wall Footwall block Hanging wall block
  • 25.  Faults caused by blocks of crust colliding under the forces of compression are called reverse faults.  Reverse faults form during continent-continent collision. Usually, there is also accompanying folding of rocks.  During reverse faulting, the hanging wall block moves upward (and over) relative to the footwall block. Faults: Reverse FaultsFaults: Reverse Faults Hanging Wall Foot Wall Footwall block Hanging wall block Relative movement of two blocks indicating a reverse fault. Diagrammatic sketch of the two types of blocks used in identifying reverse faults. Hanging Wall Footwall
  • 26. FoldingFolding  During mountain building processes, rocks can undergo folding as well as faulting.  Sometimes rocks deform ductilely, particularly if they are subjected to heat and pressure. At elevated temperature and pressure within the crust, folds can form from compressional forces.  Entire mountain rages, like the Appalachians, have extensive fold systems. Z-fold in schist with white felsic dike (hammer for scale). Near Lake Murray, South Carolina. Large fold in outcrop (geologists for scale). Near Oakridge, Tennessee, Appalachian Mtns.
  • 27. Plate Movement Over Geologic TimePlate Movement Over Geologic Time  Alfred Wegener proposed that all of the continents once formed a “supercontinent” called Pangaea.  From the Greek language, ‘pan’ meaning ALL and ‘gaea’ meaning EARTH. It was thought to have come together and formed approximately 200 million years ago.  Evidence for a supercontinent included: 1. Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica, and South America. 2. Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal could not have swum across the existing Atlantic Ocean! 3. Glacial deposits found in current warm climates and warm-climate plant fossils found in what is now the Arctic. 4. Nearly identical rock formations found on the east coast of U.S. and the west coast of Europe and on eastern South America and western Africa.
  • 28. 1. About 1,100 million years ago, a super- continent called Rodinia existed (pre- Cambrian). 2. Rodinia broke apart, and about 400 million years ago, the oceans began to close up to form a pre-Pangea (early Devonian). 3. Pangea formed around 250 million years ago and animals could migrate from the north to the south pole (Early Triassic).
  • 29. 5. Gondwananland split to form Africa, South America, Antarctica, Australia and India. Laurasia split to form North America, Eurasia (minus India) and Greenland. 4. Pangaea began to break apart into 2 halves approximately 200 million years ago (Early Jurassic). The northern half is called Laurasia and the southern half is called Gondwanaland. These two huge continents were separated by a body of water called the Tethys Sea. 6. Around 15 million years ago, the continents finally looked like they do today
  • 30. In 50 million years, it is possible that the Mediterranean could close due to the collision of Africa with Europe. Australia may eventually join Asia. It is though that in another 250 million years, another Pangea will form. Continents in the future?Continents in the future? 30
  • 31. Volcanoes  Most volcanoes form above subduction zones because as one slab is subducted beneath the other, it causes melting, forming new magma, which then rises upward. This is why most volcanoes are found near plate boundaries.  Volcanoes are constructive because they add new rock, form new islands, and create new land masses. However, they are also destructive when they erupt and change the landscape (possibly even the climate). Creation and Change of LandformsCreation and Change of Landforms Above: Diagrammatic sketch of a volcanic arc located above a subduction zone Left: Active volcanoes found along plate boundaries
  • 32. Volcanic Eruptions:Volcanic Eruptions:  Being hot, magma is less dense than the solid rock surrounding it. This enables the liquid magma to rise up through cracks in the solid rock and collect in pockets within the earth, called magma chambers. Eventually some of the magma pushes through vents and fissures in Earth's surface, causing an eruption that may be violent or quiet. Once the magma reaches the earth’s surface, it is called lava.  The explosiveness of an eruption depends on the viscosity of the magma. Viscosity is the resistance of a liquid to flow. If the magma is very thick and viscous, gases can build up within the magma. Finally, when threshold is reached, there is a violent eruption from the built up pressure of the gases in the magma chamber.  Most volcanoes form above subduction zones because as one slab is subducted beneath the other, melting occurs, forming new magma, which then rises upward along the plate boundary. Volcano Cross-sectional view looking through the side of a volcano Core Crust Upper mantle Lower mantle Magma chamber Land surface Vent
  • 33. VOLCANIC ERUPTIONS:  If magma is less viscous, or more fluid, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano and violent explosions are rare.  Sometimes, huge clouds of ash race down mountainsides destroying almost everything in their path. These are pyroclastic clouds. They can travel faster than a high-speed train. The ash produced from an eruption will fall back to the ground and suffocate humans, animals, and plants. 33
  • 34.  A famous volcano that erupted in the U.S. nearly 30 years ago was Mt. St. Helens in the Cascade Range of Washington.  The volcanic activity of Mt. St. Helens is caused by the ongoing subduction of the Juan de Fuca plate under the North American plate. It is part of the Cascade Volcanic arc, which includes some 160 active volcanoes along the west coast.  On May 18, 1980 Mount St. Helens erupted in what was the most economically destructive volcanic event in U.S. history. Nearly 60 people lost their lives and 250 homes were destroyed.  Before the volcano exploded, it was 2,950 m tall. Afterwards, it was 2,550 m tall, and instead of a sharp peak at the summit, a mile-wide horseshoe shaped crater was left. The debris avalanche associated with the eruption was nearly 3 cubic kilometers in volume. Volcanoes: Mount St. Helens Before eruption… After eruption…  When Mount St. Helens exploded, it had not erupted for 123 years. Most people thought Mount St. Helens was a beautiful, peaceful mountain and not a dangerous volcano.
