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Transcript
Learning objective Announcements • • • • Textbooks? ESCI 322 – Meet in Env. Studies Bldg Rm 60, 1 PM Marine geology problem set Today, finish plate tectonics, cover ocean margins and bathymetry, start sediments? • Thursday, finish sediments, quiz on Booth 1994 and discussion on Puget Sound formation. (Lots of odd terms in Booth paper – look them up as you read.) What constitutes the Theory of plate tectonics? • • • • 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 Testing the hypothesis The Glomar Challenger drilling ship 1 Current drilling vessels JOIDES Resolution Texas A&M and Columbia Universities 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) Chikyu JAMSTEC 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 2 Magnetic reversals in the Pacific Magnetic Reversals and Age Ages of Sediments Data from Deep-sea Drilling Project 3 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 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 4 Types of plate boundaries Types of plate boundaries: Transform faults Divergent Convergent 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 Important examples of important plate boundaries Blanco Fracture Zone Transform boundary San Andreas fault zone Convergent plate boundary: continental-continental crust Himalayan mountains All three types of plate boundaries off WA coast • Juan de Fuca ridge • Blanco Fracture zone • Cascadia subduction zone 5 “Hot spots” and island chains Island and seamount formation Do hotspots stand still? Tarduno et al. Science, August 22, 2003 Where does the hot stuff in hotspots come from? Hand, 2015, Science 6 Patterns and observations Mantle-core boundary French and Romanowicz, 2015, Nature Tale of two coastlines … Ocean margins and bathymetry Ocean margins and bathymetry Take-home messages - Know the differences between active and passive margins - Know the structures and provinces of the ocean floor from the coast to the deep sea - Understand the origin and significance of turbidite deposits - Understand the processes that create hydrothermal vents Ch. 3, The sea floor and its sediments 7 http://pubs.usgs.gov/publications/text/historical.html Appalacian Mountains: Sedimentary rock, deformed from collision between N. Am And Europe, ~480 mya Supercontinent forms ~400 mya, more mountain building Coastal rock formations similar to those in Europe and Africa. Ripped off Pangaea when Atlantic Ocean formed ~150 mya. Primarily rocky coastline Primarily sandy beaches 8 1.6 m 1.6 m Geometry of passive continental margins Primarily rocky coastline Primarily sandy beaches 9 Atlantic continental slope http://ccom.unh.edu/images/lots/atlantic/Atlantic_North_bathy.jpg Maryland Seafloor at 740 m, Block Canyon How do these canyons form? New Jersey 10 Breaking of transatlantic cables Nov 18, 1929 Nova Scotia Grand Banks Earthquake Epicenter Sediment sources for continental rise: 1: Continental erosion and transport across continental shelf 2: Slow accumulation of biogenic particles from open ocean Ancient turbidites along coast of Maine Recent turbidite from the base of Zhemchug Canyon, the largest submarine canyon in the world (Bering Sea, 3600 m water depth) Abyssal Plains Turbidite Deposits Why are these deposits restricted to passive margins? 11 Active versus passive continental margins Cascadia Subduction Zone Image from USGS Hydrothermal vents: E.g., Axial volcano of the Juan de Fuca ridge from: http://www.rsmas.miami.edu/personal/schuller/images/map_juan_de_fuca_subduction2.gif http://www.pmel.noaa.gov/vents/ 12 Hydrothermal vents “Black smoker” and tube worms at Juan de Fuca ridge hydrothermal vent system http://www.pmel.noaa.gov/vents/ HDTV photo of Juan de Fuca Ridge sediment Photo: http://www.visions05.washington.edu/ 11:00 AM http://novae.ocean.washington.edu/story/Ashes_CAMHD_Live 13 Passive margin Active margin Image from NOAA Image from NOAA Summary of margins and bathymetry I. Continental margins consist of shelf, slope, and rise II. Coastlines are shaped by tectonic history, sea level rise and (in northern latitudes) glaciations III. Submarine canyons convey coastal sediments to the deep sea forming the continental rise along passive continental margins IV. Active continental margins typically have a narrow shelf and slope and no rise due to subduction V. Geochemical processes at hydrothermal vents affect elemental cycles and support chemosynthetic communities of organisms 14