Plate Tectonics 1 New Evidence For Wegener’s Theory of Continental Drift Wegener’s theory of continental drift was out of favor with the scientific community until new technology provided evidence to support his ideas. Seismographs revealed a pattern of volcanoes and earthquakes Radiometric dating of rocks revealed a surprisingly young oceanic crust. Echo sounders revealed the shape of the Mid-Atlantic Ridge and other features on the sea floor. Locations of shallow and deep focus earthquakes 2 3 Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc. 4 Rock types and glacial features match up across the continents. Seafloor Spreading - A Key Idea Seafloor spreading was an idea proposed in 1960 to explain the features of the ocean floor. It explained the development of the seafloor at the Mid-Atlantic Ridge. Convection currents in the mantle were proposed as the force that caused the ocean to grow and the continents to move. The Mid-Atlantic Ridge conforms to the shape of the adjacent continents. The inset shows the central rift. 5 Sea-Floor Spreading Provides a mechanism for continental drift Sea floor moves away from oceanic ridge at rate of 1 - 6 cm/yr Plunges beneath continent or island arc - subduction Driving force may be mantle convection 6 7 Paleomagnetism Magnetic minerals align to magnetic field when rocks solidify Can determine orientation of magnetic field in past by measuring magnetism “locked” in rocks Orientation and dip of past magnetic fields give evidence of (apparent) polar wander Different continents give different polar wander paths 8 9 Confirmation of the Theory of Plate Tectonics Paleomagnetism: strips of alternating magnetic polarity at spreading regions. The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes. 10 Tying It All Together - The Theory Of Plate Tectonics The ideas of continental drift and seafloor spreading were tied together in the theory of plate tectonics. Main points of the theory include: • Earth’s outer layer is divided into lithospheric plates • Earth’s plates “float” on the asthenosphere • Plate movement is powered by convection currents in the asthenosphere, and the downward pull of a descending plate’s leading edge. 11 Evidence for Plate Motion Magnetic anomalies - predict rate of motion and sea floor ages Seismicity along transform faults Direct measurement The Major Lithospheric Plates 12 Types of Plate Boundaries Divergent Transform Convergent – Ocean-ocean – Ocean-continent – Continent-continent 13 Plate Boundaries The lithospheric plates interact with the neighboring plates in several ways. Divergent plate boundaries – Boundaries between plates moving apart, further classified as: Divergent oceanic crust – for example, the Mid-Atlantic Ridge Divergent continental crust - for example, the Rift Valley of East Africa. Extension of divergent boundaries causes splitting and rifting. 14 Divergent Boundary Mid-oceanic ridge develops Oceanic ridge - site of formation of new oceanic crust Initial rifting, extension, faulting Creates new ocean basin Passive continental margins face ridge; marine sediment covers rift zone edges 15 16 17 Transform Boundaries Plates slide past each other Usually occur at oceanic ridge Causes offset of ridges May intersect a continent San Andreas Fault Plate Boundaries Transform plate boundaries - locations where crustal plates move past one another, for example, the San Andreas fault. Translation at transform boundaries causes shear. 18 Plate Boundaries Convergent Plate Boundaries - Regions where plates are pushing together can be further classified as: Oceanic crust toward continental crust - for example, the west coast of South America Oceanic crust toward oceanic crust - occurring in the northern Pacific Continental crust toward continental crust – one example is the Himalayas Compression at convergent boundaries produces buckling and shortening. Convergent: Ocean-Ocean Trench and volcanic arc form - convex toward subducting plate Benioff Zone of earthquakes Andesitic volcanism in volcanic arc Accretionary wedge of ocean sediment 19 Ocean-Continent Convergence Active continental margin Subduction of oceanic lithosphere beneath continental lithosphere - trench Magmatic arc and intrusions - andesitic volcanoes on the continent Benioff Zone 20 Convergent: Continent-Continent Two continents approach each other Ocean disappears Continents collide, neither are subducted Crust is thickened Forms mountain belt in continental interior 21 22 Motion of Plate Boundaries Boundaries move as well as plates Ridge crests may jump to a new position Convergent boundaries can migrate or jump Transform boundaries can shift What Causes Plate Motions Convection in mantle Deep mantle vs. two layer convection Convection due to plate motion Ridge push; slab pull; trench suction Mantle plumes and Hot Spots Hawaii and Yellowstone volcanism 23 Hot Spots Hot Spots: Surface expression of plumes of magma. (A volcanic island chain can form when a plate passes over a hot spot and a stationary mantle plume. Atolls and Guyots Atolls and Guyots: Coral formations and submerged volcanic mountains. Guyots were once volcanic peaks above sea level. They were eroded by wave action as they sank beneath the surface of the water. 24 History of Continental Positions Pangaea split up about 200 m.y. ago New ocean formed - Atlantic Ocean Pacific Ocean is becoming smaller as Atlantic Ocean becomes larger Continents have been in motion for at least 2 b.y. Plate Tectonics Explains: Ocean Ridge System - high heat flow, basalt eruptions, rift valley, shallow focus earthquakes Oceanic Trenches - low heat flow, deep focus earthquakes, andesitic volcanism Age of the Sea Floor - youngest at ridge, oldest at trench; no sediments at ridge crest 25 Wilson Cycle refers to the3-3 sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins. Stages in basin history are: Embryonic - rift valley forms as continent begins to split. Juvenile - sea floor basalts begin forming as continental fragments diverge. Mature - broad ocean basin widens, trenches eventually develop and subduction begins. Declining - subduction eliminates much of sea floor and oceanic ridge. Terminal - last of the sea floor is eliminated and continents collide forming a continental mountain chain. 26.