Plate_Tectonics_I

Plate_Tectonics_I - PLATE TECTONICS I PLATE EARTH 2[Marshak Chapter 2 Theory of Plate Tectonics The Dynamic Planet Plate Tectonics Summary Ocean

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Unformatted text preview: PLATE TECTONICS I PLATE EARTH 2 [Marshak, Chapter 2] Theory of Plate Tectonics The Dynamic Planet Plate Tectonics Summary: Ocean floor created at mid-ocean ridges Plate is consumed at oceanic trenches. is Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics The Dynamic Planet Plate Tectonics Summary: Ocean floor created at mid-ocean ridges Plate is consumed at oceanic trenches. is Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Tectonics The paradigm of “How the Earth Works” Earth’s outer shell is broken into rigid plates that move Moving plates change the face of planet Earth A case study of a Scientific Revolution case A powerful idea based on multiple lines of evidence Allows scientists to predict events and rebuild the past Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Plate Tectonics Plate Tectonic theory is powerful It provides a unified mechanism explaining: It Igneous, sedimentary and metamorphic rocks Igneous, The distribution of earthquakes and volcanoes The origin of continents and ocean basins The distribution of fossil plants and animals The genesis and destruction of mountain chains The Continental drift Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Earth’s Layers CRUST: • Continental • Oceanic Mantle: • Upper • Lower Core: • Outer (L) • Inner (S) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Different subdivision of Earth’s Layers— more useful for discussion of Plate Tectonics Lithosphere • rigid • no convection Asthenosphere with Low-Velocity Layer • ductile • capable of convection Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics The Crust The outermost “skin” of Earth with variable thickness Thickest under mountain ranges (70 km - 40 miles) Thinnest under mid-ocean ridges (3 km – 2 miles) Mohorovicic discontinuity (“Moho”) at base of the crust The “Moho” separates the crust from the upper mantle The Andrija Mohorovičić found a change in P-wave velocity Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Two Types of Crust Two Continental crust – Granitic, underlies the continents Average rock density about 2.7 g/cm3 Average thickness 35–40 km Oceanic crust – Basaltic, underlies the ocean basins Density about 3.0 g/cm3 Average thickness 7–10 km Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Earth’s Mantle Earth’s Solid rock layer between the crust and the core Solid 2,885 km thick, the mantle is 82% of Earth’s volume Mantle composition is the ultramafic rock peridotite Mantle peridotite Below ~100–150 km, the rock is hot enough to flow Below Hot mantle rises, cold mantle sinks (convection) Hot Three subdivisions: Upper, transitional, and Lower Three Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics The Core An iron-rich sphere with a radius of 3,471 km An 2 components with differing seismic wave behavior Outer core Liquid iron-nickel-sulfur 2255 km thick. Density – 10-12 g/cm3 Inner core Inner Solid iron-nickel alloy. Radius of 1220 km. Density – 13 g/cm3. Flow in the outer core Flow generates the magnetic field. generates Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics ¡ Lithosphere – Asthenosphere ! Lithosphere – The outermost 100–150 km of Earth Non-flowing, rigid material that moves as tectonic plates Non-flowing, Made of 2 components: Crust and Upper Mantle Made Asthenosphere – Upper Mantle below lithosphere Asthenosphere below Shallow under oceans; deep under continents (“root”) Flows as a soft solid (plastic, or ductile) or ductile Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Continental Drift Continental The hypothesis that continents are mobile Proposed by German meteorologist Alfred Wegener The Origins of Oceans and Continents published in 1915 Wegener hypothesized a former supercontinent, Pangaea Wegener Idea was founded on strong evidence “Fit” of continents Location of glaciations Fossil organisms Rock type and structural similarities Paleoclimate indicators Paleoclimate preserved in rocks preserved Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Animation Pangaea Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Glacial Evidence Permian glacial till is found on four continents Permian The tills in Africa and India are now near the equator A cooler earth? No. Also tropical plants during Permian cooler Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Glacial Striations Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Paleoclimatic Evidence Placing Pangaea over the Permian South Pole He correctly predicted… Tropical coals Tropical reefs Subtropical deserts Subtropical evaporites Subtropical evaporites Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Fossil Evidence Identical fossils found on widely separated land Lystrosaurus – A non-swimming, land-dwelling