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Plate Tectonics Continental drift: An idea before its time Alfred Wegener • Proposed hypothesis in 1915 • Published The Origin of Continents and Oceans Continental drift hypothesis • Supercontinent Pangaea began breaking apart about 200 million years ago Pangaea approximately 200 million years ago What would the world have been like as a supercontinent? A) Different climate - continental interiors hotter and drier (like Mongolia today) B) Lower species diversity due to connectivity of continents and fewer shorelines C) Differences in marine environments - less shelf area D) All of the above What would the world have been like as a supercontinent? A) Different climate - continental interiors hotter and drier (like Mongolia today) B) Lower species diversity due to connectivity of continents and fewer shorelines C) Differences in marine environments - less shelf area D) All of the above Watch Pangaea Breakup 200 Million Years Ago to present QuickTime™ and a Cinepak decompressor are needed to see this picture. Which continent moves the farthest during the breakup of Pangaea? A) B) C) D) E) North America South America Africa India Australia QuickTime™ and a Cinepak decompressor are needed to see this picture. Which continent moves the farthest during the breakup of Pangaea? A) B) C) D) E) North America South America Africa India Australia QuickTime™ and a Cinepak decompressor are needed to see this picture. Ocean opens between rigid plates But oceans are parts of plates and not indepen Of continents… Continental drift hypothesis • Continents "drifted" to present positions Evidence used in support of continental drift hypothesis • Fit of continents • Fossil evidence • Rock type and mountain belts • Paleoclimatic evidence The great debate Objections to drift hypothesis • Inability to provide a mechanism capable of moving continents across globe • Wegner suggested that continents broke through the ocean crust, much like ice breakers cut through ice • Geophysics suggests this was unlikely Matching of mtn ranges on continents Once connected in one belt, now separated… Paleoclimatic evidence for Continental Drift Dismembering of an ancient ice sheet centered over the south pole Why is there no evidence for a dismembered ice sheet at the north pole at this time also? A) Continents were not located over the north pole at this time B) An ice sheet only formed over the south pole and not the north C) Global warming from greenhouse gas emissions from countries at northern latitudes melted the northern ice sheet D) Solar energy in the northern hemisphere was higher then due to sunspot activity Why is there no evidence for a dismembered ice sheet at the north pole at this time also? A) Continents were not located over the north pole at this time B) An ice sheet only formed over the south pole and not the north C) Global warming from greenhouse gas emissions from countries at northern latitudes melted the northern ice sheet D) Solar energy in the northern hemisphere was higher then due to sunspot activity The great debate Continental drift and the scientific method • Wegner’s hypothesis was correct in principle, but contained incorrect details • For any scientific viewpoint to gain wide acceptance, supporting evidence required Continental drift and paleomagnetism Renewed interest in continental drift came from rock magnetism Magnetized minerals in rocks • Show direction to Earth’s magnetic poles • Provide a means of determining their original latitude What is the orientation of the Earth’s current magnetic field in Boulder, CO? A) North, inclined upward at about 45 deg B) North, inclined down at about 45 deg C) North, inclined down at about 90 deg D) North, inclined upward at about 90 deg E) None of the above What is the orientation of the Earth’s current magnetic field in Boulder, CO? A) North, inclined upward at about 45 deg B) North, inclined down at about 45 deg C) North, inclined down at about 90 deg D) North, inclined upward at about 90 deg E) None of the above Continental drift and paleomagnetism Polar wandering • Apparent movement of magnetic poles in volcanic rocks indicates continents move • Shows Europe was closer to equator when coal-producing swamps existed Continental drift and paleomagnetism Polar wandering • Polar wandering curves for North America and Europe have similar paths but are separated by about 24 of longitude – Differences between paths reconciled if continents next to one another Apparent polar-wandering paths for Eurasia and North America The scientific revolution begins During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s The scientific revolution begins Geomagnetic reversals • Earth's magnetic field periodically reverses polarity – north magnetic pole becomes south magnetic pole, vice versa • Dates when polarity of Earth’s magnetism changed were determined from lava flows Geomagnetic reversals • Geomagnetic reversals are recorded in the ocean crust • In 1963 Fred Vine and D. Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading Paleomagnetic reversals recorded by basalt at mid-ocean ridges Maps of Magnetic Stripes in Oceanic Crust Why are seafloor magnetic anomalies symmetrically oriented about ridges? A) Rates of plate motions on the two divergent plates are the same for both plates B) Magma is intruded into the centers of spreading ridges and then pulled apart in the middle C) Convective cells in the mantle transfer heat and move rock straight up beneath ridges and then move outward D) All of the above Why are seafloor magnetic anomalies symmetrically oriented about ridges? A) Rates of plate motions on the two divergent plates are the same for both plates B) Magma is intruded into the centers of spreading ridges and then pulled apart in the middle C) Convective cells in the mantle transfer heat and move rock straight up beneath ridges and then move outward D) All of the above Structure of Juan de Fuca Plate Age of Oceanic Crust Why is there no oceanic crust older that 180Ma? A) Plate tectonics didn’t occur prior to this B) Spreading ridges ran much more slowly in the past C) Older oceanic crust has all been subducted D) Older oceanic crust probably exists somewhere, we just haven’t found it under the ocean yet. Why is there no oceanic crust older that 180Ma? A) Plate tectonics didn’t occur prior to this B) Spreading ridges ran much more slowly in the past C) Older oceanic crust has all been subducted D) Older oceanic crust probably exists somewhere, we just haven’t found it under the ocean yet. Geomagnetic reversal • Paleomagnetism was the most convincing evidence to support concepts of continental drift and seafloor spreading Plate tectonics: The new paradigm More encompassing theory than continental drift Mix of ideas that explained motion of Earth’s lithosphere by subduction and seafloor spreading Plate tectonics: The new paradigm Earth’s major plates • Associated with Earth's strong, rigid outer layer – Known as the lithosphere – Consists of uppermost mantle and overlying crust – Overlies a weaker region in the mantle called the asthenosphere Basal tractions drive plate motions Earth’s major plates • Seven major lithospheric plates • Plates are in motion and change in shape and size • Largest plate is the Pacific plate • Several plates include an entire continent plus a large area of seafloor Why did Pangaea break up? A) Supercontinents act as thermal blankets for heat escaping from the mantle, which leads to extension and breakup B) The Earth was hit by a giant asteroid at the time, pushing the plates apart C) Stresses caused by orbital forcing D) Conservative politics in Russia caused it to slide towards the right, while liberals in North America caused it to slide to the left Why did Pangaea break up? A) Supercontinents act as thermal blankets for heat escaping from the mantle, which leads to extension and breakup B) The Earth was hit by a giant asteroid at the time, pushing the plates apart C) Stresses caused by orbital forcing D) Conservative politics in Russia caused it to slide towards the right, while liberals in North America caused it to slide to the left Earth’s Tectonic Plates QuickTime™ and a Cinepak decompressor are needed to see this picture. Earth’s major plates • Plates move relative to each other at a very slow but continuous rate – Average about 5 centimeters (2 inches) per year – Cooler, denser slabs of oceanic lithosphere descend into the mantle – Motion defined by rotation around a pole Plate boundaries • Interactions among individual plates occur along their boundaries • Types of plate boundaries – Divergent plate boundaries – Convergent plate boundaries – Transform fault boundaries Types of Plate Margins Plate boundaries • Each plate bounded by combination of all three boundary types • New plate boundaries created in response to changes in forces acting on rigid slabs Divergent plate boundaries Most are located along the crests of oceanic ridges Oceanic ridges and seafloor spreading • seafloor is elevated forming oceanic ridges Oceanic ridges and seafloor spreading • Seafloor spreading occurs along the oceanic ridge system Spreading rates and ridge topography • Ridge systems exhibit topographic differences • Topographic differences are controlled by spreading rates (see map of age of oceanic crust for width of ridges relative to their age) Which ridge spreads the fastest? A)Mid Atlantic ridge B) East Pacific Rise C) Australia-Antarctic Ridge D) Arctic Ridge Which ridge spreads the fastest? A)Mid Atlantic ridge B) East Pacific Rise C) Australia-Antarctic Ridge D) Arctic Ridge Divergent boundaries are located mainly along oceanic ridges Divergent boundaries in Continents Continental rifts • Splits landmasses into two or more smaller segments Divergent boundaries Continental rifts • Example includes East African rifts • Produced by extensional forces acting on the lithospheric plates • Not all rift valleys develop into spreading centers • Otherwise Nevada would be an ocean! The East African Rift Development of Continental Rift into Ocean Basin What is the geologic evidence here (New Mexico) for rifting? A) volcanoes B) normal faults C) calderas D) basaltic lava flows E) all of the above Convergent plate boundaries Old portions of oceanic plates are returned to the mantle • Surface expression of descending plate is an ocean trench • Called subduction zones • Average angle at which oceanic lithosphere descends into the mantle is about 45 All have same basic characteristics, but can have highly variable features Types of convergent boundaries • Oceanic-continental convergence – Denser oceanic slab sinks into the asthenosphere – Bathymetry marked by trench – As plate descends, partial melting of mantle rock makes basaltic or andesitic magmas – Volcanic mountains associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades) Types of Arcs Types of convergent boundaries • Oceanic-oceanic convergence – When two oceanic slabs converge, one descends beneath the other – Often forms volcanoes on the ocean floor – If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands) Fly through the Marianas Trench: Which direction is the camera flying? A) To the West, B) To the East, C) To the North, D) To the South QuickTime™ and a QuickDraw decompressor are needed to see this picture. Fly through the Marianas Trench: Which direction is the camera flying? A) To the West, B) To the East, C) To the North, D) To the South QuickTime™ and a QuickDraw decompressor are needed to see this picture. Continental Arc Oceanic Arc Continental Collision Types of convergent boundaries • Continental-continental convergence – Continued subduction brings continents together – Less dense, buoyant continental lithosphere does not subduct – Result is a collision between two continental blocks – Process produces mountains (Himalayas, Alps, Appalachians) The collision of India and Asia produced the Himalayas Transform fault boundaries Third type of plate boundary Plates slide past one another and no new lithosphere is created or destroyed Transform faults • Most join two segments of a mid-ocean ridge as parts of linear breaks in the oceanic crust known as fracture zones • Accommodate simultaneous movement of offset ridges Transform faults accommodate movement on offset ridge segments ridge Map view of ridge-transform-ridge system transform ridge ridge Map view of ridge-transform-ridge system transform What is the sense of slip on the transform fault? A) Left lateral strike slip B) Right lateral strike slip C) Thrust D) Normal E) Oblique ridge ridge Map view of ridge-transform-ridge system transform ? left lateral fault? ridge ridge Map view of ridge-transform-ridge system transform Right lateral strike slip! ridge ridge Map view of ridge-transform-ridge system transform fracture zone fracture zone Right lateral strike slip! ridge Why aren’t there earthquakes on the transform fault away from the ridge? ridge A) B) C) D) Volcanic rocks weld plates together Plates are moving at same rate Plates are hotter here All of the above transform fracture zone fracture zone Right lateral strike slip! ridge Why aren’t there earthquakes on the transform fault away from the ridge? ridge A) B) C) D) Volcanic rocks weld plates together Plates are moving at same rate Plates are hotter here All of the above transform fracture zone fracture zone Right lateral strike slip! ridge Testing the plate tectonics model Plate tectonics and earthquakes • Plate tectonics model accounts for the global distribution of earthquakes – Absence of deep-focus earthquakes along the oceanic ridge is consistent with tectonic theory – Deep-focus earthquakes associated with subduction zones – The pattern of earthquakes along a trench provides method to track plate's descent Deep-focus earthquakes occur along convergent boundaries Earthquakes near Japan trench Locations of E-quake Hypocenters Evidence from ocean drilling • Most convincing evidence confirming seafloor spreading comes from drilling directly into ocean-floor sediment – Age of deepest sediments – Thickness of ocean-floor sediments verifies seafloor spreading Hot spots • Caused by rising plumes of mantle material • Volcanoes form over them (Hawaiian Island chain) • Mantle plumes are long-lived structures and originate at great depth, perhaps at core-mantle boundary The Hawaiian Islands form over stationary hot spot What caused hot spot Hawaii’s track to change direction at ~45Ma? A) Location of hot spot in mantle moved to north. B) Direction of plate motion changed. C) Two different hot spots formed the bent pattern D) Northern section of track is actually an old, dead spreading ridge. What caused hot spot Hawaii’s track to change direction at ~45Ma? A) Location of hot spot in mantle moved to north. B) Direction of plate motion changed. C) Two different hot spots formed the bent pattern D) Northern section of track is actually an old, dead spreading ridge. No one driving mechanism accounts for all major facets of plate tectonics Researchers agree that convective flow in 2,900 km-thick mantle is main driving force of plate tectonics (by basal traction) Other mechanisms generate forces that contribute to plate motion • Slab-pull • Ridge-push Importance of plate tectonics Provides a unified explanation of Earth’s major surface processes, especially oceans Within framework of plate tectonics, we find explanations for the distribution of earthquakes, volcanoes, and mountains Plate tectonics provides explanations for distribution/evolution of plants and animals and climate record