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Solid-earth geology: Plate tectonics, earthquakes, and volcanoes • Explain shape and structure of the ocean basins • Explain cycle of oceanic opening and closing • Explain many aspects of basic marine geology • Understand occurrence of dramatic, sometimes destructive events like quakes and volcanoes Earth’s layering: By composition Key topics today: • • • • Structure of the earth Plate tectonics: Evidence and basic theory Relate to marine geology and ocean basins Rates of plate motions How do we know about the Earth’s interior structure? • Drilling? • Seismic waves (earthquakes) • Other observations 1 Layering of the outer part of the earth: by physical properties Lithosphere and asthenosphere • Lithosphere: the rigid upper part (crust and upper mantle) • Asthenosphere: the squishy part just below lithosphere • (below these are lower mantle and core) Moving on… • Structure of the earth • Plate tectonics: – Evidence for continental drift and seafloor spreading – The basic theory – Relate to marine geologic patterns and phenomena (e.g. earthquakes, volcanoes) – Plate motion Continental drift • Proposed in 1912 by Alfred Wegener • Suggested that continents had oringially formed a single land mass, Pangaea, and had split apart and “drifted” to current positions • Based on simple geological and geographical observations (3 examples) • Lacked a believable mechanism for explaining how this could happen • He died in 1936, with few believers left 2 Evidence 1: fit of continents: Evidence 2: Distribution of fossils Evidence 3: Geological formations From continental drift to seafloor spreading • Mountains of similar age and composition on either side of the Atlantic • New technologies in the 1950s-1960’s allow better ocean measurements: – Depth – Heat flow from the bottom – Magnetic orientation of rocks – Ages of rocks • New picture of marine geology… Aha! 3 Mid-Ocean ridges Ocean evidence for seafloor spreading • Noted initially by Harry Hess • Bathymetry: Ridges look like seams between continents • Ridges are hot • Ridges are young • Ridges have magnetic reversals preserved in their minerals Heezen and Tharp Magnetic anomalies Lava magnetization • Ocean crust, like most rocks, has magnetic properties that depend on alignment of magnetic grains – Set when lava cools to align with prevailing magnetic field • Magnetic anomalies are symmetric about the ridge axis Magnetic minerals in lava respond to prevailing magnetic field. Orientation indicates both field direction and latitude 4 Magnetic anomalies preserved in seafloor Magnetic stratigraphy • Reversals have common pattern at all ridges • Can date lavas to determine ages (K/Ar) • Once known at one site, can extrapolate to other sites • Basis for magnetostratgraphy Seafloor spreading gives new life to the idea of continental drift • Oceans play an active role! • Continents carried passively as part of rigid plates • Ridges as “spreading centers” where new crust is formed • Deep trenches: regions where crust is destroyed Theory of plate tectonics emerges from this synthesis (CD + SFS) 5 Plate tectonic theory Rigid lithospheric plates make up the Earth’s surface • Earth surface consists of rigid lithospheric plates • They move as a result of heating and flow in the softer asthenosphere, below • Their interactions at plate boundaries result in earthquakes and volcanoes • Plate tectonics remakes the surface of the Earth! – Continents expanded and split apart – Ocean basins created and swallowed • It explains diverse geological observations • It is a theory that is constantly being tested by new observations Predictions of plate tectonics Age of oldest sediments overlying basalt ocean crust • Ocean sediments should be younger and thinner near ridges • Crust should cool and shrink as it moves away from ridges, deepening ocean • Specific ideas about where certain kinds of EQ and volcanoes should be found Red = young (<20 million yr); blue is oldest (>130 million yr) 6 Predictions of plate tectonics Thermal subsidence • Ocean sediments should be younger and thinner near ridges • Crust should cool and shrink as it moves away from ridges, deepening ocean • Specific ideas about where certain kinds of EQ and volcanoes should be found • Ridges stand higher than surrounding basins • (What makes a ridge a ridge!) Predictions of plate tectonics • Ocean sediments should be younger and thinner near ridges • Crust should cool and shrink as it moves away from ridges, deepening ocean • Specific ideas about where certain kinds of EQ and volcanoes should be found Where do we find earthquakes? Shallow Medium Deep What about DEEP earthquakes? 7 Where are the worlds active volcanoes? Plate boundaries - where the action is • Convergent: 3 types – (type of crust determines how plates interact) – Ocean-continent – Ocean-ocean – Continent-continent • Divergent • Transform Convergent 1: ocean-continent Cotopaxi Ecuador Ocean-continent convergence and explosive volcanism W. coast of S. America • Ocean crust subducted beneath continental (why?) • Deep earthquakes occur along the sinking slab of ocean crust • Trench forms along subduction zone, offshore • Explosive volcanoes form along continent’s edge 8 Convergent 2: Ocean-ocean Convergent 3: Continent-continent Tibetan Plateau Aleutian trench Marianas trench • Ocean crust is subducted beneath ocean crust • Trench forms along subduction zone • Island arc of volcanoes forms over subducting slab; these are not as explosive as continental volcanoes Annapurna north face (Nepal Himalaya) continent-continent convergence • Continental crust buckles and deforms, but subduction is difficult since continental crust is thick and less dense • Uplift produces high mountains (e.g. Himalaya) • No volcanism Divergent: mid-ocean ridge Ridge Lith. Asth. Plume •New crust forms at center: hot, expanded, young •Shallow earthquakes •“Mid-ocean ridge volcanism” is not explosive 9 Earthquakes at divergent boundaries Example divergent boundary: mid-Atlantic rift Formation of Atlantic from Pangaea Transform fault Divergent: can also be on a continent •Continental crust is breaking apart and stretching •Shallow quakes and volcanism that can be explosive •New crust will form that has the composition of ocean crust (basalt) •Eventually, if motion continues, ocean will fill in the rift Kilimanjaro - highest mountain in Africa Formed by divergent plate motion and hotspots - E. African rift 10 Red Sea = divergent junction Transform junction • Plates sliding past each other • Shallow quakes but no volcanism • Crust is neither created nor destroyed • Example: San Andreas fault Plate mechanics USGS map of global bathymetry/topography • Summary of key features • What drives plate motion – Ridge push and slab pull 11