Plate Tectonics PDF

Plate Tectonics: A Unifying Theory  Unifying Theory A unifying theory is one that helps  explain a broad range of diverse observations  interpret many aspects of a science on a grand scale  and relate many seemingly unrelated phenomena Plate tectonics is a unif...

Plate Tectonics: A Unifying Theory  Unifying Theory A unifying theory is one that helps  explain a broad range of diverse observations  interpret many aspects of a science on a grand scale  and relate many seemingly unrelated phenomena Plate tectonics is a unifying theory for geology.  Plate Tectonics Plate tectonics helps to explain  earthquakes  volcanic eruptions Tectonic interactions  formation of affect mountains  atmospheric and oceanic  location of circulation and climate continents  geographic distribution,  location of ocean basins  evolution and extinction of organisms  distribution and formation of resources  Looking at the world map, what do you notice about the shape of the continents? What do you notice when you look closely at this world map? The thing is…..the world did not look like what it looks now millions of years ago How is this possible?!?!?  At one time all land masses were connected into one piece called Pangaea Continental drift theory  The continents have shifted their position over geologic time o Pangaea began to split apart 200 million years ago North America Laurasia Greenland Eurasia Pangaea Africa West G. S.America Gondwanaland Antarctica East G. Australia India  Alfred Wegener and the Continental Drift Hypothesis German meteorologist Credited with hypothesis of continental drift-1912 in a scientific presentation – published a book in 1915.  Alfred Wegener and the Continental Drift Hypothesis He proposed that all landmasses  were originally united into a supercontinent  he named Pangaea from the Greek meaning “all land” He presented a series of maps  showing the breakup of Pangaea He amassed a tremendous amount of geologic, paleontologic, and climatologic evidence  Wegener’s Evidence Shorelines of continents fit together  matching marine, nonmarine and glacial rock sequences from Pennsylvanian to Jurassic age for all five Gondwana continents including Antarctica Mountain ranges and glacial deposits  match up when continents are united into a single landmass  Earth’s Magnetic Field Earth as a giant dipole magnet  magnetic poles essentially coincide with the geographic poles  may result from different rotation speeds of outer core and mantle  Magnetic Field Varies Strength and orientation of the magnetic field varies  weak and horizontal at the equator  strong and vertical at the poles  Paleomagnetism Paleomagnetism is a remnant magnetism in ancient rocks  recording the direction and the strength of Earth’s magnetic field at the time of the rock’s formation When magma cools  below the Curie point temperature  magnetic iron-bearing minerals align  with Earth’s magnetic field  Polar Wandering In 1950s, research revealed  that paleomagnetism of ancient rocks showed orientations different from the present magnetic field Magnetic poles apparently moved.  The apparent movement was called polar wandering. The best explanation  Different continents had  is stationary poles and different paths. moving continents  Polar Wandering The successive positions of dated paleo poles trace out a curving line known as the apparent polar wander path Example: Layer of basalts record magnetic change over time Inclination and declination indicate change in position  Magnetic Reversals Earth’s present magnetic field is called normal,  withmagnetic south near the north geographic pole  and magnetic north near the south geographic pole At various times in the past,  Earth’smagnetic field has completely reversed,  with magnetic south near the north geographic pole  and magnetic north near the south geographic pole This is referred to as a magnetic reversal  Magnetic Reversals Measuring paleomagnetism and dating continental lava flows led to the realization that magnetic reversals existed the establishment of a magnetic reversal time scale Jigsaw-Puzzle Fit of Continents Continental Fit Fig. 3-4, p. 39 Jigsaw-Puzzle Fit of Continents Matching mountain Matching glacial ranges evidence Matching Fossils  The Perceived Problem with Continental Drift Most previous geologists did not accept the idea of moving continents  Therewas no suitable mechanism to explain  how continents could move over Earth’s surface Interest in continental drift only revived when  new evidence from studies of Earth’s magnetic field  and oceanographic research  showed that the ocean basins were geologically young features  Mapping Ocean Basins Ocean mapping revealed a ridge system more than 55,000 km long, the most extensive mountain range in the world The Mid-Atlantic Ridge  isthe best known part of the system and divides the Atlantic Ocean basin in two nearly equal parts Atlantic Ocean Basin Mid-Atlantic Ridge  Seafloor Spreading Harry Hess, in 1962, proposed the theory of seafloor spreading:  Continents and oceanic crust move together  Seafloor separates at oceanic ridges where new crust forms from upwelling and cooling magma, and the new crust moves laterally away from the ridge  The mechanism that drives seafloor spreading was thermal convection cells in the mantle hot magma rises from mantle to form new crust cold crust subducts into the mantle at oceanic trenches, where it is heated and recycled  Confirmation of Hess’s Hypothesis In addition to mapping mid-ocean ridges,  ocean research also revealed magnetic anomalies on the sea floor A magnetic anomaly is a deviation from the average strength of Earth’s Magnetic field  Confirmation of Hess’s Hypothesis The magnetic anomalies were