Geology 101 Outline of Last Lecture Continental Drif Great Debate Continental Drif and Paleomagnetism Polar Wandering Apparent Wandering Scientific Revolution Marine Magnetic Anomalies Magnetic Reversals Piece Observation and Hypothesis Together Outline of Current Lecture Divergent Plate Boundaries Convergent Plate Boundaries Transform Plate Boundaries Plate Boundaries along West Coast Testing the Plate Tectonics Model Lecture 5 Divergent Plate Boundaries o Most are located along the crest of oceanic ridges Oceanic ridges and seafloor spreading Elevated areas of the seafloor high heat flow and volcanism up to thousands km wide o Used to think ocean floor was flat Examples o Mid Atlantic Ridge North south pattern o East Pacific Rise Seafloor spreading occurs along these ridge Spreading rates Typical 5cm yr o Very fast at East Pacific Rise 15 cm yr At ridges hot rocks less dense elevated rise o Magma o Upwelling Reason for elevation Away cool contract thermal contraction Geology 101 The thickness and age of the oceanic lithosphere increases away from ridges o Further Evidence magnetic anomalies o Continental Rifing Splits landmasses into smaller segments along a continental rife Different stages and modern examples Upwarping of lithosphere Continental rif valley o East African Rif Valley o Basin and Range o Linear sea Red Sea o Ocean Produced by extensional forces Gravity pulls downward o Move apart Moves apart magma upwells Cools and makes new seafloor Seafloor spreading Convergent Plate Boundaries o Older portions of oceanic plates are returned to the mantle at these destructive plate margins Surface expressions of the descending plate is an ocean trench Also called subduction zone o Most earthquakes occur on convergent boundaries o Descending angles of oceanic plates at trench depend on oceanic plate age density As it becomes older it becomes more dense o Deep ocean trenches are a surface manifestation of the boundary 10 million years becomes more dense than asthenosphere at 10 million year Older larger descending angle Father away from MOR o Examples Peru Chile close to MOR young small angles More severe earthquakes around small angles Largest earthquake 1960 magnitude 9 4 Marina and Tonga old large angles far from MOR o Types of Convergent Boundaries Oceanic Continental convergence The denser oceanic slab sinks into the asthenosphere Along the descending plate partial melting of mantle rock due to water generates magma o Water makes it melt at lower temperatures The resulting volcanic mountain chain is called a continental volcanic arc o Andes o Cascades Geology 101 Oceanic Oceanic convergence When two oceanic slabs converge one descends beneath the other o Depends on density Heavier is the one that subducts Ofen forms volcanoes on ocean floor If the volcanoes emerge as islands a volcanic island arc is formed o Japan o Aleutian islands o Tonga islands o Most are in Pacific Ocean Continental Continental Continued subduction can bring two continents together Less dense buoyant continental lithosphere compared to asthenosphere o does not subduct Results in collision between the two plates Produces mountains o Himalayas o Alps o Appalachians Folding and deforming of the sediment accumulation o Balances the amount of lithosphere that is created at divergent plate boundaries by destroying subducting at convergent o Move toward each other Accommodated by one plate sliding beneath the other Lithosphere descends into asthenosphere o Deep ocean trenches develops between two converging plates and indicate where the subducting plates beings its descent beneath the overriding plate o Subduction occurs because of different densities between lithosphere and asthenosphere o The subducted plate is almost always oceanic Transform Plate Boundaries o Plates slide past one another and no new lithosphere is created or destroyed Conservative o Transform faults Most join two segments of a mid ocean ridge along breaks in the oceanic crust known as fracture zones A few cut through continental crust San Andreas Alpine Mid Atlantic Ridge is not straight because of fracture zones o Active means it can create earthquakes Different direction Geology 101 o Means of transporting oceanic crust created at ridge to be transported to site of destruction Plate Boundaries along West Coast o San Andreas transform Connects spreading centers in Gulf of CA and the Cascadia subduction zone Pacific plate moves NW past N A o Cascadia Subduction Zone Juan de Fuca plate subducts beneath North America Plate o Earthquake potentials by these movements Testing the Plate Tectonics Model o Evidence from ocean drilling 70 s Some of the most convincing evidence from drilling into ocean floor sediment Sediments oldest at each site increase in age with distance from the ridge The thickness of sediments also increases with distance from the ridge All younger than 180 million years MA o Hot spots and mantle plumes Hot spots areas of volcanism Ex Hawaii Island Chain o Different ages with a different pattern Caused by mantle plumes o Long lived cylindrically shaped upwelling of hot rock o Some originate at great depth o Some along spreading centers Iceland o Measuring plate motions Mantle plumes and plate motions Rate 6000 km 50ma Direction N NW 50 ma Paleomagnetism and Plate motions Paleomagnetism stored in rocks on the ocean floor spacing of magnetic strips East Pacific Rise spreading much faster than Atlantic