Plate_Tectonics_II

Plate_Tectonics_II - PLATE TECTONICS II PLATE EARTH 2...

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Unformatted text preview: PLATE TECTONICS II PLATE EARTH 2 [Marshak, Chapter 2] Plate Tectonics - Specifics What are Plates? Types of Lithosphere Types of Plate Boundaries & Examples Hot Spots What’s Driving the Plates? Brief Overview of CA Geology Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Lithosphere Tectonic plates are fragments of lithosphere Lithosphere is made of both crust and the upper mantle The lithosphere is in motion over the LVZ (asthenosphere) Lithosphere bends elastically when loaded Asthenosphere flows plastically when loaded Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Two Types of Lithosphere Continental ~ 150 km thick. Granitic crust 35-40 km thick Lighter (less dense) More buoyant – floats higher Oceanic ~ 7 to 100 km thick. Basaltic crust. 7-10 km thick Heavier (more dense) Less buoyant – sinks lower Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Boundaries Lithosphere is fragmented into ~ 20 tectonic plates Plates move continuously at a rate of 1 to 15 cm/yr Slow on a human time scale; extremely rapid geologically Plates interact along their boundaries Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics People sometimes say People that plates move as fast as fingernails grow … as Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics … as fast as fingernails grow! as 35 years Essentials of Geology, 3rd edition, by Stephen Marshak 27 years Chapter 2: The Way the Earth Works: Plate Tectonics Plate Boundaries Plate Locations on Earth where tectonic plates meet Identified by concentrations of earthquakes Associated with many other dynamic phenomena Plate interiors are almost earthquake free Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Animation Earthquake Distribution Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Continental Margins Where land meets the ocean Margins near plate boundaries are “active” Margins far from a plate boundaries are “passive” Passive margin continental crust thins seaward Transitions into oceanic crust Traps eroded sediment Develops into the continental shelf Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Boundaries: Three Types Plate Divergent – Tectonic plates move apart Lithosphere thickens away from the ridge axis Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Boundaries: Three Types Convergent – Tectonic plates move together The process of plate consumption is called subduction Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Convergent: Continent-Continent Collision Subduction consumes ocean basins Ocean closure ends in continental collision Buoyant continental crust will not subduct Subduction ends; mountains pushed upward Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Boundaries: Three Types Transform – Tectonic plates slide sideways Plate material is neither created, nor destroyed Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics PLATE BOUNDARIES IN DETAIL Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Divergent Boundaries Sea-floor spreading causes plates to move apart Magma wells up to fill the gap Magma cools, adding material to each plate Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Divergent Boundaries - Progression Early stage: a long, thin and young ocean basin Mid-stage: Ocean begins to widen Example: The Red Sea Example: Greenland and the North Atlantic Late Stage: Mature, wide ocean basin Example: The Atlantic Ocean Note: These diagrams only depict the crust, not the entire lithosphere. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Mid-Ocean Ridges Linear mountain ranges in Earth’s ocean basins Example: The Mid-Atlantic Ridge Snakes N-S through the entire Atlantic Ocean Elevated ridge (1500 km wide) 2 km above abyssal plains Axial rift valley 500 m deep 10 km wide Symmetric Site of eruptions Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Mid-Ocean Ridges Sea-floor spreading opens the axial rift valley Rising asthenosphere melts, forming mafic magma Pooled magma solidifies into oceanic crustal rock Top: Pillow basalt – Magma quenched at the sea floor Middle” Dikes – Preserved magma conduits Bottom: Gabbro – Deeper magma Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Mid-Ocean Ridges “Black Smokers” are found at some MORs Water entering fractured rock is heated by magma Hot water dissolves minerals and cycles back out of rock When water reaches the sea, minerals precipitate quickly Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Black Smokers Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Ocean Crustal Age Oceanic crust spreads away from the ridge axis New crust is closer to the ridge; older crust farther away Oldest oceanic crust is found at the far edge of the basin Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics DIVERGENT PLATE BOUNDARIES Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Continental Rifting Example: East Africa The Arabian plate is rifting from the African plate Rifting has progressed to sea-floor spreading in… The Red Sea The Gulf of Aden Rifting continues along the East African Rift Thinned crust Elongate trough Volcanoes The rift and two spreading ridges comprise a triple junction Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics The East Side Story … Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics … may have caused different pathways of evolution different Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics From Spreading Center to Subduction From The hot asthenosphere is at the base of the MOR Aging ocean crust moves away from this heat source … Cooling, increasing in density and slowly sinking Cooling, Accumulating an increasing thickness of sediment Accumulating Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Subduction Old oceanic lithosphere is more dense than mantle A flat-lying oceanic plate doesn’t subduct easily Once bent downward, however, the leading edge sinks Once like an anchor rope like Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Convergent Boundaries Lithospheric plates move toward one another One plate dives back into the mantle (subduction) The subducting plate is always oceanic lithosphere Subduction recycles oceanic lithosphere Subduction is balanced by sea-floor spreading Earth maintains a constant circumference Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Convergences Volcanic Island Arc Essentials of Geology, 3rd edition, by Stephen Marshak Continental Volcanic Arc Chapter 2: The Way the Earth Works: Plate Tectonics Convergent Boundaries - Subduction Volcanic Arc – A chain of volcanoes on overriding plate The descending plate partially melts at ~ 150 km depth Magmas burn through overriding plate Volcanic arcs are curved because the Earth is a sphere Volcanic Arc type depends upon the overriding plate