reptile Cynognathus – A non-swimming, land-dwelling mammallike reptile These organisms could not These have crossed an ocean have Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Fossil Evidence Identical fossils found on widely separated landmasses Identical Mesosaurus – A freshwater reptile Glossopteris – A subpolar plant with heavy seeds Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Mesosaur – a freshwater reptile Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Matching Geology Geologic phenomena match across the Atlantic Geologic structures Rock types Rock ages Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Matching Geology Geologic phenomena match across the Atlantic Mountain belts: The Appalachians The Caledonides Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Why wasn’t the continental drift Why hypothesis accepted? hypothesis Wegener’s idea was debated and ridiculed Most scientists didn’t believe him— too radical of an idea Major criticism: Major Couldn’t explain mechanism for drift Couldn’t Wegener died in 1930 at the age of 40 Lacking an advocate, Lacking the drift hypothesis faded the Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics From Continental Drift to Seafloor Spreading From Seafloor The drift hypothesis needed new and different evidence His idea was revived in the 1950s Evidence provided by seafloor exploration Evidence in combination with results of paleomagnetism paleomagnetism Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Evidence from the Ocean Floor Submarine exploration during WWII and 1950s: Seafloor is not smooth -- huge under-water mountain ranges (ridges) Sedimentary cover not uniform in thickness: Sedimentary -- thicker near continents; thinner near ridges -- Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Evidence from the Ocean Floor … Topography of Ocean Basins: Ridges are dissected by fracture zones Deep ocean trenches, many parallel to continents Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Seafloor Topography Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Continental “Fit” Continental Wegener had noted that continents seem to fit together Wegener and had argued that the fit could not be coincidental and In fact, present shorelines make only a rough fit The continental shelf edges make a better fit The continental Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Paleomagnetism Paleomagnetism (fossil magnetism) 1. Magnetic Direction & Declination Position of the rock in relation to the magnetic pole 2. Magnetic Latitude / Inclination Position of the rock in relation to the position of the magnetic pole at that time 3. Magnetic Polarity Position of the magnetic pole in terms of N or S, gives magnetic chron Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics How does Magnetic Overprinting work? Above 350-550°C: Below 350-550°C: Thermal energy of atoms high Magnetic dipoles randomly oriented No magnetic signature Thermal energy slows atoms Dipoles align with Earth’s field Material permanently magnetized Fe-minerals can lock in the Earth’s Fe-minerals magnetic signal at the time formed magnetic Preserves declination and inclination Can be used to determine lat / long Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics 1. Magnetic Direction & Declination Earth’s magnetic field acts like a giant bar magnet Earth’s It’s N and S ends are currently tilted ~11° from the axis of It’s rotation rotation Measured everywhere on Earth, it extends out into space Some iron minerals in rocks align to the magnetic field This permits some rocks This to preserve magnetic information information Preserved magnetism can Preserved be read from these rocks be Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Magnetic Poles The “bar magnet” intersects Earth’s surface Magnetic North and South Pole; differs from geographic Magnetic North and South Poles (at rotational axis) The magnetic poles move constantly, but stay in the The vicinity of the N and S geographic poles Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics The Earth’s Magnetic Field Declination – Difference between magnetic north and geographic (true) north. Depends upon… geographic Longitude (also Latitude) Absolute position of the two poles (which changes over time). Geographic North Magnetic North Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics 2. Magnetic Latitude / Inclination Inclination – Tilt of a compass needle from the Inclination horizontal. Depends upon… horizontal. Polarity (normal or reverse) Latitude Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Polar Wander Paleomagnetism from ancient lavas did not align with the Paleomagnetism present magnetic field (numbers are rock ages in Ma) This lack of alignment indicates past magnetic polar This wandering wandering Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Problems with Polar Wander … Each continent had a separate polar-wander path Either the location of the magnetic pole is not fixed, or… The lava flows themselves have moved But curves align when continents are assembled! Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Apparent Polar Wander Polar wander is now known to be an artifact Polar Not the signature of a wandering pole on a fixed continent Not continent The signature of a fixed pole on a wandering continent The Apparent polar wander is strong evidence for drift! Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics 3. Magnetic Polarity Towed magnetometers measure ocean crust Magnetism varies perpendicular to Ocean Ridges These variations are pos. and neg. magnetic anomalies Anomalies are linear belts that parallel Ocean Ridges Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Animation MagStripes_Reversals Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Magnetic Reversals Layered lava flows reveal reversals in polarity Layered Magnetic polarity is the direction of magnetic north A reversed N magnetic pole is near the S geographic pole Reversals are geologically rapid Reversals can be used as time markers Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Magnetic Reversals Magnetic Radioactivity permits absolute Radioactivity age-dating of rocks age-dating Geomagnetic reversals have Geomagnetic been placed in a time scale been Reversals occur every Reversals 500-700 ka (chrons) 500-700 Shorter durations Shorter (~200 ka; subchrons) subchrons Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Newer Discoveries: By 1950, we had learned much about oceanic crust Oceanic does not make up the oldest rocks on the planet Ocean-floor ages NOT uniform Oceanic crust is mafic (rich in Mg & Fe); Oceanic mafic basalt and gabbro basalt Maximum ages around 200 Ma Oldest ages near continents (where thickest sediment Oldest covers have been found) covers No granitic rocks; no metamorphic rocks High heat flow characterizes the mid-ocean ridge. High Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Sea-Floor Spreading Drilling in the late 1960s recovered crust samples Ages increase away from the mid-ocean ridge Ages are “mirror images” across the mid-ocean ridge Strong supporting evidence for sea-floor spreading Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Additional Findings: Earthquake Distribution: NOT uniform: NOT Curvilinear belts of concentrated subsea earthquakes Curvilinear Limited to… Oceanic ridge/rise axes Deep ocean trenches Oceanic fracture zones Oceanic Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics From Continental Drift to Sea-Floor Spreading In 1960, Harry Hess published his “Essay in Geopoetry” He called his theory “sea-floor spreading” Upwelling mantle erupts at the mid-ocean ridges New crust moves away from ridges, gathering sediment At trenches, the sea-floor dives back into the mantle Provided a potential mechanism for continental drift Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Sea-Floor Spreading: Evidence Reversals in polarity explain magnetic anomalies Positive anomalies – Normal polarity crust Negative anomalies – Reversed polarity crust Magnetic anomalies are symmetrical across a mid-ocean ridge Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Sea-Floor Spreading: Evidence Magnetic anomalies mimic layered lava flows Magnetic “stripes” form as lava cools at a mid-ocean ridge Ocean crust spreads away from mid-ocean ridge Reversals are recorded within the cooled basaltic lava Sea-Floor Spreading is the mechanism of Continental Drift! Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Observation – Interpretation … Have these two ocean ridges spread at the same rate? Could the topography of the Ridges/Rise provide Could evidence for differential spreading rates (next slide)? evidence Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics Seafloor Topography Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 2: The Way the Earth Works: Plate Tectonics From Seafloor-Spreading to Plate Tectonics Previous research provided a strong foundation Wegener (1915) – Evidence supporting Continental Drift Hess / Dietz (1960) – Evidence for Sea-Floor Spreading, providing the mechanism for Wegener’s Continental Drift Concept of lithosphere and Wadati-Benioff Zone >>> >>> By 1968, the Theory of Plate Tectonics was postulated This new framework changed the view of most geologists 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 Review Continental Drift >> Seafloor Spreading >> Plate Tectonics Important Players: Lithosphere (2 types) & Low-Velocity Zone Multiple Lines of Evidence for Continental Drift Hypothesis New Evidence: Made possible to modern technology Exploration of seafloor topography Paleomagnetism Vindicates and revives older hypothesis Leads to Paradigm shift (worth 20 cents) Importance of Anomalies for driving discoveries: Ocean floor not smooth and feature-less Ocean floor younger than continents Ocean younger 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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