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Mod. E.docx

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Department
Earth and Ocean Sciences
Course Code
EOSC 116
Professor
Louise Longridge

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Description
MODULE E – Planetary Engineering: Mesozoic Tectonics Lesson 17 – Plate Tectonics 101  Introduction - Plate Tectonics = “Grand Unifying Theory” explaining relationships between processes within Earth  From Theory of Continental Drift - Alfred Wegener in 1915  The Theory of Plate Tectonics 1) Earth’s surface consists of many lithospheric plates including crust (continental or oceanic) and immediately underlying mantle, cold and rigid 2) These plates are presently moving around on Earth’s surface and interacting with one another -some lithospheric plates consist of both continental and oceanic floor ex) N.American plate = N.American continent and western half of Atlantic Ocean Indian-Australian plate = continental rocks of India, Australia and oceanic floor  Plate Motion - plate motions random rates and directions - rates of 2-18 cm/yr quite slow, but with perspective of geological time in terms of million years, rates of 100’s of km/million years means entire oceans can be destroyed in tens millions years  Significance of plate tectonics 1) Almost all earthquakes, most volcanoes, occur where lithospheric plates are interacting 2) Main mineral and hydrocarbon resources occur in specific tectonic settings 3) Plate tectonic processes happening at depth responsible for occurrences at Earth’s surface: size and shape of oceans, nature and distribution of landforms, general climatic conditions of each of Earth’s regions  Plate Boundaries - map of epicenter of significant earthquake, M > 3.5 - active boundaries marked by zones of earthquakes and volcanoes - earthquakes closely associated with active plate boundaries  Land forms associated with plate boundaries -mountain belts occur along many plate boundaries - N. America: main mountain belts stretch from Alaska to northwest N. America all the way to south along western edge of N. and S. America  Oceanic forms associated with plate boundaries - topography of ocean floors is very irregular, including what would be high mountain chains and deep canyons (if they weren’t covered by water) - much of Earth’s recent tectonic history recorded on ocean floor, so oceans just as important and interesting as land surfaces  Paleogeography - study of nature of landforms and evolution of landforms through time - critical for all life all Earth’s surface ex) if tectonic processes cause uplift of part of continent, may change from low-lying swamp area covered by shallow seas that is suitable for many landforms, to a high barren desert incapable of supporting significant life  Evolution of the western margin of North America - following description of figures outline how western margin of N. America evolved from Early Jurassic (~180 Ma) until end of Cretaceous (~65 Ma; dinos died) - subaerial = land exposed above sea level (brown) - submarine = areas covered by shallow water (pale blue) - mountainous areas = bumpy, deeper submarine areas = dark blue, red dots = individual volcanic centers 1) Early Jurassic: ~180 Ma - large block of crust (Wrangellia) was converging with western edge of N. America and would eventually collide 2) Middle Jurassic: ~160 Ma - a series of volcanic islands fringing western edge of N. America and Wrangellia continuing to move towards continent - a shallow sea now covered central part of N. American continent 3) Late Jurassic: ~145 Ma - Wrangellia actively colliding with southern edge of N. America and process caused mountain belt growth along collision zone - shape and extent of shallow water interior sea has changed considerably since 160 Ma 4) Early Cretaceous: ~125 Ma - Wrangellia now collided with most of western margin - mountain chain along most of western edge of N. America - shallow interior seaway extends from Arctic Ocean south to southern Wyoming and another in Northern Mexico 5) Late Cretaceous: ~85 Ma - interior seaway (called Cretaceous Seaway) extended full length of N. America and separated from Pacific Ocean by a continuous mountain chain 6) Cretaceous/Tertiary boundary: ~65 Ma - interior seaway, where many dinos had flourished, had largely dried up - dinos extinct  Plate tectonics: Basic concepts - 2 main types lithospheric plates: oceanic (thinner, denser) and continental - differ in composition, thickness, density - as a result, oceanic crust