Continental crust – Continental Arc Oceanic – Island Arc Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Convergent Boundaries The subducting plate descends at a range of angles Plate descent is revealed by Wadati-Benioff earthquakes Mark frictional contact and mineral transformations Mark Earthquakes deepen away from trench Most quakes cease below 660 km Most Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Various Geometries Distance between trench and volcanic arc depends on angle of subducting slab Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Wadati-Benioff Zone Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Convergent Boundaries - Subduction Features Subduction is associated with unique features. Subduction Accretionary prisms Volcanic arcs Back-arc basins Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Convergent Boundaries Accretionary Prism – A deformed sediment wedge Sediments are scraped off of subducting plates This thrusts them onto the overriding plate This Contorted prism sediments can be pushed above sea-level Olympic Peninsula Taiwan Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Convergent Boundaries – Subduction Features Back-arc basin – A marginal sea behind an arc Forms between an island arc and a continent Offshore subduction traps a piece of oceanic crust, or… Stretching may create a new spreading ridge Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics R.I.P. Plate descent may continue past the earthquake limit The lower mantle may be a “plate graveyard” The Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Collision Subduction consumes ocean basins Ocean closure ends in continental collision Buoyant continental crust will not subduct Subduction ceases and mountains are uplifted Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Collision Plate tectonic collision may involve… Plate Two continents A continent and an island arc Collision “sutures” the convergent plate boundary Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Example: Example: Himalayas Himalayas Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Transform Boundaries Lithosphere slides past; not created or destroyed Lithosphere Many transforms offset spreading ridge segments Some transforms cut through continental crust Most on oceanic crust Characterized by… Earthquakes Absence of volcanism Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Oceanic Transforms Ocean Ridge axes are offset by transform faults A geometric necessity for a line spreading across a sphere Transforms bear strong evidence of sea-floor spreading Abundant earthquakes common when offsets are opposed Abundant Earthquakes vanish when offsets are concurrent Figure 2.23a Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Slip Along a Transform Fault Slip Transform Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Transform Boundaries Continental transforms – Chop continental crust Continental Example: The San Andreas Fault Example: Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics San Andreas Fault Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics San Andreas Fault at Wallace Creek, Carrizo Plain Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Triple Junctions Places where three plate boundaries coincide Multiple boundary combinations occur Multiple Triple junctions migrate and change geometry over time Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Hot Spots Volcanic plumes independent of tectonic plates Most are located far from plate boundaries Comprised of mafic magmas from the lower mantle Tattoo overriding plates with volcanoes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Hot Spots Hot spot perforates overriding plate Hot perforates Volcano builds above sea level Volcano Plate motion pulls volcano off plume Plate Volcano goes extinct and erodes Subsidence creates a guyot Hot spots reinforce sea-floor spreading sea-floor (Iceland) (Iceland) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Hot Spots Hot-spot seamounts age away from originating hot spot Age change marks rate of plate motion (sort of) Line of seamounts indicates direction of plate motion Line Figure 2.25a Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Velocities Absolute plate velocities may be mapped by… Plotting plate motion relative to a fixed spot in the mantle Measuring volcano ages / distance along a hot-spot track Doesn’t work for oceanic ridges (are mobile, too) Doesn’t Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Velocities Plate vectors are determined by GPS measurements Global Positioning System (GPS) uses satellites Knowledge of plate motion is now accurate and precise Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics The Dynamic Planet Plate boundaries change over geologic time Oceanic plates are... Continental plates are... Created at mid-ocean ridge spreading centers Destroyed at subduction zones Torn apart at rifts Joined during collision Joined Earth looked very different in the past. Earth Earth will look very different in the future. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Tectonics: Plate The Key to Understanding Geology The It explains earthquakes, volcanoes, and shifting continents The interior and surface of Earth are in constant motion Mantle is transferred to the surface and back down again Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Remaining Questions: How do continents break up? Not clear how continents start breaking up Started by hot/weak spots perhaps? Why aren’t there any subduction margins around the Atlantic Ocean where the oceanic crust is old, cold, and dense? Discussion in EARTH magazine, July 2011 Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics What’s Driving the Plates? Are plates dragged atop a convecting mantle? Mantle convection does occur Some matching of convection currents below with plate movements at the surface Not likely to be the primary driving mechanism Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics What’s Driving the Plates? Probably several forces at work: Mainly Slab-pull from the down-going slab (gravity!) Down-going and spreading plates help pull the ridges apart Some ridge-push from the elevated MOR Probably very little; the part that is actively spreading is only about 1 km wide (quickly chilled by sea water) Spreading centers are moving around as well (East-Pacific) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics What’s Driving the Plates? Convection in the asthenosphere adds or subtracts But plates appear to be in the driver’s seat— it looks like they drag the asthenosphere around Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics ...
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This note was uploaded on 02/25/2012 for the course EARTH 2 taught by Professor Thomas during the Summer '11 term at UCSB.

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