  • 35. Mountain-building forces  When two continental plates collide at a convergent boundary, the process produces a mountain range. Compressional forces drive the mountain building process.  The Appalachians, the Alps, and the Himalayas were formed through compression.  The Himalayan mountain chain was formed approximately 150 million years ago. When we think of the Himalayas, we think of very high, steep mountains, cliffs, and, of course, Mt. Everest.  In contrast, when we consider our own Appalachians, which formed about 400 million years ago, we see more subdued topography than in the Himalayas. This is because the process of wind and water erosion have eroded hundreds of vertical feet of land surface from the area and reduced high jagged mountains into the rolling hills present today. Continent/ continent convergence The Matterhorn, Alps The Appalachian Mountains.
  • 36.  Even though approximately 70% of the earth’s surface is covered by water, it is the study of sea- floor rocks, sediment, and topography that provide most of the information used to validate the theory of plate tectonics. Ongoing research on the sea floor continues to provide clues to the Earth’s dynamic structural processes.  Research and technology have granted us access to study both the shallow continental shelves as well as the deep abyssal plains of the ocean basins. These include rock dredging, coring, drilling, geophysical studies (seismic and magnetic), and age dating.  One advancement in technology is the use echo sounders, which are used to draw profiles of submarine topography. Seismic profilers also use acoustic echoes but can penetrate the bottom of the seafloor. These profilers allow us to see a “picture” of the layered structure under the sea floor. Continental margins and deep-sea landformsContinental margins and deep-sea landforms Cross-sectional view of continents grading into deep ocean basins. Note change in topography. USA, east coast USA, west coast Africa, west coast Ocean Basin Continent
  • 37. Continental Margins: Continental Shelves and Slopes  All continents are surrounded by a shallow, relatively flat platform called a continental shelf and a sloping surface called a continental slope that gently descends down to the deep ocean floor.  Continental shelves vary in width depending on the type of continental margin. On the U.S. Pacific coast, it is only a few km wide, but off of the Atlantic Coast it is up to 500 km wide.  The continental shelf area has thick accumulations of young sediment and has water depths less than 200 meters.  The continental slope has a relatively steep slope (4-5 degrees) and it joins the edge of the shelf to the deep ocean floor. Relatively little is known about the slopes, as it is difficult to drill on the steep surfaces.  Beyond the continental shelf is the continental break where flat shelf ends and the steep continental slope begins. Although typical continental slopes have an incline of around 4 degrees, some active margins, like the Gulf of California, slope at about 20 degrees.  The continental slope may be marked by channels called submarine canyons that transport sediment from the shelf to the sea floor, sometimes in violent events called turbidity currents. The continental rise has been built up by these thick deposits of sediment. California Continental Margin looking east towards Los Angeles. Shelf Slope
  • 38. 1. Passive Continental Margins  A passive continental margin includes a continental shelf, slope, and rise and these margins gently grade into a deep abyssal plain.  The ocean floor and continent usually belong to the same continental plate.  Passive margins form on “geologically quiet” coasts, also called trailing margins, where there is no tectonic or volcanic activity, such as the eastern seaboard of the U.S.  The continental rise is a wedge of sediment that extends from the lower part of the slope to the deep sea floor sloping at about 0.5 degrees. It grades into a flat abyssal plain at around 5km in depth.  Abyssal plains are found at the base of the continental rise and are the flattest features on earth. Abyssal plains form where turbidity currents carry amounts of sediment large enough to bury and obscure the rugged relief normal found on the sea floor. Continental Shelf Continental Slope Continental break Sketch of an abyssal fan forming on the sea floor. Sediment for the fan is carried from the shelf through the canyon in violent turbidity currents and are deposited, forming a fan .
  • 39. Active continental margin with trench Passive continental margin 2. Active continental margin:  An active continental margin includes a continental shelf, slope, and an ocean trench. It is the result of an ocean plate colliding with a continental plate.  Active margins are tectonically active, characterized by earthquakes, volcanoes, and mountain belts. Tectonically active margins may have a deep trench instead of a continental rise where the oceanic plate is subducted beneath the continental plate.  The west coast of the U.S. and South America are examples of active margins.  Ocean trenches are elongate features parallel to the edge of the continent and are the deepest regions on earth at over 11 km below sea level.  Ocean trenches are associated with seismic Benioff Zones, which begin at the trench and dip underneath the continent as part of the subduction zone. Earthquakes are prevalent along the Benioff Zone, which is the site of one tectonic plate descending underneath another. Cross-sectional sketch showing the trench off the active continental margin and the rise off of the passive continental margin.
  • 40.  The Mid-Atlantic Ridge is an example of a divergent boundary found extending around the world dividing the world’s ocean basins. It spreads at about 1-2 centimeters per year and is made of basalt. It is 56,000 km long and 2,500 km wide, and it rises 2 –3 km above the sea floor.  The spreading of the ridge creates a rift valley running down the crest of the ridge. The valley is about 1-2 km deep and several kilometers wide- similar to the dimensions of the Grand Canyon!  Shallow earthquakes are frequent along these ridges and long deep fractures run perpendicular to the ridge.  Seamounts, guyots, and black smokers are other geological features that can be found on the deep sea floor. Satellite bathymetry of the East Pacific Rise spreading ridge. Sea floor spreading at the mid-ocean ridge and rift valley.
  • 41. BY, • Bhavik Shah – 130210106049 • Dharmi Shah – 130210106050 • Ruchi Shah – 130210106051 • Yash Shah – 130210106052 • Sanket Shekadiya – 130210106053 • Anjali Solanki – 130210106054 • Digvijay Solanki – 130210106055 • Vivek Solanki – 130210106056 • Tejas Chaudari – 120210106052
  • 42. THANK YOU FOR BEARING