usually submarine (underwater) whereas continental crust usually subaerial (high-standing, well above sea level) - 3 main lithospheric plate interactions 1) Divergent boundaries – plates moving apart from one another 2) Convergent boundaries – plate moving towards one another 3) Transform boundaries – plates sliding along one another  Divergent Margins - stages of process that breaks/rifts a continent apart, ending up with an ocean basin separating two continental fragments 1) Extension 2) Rift valley formation 3) Opening of ocean – continent tears in two; continent ridges faulted and uplifted; basalt magma erupts from oceanic crust 4) Young ocean widens with developing mid-ocean ridge  The Red Sea – a continent being rifted apart - red sea, separating African and Arabian plate, an example of what happens when a single continental block is rifted into two - in this case, a spreading center or ridge is forming along boundary between the two plates and new oceanic crust is created as two continental plates spread apart  Convergent Margins - when oceanic crust subducts under another plate (oceanic or continental), large volumes of molten rock (magma) produced - magma rises to shallow levels in crust of overriding plate and some erupts to surface as lava, forming volcanoes - since magmas produced along a linear plate margin, volcanoes form a linear or more commonly arcuate line (forming an arch) - 3 types of convergent margins 1) Ocean-ocean collision – oceanic crust subducts under oceanic crust - if overriding plate is oceanic, process will be mainly submarine and volcanoes form isolated volcano islands (oceanic arc; commonly referred to as Marianas-type arc) ex) Marianas islands in western Pacific Ocean 2) Ocean-continent collision – oceanic crust subducts under continental crust - if overriding plate is continental, plate will be thicker and less dense than oceanic so it’ll mainly be subaerial => form subaerial volcanoes, known as continental or Andean-type arc. ex) South American Andes mountains 3) Continent-continent collision – continent crust subducts under continent crust - special case of subduction -both plates thick and low density; therefore, buoyant and cannot be subducted -instead, continent-continent collision zone forms high mountain ranges ex) Himalayas: India and Eurasia collision  Transform Boundaries - one plate slides past another; scars ocean floor and offsets ridges ex) San Andreas Fault in California: Pacific plate slides NW along margin of N. American plate  Mantle Plumes (Hotspots) - hotspots = centers of magmatism coming from deep in mantle which “punches up” through overlying lithospheric plates - mantle plumes = unusual situation where localized sources of magma (hotspots) from deep in mantle produce large volumes of magma that rise to surface and erupt as large isolated volcanoes or chains of volcanoes  The Hawaiian Islands - Hawaiian islands in Pacific Ocean good example of volcanic chain formed over a mantle plume - Hawaiian mantle plume presently located immediately SE of the “Big Island” - Pacific plate presently moving NW and has been for past 42 Ma - over this period of time, linear chain of (now submerged) oceanic islands has formed on Pacific plate above stationary plume - Emperor seamount chain = northerly trending chain of underwater mountains created as Pacific plate moved in more northerly across Hawaii hotspot between 85 and 42 Ma - at 42 Ma, Pacific plate motion with respect to Hawaii hotspot changed from northerly to northwesterly, resulting in sharp bend in Hawaiian volcano chain Lesson 18 – Plate Motion through the Last 200 Ma  Introduction - universally accepted that lithospheric plates currently moving around surface of Earth - why do plates move?  Models of plate motion 1) Mantle convection model - heat from deep within Earth (possible core) caused large scale convection within mantle, which caused individual plates, comprising rigid crust and upper mantle, to be dragged passively along above convention cells 2) Ridge push model - at mid-ocean ridges, thin young newly-formed oceanic crust is thermally uplifted due to large amount of hot mantle material upwelling along ridges - oceanic plates essentially slide off uplifted zone (downhill) under force of gravity - oceanic plate ultimately pushed down a subduction zone 3) Slab pull model - when oceanic lithosphere subducted at considerable depth (>400 km) at a subduction zone, elevated heat and pressure converts rocks of oceanic crust (basalt; floats on mantle because less dense) into rock called eclogite - eclogite = same elemental composition as basalt but more dense than surrounding mantle; therefore put huge weight on edge of subduction plate pulls rest of plate behind it as eclogite sinks down mantle  What drive the movement of plates? 