Announcements Lithospheric plates not as brittle as previously thought ESCI 322 Meet in Env. Studies Bldg Rm 60 at 1 PM on Tuesday One week (Thursday): Quiz on Booth 1994 and discussion. (Lots of odd terms in Booth paper look them up as you read.) Kumamoto et al. 2017. Science Advances 13 September Manganese nodule distribution Plate tectonics and the formation of ocean basins Take-home messages: -physical properties of Earth s interior -understand mountain range formation, earthquakes -know what occurs at plate boundaries -understand the evidence behind plate tectonics -understand the implications of plate tectonics on the age and structure of the ocean floor More information: http://pubs.usgs.gov/gip/dynamic/dynamic.html 1
Earth s formation and evolution How was the earth formed? What are the most important characteristics of the earth s surface and interior? How do we know anything about the inside of the planet? Physical properties Lithosphere = upper mantle(95%) + crust (5%) Chemical properties Basalt Granite O, Si, Al, Fe, Ca, K, Na Mostly Fe, some Ni Mostly O, Mg, Si, Fe (Olivine) (more Fe, less Si compared to crust) Important considerations regarding the earth s structure: 1: Lithosphere (crust + mantle) is rigid 2: Asthenosphere is deformable, partially melted 3: Lower mantle is more rigid, but deformable 4: Continents are supported isostatically (not mechanically) Primary waves: p-waves fast, longitudinal pressure waves passes through solids and fluids Secondary waves: s-waves slow, transverse waves passes through solids only 5: Oceanic crust is more dense than continental crust Major features of the earth s surface and oceans and their formation? Wave speed varies with compressibility (shear and bulk modulus of the material) 2
Patterns and observations? Continental drift hypothesis Early evidence for continental drift Edward Bullard plot 60 s Similar mountains and rocks across the Atlantic 3
Distribution of earthquake epicenters E.g., turbidite deposits in Maine What constitutes the Theory of plate tectonics? Testing this hypothesis The earth s crust is made of plates New crust is created at spreading centers Old crust is destroyed at subduction zones Plates move from spreading centers toward subduction zones and mountains, volcanoes, and earthquakes form by plate motion What kinds of data would you collect? What kind of equipment would you use? 4
Testing the hypothesis The Glomar Challenger drilling ship Deep-sea drilling program: 1968-1983 Ocean drilling program: 1983-2004 Integrated ocean drilling program: 2004 - present (> 36,000 cores from > 600 locations collected through ODP) Current drilling vessels ODP Coring sites JOIDES Resolution Texas A&M and Columbia Universities Chikyu JAMSTEC Current drilling site of IODP ship JOIDES Resolution Current location of Chikyu 5
What kind of data did the ocean drilling program collect? Magnetic Reversals on Earth If crust were created at spreading centers, the age of the crust would increase as you move away from the spreading center. How do you date the seafloor crust? Normal polarity (today) Reverse polarity Magnetic Reversals and Age Magnetic reversals in the Pacific 6
Ages of Sediments Data from Deep-sea Drilling Project Major findings of Deep-sea drilling project: Sediment depth increases away from ocean ridges Sediment age and age of the crust increases away from ridges -magnetic and radiometric dating The big picture Also identified changes in climate, ocean chemistry and marine plankton assemblages over 100 m.y. time scales. Ridge Crest Sediment Oceanic crust 7
Putting the pieces together What forces cause continents to move? How can plate tectonics explain our observations about the distributions of mountains, trenches, and earthquakes? Driving force: Gravity - Leading edges of plates pulled deep into the mantle at trenches (slab pull) - Trailing edge of plates slide off the ridges (ridge push) - Convection cells inside the earth create divergent boundaries, rifts Types of plate boundaries: How are mountains formed? 1: Divergent plate boundary: spreading center where crustal formation occurs 2: Convergent plate boundary: subduction zone where crust is destroyed 3: Transform plate boundary: transform faults where plates move past each other (intense seismic activity) Figure from USGS 8
Important examples of important plate boundaries Transform boundary San Andreas fault zone Convergent plate boundary: continental-continental crust Himalayan mountains Cascadia subduction zone Hydrothermal vents Island and seamount formation 9
Hot spots and island chains Do hotspots stand still? Tarduno et al. Science, August 22, 2003 Where does the hot stuff in hotspots come from? Mantle-core boundary Hand, 2015, Science French and Romanowicz, 2015, Nature 10
Patterns and observations 11