discovered to be parallel and symmetrical with the oceanic ridges Normal polarity Reverse polarity  Oceanic Crust Is Young Seafloor spreading theory indicates that  oceanic crust is geologically young because  it forms during spreading  and is destroyed during subduction Radiometric dating confirms  the oldest oceanic crust is less than 200 million years old whereas oldest continental crust is 3.96 billion yeas old Age of Ocean Basins  Age of Ocean Basins Example old Nazca Plate subducted beneath South America Plate  Plate Tectonics Plate tectonic theory is based on the simple model that  the lithosphere is rigid  it consists of oceanic and continental crust with upper mantle  it consists of variable-sized pieces called plates  with plate regions containing continental crust up to 250 km thick  and plate regions containing oceanic crust up to 100 km thick  Plate Map Numbers represent average rates of relative movement, cm/yr Plate Tectonics and Boundaries The lithospheric plates overlie hotter and weaker semiplastic asthenosphere Movement of the plates  resultsfrom some type of heat-transfer system within the asthenosphere As plates move over the asthenosphere  they separate, mostly at oceanic ridges  they collide, in areas such as oceanic trenches  where they may be subducted back into the mantle There are three types of plate boundaries 1. Divergent plate boundary 2. Convergent plate boundary 3.Transform plate boundary  Divergent Boundaries Divergent plate boundaries  or spreading ridges, occur where plates are separating and new oceanic lithosphere is forming. Crust is extended thinned and fractured The magma  originates from partial melting of the mantle  is basaltic  intrudes into vertical fractures to form dikes  or is extruded as lava flows  Divergent Boundaries Successive injections of magma  cool and solidify  form new oceanic crust  record the intensity and orientation  of Earth’s magnetic field Divergent boundaries most commonly  occur along the crests of oceanic ridges  such as the Mid-Atlantic Ridge Ridges have  rugged topography resulting from displacement  of rocks along large fractures  shallow earthquakes  Divergent Boundaries Divergent boundaries are also present  under continents during the early stages  of continental breakup Beneath a continent,  magma wells up, and  the crust is initially elevated, stretched and thinned  Rift Valley The stretching produces fractures and rift valleys. During this stage,  magma typically  intrudes into the fractures  and flows onto the valley floor Example: East African Rift Valley  Narrow Sea As spreading proceeds, some rift valleys  will continue to lengthen and deepen until  the continental crust eventually breaks  a narrow linear sea is formed,  separating two continental blocks  Examples: Red Sea Gulf of California Modern Divergence  View looking down the Great Rift Valley of Africa. Little Magadi soda lake  Ocean As a newly created narrow sea  continues to spread,  it may eventually become  an expansive ocean basin  such as the Atlantic Ocean basin is today, separating North and South America from Europe and Africa by thousands of kilometers  Atlantic Ocean Basin North America Europe Atlantic Ocean basin South America Africa  Convergent Boundaries Older crust must be destroyed  atconvergent boundaries  so that Earth’s surface area remains the same Where two plates collide,  subduction occurs when an oceanic plate descends beneath the margin of another plate  The subducting plate moves into the asthenosphere is heated and eventually incorporated into the mantle  Convergent Boundary: plates are moving towards each other and are colliding (3 types)  Convergent Boundaries Convergent boundaries are characterized by  deformation  volcanism  mountain building  metamorphism  earthquake activity  valuable mineral deposits Convergent boundaries are of three types:  oceanic-oceanic  oceanic-continental  continental-continental 1. Ocean-Ocean plate boundary Island arcs are created (a pattern of volcanic islands created from a subduction zone that is located off the coast)  2. Oceanic-Continental Boundary Create subduction zones, trenches Create near coast volcanoes Benioff shear zones (a pattern of earthquakes as an ocean plate grinds down the underneath side of a continent)  Oceanic-Continental Boundary An oceanic-continental plate boundary  occurswhen a denser oceanic plate  subducts under less dense continental lithosphere Magma generated by subduction  risesinto the continental crust to form large igneous bodies  or erupts to form a volcanic arc of andesitic volcanoes  Example: Pacific coast of South America Oceanic-Continental Boundary Where the Nazca plate in the Pacific Ocean is subducting under South America  the Peru-Chile Trench marks subduction site  and the Andes Mountains are the volcanic arc Benioff Shear Zones the initial point where the rocks rupture in the crust is called the focus. The epicenter is the point on the land surface that is directly above the focus.  