1) Mantle Convection 2) Ridge push 3) Slab pull  Current view of plate motion - combination of slab pull and ridge push models believed to be current driving force - plate movements drive mainly by slab pull with minor contribution from ridge push - older mantle convection model disregarded after evidence suggest large scale convection does not occur within mantle  Plate motions in Earth’s history - 3 methods used to reconstruct Earth’s plate tectonic history 1) Magnetic anomalies on sea floor - magnetic striping on seafloor present in all preserved oceanic crust used to determine past plate motions - patterns of magnetic striping with knowledge of oceanic crust age in different parts of ocean used to “rewind” past plate movements - magnetic stripes form at a spreading ridge as new crust is formed - new magma constantly being erupted at spreading ridges and cools to form solid, cold volcanic rocks - entire surface and vicinity of Earth affected by Earth’s magnetic field, which changes orientation by 180 degrees every few to ten Ma, alternating between present (normal) and opposite (reversed) orientation - when magma cools through ~450°C and solidifies, magnetic field, either normal or reversed, is locked into the rock, depending on magnetism during that time - since production of magma at spreading ridges is continuous, seafloor spreading produces a set of parallel stripes on each side of spreading ridge that alternate between normal and reverse magnetism; pattern on one side is mirror image of other side - obtain sample of volcanic rocks from each of normal or reversed stripes; can deduce rate of spreading of two sides; magnetic stripe pattern allows “rewinding” effects of seafloor spreading back through time - critical problem: oldest preserved oceanic crust only 180-190 Ma; therefore, can only use magnetic stripes for last 180-190 Ma 2) Paleomagnetism - magnetic lines of force at equator parallel to Earth’s surface; farther north or south, more inclined lines of force; magnetic poles, lines of force perpendicular to surface - magma locks into magnetic polarity (normal or reversed) as well as orientation of magnetic lines of force at that time - can be used to estimate distances moved in N-S direction, not E-W 3) Hotspot tracks - chains of volcanoes produced when lithospheric plate moves over fixed mantle plume (or hotspot) ex) Hawaiian ridge, Emperor Seamount chain: Pacific plate passed over fixed Hawaiian hotspot - by determining ages of individual volcanoes along hotspot chain, can determine that from 42 Ma to today, Pacific plate was moving NW but not clear why abrupt direction change at 42 Ma  Plate reconstruction: Putting it all together - want to determine how each plate has moved with respect to one another throughout time - reconstruction process of plates:  How N.America and African plate moved with respect to one another by examining magnetic striping on floor of north Atlantic Ocean  Determine how Africa has moved with respect to various mantle hotspots by examining hotspot tracks on African continent; also reveals N. America movements relative to hotspots  How Pacific plate moved with respect to various hotspots  Unraveling complex patterns of magnetic striping on Pacific ocean floor - until recently, Pacific ocean floor consisted of not one but three individual oceanic plates: Pacific plate, Kula plate (entirely subducted), Farallon plate (mostly subducted; Juan de Fuca plate remnant, currently subducting under west coast N.America)  50 Ma into the future - North and South Atlantic oceans much wider than present but western edge of ocean basin begun to subduct under eastern edge of N.America - Mediterranean Sea and Red Sea completely closed up  Future Plate configurations (highly speculative & lots of assumptions!) - 150 Ma from now…Atlantic ocean beginning to close and much of Indian ocean subducted - 250 Ma from now… all continental plates come together to form supercontinent  An alternative model - Ian Dalziel: 100 Ma future  Wider Atlantic ocean  Compression of Africa into Europe in Mediterranean sea  Compression of India and other plates northward into Asia  NW movement of Australia to northern Asia  Australia in N. Pacific ocean - Scotese: 150 Ma future  Australia more or less where it