3. Continent-Continent Boundary Two approaching continents are initially  separatedby ocean floor that is being subducted  under one of them, which, thus, has a volcanic arc When the 2 continents collide  the continental lithosphere cannot subduct Its density is too low,  although one continent may partly slide under the other  Continent-Continent Boundary When the 2 continents collide  theyweld together at a continent-continent plate boundary,  where an interior mountain belt forms consisting of deformed sedimentary rocks igneous intrusions metamorphic rocks fragments of oceanic crust Earthquakes occur here 3.Continental-Continental Boundary Example: Himalayas in central Asia  Earth’s youngest and highest mountain system  resulted from collision between India and Asia  began 40 to 50 million years ago  and is still continuing  Himalayas  Transform Boundaries The third type of plate boundary is a transform plate boundary  where plates slide laterally past each other  roughly parallel to the direction of plate movement Movement results in fracture zone  zone of intensely shattered rock  numerous shallow earthquakes The majority of transform faults  connect two oceanic ridge segments  and are marked by fracture zones  Transform Boundaries Other kinds of transform plate boundaries  connect two trenches  or connect a ridge to a trench  or even a ridge or trench to another transform fault Transforms can also extend into continents Transform Boundaries Example: San Andreas Fault, California  separatesthe Pacific plate from the North American plate  connects ridges in Gulf of California with the Juan de Fuca and Pacific plates  Many of the earthquakes in California result from movement along this fault  Plate Movement Measurements Satellite-laser ranging  bounce laser beams from a station on one plate  off a satellite, to a station on another plate  measure the elapsed time  after sufficient time has passed to detect motion  measure the elapsed time again  use the difference in elapsed times to calculate  the rate of movement between the two plates Hot spots  determine the age of rocks and their distance from a hot spot  divide the distance by the age  this gives the motion relative to the hot spot so  (possibly) the absolute motion of the plate  Hot Spots and Mantle Plumes Hot spots are locations where  stationary columns of magma  originating deep within the mantle, called mantle plumes  slowly rise to the surface Mantle plumes remain stationary although some evidence suggests they may move When plates move over them  hot spots leave trails of extinct, progressively older volcanoes called aseismic ridges which record the movement of the plates  Hot Spots and Mantle Plumes Example: Emperor Seamount-Hawaiian Island chain Age increases plate movement Plate Movement at Hot Spot  Speed of Spreading  Atlantic Ocean – 2-3 cm/year  South Pacific Ocean – 15-18 cm/year  What Is the Driving Mechanism of Plate Tectonics? Most geologists accept some type of convective heat system  as the basic cause of plate motion In one possible model,  thermal convection cells are restricted to the asthenosphere  What Is the Driving Mechanism of Plate Tectonics? In a second model, the entire mantle is involved in thermal convection. In both models,  spreading ridges mark the rising limbs of neighboring convection cells  trenches occur where the convection cells descend back into Earth’s interior  What Is the Driving Mechanism of Plate Tectonics? In addition to a thermal convection system,  some geologists think that movement may be aided by  “slab-pull” the slab is cold and dense and pulls the plate  “ridge-push” rising magma pushes the ridges up and gravity pushes the oceanic lithosphere away from the ridge and toward the trench  How Are Plate Tectonics and Mountain Building Related? An orogeny is an episode  of intense rock deformation or mountain building It results from compressive forces  related to plate movement During subduction,  sedimentary and volcanic rocks  are folded and faulted along the plate margin Most orogenies occur along oceanic-continental  or continental-continental plate boundaries  How Does Plate Tectonics Affect the Distribution of Life? Present distribution of plants and animals  islargely controlled by climate  and geographic barriers Barriers create biotic provinces  each province is a region characterized  by a distinctive assemblage of plants and animals Plate movements largely control barriers  When continents break up, new provinces form  When continents come together, fewer provinces result  As continents move north or south they move across temperature barriers  How Does Plate Tectonics Affect the Distribution of Life? Physical barriers caused by plate movements include  Example: Isthmus  intraplate volcanoes of Panama creates a barrier to marine  island arcs organisms  mid-ocean ridges  mountain ranges  subduction zones  Pacific  Caribbea n Plate Tectonics and the Distribution of Natural Resources Plate movements influence the formation and distribution of some natural resources such as  petroleum  natural gas  some mineral deposits Metal resources related to igneous and associated hydrothermal activity include  copper  silver  gold  tin  lead  zinc Plate Tectonics and the Distribution of Natural Resources Magma generated by subduction can precipitate and concentrate metallic ores  Example: copper  Bingham Mine in Utah is a deposits in western huge open-pit copper mine Americas Plate Tectonics and the Distribution of Natural Resources Another place where hydrothermal activity  can generate rich metal deposits  is divergent boundaries Example: island of Cyprus in the Mediterranean  Copper concentrations there formed as a result  of precipitation adjacent to hydrothermal vents  along a divergent plate boundary Example: Red Sea  copper, gold, iron, lead, silver ,and zinc deposits  are currently forming as sulfides in the Red Sea,  a divergent boundary World palaeogeography in the Early Jurassic (~200 Ma) when the Middle East was part of Gondwana passive margin submerged under the warm equatorial waters of Neo-Tethys.  Acknowledged source 1.www.wvup.edu/.../Geology%20101%20chapter2%20Plate%20tectonics.ppt 2.www.kenston.k12.oh.us/khs/.../science.../seafloor-spreading.ppt 3. Lutgens, F.K. and Tarbuck, E.J. (2006). Essentials of Geology. Pearson Prentice Hall. 4. Chernicoff, S. and Whitney, D. (2007). An Introduction to Physical Geology. Pearson Prentice Hall.