is now Lesson 19 – Mountain Belts  Introduction - mountain belts exert major influence on landscapes, climate, migration patterns of surface-dwelling species - formation, extent, nature of mountain belts entirely controlled by tectonics - 24% of Earth’s land mass is mountainous - mountainous regions dominate entire region in western N. America and most of Asia - orogeny = mountain building, result of movement of lithospheric plates - orogenic belt = mountain belt; eroded down roots of ancient mountain belt - mountain belts (or chains of individual mountains) commonly found along plate margins (convergent) - 4 main mechanisms of mountain belts growth: 1) Volcanic activity (volcanic arcs along convergent margins) 2) Regions undergoing crustal extension (ie. Normal faulting) 3) Regions undergoing crustal shortening due to compression 4) Collision between two continental plates  Volcanic activity and orogeny - at convergent margins where oceanic crust being subducted, oceanic plate sinking down into upper mantle heats up => water driven of down-going plate and passes up into overlying wedge of mantle material => cause mantle material to melt => melt/magma is hot and low density so rises buoyantly into overlying crust - portion of magma makes its way up through crust and erupts on surface, building large volcanoes - in such a magmatic arc, volcanoes typically widely spaced (10s to 100s km apart) so this process produces chain of isolated volcanoes, not continuous mountain belt  Stratovolcanoes and the Pacific Ring of Fire - Mt. St. Helens in S. Washington, stretches from Mt. Lassen in N. California to Mt. Garibaldi north of Vancouver – good example of currently active arc volcano formed along convergent margin => referred to as Cascade Arc - these volcanoes called composite cones or stratovolcanoes because consist of interlayered lava flows and volcanic tuffs/ashes - Cascade Arc only one segment of nearly continuous chain of magmatic arcs that occur along edges of Pacific ocean basin, known as Pacific Ring of Fire - all volcanoes making up Ring of Fire caused by subduction of oceanic crust  Principle of Isostasy - “why are continuous high mountain belts along convergent margins if magmatism only produces isolated volcanoes?” - answer: isostasy = state of gravitational equilibrium or balance between Earth’s lithosphere and asthenosphere - lithosphere (crust and rigid upper mantle) floating on underlying, denser lower mantle - thicker and/or less dense => floats higher on mantle; increase in thickness and/or decrease in density => float even higher (project higher above Earth’s surface), potentially enough to create continuous mountain belt  Plutons, batholiths, and mountain ranges - not all magma generated at convergent margins erupt to surface to form volcanoes - majority of magma, cools and crystalizes as large bodies (plutons or intrusions) within crust - batholith = group of adjoining plutons of similar composite extent - emplacement of plutons inflates crust with relatively low density rock masses => thickens crust and reduces overall density - Principle of isostasy = portion of Earth’s crust must float higher on underlying mantle, creating continuous mountain belt  Andes Mountains and Coast Range Batholith - Andes Mountain Belt – along western margin of S. America; good example of above process in producing high mountain belt - dark peaks = older parts of continental crust; light peaks = large and relatively low density plutons emplaced from below - emplacement of plutons thicken and reduce average density of crust in the area - Coast Range Batholith – another example; extends from east of Vancouver up into SW corner of Yukon; consists vast number of plutons  Mountain ranges are ephemeral - mountain belts don’t last very long geologically - as soon as mountain is built, erosion will occur at top due to rivers, streams, glaciers - initial erosion of upper parts of mountain cause thickened crust to thin and whole area becomes uplifted => leads to more erosion => more uplift, etc.=> until mountain belt completely removed  Mountain ranges can be long-lived - continuous mountain belts exist over periods much longer than it takes to completely erode away -although mountains continually being eroded, continued subduction of oceanic crust from west continues to produce low density magmas that rise, inflate, thick crust from below - mt. ranges can be long-lived as long as subduction process continues so new magma continually inje
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